Innate IFN‐γ Production by Subsets of Natural Killer Cells,Natural Killer T Cells and γδ T Cells in Response to Dying Bacterial‐Infected Macrophages

Interferon‐γ (IFN‐γ) activation of macrophages is a crucial step in the early innate defence against bacterial infection. This innate IFN‐γ is thought to be produced mainly by natural killer (NK) cells through activation with interleukin (IL)‐12p70 secreted by macrophages and dendritic cells (DCs) that have sensed bacterial products. However, a number of reports have shown that bacterial stimuli are unable to induce macrophages and/or DCs to produce sufficient amounts of IL‐12p70 unless these cells are primed by IFN‐γ. It remains, therefore, unsettled how initial IFN‐γ is produced. In a previous study, we reported a novel IFN‐γ production pathway that was associated with cell death in macrophages caused by intracellular bacteria like Listeria monocytogenes (LM) and Shigella flexneri. In this study, we showed that cell death of bone‐marrow‐derived macrophage (BMM) cells following in vitro infection with Staphylococcus aureus (SA), an extracellular bacterium, can also stimulate this IFN‐γ production pathway. We also unequivocally demonstrated by using BMM cells from IL‐12‐deficient mice that the bacterial‐infected macrophage cell death‐mediated IFN‐γ production can occur without IL‐12 although the magnitude of the response is much smaller than that in the presence of IL‐12. The enhancing effect of IL‐12 on this response proved to be attributable to the negligible amounts (0.5～1.5 pg/ml) of IL‐12p70 but not to the large amounts of IL‐12p40 that were both secreted by SA‐ and LM‐infected macrophages. Taken all together, we propose that macrophage cell death caused by bacteria may trigger the initial IFN‐γ production at an early stage of bacterial infection.     

Human phagosome processing of Mycobacterium tuberculosis antigens is modulated by interferon‐γ and interleukin‐10

Intracellular pathogens, such as Mycobacterium tuberculosis, reside in the phagosomes of macrophages where antigenic processing is initiated. Mycobacterial antigen–MHC class II complexes are formed within the phagosome and are then trafficked to the cell surface. Interferon‐γ (IFN‐γ) and interleukin‐10 (IL‐10) influence the outcome of M. tuberculosis infection; however, the role of these cytokines with regard to the formation of M. tuberculosis peptide–MHC‐II complexes remains unknown. We analysed the kinetics and subcellular localization of M. tuberculosis peptide–MHC‐II complexes in M. tuberculosis‐infected human monocyte‐derived macrophages (MDMs) using autologous M. tuberculosis‐specific CD4⁺ T cells. The MDMs were pre‐treated with either IFN‐γ or IL‐10 and infected with M. tuberculosis. Cells were mechanically homogenized, separated on Percoll density gradients and manually fractionated. The fractions were incubated with autologous M.  tuberculosis ‐specific CD4⁺ T cells. Our results demonstrated that in MDMs pre‐treated with IFN‐γ, M. tuberculosis peptide–MHC‐II complexes were detected early mainly in the phagosomal fractions, whereas in the absence of IFN‐γ, the complexes were detected in the endosomal fractions. In MDMs pre‐treated with IL‐10, the M. tuberculosis peptide–MHC‐II complexes were retained in the endosomal fractions, and these complexes were not detected in the plasma membrane fractions. The results of immunofluorescence microscopy demonstrated the presence of Ag85B associated with HLA‐DR at the cell surface only in the IFN‐γ‐treated MDMs, suggesting that IFN‐γ may accelerate M. tuberculosis antigen processing and presentation at the cell membrane, whereas IL‐10 favours the trafficking of Ag85B to vesicles that do not contain LAMP‐1. Therefore, IFN‐γ and IL‐10 play a role in the formation and trafficking of M. tuberculosis peptide–MHC‐II complexes.     

