Interferon-β regulates proresolving lipids to promote the resolution of acute airway inflammation

Aberrant immune responses, including hyperresponsiveness to Toll-like receptor (TLR) ligands, underlie acute respiratory distress syndrome (ARDS). Type I interferons confer antiviral activities and could also regulate the inflammatory response, whereas little is known about their actions to resolve aberrant inflammation. Here we report that interferon-β (IFN-β) exerts partially overlapping, but also cooperative actions with aspirin-triggered 15-epi-lipoxin A₄ (15-epi-LXA₄) and 17-epi-resolvin D1 to counter TLR9-generated cues to regulate neutrophil apoptosis and phagocytosis in human neutrophils. In mice, TLR9 activation impairs bacterial clearance, prolongs Escherichia coli–evoked lung injury, and suppresses production of IFN-β and the proresolving lipid mediators 15-epi-LXA₄ and resolvin D1 (RvD1) in the lung. Neutralization of endogenous IFN-β delays pulmonary clearance of E. coli and aggravates mucosal injury. Conversely, treatment of mice with IFN-β accelerates clearance of bacteria, restores neutrophil phagocytosis, promotes neutrophil apoptosis and efferocytosis, and accelerates resolution of airway inflammation with concomitant increases in 15-epi-LXA₄ and RvD1 production in the lungs. Pharmacological blockade of the lipoxin receptor ALX/FPR2 partially prevents IFN-β–mediated resolution. These findings point to a pivotal role of IFN-β in orchestrating timely resolution of neutrophil and TLR9 activation–driven airway inflammation and uncover an IFN-β–initiated resolution program, activation of an ALX/FPR2-centered, proresolving lipids-mediated circuit, for ARDS.

Acute respiratory distress syndrome (ARDS) is a common syndrome associated with high mortality in patients admitted to intensive care units (1). ARDS is characterized by diffuse alveolar damage that develops in patients with known risk factors, most commonly pneumonia, sepsis, or trauma (2, 3). The initial alveolar damage leads to recruitment of neutrophils and monocytes, which further aggravate injury (3). Treatment of the underlying cause and lung-protective ventilation are the main elements of supportive therapy (4, 5). Importantly, no therapies are available to resolve the aberrant immune responses underlying ARDS.Type I interferons, IFN-α and IFN-β, are well established to confer antiviral activities to host cells and could also regulate the inflammatory response. A delayed type I interferon response triggers the generation of proinflammatory cytokines and facilitates the recruitment of monocytes to the lung, resulting in lethal pneumonia in mice infected with SARS-CoV-1 (6) or SARS-CoV-2 (7). Type I interferons break TNF-induced tolerance to Toll-like receptor (TLR) signals on monocytes/macrophages, rendering them hyperresponsive to additional TLR signals concurrent with inflammatory activation (8). For instance, bacterial DNA (CpG DNA) or mitochondrial DNA through TLR9 impairs neutrophil phagocytosis, delays neutrophil apoptosis, and perpetuates inflammation (9, 10). In contrast, IFN-β protects against lethal polymicrobial sepsis through inhibiting IL-1 production and/or induction of IL-10 (11–13). IFN-β produced by macrophages during resolution of bacterial pneumonia facilitates removal of neutrophils from inflamed tissues and reprograms macrophages to a proresolving phenotype, thereby driving inflammatory resolution in mice (14). However, the underlying mechanisms are incompletely understood; albeit these would be essential for implementing precision treatment with IFN-β.Resolution of inflammation is an active process governed by specialized proresolving lipid and protein mediators (SPMs) (15–19). These mediators converge on select receptors, including the pleiotropic lipoxin A₄ receptor/formyl peptide receptor 2 (ALX/FPR2) (20). ALX/FPR2 plays critical roles in host defense and orchestrating inflammatory resolution (20–23). ALX/FPR2 binds multiple lipid ligands, including aspirin-triggered 15-epi-lipoxin A₄ (15-epi-LXA₄) and 17-epi-resolvin D1 (17-epi-RvD1), generated within the inflammatory microenvironment (17, 18). SPMs inhibit neutrophil recruitment, promote neutrophil apoptosis and efferocytosis, and facilitate tissue repair and return to homeostasis (17, 18, 24). Activation of ALX/FPR2 with 15-epi-LXA₄ or 17-epi-RvD1 counters TLR9-generated cues, restores impaired neutrophil function, and enhances timely resolution of airway bacterial infections (9). Since resolution of inflammation is skewed toward a proresolving lipid profile (18, 25, 26), we investigated whether IFN-β can modulate ALX/FPR2-based resolution mechanisms. Here, we report that IFN-β exerts partially overlapping, but also cooperative actions with 17-epi-RvD1 to counter TLR9-generated signals to regulate neutrophil phagocytosis and apoptosis in vitro. In mice, IFN-β facilitates clearance of bacteria, neutrophil apoptosis and efferocytosis, and promotes the resolution of acute airway inflammation, in part, by stimulating generation of proresolving lipids and activation of ALX/FPR2-centered proresolving circuits. Our results uncover a hitherto unrecognized effector mechanism by which IFN-β may facilitate resolution of ARDS.     

IL-25 and type 2 innate lymphoid cells induce pulmonary fibrosis

Disease conditions associated with pulmonary fibrosis are progressive and have a poor long-term prognosis with irreversible changes in airway architecture leading to marked morbidity and mortalities. Using murine models we demonstrate a role for interleukin (IL)-25 in the generation of pulmonary fibrosis. Mechanistically, we identify IL-13 release from type 2 innate lymphoid cells (ILC2) as sufficient to drive collagen deposition in the lungs of challenged mice and suggest this as a potential mechanism through which IL-25 is acting. Additionally, we demonstrate that in human idiopathic pulmonary fibrosis there is increased pulmonary expression of IL-25 and also observe a population ILC2 in the lungs of idiopathic pulmonary fibrosis patients. Collectively, we present an innate mechanism for the generation of pulmonary fibrosis, via IL-25 and ILC2, that occurs independently of T-cell–mediated antigen-specific immune responses. These results suggest the potential of therapeutically targeting IL-25 and ILC2 for the treatment of human fibrotic diseases.Disease conditions associated with pulmonary fibrosis are often progressive and have a poor long-term prognosis (1). In the context of developing new treatments for pulmonary fibrosis, the cytokines associated with the pathogenic milieu that can lead to aberrant extracellular matrix deposition are key targets, in particular interleukin (IL)-13, TGF-β, and, more recently, IL-17A (2). However, to develop more effective therapeutics for fibrotic lung diseases a greater understanding of the pathogenesis and the underlying mechanisms that lead to pulmonary fibrosis is needed (3, 4).The cytokine IL-13 was first implicated in fibrosis using profibrotic eggs from the type 2 cytokine-inducing pathogen Schistosoma mansoni, in the presence of a soluble IL-13Rα2-Fc fusion protein (5) and in Il13^−/− mice (6). IL-13 is now widely linked to a range of fibrotic conditions (7) including asthma, where IL-13 is being targeted as a therapy (8). In the context of the cellular source of IL-13 in the generation of fibrosis, CD4⁺ T helper (h) 2 cells are implicated (9). However, more recently innate lymphoid cells (ILC) are emerging as an important source of IL-13 (10, 11). In this context, the type 2 cytokine IL-25 is implicated in the generation of the recently identified IL-13–expressing ILC, termed ILC2 (11–14).Recent studies have implicated IL-25 and ILC2 in the pathogenesis of pulmonary conditions in both murine models and human conditions such as allergic asthma (12, 13, 15, 16). In murine studies intranasal administration of IL-25 results in evidence of pulmonary tissue remodeling including development of perivascular fibrosis, and intratracheal administration results in increased pulmonary Th2 cytokines and airways hyper-reactivity (AHR) (17, 18), whereas blocking IL-25 reduces AHR severity (19). Herein we describe a potential role for IL-25 in the generation of pulmonary fibrosis in experimental mouse models, via the activation of IL-13–producing ILC2. We also observe increases in both IL-25 and ILC2 in the lung of patients with idiopathic pulmonary fibrosis (IPF). These data suggest unique mechanisms for the generation of pulmonary fibrosis and identify an interesting area for further research on the role of IL-25 and ILC2 in the treatment of pulmonary fibrosis.     

