CD40 Ligand-Mediated Interactions Are Involved in the Generation of Memory CD8+ Cytotoxic T Lymphocytes (CTL) but Are Not Required for the Maintenance of CTL Memory following Virus Infection

CD8⁺ cytotoxic T lymphocytes (CTL) play a key role in the control of many virus infections, and the need for vaccines to elicit strong CD8⁺ T-cell responses in order to provide optimal protection in such infections is increasingly apparent. However, the mechanisms involved in the induction and maintenance of CD8⁺ CTL memory are currently poorly understood. In this study, we investigated the involvement of CD40 ligand (CD40L)-mediated interactions in these processes by analyzing the memory CTL response of CD40L-deficient mice following infection with lymphocytic choriomeningitis virus (LCMV). The maintenance of memory CD8⁺ CTL precursors (CTLp) at stable frequencies over time was not impaired in CD40L-deficient mice. By contrast, the initial generation of memory CTLp was affected. CD40L-deficient mice produced lower levels of CD8⁺ CTLp during the primary immune response to LCMV than did wild-type controls, despite the fact that the LCMV-specific effector CTL response of CD40L-deficient mice was indistinguishable from that of control animals. The differentiation of naïve CD8⁺ T cells into effector and memory CTL thus involves pathways that can be discriminated from each other by their requirement for CD40L-mediated interactions. Expression of CD40L by CTLp themselves was not an essential step during their expansion and differentiation from naïve CD8⁺ cells into memory CTLp; instead, the reduction in memory CTLp generation in CD40L-deficient mice was likely a consequence of defects in the CD4⁺ T-cell response mounted by these animals. These results thus suggest a previously unappreciated role for CD40L in the generation of CD8⁺ memory CTLp, the probable nature of which is discussed.     

Loss of the orphan nuclear receptor NR2F6 enhances CD8+ T-cell memory via IFN-γ

Memory formation is a hallmark of T cell-mediated immunity, but how differentiation into either short-lived effector cells (SLECs, CD127⁻KLRG1⁺) or memory precursors cells (MPECs, CD127⁺KLRG1⁻) and subsequent regulation of long-term memory is adjusted is incompletely understood. Here, we show that loss of the nuclear orphan receptor NR2F6 in germ-line Nr2f6-deficient mice enhances antigen-specific CD8⁺ memory formation up to 70 days after bacterial infection with Listeria monocytogenes (LmOVA) and boosts inflammatory IFN-γ, TNFα, and IL-2 cytokine recall responses. Adoptive transfer experiments using Nr2f6^−/− OT-I T-cells showed that the augmented memory formation is CD8⁺ T-cell intrinsic. Although the relative difference between the Nr2f6^+/+ and Nr2f6^−/− OT-I memory compartment declines over time, Nr2f6-deficient OT-I memory T cells mount significantly enhanced IFN-γ responses upon reinfection with increased clonal expansion and improved host antigen-specific CD8⁺ T-cell responses. Following a secondary adoptive transfer into naïve congenic mice, Nr2f6-deficient OT-I memory T cells are superior in clearing LmOVA infection. Finally, we show that the commitment to enhanced memory within Nr2f6-deficient OT-I T cells is established in the early phases of the antibacterial immune response and is IFN-γ mediated. IFN-γ blocking normalized MPEC formation of Nr2f6-deficient OT-I T cells. Thus, deletion or pharmacological inhibition of NR2F6 in antigen-specific CD8⁺ T cells may have therapeutic potential for enhancing early IFN-γ production and consequently the functionality of memory CD8⁺ T cells in vivo.Subject terms: Interferons, Bacterial infection     

Priming of Salmonella enterica Serovar Typhi-Specific CD8+ T Cells by Suicide Dendritic Cell Cross-Presentation in Humans

Background

The emergence of antibiotic-resistant strains of Salmonella enterica serovar Typhi (S. Typhi), the etiologic agent of typhoid fever, has aggravated an already important public health problem and added new urgency to the development of more effective typhoid vaccines. To this end it is critical to better understand the induction of immunity to S. Typhi. CD8⁺ T cells are likely to play an important role in host defense against S. Typhi by several effector mechanisms, including killing of infected cells and IFN-γ secretion. However, how S. Typhi regulates the development of specific CD8⁺ responses in humans remains unclear. Recent studies in mice have shown that dendritic cells (DC) can either directly (upon uptake and processing of Salmonella) or indirectly (by bystander mechanisms) elicit Salmonella-specific CD8⁺ T cells.

