A role for IFN-gamma from antigen-specific CD8+ T cells in protective immunity to Listeria monocytogenes

Endogenous IFN-γ facilitates Pneumocystis infection and downregulates carbohydrate receptors in CD4+ T cell-depleted mice

IFN-γ plays a critical role in host defense against intracellular pathogens. IFN-γ is produced in the bronchoalveolar lavage fluid of mice infected with Pneumocystis, but the role of IFN-γ in host defense against Pneumocystis remains controversial. It has been previously reported that although exogenous IFN-γ has beneficial effects on eradication of Pneumocystis, endogenous IFN-γ has a negative impact on innate immunity in immunocompromised hosts. Surprisingly, CD4+ T cell-depleted IFN-γ deficient (GKO) mice exhibit resistance to Pneumocystis. Alveolar macrophages (AM) from GKO mice exhibit higher expression of macrophage mannose receptor (MMR) and Dectin-1. Concomitantly, they exhibited greater ability to phagocytize Pneumocystis, and this activity was suppressed by inhibitors of these receptors. Incubation with IFN-γ resulted in a reduction in both the expression of these receptors on AM and their Pneumocystis-phagocytic activity. These results indicate that endogenous IFN-γ facilitates Pneumocystis to escape from host innate immunity by attenuating the phagocytic activity of AM via downregulation of MMR and Dectin-1.

Inflammation Controls Susceptibility of Immune-Experienced Mice to Sepsis

Sepsis, an amplified immune response to systemic infection that leads to life-threatening organ dysfunction, affects >125,000 people/day worldwide with 20% mortality. Modest therapeutic progress for sepsis has been made, in part because of the lack of therapeutic translatability between mouse-based experimental models and humans. One potential reason for this difference stems from the extensive use of immunologically naive specific pathogen-free mice in preclinical research. To address this issue, we used sequential infections with well-defined BSL-2 pathogens to establish a novel immune-experienced mouse model (specific pathogen experienced [SPexp]) to determine the extent to which immunological experience and/or inflammation influences the host capacity to respond to subsequent infections, including sepsis. Consistent with their immunological experience, SPexp inbred or outbred mice had significant changes in the composition and activation status of multiple leukocyte populations known to influence the severity of cecal ligation and puncture-induced sepsis. Importantly, by varying the timing of sepsis induction, we found the level of basal inflammation controls sepsis-induced morbidity and mortality in SPexp mice. In addition, although a beneficial role of NK cells in sepsis was recently demonstrated in specific pathogen-free mice, NK cell depletion before cecal ligation and puncture induction in SPexp mice lead to diminished mortality, suggesting NK cells may have beneficial or detrimental roles in the response to septic insult dependent on host immune status. Thus, data highlight the importance of utilizing immune-experienced models for preclinical studies to interrogate the cellular/molecular mechanism(s) that could be therapeutically exploited during severe and dysregulated infection-induced inflammatory responses, such as sepsis.

Immune Autoregulatory CD8 T Cells Require IFN-γ Responsiveness to Optimally Suppress Central Nervous System Autoimmunity

Investigating the complex cellular interplay controlling immunopathogenic and immunoregulatory responses is critical for understanding multiple sclerosis (MS) and for developing successful immunotherapies. Our group has demonstrated that CNS myelin-specific CD8 T cells unexpectedly harbor immune regulatory capacity in both mouse and human. In particular, PLP178-191-specific CD8 T cells (PLP-CD8) robustly suppress the MS mouse model experimental autoimmune encephalomyelitis. We have recently shown that this depends on PLP-CD8 elaborating IFN-γ and perforin in a coordinated suppression program over time. However, the cellular target and downstream effects of CD8 T cell-derived IFN-γ remains poorly understood. In this study, we show that although wild-type (WT) PLP-CD8 were robustly suppressive in IFN-γR-deficient mice, IFN-γR-deficient PLP-CD8 exhibited suboptimal suppression in WT mice. Compared with WT counterparts, IFN-γR-deficient PLP-CD8 were defective in suppressing disease in IFN-γ-deficient recipients, a scenario in which the only IFN-γ available to WT PLP-CD8 is that which they produce themselves. Further, we found that IFN-γR-deficient PLP-CD8 exhibited altered granzyme/IFN-γ profiles, altered migration in recipients, and deficits in killing capacity in vivo. Collectively, this work suggests that IFN-γ responsiveness allows myelin-specific CD8 T cells to optimally perform autoregulatory function in vivo. These insights may help elucidate future adoptive immunotherapeutic approaches for MS patients.