SARS-CoV-2 spreads through cell-to-cell transmission

Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) is a highly transmissible coronavirus responsible for the global COVID-19 pandemic. Herein, we provide evidence that SARS-CoV-2 spreads through cell–cell contact in cultures, mediated by the spike glycoprotein. SARS-CoV-2 spike is more efficient in facilitating cell-to-cell transmission than is SARS-CoV spike, which reflects, in part, their differential cell–cell fusion activity. Interestingly, treatment of cocultured cells with endosomal entry inhibitors impairs cell-to-cell transmission, implicating endosomal membrane fusion as an underlying mechanism. Compared with cell-free infection, cell-to-cell transmission of SARS-CoV-2 is refractory to inhibition by neutralizing antibody or convalescent sera of COVID-19 patients. While angiotensin-converting enzyme 2 enhances cell-to-cell transmission, we find that it is not absolutely required. Notably, despite differences in cell-free infectivity, the authentic variants of concern (VOCs) B.1.1.7 (alpha) and B.1.351 (beta) have similar cell-to-cell transmission capability. Moreover, B.1.351 is more resistant to neutralization by vaccinee sera in cell-free infection, whereas B.1.1.7 is more resistant to inhibition by vaccinee sera in cell-to-cell transmission. Overall, our study reveals critical features of SARS-CoV-2 spike-mediated cell-to-cell transmission, with important implications for a better understanding of SARS-CoV-2 spread and pathogenesis.

SARS-CoV-2 is a novel beta-coronavirus that is closely related to two other highly pathogenic human coronaviruses, SARS-CoV and MERS-CoV (1). The spike (S) proteins of SARS-CoV-2 and SARS-CoV mediate entry into target cells, and both use angiotensin-converting enzyme 2 (ACE2) as the primary receptor (2–6). The spike protein of SARS-CoV-2 is also responsible for induction of neutralizing antibodies, thus playing a critical role in host immunity to viral infection (7–10).Similar to HIV and other class I viral fusion proteins, SARS-CoV-2 spike is synthesized as a precursor that is subsequently cleaved and highly glycosylated; these properties are critical for regulating viral fusion activation, native spike structure, and evasion of host immunity (11–15). However, distinct from SARS-CoV, yet similar to MERS-CoV, the spike protein of SARS-CoV-2 is cleaved by furin into S1 and S2 subunits during the maturation process in producer cells (6, 16, 17). S1 is responsible for binding to the ACE2 receptor, whereas S2 mediates viral membrane fusion (18, 19). SARS-CoV-2 spike can also be cleaved by additional host proteases, including transmembrane serine protease 2 (TMPRSS2) on the plasma membrane and several cathepsins in the endosome, which facilitate viral membrane fusion and entry into host cells (20–22).Enveloped viruses spread in cultured cells and tissues via two routes: by cell-free particles and through cell–cell contact (23–26). The latter mode of viral transmission normally involves tight cell–cell contacts, sometimes forming virological synapses, where local viral particle density increases (27), resulting in efficient transfer of virus to neighboring cells (24). Additionally, cell-to-cell transmission has the ability to evade antibody neutralization, accounting for efficient virus spread and pathogenesis, as has been shown for HIV and hepatitis C virus (HCV) (28–32). Low levels of neutralizing antibodies, as well as a deficiency in type I IFNs, have been reported for SARS-CoV-2 (18, 33–37) and may have contributed to the COVID-19 pandemic and disease progression (38–43).In this work, we evaluated cell-to-cell transmission of SARS-CoV-2 in the context of cell-free infection and in comparison with SARS-CoV. Results from this in vitro study reveal the heretofore unrecognized role of cell-to-cell transmission that potentially impacts SARS-CoV-2 spread, pathogenesis, and shielding from antibodies in vivo.     

A recombinant herpes virus expressing influenza hemagglutinin confers protection and induces antibody-dependent cellular cytotoxicity

Despite widespread yearly vaccination, influenza leads to significant morbidity and mortality across the globe. To make a more broadly protective influenza vaccine, it may be necessary to elicit antibodies that can activate effector functions in immune cells, such as antibody-dependent cellular cytotoxicity (ADCC). There is growing evidence supporting the necessity for ADCC in protection against influenza and herpes simplex virus (HSV), among other infectious diseases. An HSV-2 strain lacking the essential glycoprotein D (gD), was used to create ΔgD-2, which is a highly protective vaccine against lethal HSV-1 and HSV-2 infection in mice. It also elicits high levels of IgG2c antibodies that bind FcγRIV, a receptor that activates ADCC. To make an ADCC-eliciting influenza vaccine, we cloned the hemagglutinin (HA) gene from an H1N1 influenza A strain into the ΔgD-2 HSV vector. Vaccination with ΔgD-2::HA_PR8 was protective against homologous influenza challenge and elicited an antibody response against HA that inhibits hemagglutination (HAI⁺), is predominantly IgG2c, strongly activates FcγRIV, and protects against influenza challenge following passive immunization of naïve mice. Prior exposure of mice to HSV-1, HSV-2, or a replication-defective HSV-2 vaccine (dl5-29) does not reduce protection against influenza by ΔgD-2::HA_PR8. This vaccine also continues to elicit protection against both HSV-1 and HSV-2, including high levels of IgG2c antibodies against HSV-2. Mice lacking the interferon-α/β receptor and mice lacking the interferon-γ receptor were also protected against influenza challenge by ΔgD-2::HA_PR8. Our results suggest that ΔgD-2 can be used as a vaccine vector against other pathogens, while also eliciting protective anti-HSV immunity.

