CD26‐mediated co‐stimulation in human CD8+ T cells provokes effector function via pro‐inflammatory cytokine production

CD26 is an activation marker of human CD4⁺ T cells, and is associated with T‐cell signal transduction processes as a co‐stimulatory molecule. We have previously demonstrated that high CD26 cell surface expression on CD4⁺ T cells is correlated with the production of T helper type 1 cytokines, whereas CD26⁺ T helper cells stimulate antibody synthesis in B cells. Although the cellular and molecular mechanisms involved in CD26‐mediated CD4⁺ T‐cell activation have been extensively evaluated by our group and others, the role of CD26 in CD8⁺ T cells has not been clearly elucidated. In the present study, we examine the effector function of CD8⁺ T cells via CD26‐mediated co‐stimulation in comparison with CD28‐mediated co‐stimulation. We found that CD26^high CD8⁺ T cells belong to the early effector memory T‐cell subset, and that CD26‐mediated co‐stimulation of CD8⁺ T cells exerts a cytotoxic effect preferentially via granzyme B, tumour necrosis factor‐α, interferon‐γ and Fas ligand. The effector function associated with CD26‐mediated co‐stimulation is enhanced compared with that obtained through CD28‐mediated co‐stimulation, suggesting that the CD26 co‐stimulation pathway in CD8⁺ T cells is distinct from the CD28 co‐stimulation pathway. Targeting CD26 in CD8⁺ T cells therefore has the potential to be useful in studies of immune responses to new vaccine candidates as well as innovative therapy for immune‐mediated diseases.     

CD8+ T cells mediate ultraviolet A-induced immunomodulation in a model of extracorporeal photochemotherapy

Extracorporeal photochemotherapy (ECP) that takes advantage of the immunomodulatory effects of UV light has been extensively used for many years for the treatment of several T cell–mediated diseases, including graft-versus-host disease (GvHD) and systemic scleroderma. Immune mechanisms that lead to the establishment of T cell tolerance in ECP-treated patients remain poorly known. In this study, we have tested the effect of UV/psoralen-treated BM-derived dendritic cells, referred to as ECP-BMDCs on the outcome of an antigen-specific T cell-mediated reaction, that is, contact hypersensitivity (CHS), which is mediated by CD8⁺ effector T cells (CD8⁺T_eff). The intravenous (i.v.) injection of antigen-pulsed ECP-BMDCs in recipient C57BL/6 mice induced specific CD8⁺ T cells endowed with immunomodulatory properties (referred to as CD8⁺T_ECP), which prevented the priming of CD8⁺T_eff and the development of CHS, independently of conventional CD4⁺ regulatory T cells. CD8⁺T_ECP mediated tolerance by inhibiting the migration and functions of skin DC and subsequently the priming of CD8⁺T_eff. CD8⁺T_ECP displayed none of the phenotypes of the usual CD8⁺T regulatory cells described so far. Our results reveal an underestimated participation of CD8⁺ T cells to ECP-induced immunomodulation that could explain the therapeutic effects of ECP in T cell-mediated diseases.     

CD4+ CD25+ Treg regulate the contribution of CD8+ T-cell subsets in repopulation of the lymphopenic environment

Peripheral T‐cell expansion is of major relevance for immune function after lymphopenia. In order to promote regeneration, the process should result in a peripheral T‐cell pool with a similar subpopulation structure as before lymphopenia. We investigated the repopulation of the CD8⁺ central‐memory T cells (T_CM) and effector‐memory T cells (T_EM) pools after adoptive transfer of sorted CD8⁺ T cells from naïve, T_CM and T_EM subsets into T‐cell‐deficient hosts. We show that the initial kinetics of expansion are distinct for each subset and that the contribution to the repopulation of the CD8⁺ T‐cell pool by the progeny of each subset is not a mere function of its initial expansion. We demonstrate that CD4⁺CD25⁺ Treg play a major role in the repopulation of the CD8⁺ T‐cell pool and that CD8⁺ T‐cell subsets impact on each other. In the absence of CD4⁺CD25⁺ Treg, a small fraction of naïve CD8⁺ T cells strongly proliferates, correlating with further expansion and differentiation of co‐expanding CD8⁺ T cells. CD4⁺CD25⁺ Treg suppress these responses and lead to controlled repopulation, contributing decisively to the maintenance of recovered T_CM and T_EM fractions, and leading to repopulation of each pool with progeny of its own kind.     

