Alternatively activated dendritic cells regulate CD4+ T-cell polarization in vitro and in vivo

Interleukin-4 is a cytokine widely known for its role in CD4(+) T cell polarization and its ability to alternatively activate macrophage populations. In contrast, the impact of IL-4 on the activation and function of dendritic cells (DCs) is poorly understood. We report here that DCs respond to IL-4 both in vitro and in vivo by expression of multiple alternative activation markers with a different expression pattern to that of macrophages. We further demonstrate a central role for DC IL-4Rα expression in the optimal induction of IFNγ responses in vivo in both Th1 and Th2 settings, through a feedback loop in which IL-4 promotes DC secretion of IL-12. Finally, we reveal a central role for RELMα during T-cell priming, establishing that its expression by DCs is critical for optimal IL-10 and IL-13 promotion in vitro and in vivo. Together, these data highlight the significant impact that IL-4 and RELMα can have on DC activation and function in the context of either bacterial or helminth pathogens.     

Transforming growth factor beta-activated kinase 1 (TAK1)-dependent checkpoint in the survival of dendritic cells promotes immune homeostasis and function

Homeostatic control of dendritic cell (DC) survival is crucial for adaptive immunity, but the molecular mechanism is not well defined. Moreover, how DCs influence immune homeostasis under steady state remains unclear. Combining DC-specific and -inducible deletion systems, we report that transforming growth factor beta-activated kinase 1 (TAK1) is an essential regulator of DC survival and immune system homeostasis and function. Deficiency of TAK1 in CD11c(+) cells induced markedly elevated apoptosis, leading to the depletion of DC populations, especially the CD8(+) and CD103(+) DC subsets in lymphoid and nonlymphoid tissues, respectively. TAK1 also contributed to DC development by promoting the generation of DC precursors. Prosurvival signals from Toll-like receptors, CD40 and receptor activator of nuclear factor-κB (RANK) are integrated by TAK1 in DCs, which in turn mediated activation of downstream NF-κB and AKT-Foxo pathways and established a gene-expression program. TAK1 deficiency in DCs caused a myeloid proliferative disorder characterized by expansion of neutrophils and inflammatory monocytes, disrupted T-cell homeostasis, and prevented effective T-cell priming and generation of regulatory T cells. Moreover, TAK1 signaling in DCs was required to prevent myeloid proliferation even in the absence of lymphocytes, indicating a previously unappreciated regulatory mechanism of DC-mediated control of myeloid cell-dependent inflammation. Therefore, TAK1 orchestrates a prosurvival checkpoint in DCs that affects the homeostasis and function of the immune system.     

Natural killer cell activation enhances immune pathology and promotes chronic infection by limiting CD8+ T-cell immunity

Infections with HIV, hepatitis B virus, and hepatitis C virus can turn into chronic infections, which currently affect more than 500 million patients worldwide. It is generally thought that virus-mediated T-cell exhaustion limits T-cell function, thus promoting chronic disease. Here we demonstrate that natural killer (NK) cells have a negative impact on the development of T-cell immunity by using the murine lymphocytic choriomeningitis virus. NK cell-deficient (Nfil3(-/-), E4BP4(-/-)) mice exhibited a higher virus-specific T-cell response. In addition, NK cell depletion caused enhanced T-cell immunity in WT mice, which led to rapid virus control and prevented chronic infection in lymphocytic choriomeningitis virus clone 13- and reduced viral load in DOCILE-infected animals. Further experiments showed that NKG2D triggered regulatory NK cell functions, which were mediated by perforin, and limited T-cell responses. Therefore, we identified an important role of regulatory NK cells in limiting T-cell immunity during virus infection.     