Influenza remains a global health threat. Seasonal strains of influenza A and B cause an estimated 5 million cases of severe infections and 500,000 deaths per year (1). Influenza pandemics have caused even greater morbidity and mortality. During the H1N1 pandemic of 1918 to 1919, 500 million people, approximately one-third of the world’s population at that time, were estimated to have been infected with this strain, leading to 50 million deaths (2). The H1N1 pandemic of 2009 is estimated to have caused up to 575,000 deaths (2). Currently, three types of influenza vaccines are offered annually in the United States: a recombinant virus expressing influenza proteins, chemically inactivated virus, and live attenuated virus (3). Regardless of the vaccine type, multiple strains are included to increase the chances of developing sufficient protection against major circulating influenza strains. However, these vaccines primarily elicit a neutralizing antibody response that is sensitive to changes in the influenza virus due to antigenic drift and shift (4). Antigenic drift results from an accumulation of random mutations in influenza antigens, like hemagglutinin (HA), altering sites recognized by the immune system (4). Influenza A strains can also undergo antigenic shift, whereby two different influenza strains infect the same cell to form a reassortant virus with new antigenic properties (4). Due to limited immunity in the population, these new strains are highly virulent, causing widespread epidemics and disease (4). With antigenic drift and shift, vaccine-mediated protection against circulating strains has been insufficient (5). Influenza vaccines that elicit more robust and long-term protection are therefore needed. Notably, if an influenza vaccine with ≥75% efficacy were to be broadly used in the United States, an estimated 19,500 deaths a year could be prevented and direct healthcare costs reduced by $3.5 billion (6).For many years, efforts to improve influenza vaccines have focused on eliciting an immune response for full, broad protection against both circulating and future strains of the virus. These studies have shown that, in general, neutralizing antibodies are sufficient for homologous protection (7). However, achieving heterologous protection may require more broadly neutralizing antibodies or nonneutralizing antibodies able to activate effector immune cells (5). Previous studies have found that passively transferred nonneutralizing monoclonal antibodies can be potently protective in a mouse influenza challenge model (8–10). Several novel strategies have attempted to generate a nonneutralizing response against influenza. For example, vaccines have been created to specifically target the conserved stem region of HA (11–13).Nonneutralizing antibodies stimulate effector cell mechanisms, including antibody-mediated phagocytosis and antibody-dependent cellular cytotoxicity (ADCC), both of which require activation of the Fcγ receptors (FcγRs) (14). Specific isotypes of IgG antibodies are associated with FcγR modulation and subsequent ADCC activation, including the IgG1 and IgG3 subtypes in humans, as well as IgG2a and IgG2c subtypes in mice (15–19). IgG2a and IgG2c isotypes are functionally equivalent and mouse strain-dependent, with IgG2c present in C57BL/6J mice (20). Recent studies have demonstrated that natural infection by influenza and vaccination elicit nonneutralizing antibodies with effector functions that contribute to protection (5, 9, 21–27). In mouse and nonhuman primate challenge models, ADCC-mediating antibodies have demonstrated protection against both homologous and heterologous influenza challenge (9, 28).Recently, we developed a single-cycle herpes simplex virus (HSV) vaccine that completely protects against vaginal, skin, and ocular challenges by HSV-1 and HSV-2 (29, 30). Protection elicited by this vaccine, designated ΔgD-2 for its lack of the essential glycoprotein D (gD) gene, is transferable via passive infusion of immune sera to naïve wild-type mice but not to mice lacking the Fcγ common chain (30). The immune response elicited by ΔgD-2 primarily elicits nonneutralizing antibodies with high levels of FcγRIV-activating function.We asked whether ΔgD-2 could be used as a vaccine platform to induce broadly protective FcγRIV-activating antibodies against a heterologous antigen, such as influenza HA. In this study, we demonstrate that our recombinant vaccine, ΔgD-2::HA_PR8, elicits protection against influenza with a high proportion of FcγRIV-activating antibodies. Additionally, anticipating the use of ΔgD-2 as a vaccine vector against other pathogens, we tested whether our construct would still be protective in mice lacking interferon (IFN) function. Many humans have inborn errors in their IFN signaling pathways, leading to more lethal outcomes in infection (31). Patients with such deficiencies are disproportionately represented among HSV encephalitis cases and are often diagnosed only after presenting with serious symptoms (32–38). This at-risk population underscores the importance of eliciting protection against HSV in the absence of a functional IFN-α/β response. Additionally, many pathogens, such as dengue virus, require mouse models lacking IFN function, and for ease of testing, an efficacious vaccine should remain functional in these mice (39–41). In this study, we demonstrate that ΔgD-2 is a versatile, immunogenic vaccine vector that provides a strong FcγRIV-activating immune response against heterologous pathogens, while maintaining its protective benefit against HSV, in both wild-type and IFN-deficient mice.     

Heightened self-reactivity associated with selective survival,but not expansion,of na?ve virus-specific CD8+ T cells in aged mice