Interrupting CD28 costimulation before antigen rechallenge affects CD8+ T‐cell expansion and effector functions during secondary response in mice

The role of CD28‐mediated costimulation in secondary CD8⁺ T‐cell responses remains controversial. Here, we have used two tools — blocking mouse anti‐mouse CD28‐specific antibodies and inducible CD28‐deleting mice — to obtain definitive answers in mice infected with ovalbumin‐secreting Listeria monocytogenes. We report that both blockade and global deletion of CD28 reveal its requirement for full clonal expansion and effector functions such as degranulation and IFN‐γ production during the secondary immune response. In contrast, cell‐intrinsic deletion of CD28 in transferred TCR‐transgenic CD8⁺ T cells before primary infection leads to impaired clonal expansion but an increase in cells able to express effector functions in both primary and secondary responses. We suggest that the proliferation‐impaired CD8⁺ T cells respond to CD28‐dependent help from their environment by enhanced functional differentiation. Finally, we report that cell‐intrinsic deletion of CD28 after the peak of the primary response does not affect the establishment, maintenance, or recall of long‐term memory. Thus, if given sufficient time, the progeny of primed CD8⁺ T cells adapt to the absence of this costimulator.     

The murine buccal mucosa is an inductive site for priming class I-restricted CD8+ effector T cells in vivo

The present study shows that Langerhans cells of the buccal mucosa and the skin share a similar phenotype, including in situ expression of MHC class II, the mannose receptor DEC-205 and CD11c, and absence of the costimulatory molecules B7.1, B7.2 and CD40 as well as Fas. Application of 2,4-dinitrofluorobenzene (DNFB) onto the buccal mucosa is associated with a rapid migration of dendritic cells (DC) to the epithelium and induction of B7.2 expression on some DC. Buccal sensitization with DNFB elicited a specific contact sensitivity (CS) in response to skin challenge, mediated by class I-restricted CD8⁺ effector T cells and down-regulated by class II-restricted CD4⁺ T cells, demonstrated by the lack of priming of class I-deficient mice and the enhanced response of class II-deficient mice, respectively. CS induced by buccal immunization is associated with priming of class I-restricted CD8⁺ effector T cells endowed with hapten-specific cytotoxic activity. Thus, the buccal epithelium is an inductive site, equivalent to the epidermis, for the generation of CS independent of CD4 help, and of cytotoxic T lymphocyte (CTL) responses mediated by class I-restricted CD8⁺ T cells. We propose that immunization through the buccal mucosa, which allows antigen presentation by epithelial DC efficient for priming systemic class I-restricted CD8⁺ CTL, may be a valuable approach for single-dose mucosal vaccination with subunit vaccines.     

Diversion of CD4+ T cell development from regulatory T helper to effector T helper cells alters the contact hypersensitivity response

Cutaneous sensitization to reactive haptens and subsequent challenge results in a T cell-mediated response, contact hypersensitivity (CHS). Recent results from this laboratory have indicated that hapten sensitization induces two populations of reactive T cells: CD8⁺ T cells producing interferon (IFN)-γ which mediate the response and CD4⁺ T cells producing interleukin (IL)-4 and IL-10 which negatively regulate the magnitude and duration of the response. Since CD4⁺ T cell development to either IFN-γ- (Th1) or IL-4/IL-10- (Th2)-producing cells is dependent upon the cytokine environment during antigen priming, we hypothesized that CD4⁺ T cell induction in a Th1-promoting environment would not only alter the CD4⁺ T cell cytokine-producing phenotype but also the course of the CHS response. Administration of the Th1-promoting cytokine IL-12 during hapten sensitization resulted in a CHS response of greater magnitude following challenge and extended the duration of the response. In hapten-sensitized mice depleted of CD8⁺ T cells, treatment with IL-12 induced effector CD4⁺ T cells. Histological examination of challenged ear tissue from these mice indicated minimal edema and an acute mononuclear cell infiltration more typical of classical delayed-type hypersensitivity than CHS. Hapten-primed CD4⁺ T cells from IL-12 treated, sensitized mice produced IFN-γ, but not IL-4 in response to T cell receptor-mediated stimulation. Use of neutralizing anti-IFN-γ antibody indicated that IL-12 not only directly promoted Th1 development but also indirectly inhibited Th2 development through stimulation of IFN-γ production at the time of hapten sensitization. Overall, these results demonstrate that diversion of CD4⁺ T cell development to Th1 effector cells rather than to Th2 cells alters the efferent nature of CHS and removes a primary regulatory mechanism of the immune response.     

