Control Of Chronic Viral Infections Research Articles

PIM kinases are essential for CD8 T cell effector function and metabolism during chronic viral infection

Abstract The production of IL-21 by CD4 T cells is one mechanism by which CD8 T cells can sustain their effector functions, including increased metabolic demand, during chronic viral infection. However, signaling cascades downstream of IL-21 that control effector function in CD8 T cells remain incompletely understood. One such set of targets are the PIM kinases. This family of serine/threonine kinases functions downstream of IL-21 via JAK-STAT signaling and are involved in diverse cellular processes. Treatment of activated CD8 T cells with IL-21 induces expression of PIM1 kinase and phosphorylation of its targets S6 T372, 4EBP T37/42, and BAD S112, which promote transcription and inhibit apoptosis. Using the lymphocytic choriomeningitis virus (LCMV) Clone 13 mouse model of chronic infection, we show that virus-specific effector CD8 T cells display the highest expression of Pim1compared to exhausted and progenitor CD8 T cells at day 21 post-infection (p.i.). Infection of Pim1−/−mice with LCMV Clone 13 results in reduced virus-specific effector CD8 T cells and increased exhausted CD8 T cells at days 21–23 p.i. compared to wildtype controls. In addition, pharmacological inhibition of PIM kinases with AZD1208, a pan-PIM kinase inhibitor, in activated CD8 T cells results in compromised mitochondrial metabolism, which is essential for effector CD8 T cell function. Overall, our results suggest that PIM kinases are critical for effector CD8 T cell differentiation, function, and metabolism in the control of chronic viral infection. Supported by grants from NIH (R01 AI148403) and by an award from the Medical College of Wisconsin Center for Immunology.

Vectored antibody gene delivery restores host B and Tcell control of persistent viral infection.

Passive antibody therapy and vectored antibody gene delivery (VAGD) in particular offer an innovative approach to combat persistent viral diseases. Here, we exploit a small animal model to investigate synergies of VAGD with the host's endogenous immune defense for treating chronic viral infection. An adeno-associated virus (AAV) vector delivering the lymphocytic choriomeningitis virus (LCMV)-neutralizing antibody KL25 (AAV-KL25) establishes protective antibody titers for >200days. When therapeutically administered to chronically infected immunocompetent wild-type mice, AAV-KL25 affords sustained viral load control. In contrast, viral mutational escape thwarts therapeutic AAV-KL25 effects when mice are unable to mount LCMV-specific antibody responses or lack CD8+ Tcells. VAGD augments antiviral germinal center B cell and antibody-secreting cell responses and reduces inhibitory receptor expression on antiviral CD8+ Tcells. These results indicate that VAGD fortifies host immune defense and synergizes with B cell and CD8 Tcell responses to restore immune control of chronic viral infection.

ILC3-derived acetylcholine promotes protease-driven allergic lung pathology

ILC3-derived acetylcholine promotes protease-driven allergic lung pathology

Role of CD4+ T Cells in the Control of Viral Infections: Recent Advances and Open Questions.

CD4+ T cells orchestrate adaptive immune responses through their capacity to recruit and provide help to multiple immune effectors, in addition to exerting direct effector functions. CD4+ T cells are increasingly recognized as playing an essential role in the control of chronic viral infections. In this review, we present recent advances in understanding the nature of CD4+ T cell help provided to antiviral effectors. Drawing from our studies of natural human immunodeficiency virus (HIV) control, we then focus on the role of high-affinity T cell receptor (TCR) clonotypes in mediating antiviral CD4+ T cell responses. Last, we discuss the role of TCR affinity in determining CD4+ T cell differentiation, reviewing the at times divergent studies associating TCR signal strength to the choice of a T helper 1 (Th1) or a T follicular helper (Tfh) cell fate.