In advanced age, decreased CD8⁺ cytotoxic T-lymphocyte (CTL) responses to novel pathogens and cancer is paralleled by a decline in the number and function of naïve CTL precursors (CTLp). Although the age-related fall in CD8⁺ T-cell numbers is well established, neither the underlying mechanisms nor the extent of variation for different epitope specificities have been defined. Furthermore, naïve CD8⁺ T cells expressing high levels of CD44 accumulate with age, but it is unknown whether this accumulation reflects their preferential survival or an age-dependent driver of CD8⁺ T-cell proliferation. Here, we track the number and phenotype of four influenza A virus (IAV)-specific CTLp populations in naïve C57BL/6 (B6) mice during aging, and compare T-cell receptor (TCR) clonal diversity for the CD44hi and CD44lo subsets of one such population. We show differential onset of decline for several IAV-specific CD8⁺ T-cell populations with advanced age that parallel age-associated changes in the B6 immunodominance hierarchy, suggestive of distinct impacts of aging on different epitope-specific populations. Despite finding no evidence of clonal expansions in an aged, epitope-specific TCR repertoire, nonrandom alterations in TCR usage were observed, along with elevated CD5 and CD8 coreceptor expression. Collectively, these data demonstrate that naïve CD8⁺ T cells expressing markers of heightened self-recognition are selectively retained, but not clonally expanded, during aging.Given that CD8⁺ cytotoxic T-lymphocyte (CTL) immunity is critical for the control of viruses and tumors, specific defects are likely to contribute substantially to overall immune dysfunction. Normal aging is associated with increased health risk, as vaccine efficacy wanes and susceptibility to, and severity of, a variety of infections and malignancies is enhanced (1, 2). Whereas aging compromises a number of arms of the immune system (3), studies in mice and humans have demonstrated deficits that are intrinsic to naïve CTL populations (4–6). Thus, a complete understanding of both age-related CTL deficiencies and the underlying mechanisms is critical.The magnitude of the response, the diversity of T-cell receptor (TCR)-defined clonal recruitment, and the avidity of TCR binding to cognate peptides in complex with MHC class I glycoproteins (pMHCI) are all key determinants of CTL response efficacy (7). Each of these factors is substantially constrained by their respective characteristics in the naive CTL precursor (CTLp) pool (7–10). During aging, both the number and TCR diversity of naïve polyclonal and epitope-specific CD8⁺ T-cell sets decreases (11–13). In addition, a large proportion (∼60–80%) of the remaining naïve CD8⁺ T cells, termed virtual memory (T_VM) cells, express high levels of CD44, traditionally regarded as a marker of T-cell activation and suggestive of proliferation (14). It is unclear whether the accumulation of T_VM cells with age (15) represents preferential retention, de novo generation, or expansion of the T_VM subset. TCR repertoire analyses show emerging TCR bias with age (11, 13, 16), limiting the diversity of the TCR repertoire beyond that defined by reduced CTLp numbers. Although it is clear that TCR biases parallel age-related T-cell loss, the relative impact of selective clonal decay versus clonal expansion remains unresolved, as do the key determinants of naïve CTLp survival.The frequency and TCR usage of naïve and immune CD8⁺ T cells specific for a range of influenza A virus (IAV) epitopes is well characterized for B6 mice (17–20), providing a convenient experimental system for investigating the characteristics and drivers of age-related CD8⁺ T-cell decline. Primary responses to the D^bNP_366–374 and D^bPA_224–233 epitopes (derived from nucleoprotein and acid polymerase proteins, respectively) are immunodominant in young adult mice, whereas those to a polymerase B subunit 1 epitope (K^bPB1_703–711) and an epitope derived from a shifted reading frame of PB1 (D^bPB1-F2_62–70) are subdominant (21). This numerical immunodominance hierarchy shifts with aging, with a decrease in D^bNP₃₆₆-specific responses alongside an increase in K^bPB1₇₀₃- and D^bPB1-F2₆₂–specific responses (13, 22). It has been suggested that this reflects the stochastic decay of CTLp (13), but the rate of decline of individual epitope-specific populations has not been directly assessed during aging.Here, we directly track the numbers of immune and naive IAV epitope-specific CD8⁺ T-cell populations through the course of aging, together with the phenotypic and clonotypic characteristics of a naïve CTLp set. We show that the shift in immunodominance across D^bNP₃₆₆, D^bPA₂₂₄, K^bPB1₇₀₃, and D^bPB1-F2₆₂ for 12-mo-old versus 3-mo mice can be accounted for by differential onset of decline across naïve CTLp populations. Moreover, phenotypic changes in a naïve CTLp set indicates that age-related clonal persistence is associated with the capacity to recognize self-pMHCI, which may, in turn, alter the capacity of CTLps to responding to novel challenges.     

SARS-CoV-2 spike engagement of ACE2 primes S2′ site cleavage and fusion initiation

The COVID-19 pandemic caused by severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infection has resulted in tremendous loss worldwide. Although viral spike (S) protein binding of angiotensin-converting enzyme 2 (ACE2) has been established, the functional consequences of the initial receptor binding and the stepwise fusion process are not clear. By utilizing a cell–cell fusion system, in complement with a pseudoviral infection model, we found that the spike engagement of ACE2 primed the generation of S2′ fragments in target cells, a key proteolytic event coupled with spike-mediated membrane fusion. Mutagenesis of an S2′ cleavage site at the arginine (R) 815, but not an S2 cleavage site at arginine 685, was sufficient to prevent subsequent syncytia formation and infection in a variety of cell lines and primary cells isolated from human ACE2 knock-in mice. The requirement for S2′ cleavage at the R815 site was also broadly shared by other SARS-CoV-2 spike variants, such as the Alpha, Beta, and Delta variants of concern. Thus, our study highlights an essential role for host receptor engagement and the key residue of spike for proteolytic activation, and uncovers a targetable mechanism for host cell infection by SARS-CoV-2.

The COVID-19 pandemic caused by severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infection has exceeded 240 million cases across the globe, but the molecular mechanisms of viral infection and host pathogenesis remain elusive. The SARS-CoV-2 spike (S) glycoprotein is a class I fusion protein decorated on the viral lipid envelope and is a key determinant of viral entry (1). The SARS-CoV-2 spike monomer contains two fragments: The amino terminus S1 subunit contains a receptor binding domain (RBD) (2–5), which recognizes the host receptor angiotensin-converting enzyme 2 (ACE2) for initial docking, while the carboxyl terminus S2 subunit catalyzes the fusion of viral and cell membranes (6, 7), enabling the subsequent release of viral RNA genome and downstream replication within the infected cells (8). Although many studies have captured the stationary phases of spike binding to human ACE2 (9–11), key molecular and cellular processes downstream of receptor recognition have not been explored.Spike can be proteolytically processed (12). SARS-CoV-2 spike encodes a polybasic cleavage site at its S1/S2 junction, and is posttranslationally processed by the endopeptidase furin (13, 14); cleaved S1 and S2 subunits remain noncovalently attached and fusion competent (15). Furin-cleaved S1 also exposes a C-terminal motif recognized by the host receptor neuropilin-1 (NRP1) (16, 17), which can facilitate SARS-CoV-2 entry. Although spike protein is autoprocessed, additional proteolytic cleavage event within the S2 subunit is proposed to be responsible for the subsequent membrane fusion (18, 19). This cleavage can be mediated at the plasma membrane by the type II transmembrane serine proteases (TMPRSS2) (20–23), or processed by the lysosomal cathepsins during the endocytosis of viral particles (24). Secreted tissue proteases, such as elastase and trypsin, can also facilitate this cleavage event and promote infection (25). As a result, this proteolytic event within the S2 subunit induces the release of a highly conserved hydrophobic region, known as the fusion peptide (18), which subsequently anchors the target host cell membrane (6, 26). A conformational reconfiguration within the S2 subunit then pulls the viral and host membranes into close proximity, allowing lipid membranes to fuse (7, 27–29). The unilateral change of the S2 subunit is of the utmost importance during viral entry, but molecular events regulating the spike processing and activation have not been demonstrated.Cells infected with SARS-CoV-2 drive the fusion with adjacent ACE2-expressing cells, producing morphologically distinct multinuclear giant cells, also known as syncytia (2, 30, 31). Spike-mediated syncytia have been reported in the postmortem lung samples of severe COVID-19 patients (32, 33). Apart from virus to cell transmission, spike-driven syncytia formation may provide an additional route for cell–cell transmission of SARS-CoV-2. Here, by using a cell–cell fusion system, in complement with a pseudoviral particle infection model, we study the functional and molecular requirements of spike activation. Through analyzing the prefusion and postfusion spike protein products, we show that proteolytic cleavage at the S2′ site is triggered by human cell receptor recognition in a range of immortalized cell lines and humanized primary cells. Generation of the S2′ fragment specifically requires spike recognition of functional host ACE2 and is conserved in the several variants of concern. We highlight that arginine 815, but not arginine residues at the S1/S2 cleavage site, is indispensable for the S2′ cleavage and syncytia formation in wild type (WT), as well as the more infectious Alpha, Beta, and Delta spike variants. Hence, these data highlight that both receptor recognition and proteolytic event at the S2′ site are functionally important for spike-mediated membrane fusion and SARS-CoV-2 infection.     

