Memory CD8+ T cells require CD28 costimulation

CD8(+) T cells are a critical component of the adaptive immune response against infections and tumors. A current paradigm in immunology is that naive CD8(+) T cells require CD28 costimulation, whereas memory CD8(+) T cells do not. We show here, however, that during viral infections of mice, costimulation is required in vivo for the reactivation of memory CD8(+) T cells. In the absence of CD28 costimulation, secondary CD8(+) T cell responses are greatly reduced and this impairs viral clearance. The failure of CD8(+) T cells to expand in the absence of CD28 costimulation is CD4(+) T cell help independent and is accompanied by a failure to down-regulate Bcl-2 and by cell cycle arrest. This requirement for CD28 costimulation was shown in both influenza A and HSV infections. Thus, contrary to current dogma, memory CD8(+) T cells require CD28 costimulation to generate maximal secondary responses against pathogens. Importantly, this CD28 requirement was shown in the context of real infections were multiple other cytokines and costimulators may be up-regulated. Our findings have important implications for pathogens, such as HIV and measles virus, and tumors that evade the immune response by failing to provide CD28 costimulation. These findings also raise questions about the efficacy of CD8(+) T cell-based vaccines against such pathogens and tumors.     

Splenic dendritic cell subsets prime and boost CD8 T cells and are involved in the generation of effector CD8 T cells

The ability of the dendritic cell (DC) subsets, CD8alpha+ and CD8alpha- DCs, to initiate a CD8 T cell response or to activate memory CD8 T cells and generate effector CD8 T cells has been controversial. In this study, we analyse the capacity of splenic DC subsets to induce CD8 T cell responses to a CD8 T cell epitope (pb9) of a malaria antigen. The administration of peptide-pulsed CD8alpha- or CD8alpha+ DCs primes and boosts a primed CD8 T cell response against the malaria epitope. In vitro, depletion of CD11c(+) DCs from mouse splenocytes, immunised with recombinant vaccinia virus Ankara (MVA) expressing pb9 epitope, significantly reduced the generation of pb9-specific IFNgamma producing effector CD8 T cells, indicating that splenic DCs are involved in the development of pb9-specific IFNgamma producing effector cells. Taken together, this result shows that both DC subsets have the ability to prime and boost CD8 T cell responses and are involved in the activation of memory CD8 T cells.     

Steady state dendritic cells present parenchymal self-antigen and contribute to, but are not essential for, tolerization of naive and Th1 effector CD4 cells

Bone marrow-derived APC are critical for both priming effector/memory T cell responses to pathogens and inducing peripheral tolerance in self-reactive T cells. In particular, dendritic cells (DC) can acquire peripheral self-Ags under steady state conditions and are thought to present them to cognate T cells in a default tolerogenic manner, whereas exposure to pathogen-associated inflammatory mediators during the acquisition of pathogen-derived Ags appears to reprogram DCs to prime effector and memory T cell function. Recent studies have confirmed the critical role of DCs in priming CD8 cell effector responses to certain pathogens, although the necessity of steady state DCs in programming T cell tolerance to peripheral self-Ags has not been directly tested. In the current study, the role of steady state DCs in programming self-reactive CD4 cell peripheral tolerance was assessed by combining the CD11c-diphtheria toxin receptor transgenic system, in which DC can be depleted via treatment with diphtheria toxin, with a TCR-transgenic adoptive transfer system in which either naive or Th1 effector CD4 cells are induced to undergo tolerization after exposure to cognate parenchymally derived self-Ag. Although steady state DCs present parenchymal self-Ag and contribute to the tolerization of cognate naive and Th1 effector CD4 cells, they are not essential, indicating the involvement of a non-DC tolerogenic APC population(s). Tolerogenic APCs, however, do not require the cooperation of CD4(+)CD25(+) regulatory T cells. Similarly, DC were required for maximal priming of naive CD4 cells to vaccinia viral-Ag, but priming could still occur in the absence of DC.     