Within-Host Phenotypic Evolution and the Population-Level Control of Chronic Viral Infections by Treatment and Prophylaxis

Chronic viral infections can persist for decades spanning thousands of viral generations, leading to a highly diverse population of viruses with its own complex evolutionary history. We propose an expandable mathematical framework for understanding how the emergence of genetic and phenotypic diversity affects the population-level control of those infections by both non-curative treatment and chemo-prophylactic measures. Our frameworks allows both neutral and phenotypic evolution, and we consider the specific evolution of contagiousness, resistance to therapy, and efficacy of prophylaxis. We compute both the controlled and uncontrolled, population-level basic reproduction number accounting for the within-host evolutionary process where new phenotypes emerge and are lost in infected persons, which we also extend to include both treatment and prophylactic control efforts. We used these results to discuss the conditions under which the relative efficacy of prophylactic versus therapeutic methods of control are superior. Finally, we give expressions for the endemic equilibrium of these models for certain constrained versions of the within-host evolutionary model providing a potential method for estimating within-host evolutionary parameters from population-level genetic sequence data.

PD-1 Signaling Promotes Control of Chronic Viral Infection by Restricting Type-I-Interferon-Mediated Tissue Damage

SUMMARYImmune responses are essential for pathogen elimination but also cause tissue damage, leading to disease or death. However, it is unclear how the host immune system balances control of infection and protection from the collateral tissue damage. Here, we show that PD-1-mediated restriction of immune responses is essential for durable control of chronic LCMV infection in mice. In contrast to responses in the chronic phase, PD-1 blockade in the subacute phase of infection paradoxically results in viral persistence. This effect is associated with damage to lymphoid architecture and subsequently decreases adaptive immune responses. Moreover, this tissue damage is type I interferon dependent, as sequential blockade of the interferon receptor and PD-1 pathways prevents immunopathology and enhances control of infection. We conclude that PD-1-mediated suppression is required as an immunoregulatory mechanism for sustained responses to chronic viral infection by antagonizing type-I interferon-dependent immunopathology.

CD8+ T Cell Priming in Established Chronic Viral Infection Preferentially Directs Differentiation of Memory-like Cells for Sustained Immunity

CD8+ Tcell exhaustion impedes control of chronic viral infection; yet how new Tcell responses are mounted during chronic infection is unclear. Unlike Tcells primed at the onset of infection that rapidly differentiate into effectors and exhaust, we demonstrate that virus-specific CD8+ Tcells primed after establishment of chronic LCMV infection preferentially generate memory-like transcription factor TCF1+ cells that were transcriptionally and proteomically distinct, less exhausted, and more responsive to immunotherapy. Mechanistically, adaptations of antigen-presenting cells and diminished Tcell signaling intensity promoted differentiation of the memory-like subset at the expense of rapid effector cell differentiation, which was now highly dependent on IL-21-mediated CD4+ Tcell help for its functional generation. Chronic viral infection similarly redirected de novo differentiation of tumor-specific CD8+ Tcells, ultimately preventing cancer control. Thus, targeting these Tcell stimulatory pathways could enable strategies to control chronic infection, tumors, and enhance immunotherapeutic efficacy.

Abstract A26: De novo epigenetic programming restrains PD-1 blockade-mediated T cell rejuvenation