The role of melatonin in the cells of the innate immunity: a review

Melatonin is the major secretory product synthesized and secreted by the pineal gland and shows both a wide distribution within phylogenetically distant organisms from bacteria to humans and a great functional versatility. In recent years, a considerable amount of experimental evidence has accumulated showing a relationship between the nervous, endocrine, and immune systems. The molecular basis of the communication between these systems is the use of a common chemical language. In this framework, currently melatonin is considered one of the members of the neuroendocrine–immunological network. A number of in vivo and in vitro studies have documented that melatonin plays a fundamental role in neuroimmunomodulation. Based on the information published, it is clear that the majority of the present data in the literature relate to lymphocytes; thus, they have been rather thoroughly investigated, and several reviews have been published related to the mechanisms of action and the effects of melatonin on lymphocytes. However, few studies concerning the effects of melatonin on cells belonging to the innate immunity have been reported. Innate immunity provides the early line of defense against microbes and consists of both cellular and biochemical mechanisms. In this review, we have focused on the role of melatonin in the innate immunity. More specifically, we summarize the effects and action mechanisms of melatonin in the different cells that belong to or participate in the innate immunity, such as monocytes–macrophages, dendritic cells, neutrophils, eosinophils, basophils, mast cells, and natural killer cells.     

Roles for both CD+ and CD8+ T cells in protective immunity against Onchocerca lienalis microfilariae in the mouse

Mice inoculated with microfilariae of the filarial nematode Onchocerca lienalis clear their parasites from the skin over a period of 3 to 4 months and are highly resistant to a challenge infection. The adoptive transfer of spleen cells at various time points following primary and secondary infections of mice shows that exposures of 50 days or greater are required for the generation of lymphocytes capable of transferring protection to naive recipients. This adoptive transfer of protection with spleen cells from infection-primed mice partitions with the T lymphocyte population. In contrast, the passive transfer of protection with spleen-derived B cells, or sera taken at various time points following infection was not achieved. Moreover, there was no detectable synergistic effect when B and T cells were co-administered to recipient animals. Depletion of CD4+ and CD8+ T cells with monoclonal antibodies shows that CD8 + T cells have some regulatory effect on parasite establishment early in primary infection, but this is later superseded by CD4+ T cell reactivity that is predominant both when primary infection microfilariae are cleared and also during resistance to reinfection. Measurement of cytokines in the sera of mice undergoing primary and secondary infections support a microfilariae-induced Th2 activity, with high levels of IL-5 that are sustained upon reinfection, and low levels oflFN-γ that are negligible at the time when mice are most strongly immune.     