Central domain of IL-33 is cleaved by mast cell proteases for potent activation of group-2 innate lymphoid cells

Interleukin-33 (IL-33) is an alarmin cytokine from the IL-1 family. IL-33 activates many immune cell types expressing the interleukin 1 receptor-like 1 (IL1RL1) receptor ST2, including group-2 innate lymphoid cells (ILC2s, natural helper cells, nuocytes), the major producers of IL-5 and IL-13 during type-2 innate immune responses and allergic airway inflammation. IL-33 is likely to play a critical role in asthma because the IL33 and ST2/IL1RL1 genes have been reproducibly identified as major susceptibility loci in large-scale genome-wide association studies. A better understanding of the mechanisms regulating IL-33 activity is thus urgently needed. Here, we investigated the role of mast cells, critical effector cells in allergic disorders, known to interact with ILC2s in vivo. We found that serine proteases secreted by activated mast cells (chymase and tryptase) generate mature forms of IL-33 with potent activity on ILC2s. The major forms produced by mast cell proteases, IL-33_95–270, IL-33_107–270, and IL-33_109–270, were 30-fold more potent than full-length human IL-33_1–270 for activation of ILC2s ex vivo. They induced a strong expansion of ILC2s and eosinophils in vivo, associated with elevated concentrations of IL-5 and IL-13. Murine IL-33 is also cleaved by mast cell tryptase, and a tryptase inhibitor reduced IL-33–dependent allergic airway inflammation in vivo. Our study identifies the central cleavage/activation domain of IL-33 (amino acids 66–111) as an important functional domain of the protein and suggests that interference with IL-33 cleavage and activation by mast cell and other inflammatory proteases could be useful to reduce IL-33–mediated responses in allergic asthma and other inflammatory diseases.Interleukin-33 (IL-33), previously known as nuclear factor from high endothelial venules or NF-HEV (1, 2), is an IL-1 family cytokine (3) that signals through the interleukin 1 receptor-like 1 (IL1RL1) receptor ST2 (4, 5) and induces expression of cytokines and chemokines in various immune cell types, including mast cells, basophils, eosinophils, Th2 lymphocytes, invariant natural killer T, and natural killer cells (3, 4, 6–8). Studies in IL-33–deficient mice indicate that IL-33 plays important roles in type-2 innate immunity and innate-type allergic airway inflammation (9–13). Indeed, IL-33 is a key activator of the recently described group-2 innate lymphoid cells (ILC2s, natural helper cells, nuocytes) (14–17). These cells control eosinophil homeostasis in blood and adipose tissue (18, 19) and produce extremely high amounts of the type-2 cytokines IL-5 and IL-13 in response to IL-33 (14–16). ILC2s also play important roles in allergic airway inflammation (20–24), atopic skin disease (25–28), helminth infection in the intestine (11, 12, 14–16), and influenza virus infection in the lungs (29, 30).Based on animal model studies and analyses of diseased tissues from patients, IL-33 has been proposed as a candidate therapeutic target for several important diseases, including asthma and other allergic diseases, rheumatoid arthritis, inflammatory bowel diseases, and cardiovascular diseases (4, 6). IL-33 is likely to play a critical role in asthma because the IL33 and IL1RL1/ST2 genes have been reproducibly identified as major susceptibility loci in several independent large-scale genome-wide association studies of human asthma (31, 32).Despite these important advances into the roles of IL-33, very little is known yet about the mechanisms regulating its activity. Full-length human IL-33 is a 270 amino acid protein localized in the nucleus of endothelial and epithelial cells in blood vessels and epithelial barrier tissues (1, 2, 33, 34), which associates with chromatin (2) and histones H2A-H2B, through a short chromatin-binding motif located in its N-terminal part (amino acids 40–58) (35). IL-33 can be released in the extracellular space upon cellular damage or necrotic cell death (36, 37), and it was thus proposed to function as an alarmin (alarm signal or endogenous danger signal), which alerts the immune system to tissue injury following trauma or infection (33, 36, 37).Proteases have been shown to regulate IL-33 activity. Full-length IL-33_1–270 is biologically active but processing by caspases after residue Asp₁₇₈ in the IL-1–like cytokine domain results in its inactivation (36, 37). In contrast, inflammatory proteases from neutrophils, cathepsin G, and elastase, process full-length IL-33 into mature forms that contain an intact IL-1–like cytokine domain and that have an increased biological activity compared with full-length IL-33_1–270 (38). Although neutrophils have been implicated in virus-induced exacerbations of asthma, they are unlikely to be involved in the processing of IL-33 during allergic inflammation. We therefore investigated the possibility that other cell types may be involved in this process. Mast cells, which are widely recognized for their roles as effector cells in allergic disorders, were good candidates because they interact directly with ILC2s in vivo (26) and they are strategically positioned close to vessel walls and epithelial surfaces exposed to the environment (39), the major sites of IL-33 expression (33, 34). We now demonstrate that activated human mast cells and purified mast cell proteases, tryptase and chymase, generate mature forms of IL-33 (IL-33_95–270, IL-33_107–270, and IL-33_109–270), which are ∼30-fold more potent than full-length IL-33_1–270 for activation of ILC2s ex vivo. These IL-33 mature forms are also potent inducers of ILC2s, eosinophils, and type-2 cytokines in vivo. Our study suggests that release of the C-terminal IL-1–like cytokine domain, through proteolytic maturation within the central cleavage/activation domain (amino acids 66–111), is important for full biological activity of IL-33.     