Methodology/Principal Findings

We report here that upon infection with live S. Typhi, human DC produced high levels of pro-inflammatory cytokines IL-6, IL-8 and TNF-α, but low levels of IL-12 p70 and IFN-γ. In contrast, DC co-cultured with S. Typhi-infected cells, through suicide cross-presentation, uptake S. Typhi-infected human cells and release high levels of IFN-γ and IL-12p70, leading to the subsequent presentation of bacterial antigens and triggering the induction of memory T cells, mostly CD3⁺CD8⁺CD45RA⁻CD62L⁻ effector/memory T cells.

Conclusions/Significance

This study is the first to demonstrate the effect of S. Typhi on human DC maturation and on their ability to prime CD8⁺ cells and highlights the significance of these phenomena in eliciting adaptive immunity to S. Typhi.     

Coxsackievirus B3 Inhibits Antigen Presentation In Vivo,Exerting a Profound and Selective Effect on the MHC Class I Pathway

Many viruses encode proteins whose major function is to evade or disable the host T cell response. Nevertheless, most viruses are readily detected by host T cells, and induce relatively strong T cell responses. Herein, we employ transgenic CD4⁺ and CD8⁺ T cells as sensors to evaluate in vitro and in vivo antigen presentation by coxsackievirus B3 (CVB3), and we show that this virus almost completely inhibits antigen presentation via the MHC class I pathway, thereby evading CD8⁺ T cell immunity. In contrast, the presentation of CVB3-encoded MHC class II epitopes is relatively unencumbered, and CVB3 induces in vivo CD4⁺ T cell responses that are, by several criteria, phenotypically normal. The cells display an effector phenotype and mature into multi-functional CVB3-specific memory CD4⁺ T cells that expand dramatically following challenge infection and rapidly differentiate into secondary effector cells capable of secreting multiple cytokines. Our findings have implications for the efficiency of antigen cross-presentation during coxsackievirus infection.     

Differential Regulation of Effector- and Central-Memory Responses to Toxoplasma gondii Infection by IL-12 Revealed by Tracking of Tgd057-Specific CD8+ T Cells

Production of the pro-inflammatory cytokine IL-12 by innate phagocytes drives the differentiation of IFN-γ-producing effector T cells during Toxoplasma gondii infection. However, the role of IL-12 in the regulation of memory CD8+ T cell differentiation and function during murine toxoplasmosis is unclear. To track memory CTL development, we identified a novel H-2K^b-restricted CTL population specific for the Toxoplasma antigen tgd057. Tgd057-specific CTLs were induced by both vaccination and natural peroral infection, and were representative of the polyclonal CTL population. Tgd057-specific primary effector cells required IL-12 for the differentiation of KLRG1+ effector subpopulations and IFN-γ production in response to restimulation with parasite-infected cells, but not to restimulation with cognate peptide. The effect of IL-12 deficiency during the primary response was profoundly imprinted on memory CTLs, which continued to show defects in cell numbers, KLRG1+ effector memory subpopulation differentiation, and IFN-γ recall responses. Importantly, isolated CD62L^hi KLRG1- CD8+ T cells differentiated in the absence of IL-12 were enhanced in their ability to generate IFN-γ-producing secondary tgd057-specific effector cells. Our data, for the first time, demonstrate the negative impact of IL-12 signaling on the quality of the central memory CTL compartment. Thus, despite the beneficial role of IL-12 in promoting effector differentiation, excessive exposure to IL-12 during CTL priming may limit the development of long-term protective immunity through the decreased fitness of central memory CTL responses.     