Synergistic inhibition of natural killer cells by the nonsignaling molecule CD94

Peptide selectivity is a feature of inhibitory receptors for MHC class I expressed by natural killer (NK) cells. CD94–NKG2A operates in tandem with the polymorphic killer cell Ig-like receptors (KIR) and Ly49 systems to inhibit NK cells. However, the benefits of having two distinct inhibitory receptor–ligand systems are not clear. We show that noninhibitory peptides presented by HLA-E can augment the inhibition of NKG2A⁺ NK cells mediated by MHC class I signal peptides through the engagement of CD94 without a signaling partner. Thus, CD94 is a peptide-selective NK cell receptor, and NK cells can be regulated by nonsignaling interactions. We also show that KIR⁺ and NKG2A⁺ NK cells respond with differing stoichiometries to MHC class I down-regulation. MHC-I–bound peptide functions as a molecular rheostat controlling NK cell function. Selected peptides which in isolation do not inhibit NK cells can have different effects on KIR and NKG2A receptors. Thus, these two inhibitory systems may complement each other by having distinct responses to bound peptide and surface levels of MHC class I.Natural killer (NK) cells play an important role in the immune response to viral infections and cancer. Their responses are determined by signals integrated from activating and inhibitory receptor–ligand interactions (1). In many situations inhibitory signals dominate activating signals. Therefore, releasing NK cells from inhibition is an important mechanism of enhancing their response to target cells. Inhibitory interactions are mediated by receptors for self-MHC class I. Most species have at least two discrete gene families of inhibitory receptors for MHC class I: the CD94–NKG2A C-type lectin-like receptor system and either the related Ly49 family of receptors or the unrelated killer cell Ig-like receptors (KIR) (2). The KIR family is important in humans and other primates, having undergone extensive diversification under positive selection. In contrast, the CD94–NKG2A system has remained relatively well conserved across the species with orthologous genes in primates and a closely related functional homolog in rodents (3, 4). Consistent with the coevolution of these families and their MHC class I ligands, KIR bind polymorphic MHC class I, HLA-A, -B, and -C molecules, whereas CD94–NKG2A binds the conserved oligomorphic HLA-E molecule or the rodent homolog Qa-1 (5, 6).Both receptor families are important in the immune response to viral infections. KIR are genetic determinants in the outcome of both HIV and hepatitis C virus (HCV) infection (7–10). Expression of CD94–NKG2A is up-regulated on NK cells in HIV and HCV infection and in the latter has been associated with a poor response to treatment (11, 12). Furthermore NKG2A⁺ NK cell clones lyse vaccinia-infected targets (13), and CD94 is important in clearing mouse pox infection (14). Both KIR and CD94–NKG2A respond to MHC class I down-regulation. One hypothesis is that the KIR have evolved to recognize MHC class I-specific down-regulation (15). However, because the majority of MHC class I leader peptides bind HLA-E and are inhibitory for NKG2A, the CD94–NKG2A system also is able to recognize down-regulation of most MHC class I alleles. It has been shown that KIR⁺ NK cells can be modulated by changes in the peptide bound by MHC class I, which confers additional functionality on the KIR system (16–18). In particular peptide antagonism is a potent mechanism for activating KIR⁺ NK cells (19, 20). The CD94–NKG2A receptor also is peptide selective, with receptor binding being particularly influenced by residues 5, 6, and 8 of the peptide bound by HLA-E (21–23). These residues interact primarily with the nonsignaling CD94 moiety, which occupies the majority of the HLA-E–binding interface. CD94–NKG2A seems to be a target for viral escape, with peptides derived from CMV, HCV, HIV, and EBV binding HLA-E and subsequently inhibiting NK cells (24–27). Viral peptides that inhibit at KIR also are identifiable (28), but their relevance likely is limited to the subset of individuals who have the relevant peptide-binding MHC class I allele. Understanding differences in how the KIR and NKG2 systems respond to peptide may be important for interpreting their roles in the immune response to viral infections and tumors. Therefore we explored how HLA-E–bound peptide can influence NK cell reactivity.     

Inhibition of invariant chain expression in dendritic cells presenting endogenous antigens stimulates CD4+ T-cell responses and tumor immunity

Induction of potent and sustained antiviral or antitumor immunity is dependent on the efficient activation of CD8+ and CD4+ T cells. While dendritic cells constitute a powerful platform for stimulating cellular immunity, presentation of endogenous antigens by dendritic cells transfected with nucleic acid-encoded antigens favors the stimulation of CD8+ T cells over that of CD4+ T cells. A short incubation of mRNA-transfected dendritic cells with antisense oligonucleotides directed against the invariant chain enhances the presentation of mRNA-encoded class II epitopes and activation of CD4+ T-cell responses in vitro and in vivo. Immunization of mice with the antisense oligonucleotide-treated dendritic cells stimulates a more potent and longer lasting CD8+ cytotoxic T-cell (CTL) response and enhances the antitumor efficacy of dendritic cell-based tumor vaccination protocols. Transient inhibition of invariant chain expression represents a simple and general method to enhance the stimulation of CD4+ T-cell responses from endogenous antigens.     

In vivo transformation of mouse conventional CD8alpha+ dendritic cells leads to progressive multisystem histiocytosis

Division and proliferation of dendritic cells (DCs) have been proposed to contribute to homeostasis and to prolonged antigen presentation. Whether abnormal proliferation of dendritic cells causes Langerhans cell histiocytosis (LCH) is a highly debated topic. Transgenic expression of simian virus 40 (SV40) T antigens in mature DCs allowed their transformation in vivo while maintaining their phenotype, function, and maturation capacity. The transformed cells were differentiated splenic CD8 alpha-positive conventional dendritic cells with increased Langerin expression. Their selective transformation was correlated with higher steady-state cycling compared with CD8 alpha-negative DCs in wild-type and transgenic mice. Mice developed a DC disease involving the spleen, liver, bone marrow, thymus, and mesenteric lymph node. Surprisingly, lesions displayed key immunohistologic features of Langerhans cell histiocytosis, including expression of Langerin and absence of the abnormal mitoses observed in Langerhans cell sarcomas. Our results demonstrate that a transgenic mouse model with striking similarities to aggressive forms of multisystem histiocytosis, such as the Letterer-Siwe syndrome, can be obtained by transformation of conventional DCs. These findings suggest that conventional DCs may cause some human multisystem LCH. They can reveal shared molecular pathways for human histiocytosis between humans and mice.     