CD40 ligand and CTLA-4 are reciprocally regulated in the Th1 cell proliferative response sustained by CD8(+) dendritic cells

Subsets of murine dendritic cells (DCs) from the spleen differ in their ability to induce proliferative responses in both primary and secondary CD4(+) T cells. Recent evidence indicates that lymphoid-related CD8(+) DCs fail to provide appropriate signals to freshly isolated secondary CD4(+) T cells to sustain their proliferation in vitro. In the present study, we examined peptide-pulsed CD8(-) and CD8(+) DCs for ability to stimulate Th1 and Th2 cell clones with the same Ag specificity. Defective ability to induce proliferation was selectively shown by CD8(+) DCs presenting Ag to the Th1 clone. The deficiency in CD8(+) DCs was overcome by CD40 triggering before peptide pulsing. When exposed to CD8(+) DCs in the absence of CD40 activation, the Th1 clone expressed low levels of CD40 ligand and high levels of surface CTLA-4. Neutralization of CTLA-4 during the DC/T cell coculture resulted in increased CD40 ligand expression and proliferation of T cells. Remarkably, the activation of CD40 on DCs under conditions that would increase Th1 cell proliferation, also resulted in down-regulation of surface CTLA-4. These results confirm differential effects of CD8(+) and CD8(-) DCs in the stimulation of Ag-primed Th cells. In addition, they suggest that reciprocal regulation of CD40 ligand and CTLA-4 expression occurs in Th1 cells exposed to CD8(+) DCs.     

Requirement of mature dendritic cells for efficient activation of influenza A-specific memory CD8+ T cells

It is critical to identify the developmental stage of dendritic cells (DCs) that is most efficient at inducing CD8+ T cell responses. Immature DCs can be generated from monocytes with GM-CSF and IL-4, while maturation is accomplished by the addition of stimuli such as monocyte-conditioned medium, CD40 ligand, and LPS. We evaluated the ability of human monocytes and immature and mature DCs to induce CD8+ effector responses to influenza virus Ags from resting memory cells. We studied replicating virus, nonreplicating virus, and the HLA-A*0201-restricted influenza matrix protein peptide. Sensitive and quantitative assays were used to measure influenza A-specific immune responses, including MHC class I tetramer binding assays, enzyme-linked immunospot assays for IFN-gamma production, and generation of cytotoxic T cells. Mature DCs were demonstrated to be superior to immature DC in eliciting IFN-gamma production from CD8+ effector cells. Furthermore, only mature DCs, not immature DCs, could expand and differentiate CTL precursors into cytotoxic effector cells over 7 days. An exception to this was immature DCs infected with live influenza virus, because of the virus's known maturation effect. Finally, mature DCs pulsed with matrix peptide induced CTLs from highly purified CD8+ T cells without requiring CD4+ T cell help. These differences between DC stages were independent of Ag concentrations or the number of immature DCs. In contrast to DCs, monocytes were markedly inferior or completely ineffective stimulators of T cell immunity. Our data with several qualitatively different assays of the memory CD8+ T cell response suggest that mature cells should be considered as immunotherapeutic adjuvants for Ag delivery.     

IL-2 is required for the activation of memory CD8+ T cells via antigen cross-presentation

Dendritic cells (DCs) are capable of capturing exogenous Ag for the generation of MHC class I/peptide complexes. For efficient activation of memory CD8(+) T cells to occur via a cross-presentation pathway, DCs must receive helper signals from CD4(+) T cells. Using an in vitro system that reflects physiologic recall memory responses, we have evaluated signals that influence helper-dependent cross-priming, while focusing on the source and cellular target of such effector molecules. Concerning the interaction between CD4(+) T cells and DCs, we tested the hypothesis that CD40 engagement on DCs is critical for IL-12p70 (IL-12) production and subsequent stimulation of IFN-gamma release by CD8(+) T cells. Although CD40 engagement on DCs, or addition of exogenous IL-12 are both sufficient to overcome the lack of help, neither is essential. We next evaluated cytokines and chemokines produced during CD4(+) T cell/DC cross talk and observed high levels of IL-2 produced within the first 18-24 h of Ag-specific T cell engagement. Functional studies using blocking Abs to CD25 completely abrogated IFN-gamma production by the CD8(+) T cells. Although required, addition of exogenous IL-2 did not itself confer signals sufficient to overcome the lack of CD4(+) T cell help. Thus, these data support a combined role for Ag-specific, cognate interactions at the CD4(+) T cell/DC as well as the DC/CD8(+) T cell interface, with the helper effect mediated by soluble noncognate signals.     