Concurrent Generation of Effector and Central Memory CD8 T Cells during Vaccinia Virus Infection

It is generally thought that during the contraction phase of an acute anti-viral T cell reponse, the effector T cells that escape activation-induced cell death eventually differentiate into central memory T cells over the next several weeks. Here we report that antigen-specific CD8T cells with the phenotype and function of central memory cells develop concomitantly with effector T cells during vaccinia virus (vv) infection. As soon as 5 days after an intraperitoneal infection with vv, we could identify a subset of CD44^hi and CD62L⁺ vv-specific CD8 T cells in the peritoneal exudate lymphocytes. This population constituted approximately 10% of all antigen-specific T cells and like central memory T cells, they also expressed high levels of CCR7 and IL-7R but expressed little granzyme B. Importantly, upon adoptive transfer into naïve congenic hosts, CD62L⁺, but not CD62L⁻ CD8 T cells were able to expand and mediate a rapid recall response to a new vv challenge initiated 6 weeks after transfer, confirming that the CD62L⁺ vv-specific CD8 T cells are bonafide memory cells. Our results are thus consistent with the branched differentiation model, where effector and memory cells develop simultaneously. These results are likely to have implications in the context of vaccine design, particularly those based on vaccinia virus recombinants.     

The impact of a boosting immunogen on the differentiation of secondary memory CD8+ T cells

While recent studies have demonstrated that secondary CD8⁺ T cells develop into effector-memory cells, the impact of particular vaccine regimens on the elicitation of these cells remains poorly defined. In the present study we evaluated the effect of three different immunogens—recombinant vaccinia, recombinant adenovirus, and plasmid DNA—on the generation of memory cellular immune responses. We found that vectors that induce the rapid movement of CD8⁺ T cells into the memory compartment during a primary immune response also drive a rapid differentiation of these cells into effector-memory CD8⁺ T cells following a secondary immunization. In contrast, the functional profiles of both CD8⁺ and CD4⁺ T cells, assessed by measuring antigen-stimulated gamma interferon and interleukin-2 production, were not predominantly shaped by the boosting immunogen. We also demonstrated that the in vivo expression of antigen by recombinant vectors was brief following boosting immunization, suggesting that antigen persistence has a minimal impact on the differentiation of secondary CD8⁺ T cells. When used in heterologous or in homologous prime-boost combinations, these three vectors generated antigen-specific CD8⁺ T cells with different phenotypic profiles. Expression of the memory-associated molecule CD27 on effector CD8⁺ T cells decreased following heterologous but not homologous boosting, resulting in a phenotypic profile similar to that seen on primary CD8⁺ T cells. These data therefore suggest that the phenotype of secondary CD8⁺ T cells is determined predominantly by the boosting immunogen whereas the cytokine profile of these cells is shaped by both the priming and boosting immunogens.     

Role of PKR and Type I IFNs in Viral Control during Primary and Secondary Infection

Type I interferons (IFNs) are known to mediate viral control, and also promote survival and expansion of virus-specific CD8⁺ T cells. However, it is unclear whether signaling cascades involved in eliciting these diverse cellular effects are also distinct. One of the best-characterized anti-viral signaling mechanisms of Type I IFNs is mediated by the IFN-inducible dsRNA activated protein kinase, PKR. Here, we have investigated the role of PKR and Type I IFNs in regulating viral clearance and CD8⁺ T cell response during primary and secondary viral infections. Our studies demonstrate differential requirement for PKR, in viral control versus elicitation of CD8⁺ T cell responses during primary infection of mice with lymphocytic choriomeningitis virus (LCMV). PKR-deficient mice mounted potent CD8⁺ T cell responses, but failed to effectively control LCMV. The compromised LCMV control in the absence of PKR was multifactorial, and linked to less effective CD8⁺ T cell-mediated viral suppression, enhanced viral replication in cells, and lower steady state expression levels of IFN-responsive genes. Moreover, we show that despite normal expansion of memory CD8⁺ T cells and differentiation into effectors during a secondary response, effective clearance of LCMV but not vaccinia virus required PKR activity in infected cells. In the absence of Type I IFN signaling, secondary effector CD8⁺ T cells were ineffective in controlling both LCMV and vaccinia virus replication in vivo. These findings provide insight into cellular pathways of Type I IFN actions, and highlight the under-appreciated importance of innate immune mechanisms of viral control during secondary infections, despite the accelerated responses of memory CD8⁺ T cells. Additionally, the results presented here have furthered our understanding of the immune correlates of anti-viral protective immunity, which have implications in the rational design of vaccines.     