Live Vaccination Generates Both Disease Tolerance and Host Resistance During Chronic Pulmonary Infection With Highly Virulent Francisella tularensis SchuS4

Disease tolerance can preserve host homeostasis and limit the negative impact of infections. We report that vaccinated mice survived pulmonary challenge with the extremely virulent SchuS4 strain of Francisella tularensis for at least 100 days, despite the persistence of large numbers (~104) of organisms. Transfer of 100 of these resident bacteria to naive animals caused 100% lethality, demonstrating that virulence was maintained. Tissue damage in the lung was limited over the course of infection and was associated with increased levels of amphiregulin. Mice depleted of CD4+ cells had reduced amphiregulin and succumbed to infection. In addition, neutralization of interferon-γ or depletion of CD8+ cells resulted in increased pathogen loads, bacteremia, and death of the host. Conversely, depletion of Ly6G+ neutrophils had no effect on survival and actually resulted in reduced bacterial levels. Understanding the interplay between host resistance and disease tolerance will provide new insights into the understanding of chronic infectious diseases.

Early IFNγ-Mediated and Late Perforin-Mediated Suppression of Pathogenic CD4 T Cell Responses Are Both Required for Inhibition of Demyelinating Disease by CNS-Specific Autoregulatory CD8 T Cells

Pathogenesis of immune-mediated demyelinating diseases like multiple sclerosis (MS) is thought to be governed by a complex cellular interplay between immunopathogenic and immunoregulatory responses. We have previously shown that central nervous system (CNS)-specific CD8 T cells have an unexpected protective role in the mouse model of MS, experimental autoimmune encephalomyelitis (EAE). In this study, we interrogated the suppressive potential of PLP178-191-specific CD8 T cells (PLP-CD8). Here, we show that PLP-CD8, when administered post-disease onset, rapidly ameliorated EAE progression, and suppressed PLP178-191-specific CD4 T cell responses as measured by delayed-type hypersensitivity (DTH). To accomplish DTH suppression, PLP-CD8 required differential production of perforin and IFNγ. Perforin was not required for the rapid suppressive action of these cells, but was critical for maintenance of optimal longer term DTH suppression. Conversely, IFNγ production by PLP-CD8 was necessary for swift DTH suppression, but was less significant for maintenance of longer term suppression. These data indicate that CNS-specific CD8 T cells employ an ordered regulatory mechanism program over a number of days in vivo during demyelinating disease and have mechanistic implications for this immunotherapeutic approach.

CD160 Stimulates CD8+ T Cell Responses and Is Required for Optimal Protective Immunity to Listeria monocytogenes

CD160 promotes NK cell cytotoxicity and IFN-γ production, but the function of CD160 on CD8⁺ T cells remains unclear with some studies supporting a coinhibitory role and others a costimulatory role. In this study, we demonstrate that CD160 has a costimulatory role in promoting CD8⁺ T cell effector functions needed for optimal clearance of oral Listeria monocytogenes infection. CD160^-/- mice did not clear oral L. monocytogenes as efficiently as wild type (WT) littermates. WT RAG^-/- and CD160^-/- RAG^-/- mice similarly cleared L. monocytogenes, indicating that CD160 on NK cells does not contribute to impaired L. monocytogenes clearance. Defective L. monocytogenes clearance is due to compromised intraepithelial lymphocytes and CD8⁺ T cell functions. There was a reduction in the frequencies of granzyme B-expressing intraepithelial lymphocytes in L. monocytogenes-infected CD160^-/- mice as compared with WT littermate controls. Similarly, the frequencies of granzyme B-expressing splenic CD8⁺ T cells and IFN-γ and TNF-α double-producer CD8⁺ T cells were significantly reduced in L. monocytogenes-infected CD160^-/- mice compared with WT littermates. Adoptive transfer studies showed that RAG^-/- recipients receiving CD160^-/- CD8⁺ T cells had a higher mortality, exhibited more weight loss, and had a higher bacterial burden compared with RAG^-/- recipients receiving WT CD8⁺ T cells. These findings demonstrate that CD160 provides costimulatory signals to CD8⁺ T cells needed for optimal CD8⁺ T cell responses and protective immunity during an acute mucosal bacterial infection.

CD28 blockade controls T cell activation to prevent graft-versus-host disease in primates

