T Cell-Intrinsic IL-6R Signaling Is Required for Optimal ICOS Expression and Viral Control during Chronic Infection.

The pleiotropic cytokine IL-6 plays an integral role not only in innate inflammatory responses but also in the activation and differentiation of lymphocyte subsets. In this study, by using a conditional knockout (cKO) model with selective IL-6 receptor deletion in T cells (IL-6R-cKO), we demonstrated that T cell-specific IL-6R signaling is essential for viral control during persistent lymphocytic choriomeningitis virus clone 13 infection. Strikingly, we observed that in contrast to previous studies with ubiquitous IL-6 deletion or blockade, specific IL-6R deletion in T cells did not affect T follicular helper (Tfh) cell accumulation unless IL-6R-deficient T cells were competing with wild-type cells in mixed bone marrow chimeras. In contrast, Tfh cells from IL-6R-cKO-infected mice exhibited reduced ICOS expression in both chimeric and nonchimeric settings, and this sole identifiable Tfh defect was associated with reduced germinal centers, compromised Ig switch and low avidity of lymphocytic choriomeningitis virus-specific Abs despite intact IL-6R expression in B cells. We posit that IL-6R cis-signaling is absolutely required for appropriate ICOS expression in Tfh cells and provides a competitive advantage for Tfh accumulation, enabling generation of optimal B cell and Ab responses, and ultimately viral control during in vivo chronic infection.

IL-27 regulates the number, function and cytotoxic program of antiviral CD4 T cells and promotes cytomegalovirus persistence.

The role of IL-27 in antiviral immunity is still incompletely understood, especially in the context of chronic viruses that induce a unique environment in their infected host. Cytomegalovirus (CMV) establishes a persistent, tissue localized infection followed by lifelong latency. CMV infects the majority of people and although asymptomatic in healthy individuals, can cause serious disease or death in those with naïve or compromised immune systems. Therefore, there is an urgent need to develop a protective CMV vaccine for people at-risk and identifying key regulators of the protective immune response towards CMV will be crucial. Here we studied mouse CMV (MCMV) in IL-27 receptor deficient animals (Il27ra-/-) to assess the role of IL-27 in regulating CMV immunity. We found that IL-27 enhanced the number of antiviral CD4 T cells upon infection. However, in contrast to a well-established role for CD4 T cells in controlling persistent replication and a positive effect of IL-27 on their numbers, IL-27 promoted MCMV persistence in the salivary gland. This coincided with IL-27 mediated induction of IL-10 production in CD4 T cells. Moreover, IL-27 reduced expression of the transcription factor T-bet and restricted a cytotoxic phenotype in antiviral CD4 T cells. This is a highly intriguing result given the profound cytotoxic phenotype of CMV-specific CD4 T cells seen in humans and we established that dendritic cell derived IL-27 was responsible for this effect. Together, these data show that IL-27 regulates the number and effector functions of MCMV-specific CD4 T cells and could be targeted to enhance control of persistent/latent infection.

Interleukin-27R Signaling Mediates Early Viral Containment and Impacts Innate and Adaptive Immunity after Chronic Lymphocytic Choriomeningitis Virus Infection.

Chronic viral infections represent a major challenge to the host immune response, and a unique network of immunological elements, including cytokines, are required for their containment. By using a model persistent infection with the natural murine pathogen lymphocytic choriomeningitis virus clone 13 (LCMV Cl13) we investigated the role of one such cytokine, interleukin-27 (IL-27), in the control of chronic infection. We found that IL-27 receptor (IL-27R) signaling promoted control of LCMV Cl13 as early as days 1 and 5 after infection and that il27p28 transcripts were rapidly elevated in multiple subsets of dendritic cells (DCs) and myeloid cells. In particular, plasmacytoid DCs (pDCs), the most potent type 1 interferon (IFN-I)-producing cells, significantly increased il27p28 in a Toll-like receptor 7 (TLR7)-dependent fashion. Notably, mice deficient in an IL-27-specific receptor, WSX-1, exhibited a pleiotropy of innate and adaptive immune alterations after chronic lymphocytic choriomeningitis virus (LCMV) infection, including compromised NK cell cytotoxicity and antibody responses. While, the majority of these immune alterations appeared to be cell extrinsic, cell-intrinsic IL-27R was necessary to maintain early pDC numbers, which, alongside lower IFN-I transcription in CD11b+ DCs and myeloid cells, may explain the compromised IFN-I elevation that we observed early after LCMV Cl13 infection in IL-27R-deficient mice. Together, these data highlight the critical role of IL-27 in enabling optimal antiviral immunity early and late after infection with a systemic persistent virus and suggest that a previously unrecognized positive-feedback loop mediated by IL-27 in pDCs might be involved in this process.IMPORTANCE Persistently replicating pathogens, such as human immunodeficiency virus, hepatitis B virus, and hepatitis C virus, represent major health problems worldwide. These infections impose a long-term challenge on the host immune system, which must be heavily and continuously regulated to keep pathogen replication in check without causing fatal immunopathology. Using a persistently replicating rodent pathogen, LCMV, in its natural host, we identified the cellular sources and effects of one important regulatory pathway, interleukin-27 receptor WSX-1 signaling, that is required for both very early and late restriction of chronic (but not acute) infection. We found that WSX-1 was necessary to promote innate immunity and the development of aberrant adaptive immune responses. This not only highlights the role of IL-27 receptor signaling in regulating distinct host responses that are known to be necessary to control chronic infections, but also positions IL-27 as a potential therapeutic target for their modulation.