Abstract Immune checkpoint blockade (ICB)-mediated rejuvenation of exhausted T cells has emerged as a promising approach for treating various cancers and chronic infections. However, T cells that become fully exhausted during prolonged antigen exposure remain refractory to ICB-mediated rejuvenation. Given that many of the impaired effector properties of terminally exhausted CD8 T cells appear to be heritably maintained even in the absence of antigen, we investigated the role of de novo DNA methylation programming as a cell-intrinsic mechanism for establishing the ICB-nonresponsive state of T-cell exhaustion. We report that blocking de novo DNA methylation in activated CD8 T cells allows them to retain their effector functions despite chronic stimulation during a persistent viral infection. Whole-genome bisulfite sequencing of antigen-specific murine CD8 T cells at the effector and exhaustion stages of an immune response identified progressively acquired heritable de novo methylation programs that restrict T cell expansion and clonal diversity during PD-1 blockade treatment. Moreover, these exhaustion-associated DNA methylation programs were acquired in tumor-infiltrating PD-1hi CD8 T cells. Therapeutic approaches to reverse these programs can enhance ICB-mediated T cell rejuvenation and ultimately facilitate the control of chronic viral infections and tumor growth. These data establish de novo DNA methylation programming as a regulator of T cell exhaustion and barrier of ICB therapy. Citation Format: Hazem E. Ghoneim, Yiping Fan, Ardiana Moustaki, Hossam Abdelsamed, Pradyot Dash, Pranay Dogra, Robert Carter, Walid Awad, Geoff Neale, Paul G. Thomas, Ben Youngblood. De novo epigenetic programming restrains PD-1 blockade-mediated T cell rejuvenation [abstract]. In: Proceedings of the AACR Special Conference on Tumor Immunology and Immunotherapy; 2017 Oct 1-4; Boston, MA. Philadelphia (PA): AACR; Cancer Immunol Res 2018;6(9 Suppl):Abstract nr A26.

Cytomegalovirus-Specific T Cells Restricted by HLA-Cw*0702 Increase Markedly with Age and Dominate the CD8+ T-Cell Repertoire in Older People.

Cytomegalovirus (CMV) infection elicits a strong T-cell immune response, which increases further during aging in a process termed “memory inflation.” CMV downregulates the expression of HLA-A and HLA-B on the surface of infected cells to limit presentation of viral peptides to T-cells although HLA-C is relatively spared as it also engages with inhibitory killer immunoglobulin receptor receptors and therefore reduces lysis by natural killer cells. We investigated the magnitude and functional properties of CMV-specific CD8+ T-cells specific for 10 peptides restricted by HLA-C in a cohort of 53 donors between the age of 23 and 91 years. This was achieved via peptide stimulation of PBMCs followed by multicolor flow cytometry. Three peptides, derived from proteins generated in the immediate-early period of viral replication and restricted by HLA-Cw*0702, elicited strong immune responses, which increased substantially with age such that the average aggregate response represented 37% of the CD8+ T-cell pool within donors above 70 years of age. Remarkably, a single response represented 70% of the total CD8+ T-cell pool within a 91-year-old donor. HLA-Cw*0702-restricted CD8+ T-cell responses were immunodominant over HLA-A and HLA-B-restricted CMV-specific responses and did not show features of exhaustion such as PD-1 or CD39 expression. Indeed, such CTL exhibit a polyfunctional cytokine profile with co-expression of IFN-γ and TNF-α and a strong cytotoxic phenotype with intracellular expression of perforin and granzymeB. Functionally, HLA-Cw*0702-restricted CTL show exceptionally high avidity for cognate peptide-HLA and demonstrate very early and efficient recognition of virally infected cells. These observations indicate that CD8+ T-cells restricted by HLA-C play an important role in the control of persistent CMV infection and could represent a novel opportunity for CD8+ T-cell therapy of viral infection within immunosuppressed patients. In addition, the findings provide further evidence for the importance of HLA-C-restricted T-cells in the control of chronic viral infection.

Sustained T follicular helper cell response is essential for control of chronic viral infection.

During chronic viral infections, both CD8 and CD4 T cell responses are functionally compromised. Alongside exhaustion of CD8 T cells during chronic viral infections, it has also been documented that the CD4 T cells have an increased propensity to differentiate toward CXCR5+ T follicular helper cell (TFH) lineage. Whether these TFH cells contribute to the immune response to chronic viral infection has remained unclear. Using chronic lymphocytic choriomeningitis virus (LCMV) infection in conjunction with an in vivo system where TFH cells can be conditionally ablated, we have established that these TFH cells do in fact play an important protective function. Specifically, we demonstrate that these TFH cells are essential for the late emergence of neutralizing LCMV-specific antibodies that keep viral titers in check and ultimately allow mice to clear the virus. By supporting the generation of neutralizing antibodies, we show that sustained activity of TFH cells promotes control of the chronic infection in face of exhausted CD8 T cell responses.