Controlling graft-versus-host disease (GVHD) remains a major unmet need in stem cell transplantation, and new, targeted therapies are being actively developed. CD28-CD80/86 costimulation blockade represents a promising strategy, but targeting CD80/CD86 with CTLA4-Ig may be associated with undesired blockade of coinhibitory pathways. In contrast, targeted blockade of CD28 exclusively inhibits T cell costimulation and may more potently prevent GVHD. Here, we investigated FR104, an antagonistic CD28-specific pegylated-Fab', in the nonhuman primate (NHP) GVHD model and completed a multiparameter interrogation comparing it with CTLA4-Ig, with and without sirolimus, including clinical, histopathologic, flow cytometric, and transcriptomic analyses. We document that FR104 monoprophylaxis and combined prophylaxis with FR104/sirolimus led to enhanced control of effector T cell proliferation and activation compared with the use of CTLA4-Ig or CTLA4-Ig/sirolimus. Importantly, FR104/sirolimus did not lead to a beneficial impact on Treg reconstitution or homeostasis, consistent with control of conventional T cell activation and IL-2 production needed to support Tregs. While FR104/sirolimus had a salutary effect on GVHD-free survival, overall survival was not improved, due to death in the absence of GVHD in several FR104/sirolimus recipients in the setting of sepsis and a paralyzed INF-γ response. These results therefore suggest that effectively deploying CD28 in the clinic will require close scrutiny of both the benefits and risks of extensively abrogating conventional T cell activation after transplant.

Antigen Exposure History Defines CD8 T Cell Dynamics and Protection during Localized Pulmonary Infections

Unlike systemic infections, little is known about the role of repeated localized infections on (re)shaping pathogen-specific memory CD8 T cell responses. Here, we used primary (1°) and secondary (2°) intranasal influenza virus infections of mice as a model to study intrinsic memory CD8 T cell properties. We show that secondary antigen exposure, relative to a single infection, generates memory CD8 T cell responses of superior magnitude in multiple tissue compartments including blood, spleen, draining lymph nodes, and lung. Unexpectedly, regardless of the significantly higher number of 2° memory CD8 T cells, similar degree of protection against pulmonary challenge was observed in both groups of mice containing 1° or 2° memory CD8 T cells. Mechanistically, using pertussis toxin-induced migration block, we showed that superior antigen-driven proliferation and ability to relocate to the site of infection allowed 1° memory CD8 T cells to accumulate in the infected lung during the first few days after challenge, compensating for the initially lower cell numbers. Taken together, the history of antigen exposures to localized pulmonary infections, through altering basic cell biology, dictates dynamic properties of protective memory CD8 T cell responses. This knowledge has important implications for a design of novel and an improvement of existing vaccines and immunization strategies.

The Notch signaling pathway controls short-lived effector CD8+ T cell differentiation but is dispensable for memory generation

Following an infection, naive CD8(+) T cells expand and differentiate into two main populations of effectors: short-lived effector cells (SLECs) and memory precursor effector cells (MPECs). There is limited understanding of the molecular mechanism and cellular processes governing this cell fate. Notch is a key regulator of cell fate decision relevant in many immunological pathways. In this study, we add to the role of Notch in cell fate decision and demonstrate that the Notch signaling pathway controls the MPEC/SLEC differentiation choice following both Listeria infection and dendritic cell immunization of mice. Although fewer SLECs were generated, Notch deficiency did not alter the rate of memory CD8(+) T cell generation. Moreover, we reveal that the Notch signaling pathway plays a context-dependent role for optimal cytokine production by effector CD8(+) T cells. Together, our results unravel critical functions for the Notch signaling pathway during effector CD8(+) T cell differentiation.

Diet-induced obesity does not impact the generation and maintenance of primary memory CD8 T cells

The extent to which obesity compromises the differentiation and maintenance of protective memory CD8 T cell responses and renders obese individuals susceptible to infection remains unknown. In this study, we show that diet-induced obesity did not impact the maintenance of pre-existing memory CD8 T cells, including acquisition of a long-term memory phenotype (i.e., CD27(hi), CD62L(hi), KLRG1(lo)) and function (i.e., cytokine production, secondary expansion, and memory CD8 T cell-mediated protection). Additionally, obesity did not influence the differentiation and maintenance of newly evoked memory CD8 T cell responses in inbred and outbred hosts generated in response to different types of systemic (LCMV, L. monocytogenes) and/or localized (influenza virus) infections. Interestingly, the rate of naive-to-memory CD8 T cell differentiation after a peptide-coated dendritic cell immunization was similar in lean and obese hosts, suggesting that obesity-associated inflammation, unlike pathogen- or adjuvant-induced inflammation, did not influence the development of endogenous memory CD8 T cell responses. Therefore, our studies reveal that the obese environment does not influence the development or maintenance of memory CD8 T cell responses that are either primed before or after obesity is established, a surprising notion with important implications for future studies aiming to elucidate the role obesity plays in host susceptibility to infections.