Cell-Intrinsic gp130 Signaling on CD4+ T Cells Shapes Long-Lasting Antiviral Immunity.

The IL-6 cytokine family utilizes the common signal transduction molecule gp130, which can mediate a diverse range of outcomes. To clarify the role of gp130 signaling in vivo during acute viral infection, we infected Cd4-cre Il6st(fl/fl) mice, in which gp130 is conditionally ablated in T cells, with acute lymphocytic choriomeningitis virus. We found that by day 12, but not at day 8, after infection the number of virus-specific CD4(+) T cells was reduced in the absence of gp130, and this was sustained for up to 2 mo postinfection. Additionally, gp130-deficient T follicular helper cells had lower expression of Maf, IL-21, and ICOS, and this was accompanied by a reduction in the proportion of germinal center B cells and plasmablasts. Remarkably, at 2 mo postinfection the proportion of IgG2a/c(+) memory B cells and the systemic levels of lymphocytic choriomeningitis virus-specific IgG2 Abs were dramatically decreased, whereas there was a corresponding increase in IgG1(+) memory B cells and virus-specific IgG1 Abs. In the same animals gp130-deficient virus-specific CD8(+) T cells showed a reduced proportion of memory cells, which expressed lower levels of Tcf7, and displayed diminished recall responses on secondary infection. Mixed bone marrow chimeras revealed that the aforementioned gp130 effects on CD4(+) T cells were cell intrinsic. Overall, our data show that gp130 signaling in T cells influences the quantity and quality of long-lasting CD4(+) T cell responses as well as CD8(+) T cell- and Ab-mediated immunity after acute viral infection.

Daily Plasmodium yoelii infective mosquito bites do not generate protection or suppress previous immunity against the liver stage.

BACKGROUND: Human populations that are naturally subjected to Plasmodium infection do not acquire complete protection against the liver stage of this parasite despite prolonged and frequent exposure. However, sterile immunity against Plasmodium liver stage can be achieved after repeated exposure to radiation attenuated sporozoites. The reasons for this different response remain largely unknown, but a suppressive effect of blood stage Plasmodium infection has been proposed as a cause for the lack of liver stage protection. METHODS: Using Plasmodium yoelii 17XNL, the response generated in mice subjected to daily infective bites from normal or irradiated mosquitoes was compared. The effect of daily-infected mosquito bites on mice that were previously immunized against P. yoelii liver stage was also studied. RESULTS: It was observed that while the bites of normal infected mosquitoes do not generate strong antibody responses and protection, the bites of irradiated mosquitoes result in high levels of anti-sporozoite antibodies and protection against liver stage Plasmodium infection. Exposure to daily infected mosquito bites did not eliminate the protection acquired previously with a experimental liver stage vaccine. CONCLUSIONS: Liver stage immunity generated by irradiated versus normal P. yoelii infected mosquitoes is essentially different, probably because of the blood stage infection that follows normal mosquito bites, but not irradiated. While infective mosquito bites do not induce a protective liver stage response, they also do not interfere with previously acquired liver stage protective responses, even if they induce a complete blood stage infection. Considering that the recently generated anti-malaria vaccines induce only partial protection against infection, it is encouraging that, at least in mouse models, immunity is not negatively affected by subsequent exposure and infection with the parasite.

Plasma cells negatively regulate the follicular helper T cell program.

B lymphocytes differentiate into antibody-secreting cells under the antigen-specific control of follicular helper T cells (T(FH) cells). Here we demonstrate that isotype-switched plasma cells expressed major histocompatibility complex (MHC) class II, the costimulatory molecules CD80 and CD86, and the intracellular machinery required for antigen presentation. Antigen-specific plasma cells accessed, processed and presented sufficient antigen in vivo to induce multiple helper T cell functions. Notably, antigen-primed plasma cells failed to induce interleukin 21 (IL-21) or the transcriptional repressor Bcl-6 in naive helper T cells and actively decreased these key molecules in antigen-activated T(FH) cells. Mice lacking plasma cells showed altered T(FH) cell activity, which provided evidence of this negative feedback loop. Hence, antigen presentation by plasma cells defines a previously unknown layer of cognate regulation that limits the antigen-specific T(FH) cell program that controls ongoing B cell immunity.

A Plasmodium yoelii soluble factor inhibits the phenotypic maturation of dendritic cells.