Differential requirements of CD4+ T‐cell signals for effector cytotoxic T‐lymphocyte (CTL) priming and functional memory CTL development at higher CD8+ T‐cell precursor frequency

Increased CD8⁺ T‐cell precursor frequency (PF) precludes the requirement of CD4⁺ helper T (Th) cells for primary CD8⁺ cytotoxic T‐lymphocyte (CTL) responses. However, the key questions of whether unhelped CTLs generated at higher PF are functional effectors, and whether unhelped CTLs can differentiate into functional memory cells at higher PF are unclear. In this study, ovalbumin (OVA) ‐pulsed dendritic cells (DC_OVA) derived from C57BL/6, CD40 knockout (CD40^?/?) or CD40 ligand knockout (CD40L^?/?) mice were used to immunize C57BL/6, Ia^b?/?, CD40^?/? or CD40L^?/? mice, whose PF was previously increased with transfer of 1 × 10⁶ CD8⁺ T cells derived from OVA‐specific T‐cell receptor (TCR) transgenic OTI, OTI(CD40^?/?) or OTI(CD40L^?/?) mice. All the immunized mice were then assessed for effector and memory CTL responses. Following DC immunization, relatively comparable CTL priming occurred without CD4⁺ T‐cell help and Th‐provided CD40/CD40L signalling. In addition, the unhelped CTLs were functional effectors capable of inducing therapeutic immunity against established OVA‐expressing tumours. In contrast, the functional memory development of CTLs was severely impaired in the absence of CD4⁺ T‐cell help and CD40/CD40L signalling. Finally, unhelped memory CTLs failed to protect mice against lethal tumour challenge. Taken together, these results demonstrate that CD4⁺ T‐cell help at higher PF, is not required for effector CTL priming, but is required for functional memory CTL development against cancer. Our data may impact the development of novel preventive and therapeutic approaches in cancer patients with compromised CD4⁺ T‐cell functions.     

DC‐induced CD8+ T‐cell response is inhibited by MHC class II‐dependent DX5+CD4+ Treg

CD4⁺ T cells are important for CD8⁺ T‐cell priming by providing cognate signals for DC maturation. We analyzed the capacity of CD4⁺ T cells to influence CD8⁺ T‐cell responses induced by activated DC. Surprisingly, mice depleted for CD4⁺ cells were able to generate stronger antigen‐specific CD8⁺ T‐cell responses after DC vaccination than non‐depleted mice. The same observation was made when mice were vaccinated with MHC class II^?/? DC, indicating the presence of a MHC class II‐dependent CD4⁺ T‐cell population inhibiting CD8⁺ T‐cell responses. Recently we described the expansion of DX5⁺CD4⁺ T cells, a T‐cell population displaying immune regulatory properties, upon vaccination with DC. Intriguingly, we now observe an inverse correlation between CD8⁺ T‐cell induction and expansion of DX5⁺CD4⁺ T cells as the latter cells did not expand after vaccination with MHC class II^?/? DC. In vitro, DX5⁺CD4⁺ T cells were able to limit proliferation, modulate cytokine production and induce Foxp3⁺ expression in OVA‐specific CD8⁺ T cells. Together, our data show an inhibitory role of CD4⁺ T cells on the induction of CD8⁺ T‐cell responses by activated DC and indicate the involvement of DX5⁺CD4⁺, but not CD4⁺CD25⁺, T cells in this process.     