Host genetics play a critical role in controlling CD8 T cell function and lethal immunopathology during chronic viral infection.

Effective CD8 T cell responses are vital for the control of chronic viral infections. Many factors of the host immune response contribute to the maintenance of effector CD8 T cell responses versus CD8 T cell exhaustion during chronic infection. Specific MHC alleles and the degree of MHC heterogeneity are associated with enhanced CD8 T cell function and viral control during human chronic infection. However, it is currently unclear to what extent host genetics influences the establishment of chronic viral infection. In order to examine the impact of MHC heterogeneity versus non-MHC host genetics on the development of chronic viral infection, an F1 cross of B10.D2 x B6 (D2B6F1) and BALB.B x BALB/c (BCF1) mice were infected with the clone-13 (Cl-13) strain of lymphocytic choriomeningitis virus (LCMV). Following chronic Cl-13 infection both H-2bxd D2B6F1 and BCF1 mice demonstrated increased viral control compared to homozygous mice. Strikingly, H-2bxd D2B6F1 mice on a C57BL genetic background exhibited mortality following Cl-13 infection. CD8 T cell depletion prevented mortality in Cl-13-infected D2B6F1 mice indicating that mortality was CD8 T-cell-dependent. D2B6F1 mice maintained more CD8 T cell effector cytokine production and exhibited reduced expression of the T cell exhaustion marker PD-1. In addition, D2B6F1 mice also induced a larger Th1 response than BCF1 mice and Cl-13-induced mortality in D2B6F1 mice was also dependent on CD4 T-cell-mediated IFN-γ production. Thus, following a chronic viral infection, increased functionality of the CD8 T cell response allowed for more rapid viral clearance at the cost of enhanced immunopathology dependent on both MHC diversity and the genetic background of the host.

A Critical Role of IL-21-Induced BATF in Sustaining CD8-T-Cell-Mediated Chronic Viral Control.

Control of chronic viral infections by CD8 T cells is critically dependent on CD4 help. In particular, helper-derived IL-21 plays a key role in sustaining the CD8 T cell response; however, the molecular pathways by which IL-21 sustains CD8 T cell immunity remain unclear. We demonstrate that IL-21 causes a phenotypic switch of transcription factor expression in CD8 T cells during chronic viral infection characterized by sustained BATF expression. Importantly, BATF expression during chronic infection is both required for optimal CD8 T cell persistence and anti-viral effector function and sufficient to rescue "unhelped" CD8 T cells. Mechanistically, BATF sustains the response by cooperating with IRF4, an antigen-induced transcription factor that is also critically required for CD8 T cell maintenance, to preserve Blimp-1 expression and thereby sustain CD8 T cell effector function. Collectively, these data suggest that CD4 T cells "help" the CD8 response during chronic infection via IL-21-induced BATF expression.

Cooperativity of HIV-Specific Cytolytic CD4 T Cells and CD8 T Cells in Control of HIV Viremia.

CD4+ T cells play a pivotal role in the control of chronic viral infections. Recently, nontraditional CD4+ T cell functions beyond helper effects have been described, and a role for cytolytic CD4+ T cells in the control of HIV infection has been suggested. We define here the transcriptional, phenotypic, and functional profiles of HIV-specific cytolytic CD4+ T cells. Fluidigm BioMark and multiparameter flow cytometric analysis of HIV-specific cytolytic CD4+ T cells revealed a distinct transcriptional signature compared to Th1 CD4+ cells but shared similar features with HIV-specific cytolytic CD8+ T cells. Furthermore, HIV-specific cytolytic CD4+ T cells showed comparable killing activity relative to HIV-specific CD8+ T cells and worked cooperatively in the elimination of virally infected cells. Interestingly, we found that cytolytic CD4+ T cells emerge early during acute HIV infection and tightly follow acute viral load trajectory. This emergence was associated to the early viral set point, suggesting an involvement in early control, in spite of CD4 T cell susceptibility to HIV infection. Our data suggest cytolytic CD4+ T cells as an independent subset distinct from Th1 cells that show combined activity with CD8+ T cells in the long-term control of HIV infection. The ability of the immune system to control chronic HIV infection is of critical interest to both vaccine design and therapeutic approaches. Much research has focused on the effect of the ability of CD8+ T cells to control the virus, while CD4+ T cells have been overlooked as effectors in HIV control due to the fact that they are preferentially infected. We show here that a subset of HIV-specific CD4+ T cells cooperate in the cytolytic control of HIV replication. Moreover, these cells represent a distinct subset of CD4+ T cells showing significant transcriptional and phenotypic differences compared to HIV-specific Th1 cells but with similarities to CD8+ T cells. These findings are important for our understanding of HIV immunopathology.