CD40L expression permits CD8+ T cells to execute immunologic helper functions

CD8(+) T cells play an essential role in immunity against intracellular pathogens, with cytotoxicity being considered their major effector mechanism. However, we here demonstrate that a major part of central and effector memory CD8(+) T cells expresses CD40L, one key molecule for CD4(+) T-cell-mediated help. CD40L(+) CD8(+) T cells are detectable among human antigen-specific immune responses, including pathogens such as influenza and yellow fever virus. CD40L(+) CD8(+) T cells display potent helper functions in vitro and in vivo, such as activation of antigen-presenting cells, and exhibit a cytokine expression signature similar to CD4(+) T cells and unrelated to cytotoxic CD8(+) T cells. The broad occurrence of CD40L(+) CD8(+) T cells in cellular immunity implicates that helper functions are not only executed by major histocompatibility complex (MHC) class II-restricted CD4(+) helper T cells but are also a common feature of MHC class I-restricted CD8(+) T cell responses. Due to their versatile functional capacities, human CD40L(+) CD8(+) T cells are promising candidate cells for immune therapies, particularly when CD4(+) T-cell help or pathogen-associated molecular pattern signals are limited.

Sustained and incomplete recovery of naive CD8+ T cell precursors after sepsis contributes to impaired CD8+ T cell responses to infection

Patients who survive severe sepsis often display compromised immune function with impairment in innate and adaptive immune responses. These septic patients are highly susceptible to "secondary" infections with intracellular pathogens that are usually controlled by CD8(+) T cells. It is not known when and if this observed immunoparalysis of CD8(+) T cell immunity recovers, and the long-term consequences of sepsis on the ability of naive CD8(+) T cells to respond to subsequent infections are poorly understood. In this study, using the cecal-ligation and puncture mouse model of sepsis, we show that sepsis induces a rapid loss of naive CD8(+) T cells. However, IL-15-dependent numerical recovery is observed a month after initial septic insult. Numerical recovery is accompanied by IL-15-dependent phenotypic changes where a substantial proportion of naive (Ag-inexperienced) CD8(+) T cells display a "memory-like" phenotype (CD44(hi)/CD11a(hi)). Importantly, the impairment of naive CD8(+) T cells to respond to viral and bacterial infection was sustained for month(s) after sepsis induction. Incomplete recovery of naive CD8(+) T cell precursors was observed in septic mice, suggesting that the availability of naive precursors contributes to the sustained impairment in primary CD8(+) T cell responses. Thus, sepsis can result in substantial and long-lasting changes in the available CD8(+) T cell repertoire affecting the capacity of the host to respond to new infections.

Conditional Stat1 ablation reveals the importance of interferon signaling for immunity to Listeria monocytogenes infection

Signal transducer and activator of transcription 1 (Stat1) is a key player in responses to interferons (IFN). Mutations of Stat1 cause severe immune deficiencies in humans and mice. Here we investigate the importance of Stat1 signaling for the innate and secondary immune response to the intracellular bacterial pathogen Listeria monocytogenes (Lm). Cell type-restricted ablation of the Stat1 gene in naïve animals revealed unique roles in three cell types: macrophage Stat1 signaling protected against lethal Lm infection, whereas Stat1 ablation in dendritic cells (DC) did not affect survival. T lymphocyte Stat1 reduced survival. Type I IFN (IFN-I) signaling in T lymphocytes reportedly weakens innate resistance to Lm. Surprisingly, the effect of Stat1 signaling was much more pronounced, indicating a contribution of Stat1 to pathways other than the IFN-I pathway. In stark contrast, Stat1 activity in both DC and T cells contributed positively to secondary immune responses against Lm in immunized animals, while macrophage Stat1 was dispensable. Our findings provide the first genetic evidence that Stat1 signaling in different cell types produces antagonistic effects on innate protection against Lm that are obscured in mice with complete Stat1 deficiency. They further demonstrate a drastic change in the cell type-dependent Stat1 requirement for memory responses to Lm infection.

Signal transducer and activator of transcription 1 (Stat1) is an indispensable component of the cellular response to interferons (IFN) during immune reactions to pathogens. Stat1 deficiency leads to severe immune defects in humans and mice. The sensitivity of animals with complete Stat1 ablation to microbial pathogens prevented determining its contribution to various effector systems of the immune response. By way of tissue-restricted Stat1 ablation we now decipher the impact of Stat1 signaling in different cell populations on the innate and adaptive immune response to the intracellular pathogen Listeria monocytogenes. Our data highlight the importance of and requirement for IFNγ-activated macrophages for clearance of the pathogen during early phases of infection, and show a yet unanticipated detrimental role for T cell Stat1. During secondary responses the picture changes and Stat1 in T cells is crucial for proper clearance of L. monocytogenes. Likewise, Stat1 signaling in dendritic cells plays a fundamental role for adaptive immunity to L. monocytogenes. Exploring the local response to L. monocytogenes infection we reveal a role of Stat1 in shaping the cellular composition of inflammatory infiltrates. Furthermore, Stat1 deficiency in dendritic cells increases the proliferation of regulatory T cells, an effect likely to dampen the antibacterial response.