H2‐M3‐restricted CD8+ T cells augment CD4+ T‐cell responses by promoting DC maturation

Infection with Listeria monocytogenes triggers the activation and expansion of nonconventional CD8⁺ T cells restricted by the MHC class Ib molecule, H2‐M3. H2‐M3‐restricted CD8⁺ T cells exhibit a memory phenotype, rapidly produce cytokines, and reach peak frequencies sooner than conventional MHC class Ia‐restricted CD8⁺ T cells. In this study, we found that simultaneous in vivo priming of H2‐M3‐restricted T cells and adoptively transferred OT‐II CD4⁺ T cells on the same DC enhances the survival of OT‐II cells. Stimulation of H2‐M3‐restricted T cells were found to induce DC maturation resulting in costimulatory molecule upregulation and production of T_H1‐type cytokines, which was dependent on both cell‐to‐cell contact and soluble factors, particularly TNF‐α, produced by activated H2‐M3‐restricted T cells. Interestingly, H2‐M3‐restricted T cells were more efficient than activated NK cells in inducing DC maturation. Furthermore, we found that OVA_323–339‐coated DC matured by coculturing with peptide‐stimulated H2‐M3‐restricted T cells were more efficient in stimulating the proliferation of Ag‐activated OT‐II cells. This study indicates that H2‐M3‐restricted T cells promote immune responses by CD4⁺ T cells by inducing DC maturation and suggests novel mechanisms for vaccine development.     

Vaccine molecules targeting Xcr1 on cross‐presenting DCs induce protective CD8+ T‐cell responses against influenza virus

Targeting antigens to cross‐presenting dendritic cells (DCs) is a promising method for enhancing CD8⁺ T‐cell responses. However, expression patterns of surface receptors often vary between species, making it difficult to relate observations in mice to other animals. Recent studies have indicated that the chemokine receptor Xcr1 is selectively expressed on cross‐presenting murine CD8α⁺ DCs, and that the expression is conserved on homologous DC subsets in humans (CD141⁺ DCs), sheep (CD26⁺ DCs), and macaques (CADM1⁺ DCs). We therefore tested if targeting antigens to Xcr1 on cross‐presenting DCs using antigen fused to Xcl1, the only known ligand for Xcr1, could enhance immune responses. Bivalent Xcl1 fused to model antigens specifically bound CD8α⁺ DCs and increased proliferation of antigen‐specific T cells. DNA vaccines encoding dimeric Xcl1‐hemagglutinin (HA) fusion proteins induced cytotoxic CD8⁺ T‐cell responses, and mediated full protection against a lethal challenge with influenza A virus. In addition to enhanced CD8⁺ T‐cell responses, targeting of antigen to Xcr1 induced CD4⁺ Th1 responses and highly selective production of IgG2a antibodies. In conclusion, targeting of dimeric fusion vaccine molecules to CD8α⁺ DCs using Xcl1 represents a novel and promising method for induction of protective CD8⁺ T‐cell responses.     

IL‐15‐dependent CD8+CD122+ T cells ameliorate experimental autoimmune encephalomyelitis by modulating IL‐17 production by CD4+ T cells

Interleukin‐15 (IL‐15) is an inflammatory cytokine whose role in autoimmune diseases has not been fully elucidated. Th17 cells have been shown to play critical roles in experimental autoimmune encephalomyelitis (EAE) models. In this study, we demonstrate that blockade of IL‐15 signaling by TMβ‐1 mAb treatment aggravated EAE severity. The key mechanism was not NK‐cell depletion but depletion of CD8⁺CD122⁺ T cells. Adoptive transfer of exogenous CD8⁺CD122⁺ T cells to TMβ‐1‐treated mice rescued animals from severe disease. Moreover, transfer of preactivated CD8⁺CD122⁺ T cells prevented EAE development and significantly reduced IL‐17 secretion. Naïve effector CD4⁺CD25^? T cells cultured with either CD8⁺CD122⁺ T cells from wild‐type mice or IL‐15 transgenic mice displayed lower frequencies of IL‐17A production with lower amounts of IL‐17 in the supernatants when compared with production by effector CD4⁺CD25^? T cells cultured alone. Addition of a neutralizing antibody to IL‐10 led to recovery of IL‐17A production in Th17 cultures. Furthermore, coculture of CD8⁺CD122⁺ T cells with effector CD4⁺ T cells inhibited their proliferation significantly, suggesting a regulatory function for IL‐15 dependent CD8⁺CD122⁺ T cells. Taken together, these observations suggest that IL‐15, acting through CD8⁺CD122⁺ T cells, has a negative regulatory role in reducing IL‐17 production and Th17‐mediated EAE inflammation.     