Influenza hemagglutinin stem-fragment immunogen elicits broadly neutralizing antibodies and confers heterologous protection

Influenza hemagglutinin (HA) is the primary target of the humoral response during infection/vaccination. Current influenza vaccines typically fail to elicit/boost broadly neutralizing antibodies (bnAbs), thereby limiting their efficacy. Although several bnAbs bind to the conserved stem domain of HA, focusing the immune response to this conserved stem in the presence of the immunodominant, variable head domain of HA is challenging. We report the design of a thermotolerant, disulfide-free, and trimeric HA stem-fragment immunogen which mimics the native, prefusion conformation of HA and binds conformation specific bnAbs with high affinity. The immunogen elicited bnAbs that neutralized highly divergent group 1 (H1 and H5 subtypes) and 2 (H3 subtype) influenza virus strains in vitro. Stem immunogens designed from unmatched, highly drifted influenza strains conferred robust protection against a lethal heterologous A/Puerto Rico/8/34 virus challenge in vivo. Soluble, bacterial expression of such designed immunogens allows for rapid scale-up during pandemic outbreaks.Seasonal influenza outbreaks across the globe cause an estimated 250,000–500,000 deaths annually (1). Current influenza vaccines need to be updated every few years because of antigenic drift (2). Despite intensive monitoring, strain mismatch between vaccine formulation and influenza viruses circulating within the population has occurred in the past (2). Public health is further compromised when an unpredictable mixing event among influenza virus genomes leads to antigenic shift facilitating a potential pandemic outbreak. These concerns have expedited efforts toward developing a universal influenza vaccine.Neutralizing antibodies (nAbs) against hemagglutinin (HA) are the primary correlate for protection in humans and hence HA is an attractive target for vaccine development (3). The precursor polypeptide, HA0, is assembled into a trimer along the secretory pathway and transported to the cell surface. Cleavage of HA0 generates the disulfide-linked HA1 and HA2 subunits. Mature HA has a globular head domain which mediates receptor binding and is primarily composed of the HA1 subunit, whereas the stem domain predominantly comprises the HA2 subunit. The HA stem is trapped in a metastable state and undergoes an extensive low-pH-induced conformational rearrangement in the host-cell endosomes to adopt the virus–host membrane fusion-competent state (4, 5).The antigenic sites on the globular head of HA are subjected to heightened immune pressure resulting in escape variants, thereby limiting the breadth of head-directed nAbs (6). However, extensive efforts have resulted in the isolation of monoclonal antibodies (mAbs) that bind within the globular head and inhibit receptor attachment, which neutralize drifted variants of an HA subtype or heterosubtypic HA (7–16). The HA stem is targeted by several broadly neutralizing antibodies (bnAbs) with neutralizing activity against diverse influenza A virus subtypes (17). The epitopes of these bnAbs in the HA stem are more conserved across different influenza HA subtypes compared with the antigenic sites in the HA globular head (18).During a primary infection, the immunodominant globular head domain suppresses the response toward the conserved stem. Several efforts have been made to circumvent this problem. Repeated immunizations with full-length, chimeric HAs (cHAs) in a protracted vaccination regimen have been shown to boost stem-directed responses in mice (19). Alternatively, full-length HA presented on nanoparticles (np) has been shown to elicit stem-directed nAbs (20). Attempts have also been made to steer the immune response toward the conserved HA stem by hyperglycosylating the head domain (21). Although the aforementioned strategies need to be further evaluated and provide novel alternatives, detrimental interference from the highly variable immunodominant head domain in eliciting a broad functional response cannot be completely evaded. A “headless” stem domain immunogen offers an attractive solution. However, early attempts at expressing the HA2 subunit independently in a native, prefusion conformation were unsuccessful. In the absence of the head domain, the HA2 subunit expressed in Escherichia coli spontaneously adopted the low-pH conformation (22) in which the functional epitopes of stem-directed bnAbs are disrupted. More recently, the entire HA stem region has been expressed in a prefusion, native-like conformation in both prokaryotic and eukaryotic systems adopting multiple strategies (23–26).Design of independently folding HA stem fragments which adopt the prefusion HA conformation presents another approach to elicit bnAbs against influenza (27, 28). The A helix of the HA2 subunit contributes substantial contact surface to the epitope of stem-directed bnAbs such as CR6261, F10, and others. Although multivalent display of A helix on the flock house virus as a virus-like particle platform elicited cross-reactive antibodies, it conferred only minimal protection (20%) against virus challenge in mice (29).We report the design and characterization of engineered headless HA stem immunogens based on the influenza A/Puerto Rico/8/34 (H1N1) subtype. H1HA10-Foldon, a trimeric derivative of our parent construct (H1HA10), bound conformation-sensitive, stem-directed bnAbs such as CR6261 (30), F10 (31), and FI6v3 (32) with a high-affinity [equilibrium dissociation constant (K_D) of 10–50 nM]. The designed immunogens elicited broadly cross-reactive antiviral antibodies which neutralized highly drifted influenza virus strains belonging to both group 1 (H1 and H5 subtypes) and 2 (H3 subtype) in vitro. Significantly, stem immunogens designed from unmatched, highly drifted influenza strains conferred protection against a lethal (2LD₉₀) heterologous A/Puerto Rico/8/34 virus challenge in mice. Our immunogens confer robust subtype-specific and modest heterosubtypic protection in vivo. In contrast to previous stem domain immunogens (23–25), the designed immunogens were purified from the soluble fraction in E. coli. The HA stem-fragment immunogens do not aggregate even at high concentrations and are cysteine-free, which eliminates the complications arising from incorrect disulfide-linked, misfolded conformations. The aforementioned properties of the HA stem-fragment immunogens make it amenable for scalability at short notice which is vital during pandemic outbreaks.     