Inflammatory monocytes and neutrophils are licensed to kill during memory responses in vivo

Immunological memory is a hallmark of B and T lymphocytes that have undergone a previous encounter with a given antigen. It is assumed that memory cells mediate better protection of the host upon re-infection because of improved effector functions such as antibody production, cytotoxic activity and cytokine secretion. In contrast to cells of the adaptive immune system, innate immune cells are believed to exhibit a comparable functional effector response each time the same pathogen is encountered. Here, using mice infected by the intracellular bacterium Listeria monocytogenes, we show that during a recall bacterial infection, the chemokine CCL3 secreted by memory CD8⁺ T cells drives drastic modifications of the functional properties of several populations of phagocytes. We found that inflammatory ly6C⁺ monocytes and neutrophils largely mediated memory CD8⁺ T cell bacteriocidal activity by producing increased levels of reactive oxygen species (ROS), augmenting the pH of their phagosomes and inducing antimicrobial autophagy. These events allowed an extremely rapid control of bacterial growth in vivo and accounted for protective immunity. Therefore, our results provide evidence that cytotoxic memory CD8⁺ T cells can license distinct antimicrobial effector mechanisms of innate cells to efficiently clear pathogens.

The immune system comprises white blood cells that belong to the innate or the adaptive immune arms. Adaptive immune cells such as T and B lymphocytes can give rise to memory cells which mediate long-lived immunity against pathogens. During a recall infection, innate immune phagocytic cells such as monocytes and neutrophils can be critical to kill microbial pathogens inside infected tissues. Whether and how such antimicrobial features of phagocytic cells of the innate immune system are modulated during a memory response in a vaccinated host is not known. The present report shows that cytolytic memory T lymphocytes, an important subpopulation of effector T cells, can drastically enhance the functional killing capacities of monocytes and neutrophils for optimized pathogen clearance from infected hosts. These phagocytes exhibit enhanced generation of oxidative burst, increased phagosomal pH and autophagy, three mechanisms that lead to intracellular pathogen death. This result is important since it suggests that modulating innate immune cells effector activities could be an interesting strategy to enhance vaccine efficacy.

IFN-γ promotes graft-versus-leukemia effects without directly interacting with leukemia cells in mice after allogeneic hematopoietic cell transplantation

The ability of IFN-γ to enhance graft-versus-leukemia (GVL) activity without direct interaction with leukemia cells was examined by comparing GVL effects against IFN-γ receptor-deficient (GRKO) leukemia between wild-type (WT) and IFN-γ-deficient (GKO) allogeneic hematopoietic cell transplantation (allo-HCT). We established a primary IFN-γ-unresponsive T-cell leukemia model using virally-transduced GRKO B6 mouse bone marrow cells overexpressing Notch1. We first assessed GVL effects in lethally-irradiated B6 mice receiving CD4-depleted allo-HCT from WT or GKO BALB/c donors. Administration of CD4(+) cell-depleted allo-HCT from WT, but not GKO, BALB/c donors mediated significant GVL effects against GRKO leukemia. Similar results were obtained in pre-established allogeneic chimeras receiving delayed donor lymphocyte infusion (DLI). Although both WT and GKO DLI achieved significant anti-tumor responses, the former was markedly stronger than the latter. These data indicate that IFN-γ is capable of promoting GVL effects via mechanisms independent of its interaction with leukemia cells.

Increased apoptosis, curtailed expansion and incomplete differentiation of CD8+ T cells combine to decrease clearance of L. monocytogenes in old mice

Aging is accompanied by altered immunity, resulting in a variable state of poorly understood immunodeficiency. While both the numbers and the functionality of naïve T cells are decreased by aging, the impact of these changes upon immune defense against bacterial pathogens in vivo remains understudied. Using a model of Listeria monocytogenes (Lm), where the primary CD8(+) T-cell response is critically important for immune defense, we show that C57BL/6 (B6) mice exhibit an age-dependent reduction in survival, with delayed bacterial clearance in old animals. Kinetic analysis of antigen-specific CD8(+) T-cell expansion showed that CD8(+) effectors begin dividing at the same time in old and adult mice, but that the proliferative burst remained incomplete during discrete windows of time and was coupled with increased effector apoptosis in old mice. Further, antilisterial CD8(+) T cells in old mice showed altered expression of key phenotypic and effector molecules and diminished polyfunctionality, measured by the ability to simultaneously produce multiple effector molecules. These results suggest that defects in functional maturation of CD8(+) cells in aged mice, compounded by (or perhaps coupled to) their reduced expansion in response to infection, yield effector CD8(+) T-cell populations insufficient in size and capability to effectively clear newly encountered intracellular pathogens.