CD40L expression by CD4+ but not CD8+ T cells regulates antiviral immune responses in acute LCMV infection in mice

CD40‐CD40 ligand (CD40L) signaling plays multiple indispensable roles in cellular and humoral immunity. Impaired memory T‐cell responses in the absence of CD40L have been well documented, but the requirement of this interaction for efficient priming of CD8⁺ T cells especially under inflammatory conditions has been under debate. In contrast to previous publications, we report here that virus‐specific CD8⁺ T‐cell responses as well as viral clearance are affected not only in the memory but also in the effector phase in CD40L^?/? mice infected with lymphocytic choriomeningitis virus (LCMV) Armstrong strain. Interestingly, a considerable part of the LCMV‐specific effector and memory T cells consists of CD40L⁺ CD8⁺ T cells. However, deficiency of CD40L in CD8⁺ T cells did influence neither the quantity nor the quality of primary T‐cell responses in LCMV infection. Virus‐specific CD8⁺ T cells in conditional knockout mice, with a selective deletion of the CD40L in CD8⁺ T cells, were fully functional regarding cytokine production and efficient pathogen clearance. Thus, our results unambiguously demonstrate that while CD40L is critical to generate effective primary CD8⁺ T‐cell responses also under inflammatory conditions, CD40L expression by CD8⁺ T cells themselves is dispensable in acute LCMV infection.     

UV inhibits allergic airways disease in mice by reducing effector CD4+ T cells

Background In human asthma, and experimental allergic airways disease in mice, antigen‐presenting cells and CD4⁺ effector cells at the airway mucosa orchestrate, and CD4⁺CD25⁺ regulatory T cells attenuate, allergen immunity. UV irradiation of skin before sensitization with ovalbumin (OVA) causes significantly reduced asthma‐like responses in respiratory tissues. Objective To determine whether UV‐induced changes in CD11c⁺ cells, CD4⁺CD25⁺ effector cells or CD4⁺CD25⁺ regulatory cells in the trachea and airway draining lymph nodes (ADLNs) were responsible for reduced allergic airways disease. Methods The phenotype and function of CD11c⁺ cells and CD4⁺CD25⁺ cells in the trachea and ADLNs of UV‐ and non‐irradiated, OVA‐sensitized mice was examined 24 h after a single exposure to aerosolized OVA. Results No changes in the function of CD11c⁺ cells from UV‐irradiated mice were observed. CD4⁺CD25⁺ cells from UV‐irradiated, OVA‐sensitized mice harvested 24 h after OVA aerosol proliferated less in response to OVA in vitro and were unable to suppress the proliferation of OVA‐sensitized responder cells. This result suggested reduced activation of effector T cells in the airway mucosa of UV‐irradiated, OVA‐sensitized mice. To exclude regulatory cells of any type, there was similar proliferation in vivo to aerosolized OVA by CFSE‐loaded, OVA‐TCR‐specific CD4⁺ cells adoptively transferred into UV‐ and non‐irradiated, OVA‐sensitized mice. In addition, there was no difference in the expression of regulatory T cell markers (Foxp3, IL‐10, TGF‐β mRNA). To examine effector T cells, ADLN cells from UV‐irradiated, OVA‐sensitized and ‐challenged mice were cultured with OVA. There was reduced expression of the early activation marker CD69 by CD4⁺CD25⁺ cells, and reduced proliferation in the absence of the regulatory cytokine, IL‐10. Conclusion Reduced allergic airways disease in UV‐irradiated mice is due to fewer effector CD4⁺CD25⁺ cells in the trachea and ADLNs, and not due to UV‐induced regulatory cells. Cite this as: J. P. McGlade, D. H. Strickland, M. J. M. Lambert, S. Gorman, J. A. Thomas, M. A. Judge, J. T. Burchell, G. R. Zosky and P. H. Hart, Clinical & Experimental Allergy, 2010 (40) 772–785.     