Murine plasmacytoid dendritic cells induce effector/memory CD8+ T-cell responses in vivo after viral stimulation

Like their human counterparts, mouse plasmacytoid dendritic cells (pDCs) play a central role in innate immunity against viral infections, but their capacity to prime T cells in vivo remains unknown. We show here that virus-activated pDCs differentiate into antigen-presenting cells able to induce effector/memory CD8(+) T-cell responses in vivo against both epitopic peptides and endogenous antigen, whereas pDCs activated by synthetic oligodeoxynucleotides containing unmethylated cytosine-guanine motifs (CpG) acquire only the ability to recall antigen-experienced T-cell responses. We also show that immature pDCs are unable to induce effector or regulatory CD8(+) T-cell responses. Thus, murine pDCs take part in both innate and adaptive immune responses by directly priming naive CD8(+) T cells during viral infection.     

Selective ex vivo expansion of cytomegalovirus-specific CD4+ and CD8+ T lymphocytes using dendritic cells pulsed with a human leucocyte antigen A*0201-restricted peptide

Adoptive transfer of ex vivo-generated cytomegalovirus (CMV)-specific T lymphocytes may be effective in preventing CMV disease in allogeneic haematopoietic stem cell transplantation (HSCT) recipients. We developed a procedure for expansion of CMV-specific T lymphocytes based on the antigen-presenting function of donor dendritic cells (DCs), pulsed with a human leucocyte antigen A*0201-restricted pp65 nonamer peptide. CMV-specific T lymphocytes were identified following induction of interferon gamma (IFN-gamma) secretion prompted by peptide exposure. Both CD8+ and CD4+ CMV-specific T lymphocytes were selectively produced in these cultures and showed CMV-restricted cytotoxicity. The simultaneous and selective expansion of CD4+ and CD8+ CMV-specific lymphocytes might be instrumental for more efficient in vivo function of infused CMV-specific lymphocytes.     

The role of CD4 cells in protective immunity to Brugia pahangi

The BALB/c mouse immunized sub-cutaneously (s.c.) with 45 kRad attenuated third stage larvae (L₃) of the lymphatic filarial nematode Brugia pahangi is strongly immune to a challenge infection (75–100% reduction in recovery at day six post challenge). Analysis of spleen cell supernatants from immunized mice re-stimulated in vitro, with parasite antigen or the non specific T cell mitogen Con-A reveals a cytokine profile (IL-4, IL-5 and IL-9) which indicates that the Th2 subset of CD4 cells has been expanded. In an attempt to formally prove a critical role for CD4 cells in immunity in this model system, immunized mice were given either anti-CD4 or anti-CD8 neutralizing antibodies. Administration of anti-CD4 antibody had a significant and detrimental effect on the immune response whereas anti-CD8 antibody had a negligible effect on immunity. The efficacy of antibody in neutralizing their target cells was determined by fluorescence activated cell sorting analysis (FACS). Spleen cells from anti-CD4 treated immunized mice, when re-stimulated with parasite antigen had a significantly reduced potential to secrete IL-4, IL-5 and IL-9 in vitro and serum from these mice had reduced levels of parasite specific IgG and IgE. These results demonstrate a critical role for CD4 T cells in host protective immunity to B. pahangi in vivo and strongly suggest that some component of the Th2 response plays an important role in the immune response elicited in this model system.     