Visualizing early splenic memory CD8+ T cells reactivation against intracellular bacteria in the mouse

Memory CD8(+) T cells represent an important effector arm of the immune response in maintaining long-lived protective immunity against viruses and some intracellular bacteria such as Listeria monocytogenes (L.m). Memory CD8(+) T cells are endowed with enhanced antimicrobial effector functions that perfectly tail them to rapidly eradicate invading pathogens. It is largely accepted that these functions are sufficient to explain how memory CD8(+) T cells can mediate rapid protection. However, it is important to point out that such improved functional features would be useless if memory cells were unable to rapidly find the pathogen loaded/infected cells within the infected organ. Growing evidences suggest that the anatomy of secondary lymphoid organs (SLOs) fosters the cellular interactions required to initiate naive adaptive immune responses. However, very little is known on how the SLOs structures regulate memory immune responses. Using Listeria monocytogenes (L.m) as a murine infection model and imaging techniques, we have investigated if and how the architecture of the spleen plays a role in the reactivation of memory CD8(+) T cells and the subsequent control of L.m growth. We observed that in the mouse, memory CD8(+) T cells start to control L.m burden 6 hours after the challenge infection. At this very early time point, L.m-specific and non-specific memory CD8(+) T cells localize in the splenic red pulp and form clusters around L.m infected cells while naïve CD8(+) T cells remain in the white pulp. Within these clusters that only last few hours, memory CD8(+) T produce inflammatory cytokines such as IFN-gamma and CCL3 nearby infected myeloid cells known to be crucial for L.m killing. Altogether, we describe how memory CD8(+) T cells trafficking properties and the splenic micro-anatomy conjugate to create a spatio-temporal window during which memory CD8(+) T cells provide a local response by secreting effector molecules around infected cells.

The roles of IL-12 and IL-23 in CD8+ T cell-mediated immunity against Listeria monocytogenes: Insights from a DC vaccination model

Listeria monocytogenes infection induces a strong inflammatory response characterized by the production of IL-12 and IFN-gamma and protective immunity against this pathogen is dependent on CD8+ T cells (CTL). Recent studies have suggested that these inflammatory cytokines affect the rate of memory CD8+ T cell generation as well as the number of short-lived effector cells generated. The role of the closely related cytokine, IL-23, in this response has not been examined. We hypothesized that IL-12 and IL-23 produced by dendritic cells collectively enhance the generation and function of memory cells. To test this hypothesis, we employed a DC vaccination approach. Mice lacking IL-12 and IL-23 were vaccinated with wild-type (WT), IL-12(-/-), or IL-12/23(-/-) DC and protection to Lm was monitored. Mice vaccinated with WT and IL-12(-/-) DC were resistant to lethal challenge with Lm. Surprisingly, mice vaccinated with IL-12/23(-/-) DC exhibited significantly reduced protection when challenged. Protection correlated with the relative size of the memory pools generated. In summary, these data indicate that IL-23 can partially compensate for the lack of IL-12 in the generation protective immunity against Lm.

CD4+CD25+Foxp3+ regulatory T cells optimize diversity of the conventional T cell repertoire during reconstitution from lymphopenia

The functional capacity of the adaptive immune system is dependent on the size and the diversity of the T cell population. In states of lymphopenia, T cells are driven to proliferate to restore the T cell population size. However, different T cell clones proliferate at different rates, and some T cells experience burst-like expansion called spontaneous lymphopenia-induced proliferation (LIP). These T cells are likely receiving stimulation from cognate Ags and are most responsible for inflammatory pathology that can emerge in lymphopenic states. Foxp3(+) regulatory T cells (Tregs) selectively inhibit spontaneous LIP, which may contribute to their ability to prevent lymphopenia-associated autoimmunity. We hypothesized that another potential negative consequence of unrestrained spontaneous LIP is constriction of the total T cell repertoire. We demonstrate that the absence of Foxp3(+) Tregs during the period of immune reconstitution results in the development of TCR repertoire "holes" and the loss of Ag-specific responsiveness to infectious microorganisms. In contrast, the presence of Tregs during the period of immune reconstitution preserves optimal TCR diversity and foreign Ag responsiveness. This finding contrasts with the generally accepted immunosuppressive role of Tregs and provides another example of Treg activity that actually enhances immune function.