CD70 and IFN‐1 selectively induce eomesodermin or T‐bet and synergize to promote CD8+ T‐cell responses

CD70‐mediated stimulation of CD27 is an important cofactor of CD4⁺ T‐cell licensed dendritic cells (DCs). However, it is unclear how CD70‐mediated stimulation of T cells is integrated with signals that emanate from signal 3 pathways, such as type‐1 interferon (IFN‐1) and IL‐12. We find that while stimulation of CD27 in isolation drives weak Eomesodermin^hiT‐bet^lo CD8⁺ T‐cell responses to OVA immunization, profound synergistic expansion is achieved by cotargeting TLR. This cooperativity can substantially boost antiviral CD8⁺ T‐cell responses during acute infection. Concomitant stimulation of TLR significantly increases per cell IFN‐γ production and the proportion of the population with characteristics of short‐lived effector cells, yet also promotes the ability to form long‐lived memory. Notably, while IFN‐1 contributes to the expression of CD70 on DCs, the synergy between CD27 and TLR stimulation is dependent upon IFN‐1's effect directly on CD8⁺ T cells, and is associated with the increased expression of T‐bet in T cells. Surprisingly, we find that IL‐12 fails to synergize with CD27 stimulation to promote CD8⁺ T‐cell expansion, despite its capacity to drive effector CD8⁺ T‐cell differentiation. Together, these data identify complex interactions between signal 3 and costimulatory pathways, and identify opportunities to influence the differentiation of CD8⁺ T‐cell responses.     

CD8+ T cells produce IL-2, which is required for CD(4+)CD25+ T cell regulation of effector CD8+ T cell development for contact hypersensitivity responses

Interleukin (IL)-2 functions to promote, as well as down-regulate, expansion of antigen-reactive CD4+ and CD8+ T cells, but the role of IL-2 in hapten-specific CD8+ T cell priming for contact hypersensitivity (CHS) responses remains untested. Using enzyme-linked immunospot to enumerate numbers of hapten-specific CD4+ and CD8+ T cells producing IL-2 in hapten-sensitized mice, the number of IL-2-producing CD8+ T cells was tenfold that of CD4+ T cells. Hapten-primed CD4+ T cells produced low amounts of IL-2 during culture with hapten-presenting Langerhans cells, whereas production by hapten-primed CD8+ T cells was fivefold greater. CD8+ T cells did not express CD25 during hapten priming, but treatment with anti-IL-2 or anti-CD25 monoclonal antibodies during hapten sensitization increased hapten-specific effector CD8+ T cells as well as the magnitude and duration of the CHS response. These results indicate that CD8+ T cells are the primary source of IL-2 and that this IL-2 is required for the function of a population of CD(4+)CD25+ T cells to restrict the development of the hapten-reactive effector CD8+ T cells that mediate CHS responses.     

Steady‐state antigen‐expressing dendritic cells terminate CD4+ memory T‐cell responses

CD4⁺ T cells are important effectors of inflammation and tissue destruction in many diseases of immune dysregulation. As memory T cells develop early during the preclinical stages of autoimmune and inflammatory diseases, immunotherapeutic approaches to treatment of these diseases, once established, must include the means to terminate memory T‐cell responses. Traditionally, it has been considered that, due to their terminally differentiated nature, memory T cells are resistant to tolerance induction, although emerging evidence indicates that some immunotherapeutic approaches can terminate memory T‐cell responses. Here, we demonstrate that CD4⁺ memory T‐cell responses can be terminated when cognate antigen is transgenically expressed in steady‐state DC. Transfer of in‐vitro‐generated CD4⁺ memory T cells establishes, in nontransgenic recipients, a stable and readily recalled memory response to cognate antigen. In contrast, upon transfer to mice expressing cognate antigen targeted to DC, memory CD4⁺ T cells undergo a phase of limited proliferation followed by substantial deletion, and recall responses are effectively silenced. This finding is important in understanding how to effectively apply immunotherapy to ongoing T‐cell‐mediated autoimmune and inflammatory diseases.     