Multi‐functional flow cytometry analysis of CD4+ T cells as an immune biomarker for latent tuberculosis status in patients treated with tumour necrosis factor (TNF) antagonists

Although monitoring tuberculosis (TB) infection during long‐term treatment with tumour necrosis factor (TNF) antagonists is of great importance, no monitoring strategy has yet proved successful. Indeed, even the newly proposed interferon‐gamma release assays (IGRAs) are known to produce dynamic changes in IFN‐γ plasma levels, making them unreliable indicators of patients' pathological/clinical status. We used intracellular cytokine flow cytometry (ICCFC) to investigate the performance of multi‐functional CD4⁺ T cells producing IFN‐γ, interleukin (IL)‐2 and/or TNF in response to Mycobacterium tuberculosis‐specific antigens in subjects treated with TNF antagonists. Patients were classified into three groups based on their TB status before commencement of treatment and on IFN‐γ level fluctuations evaluated by IGRA during a 36‐month follow‐up period. The cytokine profile of M. tuberculosis‐specific CD4⁺ T cells showed that latent tuberculosis infection (LTBI) subjects had a higher frequency of double‐positive IFN‐γ⁺ IL‐2⁺ CD4⁺ T cells and triple‐positive IFN‐γ⁺ IL‐2⁺ TNF⁺ CD4⁺ T cells compared to those without LTBI, who showed IFN‐γ‐level fluctuations over time. In contrast, this latter group of patients showed similar proportions of cells producing IFN‐γ alone, IL‐2 alone and IL‐2 in combination with TNF in response to M. tuberculosis‐specific antigens. It therefore appears that patients with and without LTBI infection are characterized by different intracellular cytokine profiles. This is the first study evaluating ICCFC in patients treated with TNF antagonists, and suggests that multi‐functional analysis of CD4⁺ T cells could be useful for ruling out TB infection in patients classified at screening as LTBI‐negative but who show IGRA fluctuations under long‐term TNF antagonist treatment.     

Multifunctional CD4+ T cells correlate with active Mycobacterium tuberculosis infection

Th1 CD4⁺ T cells and their derived cytokines are crucial for protection against Mycobacterium tuberculosis. Using multiparametric flow cytometry, we have evaluated the distribution of seven distinct functional states (IFN‐γ/IL‐2/TNF‐α triple expressors, IFN‐γ/IL‐2, IFN‐γ/TNF‐α or TNF‐α/IL‐2 double expressors or IFN‐γ, IL‐2 or TNF‐α single expressors) of CD4⁺ T cells in individuals with latent M. tuberculosis infection (LTBI) and active tuberculosis (TB). We found that triple expressors, while detectable in 85–90%TB patients, were only present in 10–15% of LTBI subjects. On the contrary, LTBI subjects had significantly higher (12‐ to 15‐fold) proportions of IL‐2/IFN‐γ double and IFN‐γ single expressors as compared with the other CD4⁺ T‐cell subsets. Proportions of the other double or single CD4⁺ T‐cell expressors did not differ between TB and LTBI subjects. These distinct IFN‐γ, IL‐2 and TNF‐α profiles of M. tuberculosis‐specific CD4⁺ T cells seem to be associated with live bacterial loads, as indicated by the decrease in frequency of multifunctional T cells in TB‐infected patients after completion of anti‐mycobacterial therapy. Our results suggest that phenotypic and functional signatures of CD4⁺ T cells may serve as immunological correlates of protection and curative host responses, and be a useful tool to monitor the efficacy of anti‐mycobacterial therapy.     

Micro‐RNA‐155 inhibits IFN‐γ signaling in CD4+ T cells

Micro‐RNA (miR) are increasingly recognized as critical regulators of tissue‐specific patterns of gene expression. CD4⁺ T cells lacking miR‐155, for example, exhibit bias towards Th2 differentiation, indicating that the absence of individual miR could alter CD4⁺ T‐cell differentiation. We now show that miR‐155 is induced upon T‐cell activation and that it promotes Th1 differentiation when over‐expressed in activated CD4⁺ T cells. Antagonism of miR‐155 leads to induction of IFN‐γ receptor α‐chain (IFN‐γRα, and a functional miR‐155 target site is identified within the 3′ untranslated region of IFN‐γRα. These results identify IFN‐γRα as a second miR‐155 target in T cells and suggest that miR‐155 contributes to Th1 differentiation in CD4⁺ T cells by inhibiting IFN‐γ signaling.     