MyD88 and interferon-alpha/beta are differentially required for dendritic cell maturation but dispensable for development of protective memory against Listeria

Signalling pathways mediated by MyD88 are important for sensing Toll-like receptor (TLR) ligands and directing an immune response. However, the influence of MyD88-derived cytokines and interferon (IFN)-alpha/beta, the latter being made by both MyD88-dependent and -independent pathways, in phenotypic and functional dendritic cell (DC) maturation during infection is poorly understood. Here we investigate the contribution of MyD88-dependent and -independent pathways to DC maturation, CD8 T-cell activation and the generation of protective memory against Listeria monocytogenes. We show that neither MyD88 deficiency alone nor MyD88/IFN-alphabetaR double deficiency alters Listeria-induced costimulatory molecule up-regulation on DCs in vivo. In contrast, DCs from infected IFN-alphabetaR(-/-) mice had higher CD80 and CD86 expression than wild-type DCs. We then examined the function of DCs matured in infected knockout mice. We found that DCs from Listeria-infected MyD88(-/-) and MyD88(-/-) IFN-alphabetaR(-/-) mice induced little or no IFN-gamma by CD8 T cells, respectively. In contrast, DCs from infected IFN-alphabetaR(-/-) mice had a greater capacity to induce IFN-gamma compared with DCs from infected wild-type mice. When the CD8 T-cell memory response was analysed, infected MyD88(-/-) and MyD88(-/- )IFN-alphabetaR(-/-) mice were found to have fewer bacteria-specific memory CD8 T cells than wild-type mice. However, the fraction of bacteria-specific CD8 T cells making IFN-gamma was similar in all mouse strains, and MyD88(-/-) and MyD88(-/- )IFN-alphabetaR(-/-) mice survived lethal challenge. Together the data suggest an inhibitory effect of IFN-alpha/beta on functional DC maturation during Listeria infection and reveal overlapping roles of MyD88-induced cytokines and IFN-alpha/beta in DC maturation and protective anti-Listeria immunity.

Constitutive activation of Wnt signaling favors generation of memory CD8 T cells

T cell factor-1 (TCF-1) and lymphoid enhancer-binding factor 1, the effector transcription factors of the canonical Wnt pathway, are known to be critical for normal thymocyte development. However, it is largely unknown if it has a role in regulating mature T cell activation and T cell-mediated immune responses. In this study, we demonstrate that, like IL-7Ralpha and CD62L, TCF-1 and lymphoid enhancer-binding factor 1 exhibit dynamic expression changes during T cell responses, being highly expressed in naive T cells, downregulated in effector T cells, and upregulated again in memory T cells. Enforced expression of a p45 TCF-1 isoform limited the expansion of Ag-specific CD8 T cells in response to Listeria monocytogenes infection. However, when the p45 transgene was coupled with ectopic expression of stabilized beta-catenin, more Ag-specific memory CD8 T cells were generated, with enhanced ability to produce IL-2. Moreover, these memory CD8 T cells expanded to a larger number of secondary effectors and cleared bacteria faster when the immunized mice were rechallenged with virulent L. monocytogenes. Furthermore, in response to vaccinia virus or lymphocytic choriomeningitis virus infection, more Ag-specific memory CD8 T cells were generated in the presence of p45 and stabilized beta-catenin transgenes. Although activated Wnt signaling also resulted in larger numbers of Ag-specific memory CD4 T cells, their functional attributes and expansion after the secondary infection were not improved. Thus, constitutive activation of the canonical Wnt pathway favors memory CD8 T cell formation during initial immunization, resulting in enhanced immunity upon second encounter with the same pathogen.

Neonatal immunization with Listeria monocytogenes induces T cells with an adult-like avidity, sensitivity, and TCR-Vbeta repertoire, and does not adversely impact the response to boosting

Listeria monocytogenes (Lm) holds promise as a neonatal vaccine vehicle. Here we show that Lm immunized neonatal mice reached maximal Ag-specific CD8(+) T cell expansion after only a single immunization, while adults required two doses. Ag-specific CD4(+) T cell expansion in both age groups required a boost to reach its peak. Neither functional avidity, sensitivity, nor the TCR-Vbeta repertoire of the Ag-specific T cells differed between mice immunized as neonates or adults. Lastly, neonatal immunization did not decrease protection or preclude a booster response. Overall, our data provide further evidence in support of immunization at birth as a feasible public health strategy to combat early life infections.

Dendritic cells require STAT-1 phosphorylated at its transactivating domain for the induction of peptide-specific CTL

Phosphorylation of transcription factor STAT-1 on Y701 regulates subcellular localization whereas phosphorylation of the transactivating domain at S727 enhances transcriptional activity. In this study, we investigate the impact of STAT-1 and the importance of transactivating domain phosphorylation on the induction of peptide-specific CTL in presence of the TLR9-dependent immune adjuvant IC31. STAT-1 deficiency completely abolished CTL induction upon immunization, which was strongly reduced in animals carrying the mutation of the S727 phospho-acceptor site. A comparable reduction of CTL was found in mice lacking the type I IFN (IFN-I) receptor, whereas IFN-gamma-deficient mice behaved like wild-type controls. This finding suggests that S727-phosphorylated STAT-1 supports IFN-I-dependent induction of CTL. In adoptive transfer experiments, IFN-I- and S727-phosphorylated STAT-1 were critical for the activation and function of dendritic cells. Mice with a T cell-specific IFN-I receptor ablation did not show impaired CTL responses. Unlike the situation observed for CTL development S727-phosphorylated STAT-1 restrained proliferation of naive CD8(+) T cells both in vitro and following transfer into Rag-deficient mice. In summary, our data reveal a dual role of S727-phosphorylated STAT-1 for dendritic cell maturation as a prerequisite for the induction of CTL activity and for T cell autonomous control of activation-induced or homeostatic proliferation.