Paneth cells directly sense gut commensals and maintain homeostasis at the intestinal host-microbial interface

The intestinal epithelium is in direct contact with a vast microbiota, yet little is known about how epithelial cells defend the host against the heavy bacterial load. To address this question we studied Paneth cells, a key small intestinal epithelial lineage. We found that Paneth cells directly sense enteric bacteria through cell-autonomous MyD88-dependent toll-like receptor (TLR) activation, triggering expression of multiple antimicrobial factors. Paneth cells were essential for controlling intestinal barrier penetration by commensal and pathogenic bacteria. Furthermore, Paneth cell-intrinsic MyD88 signaling limited bacterial penetration of host tissues, revealing a role for epithelial MyD88 in maintaining intestinal homeostasis. Our findings establish that gut epithelia actively sense enteric bacteria and play an essential role in maintaining host-microbial homeostasis at the mucosal interface.     

TLR8 on dendritic cells and TLR9 on B cells restrain TLR7-mediated spontaneous autoimmunity in C57BL/6 mice

Systemic lupus erythematosus (SLE) is a complex autoimmune disease with diverse clinical presentations characterized by the presence of autoantibodies to nuclear components. Toll-like receptor (TLR)7, TLR8, and TLR9 sense microbial or endogenous nucleic acids and are implicated in the development of SLE. In mice TLR7-deficiency ameliorates SLE, but TLR8- or TLR9-deficiency exacerbates the disease because of increased TLR7 response. Thus, both TLR8 and TLR9 control TLR7 function, but whether TLR8 and TLR9 act in parallel or in series in the same or different cell types in controlling TLR7-mediated lupus remains unknown. Here, we reveal that double TLR8/9-deficient (TLR8/9^−/−) mice on the C57BL/6 background showed increased abnormalities characteristic of SLE, including splenomegaly, autoantibody production, frequencies of marginal zone and B1 B cells, and renal pathology compared with single TLR8^−/− or TLR9^−/− mice. On the cellular level, TLR8^−/− and TLR8/9^−/− dendritic cells were hyperesponsive to TLR7 ligand R848, but TLR9^−/− cells responded normally. Moreover, B cells from TLR9^−/− and TLR8/9^−/− mice were hyperesponsive to R848, but TLR8^−/− B cells were not. These results reveal that TLR8 and TLR9 have an additive effect on controlling TLR7 function and TLR7-mediated lupus; however, they act on different cell types. TLR8 controls TLR7 function on dendritic cells, and TLR9 restrains TLR7 response on B cells.Systemic lupus erythematosus (SLE) is a complex chronic autoimmune disease that arises spontaneously and is characterized by production of autoantibodies against self-nucleic acids and associated proteins (1). These autoantibodies bind self-nucleic acids released by dying cells and form immune complexes that accumulate in different parts of the body, leading to inflammation and tissue damage. The kidneys, skin, joints, lungs, serous membranes, as well as, the cardiovascular, nervous and musculoskeletal system become targets of inflammation at onset or during the course of the disease (2). The etiology of SLE is unknown, yet genetics, sex, infectious agents, environmental factors, and certain medications may play a role in the initiation of the disease by causing alterations in lymphoid signaling, antigen presentation, apoptosis, and clearance of immune complexes (3, 4).Toll-like receptors (TLRs) detect specific microbial components widely expressed by bacteria, fungi, protozoa, and viruses, and initiate signaling pathways critical for induction of immune responses to infection (5). In contrast to the cell surface TLRs that detect bacterial cell wall components and viral particles, nucleic acid-sensing TLRs are localized mainly within endosomal compartments (6). Human endosomal TLRs consist of TLR3, which senses viral double-stranded RNA (dsRNA) (7), TLR7 and TLR8, which recognize viral single-stranded RNA (8–10), and TLR9, which detects bacterial and viral unmethylated CpG-containing DNA motifs (11). Interestingly, these endosomal TLRs are also able to detect self-nucleic acids (12–14). Although the endosomal localization isolate TLR3, TLR7, TLR8, and TLR9 away from self-nucleic acids in the extracellular space, still self-RNA or -DNA can become a potent trigger of cell activation when transported into TLR-containing endosomes, and such recognition can result in sterile inflammation and autoimmunity, including SLE (4, 15, 16). The connection of the