Incomplete activation of CD4 T cells by antigen-presenting transitional immature B cells: implications for peripheral B and T cell responsiveness

B cells leave the bone marrow as transitional B cells. Transitional B cells represent a target of negative selection and peripheral tolerance, both of which are abrogated in vitro by mediators of T cell help. In vitro, transitional and mature B cells differ in their responses to B cell receptor ligation. Whereas mature B cells up-regulate the T cell costimulatory molecule CD86 (B7.2) and are activated, transitional B cells do not and undergo apoptosis. The ability of transitional B cells to process and present Ag to CD4 T cells and to elicit protective signals in the absence of CD86 up-regulation was investigated. We report that transitional B cells can process and present Ag as peptide:MHC class II complexes. However, their ability to activate T cells and elicit help signals from CD4-expressing Th cells was compromised compared with mature B cells, unless exogenous T cell costimulation was provided. A stringent requirement for CD28 costimulation was not evident in interactions between transitional B cells and preactivated CD4-expressing T cells, indicating that T cells involved in vivo in an ongoing immune response might rescue Ag-specific transitional B cells from negative selection. These data suggest that during an immune response, immature B cells may be able to sustain the responses of preactivated CD4(+) T cells, while being unable to initiate activation of naive T cells. Furthermore, the ability of preactivated, but not naive T cells to provide survival signals to B cell receptor-engaged transitional immature B cells argues that these B cells may be directed toward activation rather than negative selection when encountering Ag in the context of a pre-existing immune response.     

Differential CD40/CD40L expression results in counteracting antitumor immune responses

Establishment of host-protective memory T cells against tumors is the objective of an antitumor immunoprophylactic strategy such as reinforcing T cell costimulation via CD40-CD40L interaction. Previous CD40-targeted strategies assumed that T cell costimulation is an all-or-none phenomenon. It was unknown whether different levels of CD40L expression induce quantitatively and qualitatively different effector T cell responses. Using mice expressing different levels of CD40L, we demonstrated that the greater the T cell CD40L expression the less tumor growth occurred; the antitumor T cell response was host-protective. Lower levels of CD40L expression on T cells induced IL-10-mediated suppression of tumor-regressing effector CD8(+) T cells and higher productions of IL-4 and IL-10. Using mice expressing different levels of CD40 or by administering different doses of anti-CD40 Ab, similar observations were recorded implying that the induction of protumor or antitumor T cell responses was a function of the extent of CD40 cross-linking. IL-10 neutralization during priming with tumor Ags resulted in a stronger tumor-regressing effector T cell response. Using IL-10(-/-) DC for priming of mice expressing different levels of CD40L and subsequent transfer of the T cells from the primed mice to nu/nu mice, we demonstrated the protumor role of IL-10 in the induction of tumor-promoting T cells. Our results demonstrate that a dose-dependent cross-linking of a costimulatory molecule dictates the functional phenotype of the elicited effector T cell response. The T cell costimulation is a continuum of a function that induces not only graded T cell responses but also two counteracting responses at two extremes.     

Autocrine activation-induced cell death of T cells by human peripheral blood monocyte-derived CD4+ dendritic cells

Mature T cells activated by antigen (Ag)-presenting cells are subject to various downmodulatory processes designed to maintain T cell homeostasis. Here we describe experiments in which mature T cells were subjected to apoptosis following stimulation with CD4(+) dendritic cells (DCs) during Ag presentation. The proliferative response of allogeneic T cells was increased by DCs at stimulator to responder (S/R) ratios ranging from 10(-3) to 1, whereas this response was decreased at S/R ratios ranging from 2 to 10. Allogeneic T cells stimulated with DCs at an S/R ratio of 5 underwent apoptosis, whereas this event was not observed in allogeneic T cells stimulated with DCs at an S/R ratio of 0.5. Stimulation of T cells with DCs at an S/R ratio of 5 induced a higher level of expression of CD95 ligand (CD95L) than stimulation of T cells cultured with DCs at an S/R ratio of 0.5, whereas similar levels of expression of CD28 and CD154 were observed in both cells. The abortive proliferation of mature T cells stimulated with DCs was prevented by blocking the CD95-CD95L system. Our results suggest that the CD4(+) DCs play counterregulatory roles in dictating T cell responses during Ag presentation.     