Dysfunctional CD8+ T cells in hepatitis B and C are characterized by a lack of antigen-specific T-bet induction.

The transcription factor T-bet regulates the production of interferon-γ and cytotoxic molecules in effector CD8 T cells, and its expression correlates with improved control of chronic viral infections. However, the role of T-bet in infections with differential outcome remains poorly defined. Here, we report that high expression of T-bet in virus-specific CD8 T cells during acute hepatitis B virus (HBV) and hepatitis C virus (HCV) infection was associated with spontaneous resolution, whereas T-bet deficiency was more characteristic of chronic evolving infection. T-bet strongly correlated with interferon-γ production and proliferation of virus-specific CD8 T cells, and its induction by antigen and IL-2 stimulation partially restored functionality in previously dysfunctional T-bet-deficient CD8 T cells. However, restoration of a strong interferon-γ response required additional stimulation with IL-12, which selectively induced the phosphorylation of STAT4 in T-bet(+) CD8 T cells. The observation that T-bet expression rendered CD8 T cells responsive to IL-12 suggests a stepwise mechanism of T cell activation in which T-bet facilitates the recruitment of additional transcription factors in the presence of key cytokines. These findings support a critical role of T-bet for viral clearance and suggest T-bet deficiency as an important mechanism behind chronic infection.

Baseline and Dynamic Expression of Activating NK Cell Receptors in the Control of Chronic Viral Infections: The Paradigm of HIV-1 and HCV.

Natural killer (NK) cell function is regulated by a balance between the triggering of activating and inhibitory receptors expressed on their surface. A relevant effort has been focused so far on the study of KIR carriage/expression setting the basis for NK cell education and self-tolerance. Focus on the evolution and regulation of activating NK receptors has lagged behind so far. Our understanding of activating receptor expression and regulation has recently improved by evidences derived from in vitro and in vivo studies. Virus infection – either acute or chronic – determines preferential expansion of NK cells with specific phenotype, activating receptors, and with recall-like functional activity. Studies on patients with viral infection (HIV and HCV) and specific diverging clinical courses confirm that inter-individual differences may exist in baseline expression of natural cytotoxicity receptors (NKp46 and NKp30). The findings that patients with divergent clinical courses have different kinetics of activating receptor density expression upon NK cell activation in vitro provide an additional, time-dependent, functional parameter. Kinetic changes in receptor expression thus represent an additional parameter to basal receptor density expression. Different expression and inducibilities of activating receptors on NK cells contribute to the high diversity of NK cell populations and may help our understanding of the inter-individual differences in innate responses that underlie divergent disease courses.