BACKGROUND: Infection with the protozoan parasite Plasmodium is the cause of malaria. Plasmodium infects host erythrocytes causing the pathology of the disease. Plasmodium-infected erythrocytes can modulate the maturation of dendritic cells (DCs) and alter their capacity to activate T cells. METHODS: Mice infected with Plasmodium yoelii and isolated P. yoelii-infected erythrocytes were used to study their effect on the maturation of mouse dendritic cells. RESULTS: DCs are not able to mature in response to LPS injection during the late stage of P. yoelii infection in mice, indicating impaired functionality of these cells in vivo. P. yoelii- infected erythrocytes inhibit the maturation of DCs in vitro in a dose-dependent manner, which is consistent with the inhibition found during late infection when parasite burden is highest. The inhibition of DC maturation and the cytokine secretion profile of DCs are modulated by soluble factors released by P. yoelii-infected erythrocytes. A small, heat-stable, non-hydrophobic molecule of P. yoelii-infected erythrocytes rapidly inhibits the LPS induced phenotypic maturation of DCs in a reversible manner. CONCLUSION: These findings add evidence to the malaria associated immune suppression in vivo and in vitro and provide insight into the nature and mechanism of the Plasmodium factor(s) responsible for altering DC functions.

Chemically attenuated Plasmodium sporozoites induce specific immune responses, sterile immunity and cross-protection against heterologous challenge.

Vaccination with Plasmodium sporozoites attenuated by irradiation or genetic manipulation induces a protective immune response in rodent malaria models. Recently, vaccination with chemically attenuated P. berghei sporozoites (CAS) has also been shown to elicit sterile immunity in mice. Here we show that vaccination with CAS of P. yoelii also protects against homologous infection and that a P. berghei CAS vaccine cross protects against heterologous challenge with P. yoelii sporozoites. Vaccination with P. yoelii or P. berghei CAS induced parasite-specific antibodies and IFN-gamma-producing CD8(+) T cells at levels not significantly different from radiation-attenuated sporozoites. Our findings provide an initial characterization of the immune response generated by CAS vaccination and suggest that this attenuation process could be used in the production of an effective cross-protective liver stage vaccine for malaria.

Interactions between dendritic cells and CD4+ T cells during Plasmodium infection.

BACKGROUND: During infection, dendritic cells (DCs) encounter pathogenic microorganisms that can modulate their function and shape the T cell responses generated. During the process of T cell activation, DCs establish strong, long-lasting interactions with naïve T cells. METHODS: Using a mouse malaria model, the interactions of DCs and naïve CD4+ T cells have been analysed. RESULTS: DCs, either incubated in vitro with infected erythrocytes or isolated from infected mice, are able to present exogenous antigens by MHC-II, but are not able to establish prolonged effective interactions with naïve CD4+ T cells and do not induce T cell activation. It was also found that effective T cell activation of naïve CD4+ T cells is impaired during late Plasmodium yoelii infection. CONCLUSION: These data may provide a mechanism for the lack of effective adaptive immune responses induced by the Plasmodium parasite.

Plasmodium infection and endotoxic shock induce the expansion of regulatory dendritic cells.

During an acute Plasmodium infection, uncontrolled proinflammatory responses can cause morbidity and mortality. Regulation of this response is required to prevent immunopathology. We therefore decided to investigate a recently characterized subset of regulatory dendritic cells (DCs) that expresses low levels of CD11c and high levels of CD45RB. During a Plasmodium yoelii infection, these regulatory CD11clowCD45RBhigh DCs become the prevalent CD11c-expressing cells in the spleen, overtaking the conventional CD11chigh DCs. Furthermore, the regulatory CD11clowCD45RBhigh DCs induce IL-10-expressing CD4 T cells. A similar change in splenic DC subsets is seen when mice are injected with sublethal doses of LPS, suggesting that shifting the splenic DC subsets in favor of regulatory CD11clowCD45RBhigh DCs can be triggered solely by a high inflammatory stimulus. This is the first time regulatory DCs have been observed in a natural immune response to an infectious disease or endotoxic shock.

Role of TGF-beta and PGE2 in T cell responses during Plasmodium yoelii infection.

During an acute blood-stage malaria infection, T cell responses to malaria and other bystander antigens are inhibited. Plasmodium infection induces strong cytokine responses that facilitate parasite clearance but may interfere with T cell functions, as some of the soluble immune mediators induced are also general inhibitors of T cell responses. Using a malaria mouse model, we have analyzed the cytokines produced by dendritic cells in response to P. yoelii infection that have potential T cell inhibitory activity. We found that during acute infection DC migrate to the spleen and secrete TGF-beta, prostaglandin E2 (PGE2) and IL-10. We have analyzed the role of these general T cell inhibitors in a particular T cell response of evident importance in malaria infections: the CD8+ T cells generated against the liver-stage of the disease. During blood-stage infection, inhibition of the activity of TGF-beta and PGE2 restores the CD8+ T cell responses generated by sporozoites, increasing protection against re-infection. Our findings suggest that the strong cytokine response induced by blood-stage P. yoelii infection affects host T cell responses, inhibiting protective CD8+ T cells against the liver-stage of the disease.