Identification of an IFN-gamma-producing neutrophil early in the response to Listeria monocytogenes

IFN-gamma plays a critical role during the immune response to infection with Listeria monocytogenes. Early in the innate response NK cells are thought to be a primary source of IFN-gamma; however, protection can be mediated by the presence of significant numbers of primed IFN-gamma-secreting CD8(+) T cells. In this report, we examined the early response to Listeria and found that 18 h after infection spleens contain CD11b(+), Gr-1(high), or Ly6G(+) cells that produce significant IFN-gamma. Morphological analysis of sorted Gr-1(high)IFN-gamma(+) and Gr-1(low)IFN-gamma(+) or Ly6G(+)IFN-gamma(+) cells confirmed that these cells were neutrophils. The importance of IFN-gamma production by these cells was further tested using adoptive transfer studies. Transfer of purified neutrophils from Ifng(+/+) mice led to increased bacterial clearance in Ifng(-/-) mice. Transfer of Ifng(-/-) neutrophils provided no such protection. We conclude that neutrophils are an early source of IFN-gamma during Listeria infection and are important in providing immune protection.

IL-12 produced by dendritic cells augments CD8+ T cell activation through the production of the chemokines CCL1 and CCL17

IL-12 family members are an important link between innate and adaptive immunity. IL-12 drives Th1 responses by augmenting IFN-gamma production, which is key for clearance of intracellular pathogens. IL-23 promotes the development of IL-17-producing CD4(+) T cells that participate in the control of extracellular pathogens and the induction of autoimmunity. However, recent studies have shown that these cytokines can modulate lymphocyte migration and cellular interactions. Therefore, we sought to determine the individual roles of IL-12 and IL-23 in naive CD8(+) T cell activation by addressing their ability to influence IFN-gamma production and cellular interaction dynamics during priming by Listeria monocytogenes-infected dendritic cells (DC). We found that IL-12 was the major cytokine influencing the level of IFN-gamma production by CD8(+) T cells while IL-23 had little effect on this response. In addition, we observed that IL-12 promoted longer duration conjugation events between CD8(+) T cells and DC. This enhanced cognate interaction time correlated with increased production of the chemokines CCL1 and CCL17 by WT but not IL-12-deficient DC. Neutralization of both chemokines resulted in reduced interaction time and IFN-gamma production, demonstrating their importance in priming naive CD8(+) T cells. Our study demonstrates a novel mechanism through which IL-12 augments naive CD8(+) T cell activation by facilitating chemokine production, thus promoting more stable cognate interactions during priming.

Mage-b vaccine delivered by recombinant Listeria monocytogenes is highly effective against breast cancer metastases

New therapies are needed that target breast cancer metastases. In previous studies, we have shown that vaccination with pcDNA3.1-Mage-b DNA vaccine is effective against breast cancer metastases. In the study presented here, we have further enhanced the efficacy of Mage-b vaccination through the improved delivery of the vaccine using recombinant Listeria monocytogenes (LM). Three overlapping fragments of Mage-b as well as the complete protein-encoding region of Mage-b have been expressed as a fusion protein with a truncated non-cytolytic form of listeriolysin O (LLO) in recombinant LM. These different Mage-b vaccine strains were preventively tested for their efficacy against breast cancer metastases in a syngeneic mouse tumour model 4T1. The LM-LLO-Mage-b/2nd, expressing position 311–660 of the cDNA of Mage-b, was the most effective vaccine strain against metastases in the 4T1 mouse breast tumour model. Vaccination with LM-LLO-Mage-b/2nd dramatically reduced the number of metastases by 96% compared with the saline group and by 88% compared with the vector control group (LM-LLO), and this correlated with strong Mage-b-specific CD8 T-cell responses in the spleen, after restimulation with Mage-b. However, no effect of LM-LLO-Mage-b/2nd was observed on 4T1 primary tumours, which may be the result of a complete absence of Mage-b-specific immune responses in the draining lymph nodes. Vaccination with LM-LLO-Mage-b/2nd could be an excellent follow-up after removal of the primary tumour, to eliminate metastases and residual tumour cells.