Preferentially expanding Vγ1+ γδ T cells are associated with protective immunity against Plasmodium infection in mice

γδ T cells play a crucial role in controlling malaria parasites. Dendritic cell (DC) activation via CD40 ligand (CD40L)‐CD40 signaling by γδ T cells induces protective immunity against the blood‐stage Plasmodium berghei XAT (PbXAT) parasites in mice. However, it is unknown which γδ T‐cell subset has an effector role and is required to control the Plasmodium infection. Here, using antibodies to deplete TCR Vγ1⁺ cells, we saw that Vγ1⁺ γδ T cells were important for the control of PbXAT infection. Splenic Vγ1⁺ γδ T cells preferentially expand and express CD40L, and both Vγ1⁺ and Vγ4⁺ γδ T cells produce IFN‐γ during infection. Although expression of CD40L on Vγ1⁺ γδ T cells is maintained during infection, the IFN‐γ positivity of Vγ1⁺ γδ T cells is reduced in late‐phase infection due to γδ T‐cell dysfunction. In Plasmodium‐infected IFN‐γ signaling‐deficient mice, DC activation is reduced, resulting in the suppression of γδ T‐cell dysfunction and the dampening of γδ T‐cell expansion in the late phase of infection. Our data suggest that Vγ1⁺ γδ T cells represent a major subset responding to PbXAT infection and that the Vγ1⁺ γδ T‐cell response is dependent on IFN‐γ‐activated DCs.     

Mycobacterium tuberculosis PstS1 amplifies IFN‐γ and induces IL‐17/IL‐22 responses by unrelated memory CD4+ T cells via dendritic cell activation

The immunological mechanisms that modulate protection during Mycobacterium tuberculosis (Mtb) infection or vaccination are not fully understood. Secretion of IFN‐γ and, to a lesser extent, of IL‐17 by CD4⁺ T cells plays a major role both in protection and immunopathology. Few Mtb Ags interacting with DCs affect priming, activation, and regulation of Ag‐unrelated CD4⁺ T‐cell responses. Here we demonstrate that PstS1, a 38 kDa‐lipoprotein of Mtb, promotes Ag‐independent activation of memory T lymphocytes specific for Ag85B or Ag85A, two immunodominant protective Ags of Mtb. PstS1 expands CD4⁺ and CD8⁺ memory T cells, amplifies secretion of IFN‐γ and IL‐22 and induces IL‐17 production by effector memory cells in an Ag‐unrelated manner in vitro and in vivo. These effects were mediated through the stimulation of DCs, particularly of the CD8α^? subtype, which respond to PstS1 by undergoing phenotypic maturation and by secreting IL‐6, IL‐1β and, to a lower extent, IL‐23. IL‐6 secretion by PstS1‐stimulated DCs was required for IFN‐γ, and to a lesser extent for IL‐22 responses by Ag85B‐specific memory T cells. These results may open new perspectives for immunotherapeutic strategies to control Th1/Th17 immune responses in Mtb infections and in vaccinations against tuberculosis.     

IFN‐γR1 defects: Mutation update and description of the IFNGR1 variation database

IFN‐γ signaling is essential for the innate immune defense against mycobacterial infections. IFN‐γ signals through the IFN‐γ receptor, which consists of a tetramer of two IFN‐γR1 chains in complex with two IFN‐γR2 chains, where IFN‐γR1 is the ligand‐binding chain of the interferon‐γ receptor and IFN‐γR2 is the signal‐transducing chain of the IFN‐γ receptor. Germline mutations in the gene IFNGR1 encoding the IFN‐γR1 cause a primary immunodeficiency that mainly leads to mycobacterial infections. Here, we review the molecular basis of this immunodeficiency in the 130 individuals described to date, and report mutations in five new individuals, bringing the total number to 135 individuals from 98 kindreds. Forty unique IFNGR1 mutations have been reported and they exert either an autosomal dominant or an autosomal recessive effect. Mutations resulting in premature stopcodons represent the majority of IFNGR1 mutations (60%; 24 out of 40), followed by amino acid substitutions (28%, 11 out of 40). All known mutations, as well as 287 other variations, have been deposited in the online IFNGR1 variation database ( www.LOVD.nl/IFNGR1 ). In this article, we review the function of IFN‐γR1 and molecular genetics of human IFNGR1.     