CD4+ T Cell-Derived IL-2 Signals during Early Priming Advances Primary CD8+ T Cell Responses

Stimulating naïve CD8⁺ T cells with specific antigens and costimulatory signals is insufficient to induce optimal clonal expansion and effector functions. In this study, we show that the activation and differentiation of CD8⁺ T cells require IL-2 provided by activated CD4⁺ T cells at the initial priming stage within 0–2.5 hours after stimulation. This critical IL-2 signal from CD4⁺ cells is mediated through the IL-2Rβγ of CD8⁺ cells, which is independent of IL-2Rα. The activation of IL-2 signaling advances the restriction point of the cell cycle, and thereby expedites the entry of antigen-stimulated CD8⁺ T-cell into the S phase. Besides promoting cell proliferation, IL-2 stimulation increases the amount of IFNγ and granzyme B produced by CD8⁺ T cells. Furthermore, IL-2 at priming enhances the ability of P14 effector cells generated by antigen activation to eradicate B16.gp33 tumors in vivo. Therefore, our studies demonstrate that a full CD8⁺ T-cell response is elicited by a critical temporal function of IL-2 released from CD4⁺ T cells, providing mechanistic insights into the regulation of CD8⁺ T cell activation and differentiation.     

iNOS-Producing Inflammatory Dendritic Cells Constitute the Major Infected Cell Type during the Chronic Leishmania major Infection Phase of C57BL/6 Resistant Mice

Leishmania major parasites reside and multiply in late endosomal compartments of host phagocytic cells. Immune control of Leishmania growth absolutely requires expression of inducible Nitric Oxide Synthase (iNOS/NOS2) and subsequent production of NO. Here, we show that CD11b⁺ CD11c⁺ Ly-6C⁺ MHC-II⁺ cells are the main iNOS-producing cells in the footpad lesion and in the draining lymph node of Leishmania major-infected C57BL/6 mice. These cells are phenotypically similar to iNOS-producing inflammatory DC (iNOS-DC) observed in the mouse models of Listeria monocytogenes and Brucella melitensis infection. The use of DsRed-expressing parasites demonstrated that these iNOS-producing cells are the major infected population in the lesions and the draining lymph nodes. Analysis of various genetically deficient mouse strains revealed the requirement of CCR2 expression for the recruitment of iNOS-DC in the draining lymph nodes, whereas their activation is strongly dependent on CD40, IL-12, IFN-γ and MyD88 molecules with a partial contribution of TNF-α and TLR9. In contrast, STAT-6 deficiency enhanced iNOS-DC recruitment and activation in susceptible BALB/c mice, demonstrating a key role for IL-4 and IL-13 as negative regulators. Taken together, our results suggest that iNOS-DC represent a major class of Th1-regulated effector cell population and constitute the most frequent infected cell type during chronic Leishmania major infection phase of C57BL/6 resistant mice.     

Inflammasome-Mediated Inhibition of Listeria monocytogenes-Stimulated Immunity Is Independent of Myelomonocytic Function

Activation of the Nlrc4 inflammasome results in the secretion of IL-1β and IL-18 through caspase-1 and induction of pyroptosis. L. monocytogenes engineered to activate Nlrc4 by expression of Legionella pneumophilia flagellin (L. monocytogenes L.p.FlaA) are less immunogenic for CD8⁺ T cell responses than wt L. monocytogenes. It is also known that IL-1β orchestrates recruitment of myelomonocytic cells (MMC), which have been shown to interfere with T cell-dendritic cells (DC) interactions in splenic white pulp (WP), limiting T cell priming and protective immunity. We have further analyzed the role of MMCs in the immunogenicity of L. monocytogenes L.p.FlaA. We confirmed that MMCs infiltrate the WP between 24–48 hours in response to wt L. monocytogenes infection and that depletion of MMCs enhances CD8⁺ T cell priming and protective memory. L. monocytogenes L.p.FlaA elicited accelerated recruitment of MMCs into the WP. While MMCs contribute to control of L. monocytogenes L.p.FlaA, MMC depletion did not increase immunogenicity of L.p.FlaA expressing strains. There was a significant decrease in L. monocytogenes L.p.FlaA in CD8α⁺ DCs independent of MMCs. These findings suggest that limiting inflammasome activation is important for bacterial accumulation in CD8α⁺ DCs, which are known to be critical for T cell response to L. monocytogenes.     