endosomal TLRs with SLE originates mainly from mouse models, where TLR7 signaling seems to play a central role. TLR7 gene duplication is the cause for the development of lupus in mice bearing the Y chromosome-linked autoimmune accelerating (Yaa) locus that harbors 17 genes, including TLR7 (17, 18). In TLR7 transgenic mouse lines, a modest increase in TLR7 expression promotes autoreactive lymphocytes with RNA specificities and myeloid cell proliferation, but a substantial increase in TLR7 expression causes fatal acute inflammatory pathology and profound dendritic cell (DC) dysregulation (17). In addition, studies in several lupus-prone mouse strains have revealed that TLR7-deficiency ameliorates disease, but TLR9-deficiency exacerbates it. Interestingly, this controversy can be explained by the enhanced TLR7 activity in the TLR9-deficient lupus mice (19, 20). Although murine TLR8 does not seem so far to be able to sense a ligand (21, 22), we have shown previously that it plays an important biological role in controlling TLR7-mediated lupus. Indeed, TLR8-deficiency in mice (on the C57BL/6 background that is not prone to lupus) leads to lupus development because of increased TLR7 expression and signaling in DCs (23). Thus, tight control and regulation of TLR7 is pivotal for avoiding SLE and inflammatory pathology in mice. Recent studies in humans have also revealed that increased expression of TLR7 is associated with increased risk for SLE (24–26).Nucleic acid TLRs are expressed in many cell types, including DCs, plasmacytoid DCs (pDCs) and B cells, all of which play a central role in SLE development. TLR7, TLR8, and TLR9 signal through the adaptor molecule myeloid differentiation primary response gene 88 (MyD88), whereas TLR3 signals via the adaptor TRIF (Toll/IL-1 receptor domain-containing adaptor inducing IFN-β) (5). MyD88-deficiency abrogates most attributes of lupus in several lupus-prone mouse strains (19, 27–29). Moreover, deficiency for Unc93B1, a multipass transmembrane protein that controls trafficking of TLRs from the endoplasmic reticulum to endolysosomes and is required for nucleic acid-sensing TLR function (30), also abrogates many clinical parameters of disease in mouse lupus strains, suggesting that endosomal TLRs are critical in this disease (31). Interestingly, TLR9 competes with TLR7 for Unc93B1-dependent trafficking and predominates over TLR7 (32). TLR9 predominance is reversed to TLR7 by a D34A mutation in Unc93B1 and mice that carry this mutation show TLR7-dependent, systemic lethal inflammation (32).Thus, in mice both TLR8 and TLR9 control TLR7-mediated lupus, but it is unknown if these TLRs act in parallel or in series in the same or different cell types and if they have an additive effect or not in controlling TLR7. To address these issues, we generated double TLR8/TLR9-deficient (TLR8/9^−/−) mice and analyzed and compared the lupus phenotype in TLR8^−/−, TLR9^−/−, and TLR8/9^−/− mice. Our data revealed that TLR8/9^−/− mice have increased abnormalities characteristic of SLE and that both TLR8 and TLR9 keep TLR7-mediated lupus under control, but they act in different cell types. On DCs TLR7 function is ruled by TLR8, whereas on B cells TLR7 is mastered by TLR9.     

Activation of influenza virus-specific CD4+ and CD8+ T cells: a new role for plasmacytoid dendritic cells in adaptive immunity

Plasmacytoid dendritic cells (pDCs) contribute to innate antiviral immune responses by producing type I interferons (IFNs) upon exposure to enveloped viruses. However, their role in adaptive immune responses, such as the initiation of antiviral T-cell responses, is not known. In this study, we examined interactions between blood pDCs and influenza virus with special attention to the capacity of pDCs to activate influenza-specific T cells. pDCs were compared with CD11c(+) DCs, the most potent antigen-presenting cells (APCs), for their capacity to activate T-cell responses. We found that like CD11c(+) DCs, pDCs mature following exposure to influenza virus, express CCR7, and produce proinflammatory chemokines, but differ in that they produce type I IFN and are resistant to the cytopathic effect of the infection. After influenza virus exposure, both DC types exhibited an equivalent efficiency to expand anti-influenza virus cytotoxic T lymphocytes (CTLs) and T helper 1 (TH1) CD4(+) T cells. Our results pinpoint a new role of pDCs in the induction of antiviral T-cell responses and suggest that these DCs play a prominent role in the adaptive immune response against viruses.     