T‐cell help dependence of memory CD8+ T‐cell expansion upon vaccinia virus challenge relies on CD40 signaling

Due to their capacity to differentiate into long‐lived memory cells, CD8⁺ T cells are able to resolve subsequent infections faster than during the primary response. Among other factors, CD4⁺ T cells play a crucial role during primary and secondary CD8⁺ T‐cell responses. However, the timing and mechanisms by which they influence CD8⁺ T cells may differ in primary and secondary responses. Here, we demonstrate that during both primary and secondary vaccinia virus infection, CD4⁺ T cells are necessary to promote CD8⁺ T‐cell responses. While CD4⁺ T cells contributed to memory CD8⁺ T‐cell development, they were even more important during memory recall responses during challenge, as absence of CD4⁺ T cells during challenge resulted in markedly decreased proliferation and increased apoptosis. T‐cell help during primary and secondary responses was mediated via CD40 signaling, with DCs being an integral part of that pathway. As opposed to primary CD8⁺ T‐cell responses where only a combination of agonistic CD40 signaling and provision of IL‐2 could substitute for T‐cell help, agonistic CD40 triggering alone was sufficient to rescue memory CD8⁺ T‐cell responses in absence of T‐cell help in the context of vaccinia virus infection.     

IL‐12‐mediated STAT4 signaling and TCR signal strength cooperate in the induction of CD40L in human and mouse CD8+ T cells

CD40L is one of the key molecules bridging the activation of specific T cells and the maturation of professional and nonprofessional antigen‐presenting cells including B cells. CD4⁺ T cells have been regarded as the major T‐cell subset that expresses CD40L upon cognate activation; however, we demonstrate here that a putative CD8⁺ helper T‐cell subset expressing CD40L is induced in human and murine CD8⁺ T cells in vitro and in mice immunized with antigen‐pulsed dendritic cells. IL‐12 and STAT4‐mediated signaling was the major instructive cytokine signal boosting the ability of CD8⁺ T cells to express CD40L both in vitro and in vivo. Additionally, TCR signaling strength modulated CD40L expression in CD8⁺ T cells after primary differentiation in vitro as well as in vivo. The induction of CD40L in CD8⁺ T cells regulated by IL‐12 and TCR signaling may enable CD8⁺ T cells to respond autonomously of CD4⁺ T cells. Thus, we propose that under proinflammatory conditions, a self‐sustaining positive feedback loop could facilitate the efficient priming of T cells stimulated by high affinity peptide displaying APCs.     

CD4+CD25+ Treg induction by an HSP60‐derived peptide SJMHE1 from Schistosoma japonicum is TLR2 dependent

Chronic schistosome infection results in the suppression of host immune responses, allowing long‐term schistosome survival and restricting pathology. Current theories suggest that Treg play an important role in this regulation. However, the mechanism of Treg induction during schistosome infection is still unknown. The aim of this study was to determine the mechanism behind the induction of CD4⁺CD25⁺ T cells by Schistosoma japonicum HSP60 (SjHSP60)‐derived peptide SJMHE1 as well as to elucidate the cellular and molecular basis for the induction of CD4⁺CD25⁺ T cells during S. japonicum infection. Mice immunized with SJMHE1 or spleen and LN cells from naïve mice pretreated with SJMHE1 in vitro all displayed an increase in CD4⁺CD25⁺ T‐cell populations. Release of IL‐10 and TGF‐β by SJMHE1 stimulation may contribute to suppression. Adoptively transferred SJMHE1‐induced CD4⁺CD25⁺ T cells inhibited delayed‐type hypersensitivity in BALB/c mice. Additionally, SJMHE1‐treated APC were tolerogenic and induced CD4⁺ cells to differentiate into suppressive CD4⁺CD25⁺ Treg. Furthermore, our data support a role for TLR2 in SJMHE1‐mediated CD4⁺CD25⁺ Treg induction. These findings provide the basis for a more complete understanding of the S. japonicum–host interactions that contribute to host homeostatic mechanisms, preventing an excessive immune response.