Memory CD8+ T cells protect dendritic cells from CTL killing

CD8(+) T cells have been shown to be capable of either suppressing or promoting immune responses. To reconcile these contrasting regulatory functions, we compared the ability of human effector and memory CD8(+) T cells to regulate survival and functions of dendritic cells (DC). We report that, in sharp contrast to the effector cells (CTLs) that kill DCs in a granzyme B- and perforin-dependent mechanism, memory CD8(+) T cells enhance the ability of DCs to produce IL-12 and to induce functional Th1 and CTL responses in naive CD4(+) and CD8(+) T cell populations. Moreover, memory CD8(+) T cells that release the DC-activating factor TNF-alpha before the release of cytotoxic granules induce DC expression of an endogenous granzyme B inhibitor PI-9 and protect DCs from CTL killing with similar efficacy as CD4(+) Th cells. The currently identified DC-protective function of memory CD8(+) T cells helps to explain the phenomenon of CD8(+) T cell memory, reduced dependence of recall responses on CD4(+) T cell help, and the importance of delayed administration of booster doses of vaccines for the optimal outcome of immunization.     

Quantitative and Qualitative Deficits in Neonatal Lung-Migratory Dendritic Cells Impact the Generation of the CD8+ T Cell Response

CD103+ and CD11b+ populations of CD11c+MHCIIhi murine dendritic cells (DCs) have been shown to carry antigens from the lung through the afferent lymphatics to mediastinal lymph nodes (MLN). We compared the responses of these two DC populations in neonatal and adult mice following intranasal infection with respiratory syncytial virus. The response in neonates was dominated by functionally-limited CD103+ DCs, while CD11b+ DCs were diminished in both number and function compared to adults. Infecting mice at intervals through the first three weeks of life revealed an evolution in DC phenotype and function during early life. Using TCR transgenic T cells with two different specificities to measure the ability of CD103+ DC to induce epitope-specific CD8+ T cell responses, we found that neonatal CD103+ DCs stimulate proliferation in a pattern distinct from adult CD103+ DCs. Blocking CD28-mediated costimulatory signals during adult infection demonstrated that signals from this costimulatory pathway influence the hierarchy of the CD8+ T cell response to RSV, suggesting that limited costimulation provided by neonatal CD103+ DCs is one mechanism whereby neonates generate a distinct CD8+ T cell response from that of adults.     

Late signals from CD27 prevent Fas-dependent apoptosis of primary CD8+ T cells

The role of costimulation has previously been confined to the very early stages of the CD8+ T cell response. In this study, we demonstrate the requirement for CD27 costimulation during the later phase, but not programming of the primary CD8+ T cell response to influenza virus and reveal a novel mechanism of action for CD27 costimulation. CD27 signals, during the later phase of the primary CD8+ T cell response, prevent apoptosis of Ag-specific CD8+ T cells. Blocking CD27L (CD70) on days 6 and 8 after infection reduces the number of NP(366-374)-specific CD8+ T cells, increases their sensitivity to CD95/Fas-mediated apoptosis, and up-regulates FasL on CD4+ T cells. This reduction of NP(366-374)-specific CD8+ T cells requires the presence of CD4+ T cells and Fas signaling. Lack of CD27 signals also decreases the quality of memory CD8+ T cell responses. Memory CD8+ T cells, which express surface CD27 similar to naive cells, however, do not require CD27 costimulation during a secondary response. Thus, CD27 acts indirectly to regulate primary Ag-specific CD8+ T cell responses by preventing apoptosis of CD8+ T cells during the later phase of the primary response and is required for optimal quality of memory cells, but is not required during normally primed secondary CD8+ T cell responses.     

Acquisition of MHC:Peptide Complexes by Dendritic Cells Contributes to the Generation of Antiviral CD8+ T Cell Immunity In Vivo

There is an increasing body of evidence suggesting that the transfer of preformed MHC class I:peptide complexes between a virus-infected cell and an uninfected APC, termed cross-dressing, represents an important mechanism of Ag presentation to CD8(+) T cells in host defense. However, although it has been shown that memory CD8(+) T cells can be activated by uninfected dendritic cells (DCs) cross-dressed by Ag from virus-infected parenchymal cells, it is unknown whether conditions exist during virus infection in which naive CD8(+) T cells are primed and differentiate to cytolytic effectors through cross-dressing, and indeed which DC subset would be responsible. In this study, we determine whether the transfer of MHC class I:peptide complexes between infected and uninfected murine DC plays a role in CD8(+) T cell priming to viral Ags in vivo. We show that MHC class I:peptide complexes from peptide-pulsed or virus-infected DCs are indeed acquired by splenic CD8α(-) DCs in vivo. Furthermore, the acquired MHC class I:peptide complexes are functional in that they induced Ag-specific CD8(+) T cell effectors with cytolytic function. As CD8α(-) DCs are poor cross-presenters, this may represent the main mechanism by which CD8α(-) DCs present exogenously encountered Ag to CD8(+) T cells. The sharing of Ag as preformed MHC class I:peptide complexes between infected and uninfected DCs without the restraints of Ag processing may have evolved to accurately amplify the response and also engage multiple DC subsets critical in the generation of strong antiviral immunity.     