Perforin-dependent elimination of dendritic cells regulates the expansion of antigen-specific CD8+ T cells in vivo

The lifespan and survival of dendritic cells (DC) in vivo are potentially critical to the expansion of T cell immune responses. We have previously reported that DC loaded with specific antigen are rapidly eliminated by cytotoxic T lymphocytes (CTL) in vivo, but the site, mechanism, and consequences of DC elimination were not defined. In this article we show that DC elimination in vivo occurs in a perforin-dependent manner and does not require IFN-gamma or the presence of CD4(+)CD25(+) regulatory T cells. Most importantly, failure to eliminate DC had profound consequences on the CTL immune response. Perforin-deficient mice showed a progressive increase in the numbers of antigen-specific CD8(+) T cells after repeated immunizations with DC. In contrast, in control mice the number of antigen-specific CD8(+) T cells did not notably increase with repeated immunizations. Lastly, we also show that CTL-mediated elimination of DC occurs in peripheral tissues but not in the lymph node. Our data suggest that CTL act as "gatekeepers" that control access of antigen-loaded DC into the lymph node, thereby preventing continued expansion of antigen-specific T cells.     

Blockade of A2A receptors potently suppresses the metastasis of CD73+ tumors

CD73 inhibits antitumor immunity through the activation of adenosine receptors expressed on multiple immune subsets. CD73 also enhances tumor metastasis, although the nature of the immune subsets and adenosine receptor subtypes involved in this process are largely unknown. In this study, we revealed that A_2A/A_2B receptor antagonists were effective in reducing the metastasis of tumors expressing CD73 endogenously (4T1.2 breast tumors) and when CD73 was ectopically expressed (B16F10 melanoma). A_2A^−/− mice were strongly protected against tumor metastasis, indicating that host A_2A receptors enhanced tumor metastasis. A_2A blockade enhanced natural killer (NK) cell maturation and cytotoxic function in vitro, reduced metastasis in a perforin-dependent manner, and enhanced NK cell expression of granzyme B in vivo, strongly suggesting that the antimetastatic effect of A_2A blockade was due to enhanced NK cell function. Interestingly, A_2B blockade had no effect on NK cell cytotoxicity, indicating that an NK cell-independent mechanism also contributed to the increased metastasis of CD73⁺ tumors. Our results thus revealed that CD73 promotes tumor metastasis through multiple mechanisms, including suppression of NK cell function. Furthermore, our data strongly suggest that A_2A or A_2B antagonists may be useful for the treatment of metastatic disease. Overall, our study has potential therapeutic implications given that A_2A/A_2B receptor antagonists have already entered clinical trials in other therapeutic settings.     

Control of Toxoplasma reactivation by rescue of dysfunctional CD8+ T-cell response via PD-1-PDL-1 blockade

In this study, we document that Toxoplasma gondii differentiation and reactivation are mediated by systemic CD8 T-cell dysfunction during chronic infection. We demonstrate that CD8(+) T-cell exhaustion occurs despite control of parasitemia during early-chronic toxoplasmosis. During later phases, these cells become exhausted, leading to parasite reactivation and mortality. Concomitant with increased CD8(+) T-cell apoptosis and decreased effector response, this dysfunction is characterized by a graded elevation in expression of inhibitory receptor PD-1 on these cells in both lymphoid and nonlymphoid tissue. Blockade of the PD-1-PDL-1 pathway reinvigorates this suboptimal CD8(+) T-cell response, resulting in control of parasite reactivation and prevention of mortality in chronically infected animals. To the best of our knowledge, this report is unique in showing that exposure to a persistent pathogen despite initial control of parasitemia can lead to CD8(+) T-cell dysfunction and parasite reactivation.