Siglec‐1 inhibits RSV‐induced interferon gamma production by adult T cells in contrast to newborn T cells

Interferon gamma (IFN‐γ) plays an important role in the antiviral immune response during respiratory syncytial virus (RSV) infections. Monocytes and T cells are recruited to the site of RSV infection, but it is unclear whether cell‐cell interactions between monocytes and T cells regulate IFN‐γ production. In this study, micro‐array data identified the upregulation of sialic acid‐binding immunoglobulin‐type lectin 1 (Siglec‐1) in human RSV‐infected infants. In vitro, RSV increased expression of Siglec‐1 on healthy newborn and adult monocytes. RSV‐induced Siglec‐1 on monocytes inhibited IFN‐γ production by adult CD4⁺ T cells. In contrast, IFN‐γ production by RSV in newborns was not affected by Siglec‐1. The ligand for Siglec‐1, CD43, is highly expressed on adult CD4⁺ T cells compared to newborns. Our data show that Siglec‐1 reduces IFN‐γ release by adult T cells possibly by binding to the highly expressed CD43. The Siglec‐1‐dependent inhibition of IFN‐γ in adults and the low expression of CD43 on newborn T cells provides a better understanding of the immune response against RSV in early life and adulthood.     

IL‐18‐induced expression of high‐affinity IL‐2R on murine NK cells is essential for NK‐cell IFN‐γ production during murine Plasmodium yoelii infection

Early production of pro‐inflammatory cytokines, including IFN‐γ, is essential for control of blood‐stage malaria infections. We have shown that IFN‐γ production can be induced among human natural killer (NK) cells by coculture with Plasmodium falciparum infected erythrocytes, but the importance of this response is unclear. To further explore the role of NK cells during malaria infection, we have characterized the NK‐cell response of C57BL/6 mice during lethal (PyYM) or nonlethal (Py17XNL) P. yoelii infection. Ex vivo flow cytometry revealed that NK cells are activated within 24 h of Py17XNL blood‐stage infection, expressing CD25 and producing IFN‐γ; this response was blunted and delayed during PyYM infection. CD25 expression and IFN‐γ production were highly correlated, suggesting a causal relationship between the two responses. Subsequent in vitro experiments revealed that IL‐18 signaling is essential for induction of CD25 and synergizes with IL‐12 to enhance CD25 expression on splenic NK cells. In accordance with this, Py17XNL‐infected erythrocytes induced NK‐cell CD25 expression and IFN‐γ production in a manner that is completely IL‐18‐ and partially IL‐12‐dependent, and IFN‐γ production is enhanced by IL‐2. These data suggest that IL‐2 signaling via CD25 amplifies IL‐18‐ and IL‐12‐mediated NK‐cell activation during malaria infection.     

T‐cell receptor activation of human CD4+ T cells shifts the innate TLR response from CXCL8hiIFN‐γnull to CXCL8loIFN‐γhi

Toll‐like receptors (TLRs) play a major part in providing innate immunity against pathogenic microorganisms. Recent studies show that these receptors are also expressed on T cells, which are the sentinels of adaptive immunity. Here, we have investigated the regulatory role of the T‐cell receptor in the functioning of these innate receptors in T cells. We show that freshly isolated human CD4⁺ T cells readily secrete the neutrophil chemoattractant CXCL8 upon activation with the TLR ligands Pam3CSK and flagellin. In contrast, TCR‐activated cells secrete considerably less CXCL8 but start producing IFN‐γ upon stimulation with TLR agonists in the absence of concomitant TCR engagement. These T cells show increased activation of p38 and JNK MAP‐kinases in response to TLR stimulation, and inhibition of p38 abrogates TLR‐induced IFN‐γ secretion. The shifting of the T‐cell innate immune response from CXCL8^hiIFN‐γ^null in freshly isolated to CXCL8^loIFN‐γ^hi in activated T cells is also observed in response to endogenous innate stimulus, IL‐1. These results suggest that the innate immune response of human CD4⁺ T cells switches from a proinflammatory to an effector type following activation of these cells through the antigen receptor.     