Immune functions during chronic viral infections

Chronic viral infections display a major health burden worldwide. Immune control of chronic viral infections requires a balanced interplay between immune activation promoting virus clearance and deactivation of antiviral immunity to prevent tissue damage. Our research is focused on identifying mechanisms regulating the balanced immune control of viral infections. Innate type I interferon production can limit viral replication in infected cells thereby contributing to elimination of viruses. Our data demonstrate that IRF7 is a key transcription factor for systemic type I interferon production. Furthermore, macrophages play a key role in limiting a viral infection after exposure to type I interferon and prevent the infection from becoming chronic [1]. Type I Interferon can activate also other immune cells such as natural killer (NK) cells. However, the role of NK cells during chronic viral infection remains insufficiently understood. Also during infection with lymphocytic choriomeningitis virus, NK cell cytotoxicity can be detected. Furthermore, while anti-viral CD8 + T cells can lose their function during establishment of a chronic viral infection, a mechanism termed T cell exhaustion, NK cell depletion restored T cell function in virus infected animals and prevented chronic viral infection. These effects were mediated by the effector protein perforin [2]. Furthermore, during viral induced hepatitis using the lymphocytic choriomeningitis virus, platelet infiltrates could be observed in snap frozen liver sections. Platelets readily release serotonin after activation. Consistently, liver blood flow was highly reduced during viral infection in control animals, but not in serotonin deficient mice. Consequently, CD8 + T cell immunity in liver tissue of serotonin deficient mice was enhanced and able to eliminate the viral infection faster than corresponding control animals. This resulted in reduced virus induced T cell mediated liver cell damage during viral infection [3]. Moreover, during virus induced liver infection, macrophages and neutrophils exhibited production of reactive oxygen species (ROS), which was dependent on the subunit of the NAPDH oxidase p47phox. Limited ROS production in p47phox deficient mice resulted in enhanced T cell survival and elevated T cell function in liver tissue. Consequently, liver cell damage was highly reduced in absence of the NADPH oxidase regulating subunit p47phox [4]. Taken together these data indicates novel mechanisms during establishment of a chronic viral infection as well as viral induced hepatitis.

Patients with tuberculosis disease have Mycobacterium tuberculosis-specific CD8 T cells with a pro-apoptotic phenotype and impaired proliferative capacity, which is not restored following treatment.

CD8 T cells play a critical role in control of chronic viral infections; however, the role of these cells in containing persistent bacterial infections, such as those caused by Mycobacterium tuberculosis (Mtb), is less clear. We assessed the phenotype and functional capacity of CD8 T cells specific for the immunodominant Mtb antigens CFP-10 and ESAT-6, in patients with pulmonary tuberculosis (TB) disease, before and after treatment, and in healthy persons with latent Mtb infection (LTBI). In patients with TB disease, CFP-10/ESAT-6-specific IFN-γ+ CD8 T cells had an activated, pro-apoptotic phenotype, with lower Bcl-2 and CD127 expression, and higher Ki67, CD57, and CD95 expression, than in LTBI. When CFP-10/ESAT-6-specific IFN-γ+ CD8 T cells were detectable, expression of distinct combinations of these markers was highly sensitive and specific for differentiating TB disease from LTBI. Successful treatment of disease resulted in changes of these markers, but not in restoration of CFP-10/ESAT-6-specific CD8 or CD4 memory T cell proliferative capacity. These data suggest that high mycobacterial load in active TB disease is associated with activated, short-lived CFP-10/ESAT-6-specific CD8 T cells with impaired functional capacity that is not restored following treatment. By contrast, LTBI is associated with preservation of long-lived CFP-10/ESAT-6-specific memory CD8 T cells that maintain high Bcl-2 expression and which may readily proliferate.

Serum IL-21 levels associated with chronic hepatitis B and hepatitis B-related liver failure