Impaired hepatitis C virus (HCV)-specific effector CD8+ T cells undergo massive apoptosis in the peripheral blood during acute HCV infection and in the liver during the chronic phase of infection

A majority of patients infected with hepatitis C virus (HCV) do not sustain an effective T-cell response, and viremia persists. The mechanism leading to failure of the HCV-specific CD8⁺ T-cell response in patients developing chronic infection is unclear. We investigated apoptosis susceptibility of HCV-specific CD8⁺ T cells during the acute and chronic stages of infection. Although HCV-specific CD8⁺ T cells in the blood during the acute phase of infection and in the liver during the chronic phase were highly activated and expressed an effector phenotype, the majority was undergoing apoptosis. In contrast, peripheral blood HCV-specific CD8⁺ T cells during the chronic phase expressed a resting memory phenotype. Apoptosis susceptibility of HCV-specific CD8⁺ T cells was associated with very high levels of programmed death-1 (PD-1) and low CD127 expression and with significant functional T-cell deficits. Further evaluation of the “death phase” of HCV-specific CD8⁺ T cells during acute HCV infection showed that the majority of cells were dying by a process of cytokine withdrawal, mediated by activated caspase 9. Contraction during the acute phase occurred rapidly via this process despite the persistence of the virus. Remarkably, in the chronic phase of HCV infection, at the site of infection in the liver, a substantial frequency of caspase 9-mediated T-cell death was also present. This study highlights the importance of cytokine deprivation-mediated apoptosis with consequent down-modulation of the immune response to HCV during acute and chronic infections.     

Allogeneic differences in the dependence on CD4+ T-cell help for virus-specific CD8+ T-cell differentiation

CD4⁺ T-cell help enables antiviral CD8⁺ T cells to differentiate into fully competent memory cells and sustains CD8⁺ T-cell-mediated immunity during persistent virus infection. We recently reported that mice of C57BL/6 and C3H strains differ in their dependence on CD28 and CD40L costimulation for long-term control of infection by polyoma virus, a persistent mouse pathogen. In this study, we asked whether mice of these inbred strains also vary in their requirement for CD4⁺ T-cell help for generating and maintaining polyoma virus-specific CD8⁺ T cells. CD4⁺ T-cell-depleted C57BL/6 mice mounted a robust antiviral CD8⁺ T-cell response during acute infection, whereas unhelped CD8⁺ T-cell effectors in C3H mice were functionally impaired during acute infection and failed to expand upon antigenic challenge during persistent infection. Using (C57BL/6 × C3H)F₁ mice, we found that the dispensability for CD4⁺ T-cell help for the H-2^b-restricted polyoma virus-specific CD8⁺ T-cell response during acute infection extends to the H-2^k-restricted antiviral CD8⁺ T cells. Our findings demonstrate that dependence on CD4⁺ T-cell help for antiviral CD8⁺ T-cell effector differentiation can vary among allogeneic strains of inbred mice.     

CD4+ T cells modulate expansion and survival but not functional properties of effector and memory CD8+ T cells induced by malaria sporozoites

CD4⁺ helper T cells are critical orchestrators of immune responses to infection and vaccination. During primary responses, naïve CD8⁺ T cells may need “CD4 help” for optimal development of memory populations. The immunological factors attributed to CD4 help depend on the context of immunization and vary depending on the priming system. In response to immunization with radiation-attenuated Plasmodium yoelii sporozoites, CD8⁺ T cells in BALB/c mice fail to generate large numbers of effector cells without help from CD4⁺ T cells – a defect not observed in most systems. Given this unique early dependence on CD4 help, we evaluated the effects of CD4⁺ cells on the development of functional properties of CD8⁺ T cells and on their ability to abolish infection. First, we determined that this effect was not mediated by CD4⁺ non-T cells and did not involve CD1d-restricted NKT cells. We found that CD8⁺ T cells induced by sporozoites without CD4 help formed memory populations severely reduced in magnitude that could not limit parasite development in the liver. The inability of these “helpless” memory T cells to protect is not a result of defects in effector function, as their capacity to produce cytokines and undergo cytotoxic degranulation was indistinguishable from control memory T cells. These data indicate that CD4⁺ T help may not be necessary to develop the functional attributes of CD8⁺ T cells; however they are crucial to ensure the survival of effector and memory cells induced in primary responses.     