Type I interferons contribute to experimental cerebral malaria development in response to sporozoite or blood‐stage Plasmodium berghei ANKA

Cerebral malaria is a severe complication of Plasmodium falciparum infection. Although T‐cell activation and type II IFN‐γ are required for Plasmodium berghei ANKA (PbA)‐induced murine experimental cerebral malaria (ECM), the role of type I IFN‐α/β in ECM development remains unclear. Here, we address the role of the IFN‐α/β pathway in ECM devel‐opment in response to hepatic or blood‐stage PbA infection, using mice deficient for types I or II IFN receptors. While IFN‐γR1^?/? mice were fully resistant, IFNAR1^?/? mice showed delayed and partial protection to ECM after PbA infection. ECM resistance in IFN‐γR1^?/? mice correlated with unaltered cerebral microcirculation and absence of ischemia, while WT and IFNAR1^?/? mice developed distinct microvascular pathologies. ECM resistance appeared to be independent of parasitemia. Instead, key mediators of ECM were attenuated in the absence of IFNAR1, including PbA‐induced brain sequestration of CXCR3⁺‐activated CD8⁺ T cells. This was associated with reduced expression of Granzyme B, IFN‐γ, IL‐12Rβ2, and T‐cell‐attracting chemokines CXCL9 and CXCL10 in IFNAR1^?/? mice, more so in the absence of IFN‐γR1. Therefore, the type I IFN‐α/β receptor pathway contributes to brain T‐cell responses and microvascular pathology, although it is not as essential as IFN‐γ for the development of cerebral malaria upon hepatic or blood‐stage PbA infection.     

Evaluation of a whole‐blood chemiluminescent immunoassay of IFN‐γ, IP‐10, and MCP‐1 for diagnosis of active pulmonary tuberculosis and tuberculous pleurisy patients

The study explored the use of IP‐10, MCP‐1, and IFN‐γ as biomarkers to improve the diagnoses of active pulmonary tuberculosis and tuberculous pleurisy. We enrolled 267 individuals, including 134 TB patients, 93 patients with non‐tuberculous pulmonary diseases, and 40 healthy controls. Whole bloods were stimulated in vitro with rCFP‐10/ESAT‐6 protein antigen of Mycobacterium tuberculosis. The levels of IFN‐γ, IP‐10, and MCP‐1 in cultured supernatants of whole bloods were detected by a chemiluminescence immunoassay. A receiver operating characteristic (ROC) curve was drawn to determine the cutoff value for diagnosing TB and to evaluate the diagnostic efficacies of the IFN‐γ, IP‐10, and MCP‐1 for TB. The antigen‐specific release of each cytokine, IFN‐γ, IP‐10, and MCP‐1, was significantly higher in the TB groups than in either the non‐tuberculous pulmonary disease group (p < 0.001) or the healthy control group (p < 0.001). The ROC curves indicated cutoff values for IFN‐γ, IP‐10, and MCP‐1 at 147.8, 160.4, and 496.4 pg/mL, respectively. The sensitivity, specificity, PPV, NPV, and diagnostic efficiency for IFN‐γ were 85.8%, 70.7%, 74.7%, 83.2%, and 78.3%, respectively; for IP‐10 were 72.4%, 75.9%, 75.2%, 73.2%, and 74.2%, respectively; and for MCP‐1 were 90.3%, 97.0%, 96.8%, 90.8%, and 93.6%, respectively. IFN‐γ combined MCP‐1 improved the sensitivity to 97.8% compared with IFN‐γ (p < 0.001). Our findings indicate high sensitivity and specificity of MCP‐1 as novel biomarkers for the diagnosis of active pulmonary tuberculosis and tuberculous pleurisy.     