A common solution to group 2 influenza virus neutralization

The discovery and characterization of broadly neutralizing antibodies (bnAbs) against influenza viruses have raised hopes for the development of monoclonal antibody (mAb)-based immunotherapy and the design of universal influenza vaccines. Only one human bnAb (CR8020) specifically recognizing group 2 influenza A viruses has been previously characterized that binds to a highly conserved epitope at the base of the hemagglutinin (HA) stem and has neutralizing activity against H3, H7, and H10 viruses. Here, we report a second group 2 bnAb, CR8043, which was derived from a different germ-line gene encoding a highly divergent amino acid sequence. CR8043 has in vitro neutralizing activity against H3 and H10 viruses and protects mice against challenge with a lethal dose of H3N2 and H7N7 viruses. The crystal structure and EM reconstructions of the CR8043-H3 HA complex revealed that CR8043 binds to a site similar to the CR8020 epitope but uses an alternative angle of approach and a distinct set of interactions. The identification of another antibody against the group 2 stem epitope suggests that this conserved site of vulnerability has great potential for design of therapeutics and vaccines.Influenza viruses are a significant and persistent threat to human health worldwide. Annual epidemics cause 3–5 million cases of severe illness and up to 0.5 million deaths (1), and periodic influenza pandemics have the potential to kill millions (2). Inhibitors against the viral surface glycoprotein neuraminidase are widely used for the treatment of influenza infections, but their efficacy is being compromised by the emergence of drug-resistant viral strains (3). Vaccination remains the most effective strategy to prevent influenza virus infection. However, protective efficacy is suboptimal in the highest risk groups: infants, the elderly, and the immunocompromised (1). Furthermore, because immunity after vaccination is typically strain-specific and influenza viruses evolve rapidly, vaccines must be updated almost annually. The antigenic composition of the vaccine is based on a prediction of strains likely to circulate in the coming year, therefore, mismatches between vaccine strains and circulating strains occur that can render the vaccine less effective (4). Consequently, there is an urgent need for new prophylactic and therapeutic interventions that provide broad protection against influenza.Immunity against influenza viruses is largely mediated by neutralizing antibodies that target the major surface glycoprotein hemagglutinin (HA) (5, 6). Identification of antigenic sites on HA indicates that influenza antibodies are primarily directed against the immunodominant HA head region (7), which mediates endosomal uptake of the virus into host cells by binding to sialic acid receptors (8). Because of high mutation rates in the HA head region and its tolerance for antigenic changes, antibodies that target the HA head are typically only effective against strains closely related to the strain(s) by which they were elicited, although several receptor binding site-targeting antibodies with greater breadth have been structurally characterized (9–15). In contrast, antibodies that bind to the membrane-proximal HA stem region tend to exhibit much broader neutralizing activity and can target strains within entire subtypes and groups (16–25) as well as across influenza types (24). These stem-directed antibodies inhibit major structural rearrangements in HA that are required for the fusion of viral and host endosomal membranes and thus, prevent the release of viral contents into the cell (8). The stem region is less permissive for mutations than the head and relatively well-conserved across divergent influenza subtypes.Anti-stem antibodies are elicited in some, but not all, individuals during influenza infection or vaccination (20, 26) and thus, hold great promise as potential broad spectrum prophylactic or therapeutic agents and for the development of a universal influenza vaccine (27–29). The majority of the known heterosubtypic stem binding antibodies neutralize influenza A virus subtypes belonging to group 1 (17–20, 23, 25). Furthermore, two antibodies that target a similar epitope in the HA stem, like most heterosubtypic group 1 antibodies, are able to more broadly recognize both group 1 and 2 influenza A viruses (22) or influenza A and B viruses (24). Strikingly, group 2-specific broadly neutralizing Abs (bnAbs) seem to be rare, because only one has been reported to date (21). CR8020 uniquely targets a distinct epitope in the stem in close proximity to the viral membrane at the HA base and binds lower down the stem than any other influenza HA antibody (21).In the discovery process that led to the isolation of bnAb CR8020, we recovered additional group 2-specific bnAbs. Here, we describe one such bnAb, CR8043, which recognizes a similar but nonidentical footprint on the HA as CR8020 and approaches the HA from a different angle. Furthermore, these two bnAbs are derived from different germ-line genes and, consequently, use distinct sets of interactions for HA recognition. Thus, the human immune system is able to recognize this highly conserved epitope in different ways using different germ-line genes. Hence, this valuable information can be used for the design of therapeutics and vaccines targeting this site of vulnerability in group 2 influenza A viruses that include the pandemic H3N2 subtype.     

RabGDIα is a negative regulator of interferon-γ–inducible GTPase-dependent cell-autonomous immunity to Toxoplasma gondii

IFN-γ orchestrates cell-autonomous host defense against various intracellular vacuolar pathogens. IFN-γ–inducible GTPases, such as p47 immunity-related GTPases (IRGs) and p65 guanylate-binding proteins (GBPs), are recruited to pathogen-containing vacuoles, which is important for disruption of the vacuoles, culminating in the cell-autonomous clearance. Although the positive regulation for the proper recruitment of IRGs and GBPs to the vacuoles has been elucidated, the suppressive mechanism is unclear. Here, we show that Rab GDP dissociation inhibitor α (RabGDIα), originally identified as a Rab small GTPase inhibitor, is a negative regulator of IFN-γ–inducible GTPases in cell-autonomous immunity to the intracellular pathogen Toxoplasma gondii. Overexpression of RabGDIα, but not of RabGDIβ, impaired IFN-γ–dependent reduction of T. gondii numbers. Conversely, RabGDIα deletion in macrophages and fibroblasts enhanced the IFN-γ–induced clearance of T. gondii. Furthermore, upon a high dose of infection by T. gondii, RabGDIα-deficient mice exhibited a decreased parasite burden in the brain and increased resistance in the chronic phase than did control mice. Among members of IRGs and GBPs important for the parasite clearance, Irga6 and Gbp2 alone were more frequently recruited to T. gondii-forming parasitophorous vacuoles in RabGDIα-deficient cells. Notably, Gbp2 positively controlled Irga6 recruitment that was inhibited by direct and specific interactions of RabGDIα with Gbp2 through the lipid-binding pocket. Taken together, our results suggest that RabGDIα inhibits host defense against T. gondii by negatively regulating the Gbp2–Irga6 axis of IFN-γ–dependent cell-autonomous immunity.IFN-γ is an important T-helper 1 (Th1) cytokine that inhibits the survival and growth of a wide range of intracellular pathogens, such as viruses, bacteria, and parasites (1). Stimulation of innate immune cells, such as macrophages, by IFN-γ up-regulates almost 2,000 effector genes encoding various IFN-γ–inducible proteins, including immunity-related GTPases such as the MX proteins, p47 immunity-related GTPases (IRGs), and p65 guanylate-binding proteins (GBPs) (2). MX proteins and GBPs have been shown to restrict replication of viruses (3). Moreover, IRGs and GBPs play roles in host defense against vacuole-forming bacteria, including Salmonella, Chlamydia, Mycobacteria, and Listeria, by induction of antibacterial responses involving autophagic effectors, inflammasomes, and phagocytic oxidases (4–6).Not only viruses and bacteria but also the vacuolar parasite Toxoplasma gondii is targeted by IFN-γ–inducible GTPases. T. gondii is an obligatory protozoan parasite that causes a life-threatening toxoplasmosis in humans and animals (7). After the active invasion of host cells, T. gondii forms a nonfusogenic cytoplasmic membranous structure called the parasitophorous vacuole (PV), in which the parasite efficiently proliferates (8, 9). In terms of cellular host defense against T. gondii, interleukin-12 (IL-12) is mainly produced from macrophages and dendritic cells, in which Toll-like receptors and the chemokine receptor CCR5 recognize T. gondii-derived ligands. Also, IL-12 promotes development of IFN-γ–producing Th1 cells (10–15). IFN-γ is critically required for suppression of T. gondii replication inside PVs and cell-autonomous clearance. Nitric oxide that is produced by inducible nitric oxide synthase (iNOS) in the infected cells mainly inhibits the replication (16, 17). On the other hand, T. gondii survival within infected cells is suppressed by cooperative action between IRGs and GBPs (18). Indeed, various types of cells (such as macrophages, fibroblasts, and astrocytes) derived from mice lacking IRGs [such as Irgm1 (also known as LRG-47), Irgm3 (IGTP), and Irga6 (IIGP1)] or GBPs [such as Gbp1, Gbp2, and a cluster of GBPs on murine chromosome 3 (GBP^chr3; Gbp1, Gbp2, Gbp3, Gbp5, and Gbp7)] were defective for IFN-γ–mediated intracellular killing of T. gondii (19–25). After the formation of PVs, GBPs and a subfamily of IRG members called GKS-IRGs [such as Irga6, Irgb6 (TGTP), and Irgb10] are shown to accumulate on PV membrane (PVM) and oligomerize dependently on GTP binding to destroy PV membrane integrity and structure (26, 27), resulting in cell-autonomous clearance by intracellular digestive pathways (20, 21, 28). The IFN-γ–mediated clearance by these GTPases is T. gondii strain-specific. Most T. gondii in North America and Europe belong to type I, type II, and type III (29). Virulent type I strain inactivates IFN-γ–inducible GTPases by effectors, such as ROP18 and ROP5, during the parasite infection (30). On the other hand, avirulent type II and type III strains are susceptible to IFN-γ–dependent clearance due to polymorphisms or reduced expression of the effectors (31–34).The regulatory mechanism of how IFN-γ–induced GTPases are recruited to PVs has gradually been elucidated. In the absence of essential autophagy-related proteins Atg3, Atg5, Atg7, and Atg16L1 and of another subfamily of IRGs called GMS-IRGs, such as Irgm1 and Irgm3, the recruitment of IFN-γ–inducible GTPases and the killing of T. gondii are severely impaired (35–39). Thus, Atg3/Atg5/Atg7/Atg16L1 and Irgm1/Irgm3 are required for proper targeting of GKS-IRGs and GBPs to T. gondii PVM and play positive roles in the cell-autonomous resistance to the pathogen. On the other hand, the inhibitory mechanism for the IFN-γ–inducible GTPase-dependent immunity remains unclear.To explore the molecular mechanism to control the action of IFN-γ–inducible GTPases, we have attempted to identify binding partners of Gbp2 because a single deletion of Gbp2 in mice has been shown to result in impaired in vitro and in vivo resistance to type II T. gondii (22). In the present study, we identify Rab GDP dissociation inhibitor α (RabGDIα) as a Gbp2-interacting protein. We have an interest in this protein for two reasons: One is because RabGDIα has been shown to participate in the regulation of Rab proteins, which, like GBPs, belong to another family of GTPases (40, 41), and the other is because we demonstrate that overexpression of RabGDIα in cells impairs IFN-γ–induced reduction of T. gondii numbers. We have tested whether RabGDIα acts as a regulator of IFN-γ–inducible GTPases under physiological conditions. Macrophages and fibroblasts from RabGDIα-deficient mice exhibit enhanced IFN-γ–dependent clearance of T. gondii. Moreover, the enhanced clearance by RabGDIα deficiency is accompanied by increased recruitment of Irga6 and Gbp2 to the parasite. Notably, Gbp2 is required for Irga6 recruitment, which is suppressed by direct and specific interactions of RabGDIα with Gbp2 through a lipid-binding pocket. Furthermore, a high dose of type II T. gondii infection in RabGDIα-deficient mice results in increased resistance, which is characterized by a decreased parasite burden in the brain. Taken together, our data indicate that RabGDIα plays a negative role in the Gbp2–Irga6 axis of IFN-γ–inducible GTPase-dependent cell-autonomous resistance to T. gondii.     