The aim of the present study was to investigate the role of interleukin (IL)-21 in chronic hepatitis B virus (HBV) infection. IL-21 stimulates T and B cell responses and plays a role in the control of chronic viral infections. Serum IL-21 levels were measured by enzyme immunoassay in 109 patients with chronic HBV infection at various clinical stages, as well as in 19 healthy controls (HCs). The proportion of T cells producing IL-21 in the peripheral blood was assessed by intracellular cytokine staining and flow cytometry. Mean serum IL-21 levels in patients with chronic hepatitis B (CHB) and the HCs were 303.54±152.77 pg/ml and 68.24±9.06 pg/ml, respectively (P=0.003). In addition, the mean serum IL-21 level in patients with hepatitis B-related acute-on-chronic liver failure (HB-ACLF) was 455.38±412.38 pg/ml, which exhibited a statistically significant difference when compared with the HCs (P=0.000). Serum IL-21 levels were highest in the patients with HB-ACLF (455.38±412.38 pg/ml) and exhibited a significant difference when compared with the CHB patients (P=0.04). The mean serum IL-21 levels in patients with cirrhosis also increased, but there was no statistically significant difference when compared with the HCs (P=0.82). The frequency of IL-21+CD4+ cells also increased compared with the HCs and correlated with the number and percentage of lymphocytes in the peripheral blood. Serum IL-21 levels increased in CHB and HB-ACLF patients. Relatively low serum IL-21 levels in CHB may have a causal role in the persistence of HBV infection. Higher serum levels in HB-ACLF may activate T and B cells to eliminate the virus or injure the liver via the release of inflammatory cytokines.

Costimulation Through GITR Increases Follicular Helper T Cell Formation and Leads To Control Of A Chronic Viral Infection

The glucocorticoid-induced TNF receptor family-related protein (GITR) is an important costimulatory receptor on T cells. We have previously shown that enhanced costimulation through GITR increases the formation of both effector and regulatory CD4+ T cells. Here we explored whether it could also affect humoral immunity and T cell help to B cells. Although development of mature B cells was not affected in GITRL transgenic (tg) mice, we found that the number of follicular helper T cells (CXCR5+ PD1+ CD4+ T cells, Tfh) was significantly increased, including the absolute number of Tfh-B cell conjugates, as revealed by ImageStream analysis. Tfh from GITRL tg mice had normal expression levels of ICOS, SLAM and CD44 and slightly lower levels of CD62L and CCR7 compared to wild-type (WT) littermates. Interestingly, Tfh from GITRL tg mice produced more IFN-g and IL-10, which was accompanied by a biased antibody repertoire (decreased IgG3 and increased IgA, IgG2a and IgG2b). Since Tfh have been implicated in the late control of viral replication, we infected WT and GITRL tg mice with LCMV Clone 13. Surprisingly, at day 30 after infection, we could not detect viral genome in spleen and liver from GITRL tg mice, while WT mice were still infected. Also, PD-1 expression was strongly decreased on virus-specific CD8+ T cells, which correlated with faster viral clearance. All in all, these results indicate that GITR-mediated costimulation enhances the control of chronic viral infections, by boosting and modulating Tfh cell responses. Disclosures:No relevant conflicts of interest to declare.

Costimulation through GITR increases follicular helper T cell formation and leads to control of a chronic viral infection (P1021)

Abstract We have previously shown that enhanced costimulation through GITR by overexpression of GITRL increases the formation of both effector and regulatory CD4+ T cells. Here we explored whether it could also affect humoral immunity and T cell help to B cells. Although development of mature B cells was not affected in GITRL transgenic (tg) mice, we found that the number of follicular helper T cells (CXCR5+ PD1+ CD4+ T cells, Tfh) was significantly increased, including the absolute number of Tfh-B cell conjugates, as revealed by ImageStream analysis. Tfh from GITRL tg mice had normal expression levels of ICOS, SLAM and CD44 and slightly lower levels of CD62L and CCR7 compared to wild-type (WT) littermates. Interestingly, Tfh from GITRL tg mice produced more IFN-γ and IL-10, which was accompanied by a biased antibody repertoire (decreased IgG3 and increased IgA, IgG2a and IgG2b). Since Tfh have been implicated in the late control of viral replication, we infected WT and GITRL tg mice with LCMV Clone 13. Surprisingly, at day 30 after infection, we could not detect viral genome in spleen and liver from GITRL tg mice, while WT mice were still infected. Also, PD-1 expression was strongly decreased on virus-specific CD8+ T cells, which correlated with faster viral clearance. All in all, these results indicate that GITRL signaling enhances the control of chronic viral infections, by boosting and modulating Tfh cell responses.