Alpha tumor necrosis factor contributes to CD8(+) T cell survival in the transition phase

Cytokine and costimulation signals determine CD8(+) T cell responses in proliferation phase. In this study, we assessed the potential effect of cytokines and costimulations to CD8(+) T cell survival in transition phase by transferring in vitro ovalbumin (OVA)-pulsed dendritic cell-activated CD8(+) T cells derived from OVA-specific T cell receptor transgenic OT I mice into wild-type C57BL/6 mice or mice with designated gene knockout. We found that deficiency of IL-10, IL-12, IFN-gamma, CD28, CD40, CD80, CD40L, and 41BBL in recipients did not affect CD8(+) T cell survival after adoptive transfer. In contrast, TNF-alpha deficiency in both recipients and donor CD8(+) effector T cells significantly reduced CD8(+) T cell survival. Therefore, our data demonstrate that the host- and T cell-derived TNF-alpha signaling contributes to CD8(+) effector T cell survival and their transition to memory T cells in the transition phase, and may be useful information when designing vaccination.     

Preferential costimulation by CD80 results in IL-10-dependent TGF-beta1(+) -adaptive regulatory T cell generation

Costimulatory ligands CD80 and CD86 have different binding preferences and affinities to their receptors, CD28 and CTLA-4. Earlier, we demonstrated that CD80 binds to CTLA-4 with higher affinity and has a role in suppressing T cell response. The current study demonstrates that not only did blockade of CD86 upon Ag presentation by bone marrow-derived dendritic cells (DC) to OVA-specific T cells result in induction of hyporesponsive T cells but also that these T cells could suppress the proliferative response of effector T cells. These T cells showed TGF-beta1 on their surface and secreted TGF-beta1 and IL-10 upon restimulation. Although blockade of CTLA-4 and neutralization of IL-10 profoundly inhibited the induction of these TGF-beta1(+) T cells, their ability to suppress the effector T cell proliferation was abrogated by neutralization of TGF-beta1 alone. Induction of TGF-beta1(+) and IL-10(+) T cells was found to be independent of natural CD4(+)CD25(+) regulatory T cells, demonstrating that preferential ligation of CTLA-4 by CD80 induced IL-10 production by effector T cells, which in turn promoted the secretion of TGF-beta1. Treatment of prediabetic NOD mice with islet beta cell Ag-pulsed CD86(-/-) DCs, but not CD80(-/-) DCs, resulted in the induction of TGF-beta1- and IL-10-producing cells, significant suppression of insulitis, and delay of the onset of hyperglycemia. These observations demonstrate not only that CD80 preferentially binds to CTLA-4 but also that interaction during Ag presentation can result in IL-10-dependent TGF-beta1(+) regulatory T cell induction, reinstating the potential of approaches to preferentially engage CTLA-4 through CD80 during self-Ag presentation in suppressing autoimmunity.     

Glucocorticoid-induced TNFR-related protein lowers the threshold of CD28 costimulation in CD8+ T cells

CD28 is well characterized as a costimulatory molecule in T cell activation. Recent evidences indicate that TNFR superfamily members, including glucocorticoid-induced TNFR-related protein (GITR), act as costimulatory molecules. In this study, the relationship between GITR and CD28 has been investigated in murine CD8(+) T cells. When suboptimal doses of anti-CD3 Ab were used, the absence of GITR lowered CD28-induced activation in these cells whereas the lack of CD28 did not affect the response of CD8(+) T cells to GITR costimulus. In fact, costimulation of CD28 in anti-CD3-activated GITR(-/-) CD8(+) T cells resulted in an impaired increase of proliferation, impaired protection from apoptosis, and an impaired rise of activation molecules such as IL-2R, IL-2, and IFN-gamma. Most notably, CD28-costimulated GITR(-/-) CD8(+) T cells revealed lower NF-kappaB activation. As a consequence, up-regulation of Bcl-x(L), one of the major target proteins of CD28-dependent NF-kappaB activation, was defective in costimulated GITR(-/-) CD8(+) T cells. What contributed to the response to CD28 ligation in CD8(+) T cells was the early up-regulation of GITR ligand on the same cells, the effect of which was blocked by the addition of a recombinant GITR-Fc protein. Our results indicate that GITR influences CD8(+) T cell response to CD28 costimulation, lowering the threshold of CD8(+) T cell activation.     