Thermodynamics of formation of coffinite,USiO4

Coffinite, USiO₄, is an important U(IV) mineral, but its thermodynamic properties are not well-constrained. In this work, two different coffinite samples were synthesized under hydrothermal conditions and purified from a mixture of products. The enthalpy of formation was obtained by high-temperature oxide melt solution calorimetry. Coffinite is energetically metastable with respect to a mixture of UO₂ (uraninite) and SiO₂ (quartz) by 25.6 ± 3.9 kJ/mol. Its standard enthalpy of formation from the elements at 25 °C is −1,970.0 ± 4.2 kJ/mol. Decomposition of the two samples was characterized by X-ray diffraction and by thermogravimetry and differential scanning calorimetry coupled with mass spectrometric analysis of evolved gases. Coffinite slowly decomposes to U₃O₈ and SiO₂ starting around 450 °C in air and thus has poor thermal stability in the ambient environment. The energetic metastability explains why coffinite cannot be synthesized directly from uraninite and quartz but can be made by low-temperature precipitation in aqueous and hydrothermal environments. These thermochemical constraints are in accord with observations of the occurrence of coffinite in nature and are relevant to spent nuclear fuel corrosion.In many countries with nuclear energy programs, spent nuclear fuel (SNF) and/or vitrified high-level radioactive waste will be disposed in an underground geological repository. Demonstrating the long-term (10⁶–10⁹ y) safety of such a repository system is a major challenge. The potential release of radionuclides into the environment strongly depends on the availability of water and the subsequent corrosion of the waste form as well as the formation of secondary phases, which control the radionuclide solubility. Coffinite (1), USiO₄, is expected to be an important alteration product of SNF in contact with silica-enriched groundwater under reducing conditions (2–8). It is also found, accompanied by thorium orthosilicate and uranothorite, in igneous and metamorphic rocks and ore minerals from uranium and thorium sedimentary deposits (2, 4, 5, 8–16). Under reducing conditions in the repository system, the uranium solubility (very low) in aqueous solutions is typically derived from the solubility product of UO₂. Stable U(IV) minerals, which could form as secondary phases, would impart lower uranium solubility to such systems. Thus, knowledge of coffinite thermodynamics is needed to constrain the solubility of U(IV) in natural environments and would be useful in repository assessment.In natural uranium deposits such as Oklo (Gabon) (4, 7, 11, 12, 14, 17, 18) and Cigar Lake (Canada) (5, 13, 15), coffinite has been suggested to coexist with uraninite, based on electron probe microanalysis (EPMA) (4, 5, 7, 11, 13, 17, 19, 20) and transmission electron microscopy (TEM) (8, 15). However, it is not clear whether such apparent replacement of uraninite by a coffinite-like phase is a direct solid-state process or occurs mediated by dissolution and reprecipitation.The precipitation of USiO₄ as a secondary phase should be favored in contact with silica-rich groundwater (21) [silica concentration >10⁻⁴ mol/L (22, 23)]. Natural coffinite samples are often fine-grained (4, 5, 8, 11, 13, 15, 24), due to the long exposure to alpha-decay event irradiation (4, 6, 25, 26) and are associated with other minerals and organic matter (6, 8, 12, 18, 27, 28). Hence the determination of accurate thermodynamic data from natural samples is not straightforward. However, the synthesis of pure coffinite also has challenges. It appears not to form by reacting the oxides under dry high-temperature conditions (24, 29). Synthesis from aqueous solutions usually produces UO₂ and amorphous SiO₂ impurities, with coffinite sometimes being only a minor phase (24, 30–35). It is not clear whether these difficulties arise from kinetic factors (slow reaction rates) or reflect intrinsic thermodynamic instability (33). Thus, there are only a few reported estimates of thermodynamic properties of coffinite (22, 36–40) and some of them are inconsistent. To resolve these uncertainties, we directly investigated the energetics of synthetic coffinite by high-temperature oxide melt solution calorimetry to obtain a reliable enthalpy of formation and explored its thermal decomposition.