Immunophenotypic Lymphocyte Profiles in Human African Trypanosomiasis

Human African trypanosomiasis (HAT) is a deadly vector-born disease caused by an extracellular parasite, the trypanosome. Little is known about the cellular immune responses elicited by this parasite in humans. We used multiparameter flow cytometry to characterize leukocyte immunophenotypes in the blood and cerebrospinal fluid (CSF) of 33 HAT patients and 27 healthy controls identified during a screening campaign in Angola and Gabon. We evaluated the subsets and activation markers of B and T lymphocytes. Patients had a higher percentage of CD19⁺ B lymphocytes and activated B lymphocytes in the blood than did controls, but lacked activated CD4+ T lymphocytes (CD25⁺). Patients displayed no increase in the percentage of activated CD8+ T cells (HLA-DR⁺, CD69⁺ or CD25⁺), but memory CD8 T-cell levels (CD8⁺CD45RA⁻) were significantly lower in patients than in controls, as were effector CD8 T-cell levels (CD8⁺CD45RA⁺CD62L⁻). No relationship was found between these blood immunophenotypes and disease severity (stage 1 vs 2). However, CD19⁺ B-cell levels in the CSF increased with disease severity. The patterns of T and B cell activation in HAT patients suggest that immunomodulatory mechanisms may operate during infection. Determinations of CD19⁺ B-cell levels in the CSF could improve disease staging.     

CD8+ T-Cells Expressing Interferon Gamma or Perforin Play Antagonistic Roles in Heart Injury in Experimental Trypanosoma Cruzi-Elicited Cardiomyopathy

In Chagas disease, CD8⁺ T-cells are critical for the control of Trypanosoma cruzi during acute infection. Conversely, CD8⁺ T-cell accumulation in the myocardium during chronic infection may cause tissue injury leading to chronic chagasic cardiomyopathy (CCC). Here we explored the role of CD8⁺ T-cells in T. cruzi-elicited heart injury in C57BL/6 mice infected with the Colombian strain. Cardiomyocyte lesion evaluated by creatine kinase-MB isoenzyme activity levels in the serum and electrical abnormalities revealed by electrocardiogram were not associated with the intensity of heart parasitism and myocarditis in the chronic infection. Further, there was no association between heart injury and systemic anti-T. cruzi CD8⁺ T-cell capacity to produce interferon-gamma (IFNγ) and to perform specific cytotoxicity. Heart injury, however, paralleled accumulation of anti-T. cruzi cells in the cardiac tissue. In T. cruzi infection, most of the CD8⁺ T-cells segregated into IFNγ⁺ perforin (Pfn)^neg or IFNγ^negPfn⁺ cell populations. Colonization of the cardiac tissue by anti-T. cruzi CD8⁺Pfn⁺ cells paralleled the worsening of CCC. The adoptive cell transfer to T. cruzi-infected cd8 ^−/− recipients showed that the CD8⁺ cells from infected ifnγ^−/− pfn ^+/+ donors migrate towards the cardiac tissue to a greater extent and caused a more severe cardiomyocyte lesion than CD8⁺ cells from ifnγ ^+/+ pfn ^−/− donors. Moreover, the reconstitution of naïve cd8 ^−/− mice with CD8⁺ cells from naïve ifnγ ^+/+ pfn ^−/− donors ameliorated T. cruzi-elicited heart injury paralleled IFNγ⁺ cells accumulation, whereas reconstitution with CD8⁺ cells from naïve ifnγ ^−/− pfn ^+/+ donors led to an aggravation of the cardiomyocyte lesion, which was associated with the accumulation of Pfn⁺ cells in the cardiac tissue. Our data support a possible antagonist effect of CD8⁺Pfn⁺ and CD8⁺IFNγ⁺ cells during CCC. CD8⁺IFNγ⁺ cells may exert a beneficial role, whereas CD8⁺Pfn⁺ may play a detrimental role in T. cruzi-elicited heart injury.