Activation‐dependent cell death of human monocytes is a novel mechanism of fine‐tuning inflammation and autoimmunity

In patients with juvenile idiopathic arthritis (JIA), increased release of IFN‐γ and GM‐CSF in cells infiltrating synovial tissue can be a potent driver of monocyte activation. Given the fundamental role of monocyte activation in remodeling the early phases of inflammatory responses, here we analyze the GM‐CSF/IFN‐γ induced activity of human monocytes in such a situation in vitro and in vivo. Monocytes from healthy donors were isolated and stimulated with GM‐CSF ± IFN‐γ. Monocyte activation and death were analyzed by flow cytometry, immunofluorescence microscopy, ELISA, and qPCR. T‐cell GM‐CSF/IFN‐γ expression and monocyte function were determined in synovial fluid and peripheral blood from 15 patients with active JIA and 21 healthy controls. Simultaneous treatment with GM‐CSF and IFN‐γ induces cell death of monocytes. This cell death is partly cathepsin B‐associated and has morphological characteristics of necrosis. Monocytes responding to costimulation with strong proinflammatory activities are consequently eliminated. Monocytes surviving this form of hyperactivation retain normal cytokine production. Cathepsin B activity is increased in monocytes isolated from synovial fluid from patients with active arthritis. Our data suggest GM‐CSF/IFN‐γ induced cell death of monocytes as a novel mechanism to eliminate overactivated monocytes, thereby potentially balancing inflammation and autoimmunity in JIA.     

Cover Picture: Eur. J. Immunol. 4'14

Our colorful cover features an immunofluorescent image of CD5ΔCK2bd.OT2 CD4⁺ T cells that had been stimulated under Th17‐polarizing conditions but without anti‐IFN‐γ, with OVA323–339 and irradiated APCs. Antibodies against ROR‐γt (red) and CD3 (green) as well as Hoechst staining (blue) show that T cells lacking CD5–CK2 signaling have lower nuclear levels of ROR‐γt after Th17 polarization. The image is taken from the article by McGuire et al. (pp. 1137–1142) in which the authors show that CD5–CK2 signaling enhances AKT activation and GSK3 phosphorylation, impacting on nuclear ROR‐γt localization and Th17‐cell differentiation. These findings show that the CD5–CK2 signaling pathway plays an important role in the GSK3–IFN‐γ axis in regulating Th‐cell differentiation.     

IFN‐γ regulates survival and function of tumor‐induced CD11b+Gr‐1high myeloid derived suppressor cells by modulating the anti‐apoptotic molecule Bcl2a1

Myeloid derived suppressor cells (MDSCs) play a critical role in suppression of immune responses in cancer and inflammation. Here, we describe how regulation of Bcl2a1 by cytokines controls the suppressor function of CD11b⁺Gr‐1^high granulocytic MDSCs. Coculture of CD11b⁺Gr‐1^high granulocytic MDSCs with antigen‐stimulated T cells and simultaneous blockade of IFN‐γ by the use of anti‐IFN‐γ blocking antibody, IFN‐γ^?/? effector T cells, IFN‐γR^?/? MDSCs or STAT1^?/? MDSCs led to upregulation of Bcl2a1 in CD11b⁺Gr‐1^high cells, improved survival, and enhanced their suppressor function. Molecular studies revealed that GM‐CSF released by antigen‐stimulated CD8⁺ T cells induced Bcl2a1 upregulation, which was repressed in the presence of IFN‐γ by a direct interaction of phosphorylated STAT‐1 with the Bcl2a1 promotor. Bcl2a1 overexpressing granulocytic MDSCs demonstrated prolonged survival and enhanced suppressor function in vitro. Our data suggest that IFN‐γ/ STAT1‐dependent regulation of Bcl2a1 regulates survival and thereby suppressor function of granulocytic MDSCs.