Cutting edge: CD4+ T cell help can be essential for primary CD8+ T cell responses in vivo

Recent studies have shown that CD4(+) T cell help is required for the generation of memory CD8(+) T cells that can proliferate and differentiate into effector cells on Ag restimulation. The importance of help for primary CD8(+) T cell responses remains controversial. It has been suggested that help is not required for the initial proliferation and differentiation of CD8(+) T cells in vivo and that classical models of helper-dependent responses describe impaired secondary responses to Ag in vitro. We have measured primary CD8(+) T cell responses to peptide-pulsed dendritic cells in mice by cytokine ELISPOT and tetramer staining. No responses were detected in the absence of help, either when normal dendritic cells were injected into MHC II-deficient mice or when MHC II-deficient dendritic cells were injected into normal mice. Thus, the primary in vivo CD8(+) T cell response depends absolutely on help from CD4(+) T cells in our experimental system.     

Ox40 costimulation enhances the development of T cell responses induced by dendritic cells in vivo.

Dendritic cells (DCs) are bone marrow-derived APCs that display unique properties aimed at stimulating naive T cells. Several members of the TNF/TNFR families have been implicated in T cell functions. In this study, we examined the role that Ox40 costimulation might play on the ability of DCs to regulate CD4(+) and CD8(+) T cell responses in vivo. Administration of anti-mouse Ox40 mAb enhanced the Th response induced by immunization with Ag-pulsed DCs, and introduced a bias toward a Th1 immune response. However, anti-Ox40 treatment enhanced the production of Th2 cytokines in IFN-gamma(-/-) mice after immunization with Ag-pulsed DCs, suggesting that the production of IFN-gamma during the immune response could interfere with the development of Th2 lymphocytes induced by DCs. Coadministration of anti-Ox40 with DCs during Ag rechallenge enhanced both Th1 and Th2 responses induced during a primary immunization with DCs, and did not reverse an existing Th2 response. This suggests that Ox40 costimulation amplifies an ongoing immune response, regardless of Th differentiation potential. In an OVA-TCR class II-restricted adoptive transfer system, anti-Ox40 treatment greatly enhanced the level of cytokine secretion per Ag-specific CD4(+) T cell induced by immunization with DCs. In an OVA-TCR class I-restricted adoptive transfer system, administration of anti-Ox40 strongly enhanced expansion, IFN-gamma secretion, and cytotoxic activity of Ag-specific CD8(+) T cells induced by immunization with DCs. Thus, by enhancing immune responses induced by DCs in vivo, the Ox40 pathway might be a target for immune intervention in therapeutic settings that use DCs as Ag-delivery vehicles.     

Differential requirement for the CD40-CD154 costimulatory pathway during Th cell priming by CD8 alpha+ and CD8 alpha- murine dendritic cell subsets

Dendritic cells (DCs) regulate the development of distinct Th populations and thereby provoke appropriate immune responses to various kinds of Ags. In the present work, we investigated the role CD40-CD154 interactions play during the process of Th cell priming by CD8 alpha(+) and CD8 alpha(-) murine DC subsets, which have been reported to differently regulate the Th response. Adoptive transfer of Ag-pulsed CD8 alpha(+) DCs induced a Th1 response and the production of IgG2a Abs, whereas transfer of CD8 alpha(-) DCs induced Th2 cells and IgE Abs in vivo. Induction of distinct Th populations by each DC subset was also confirmed in vitro. Although interruption of CD80/CD86-CD28 interactions inhibited Th cell priming by both DC subsets, disruption of CD40-CD154 interactions only inhibited the induction of the Th1 response by CD8 alpha(+) DCs in vivo. CD40-CD154 interactions were not required for the proliferation of Ag-specific naive Th cells stimulated by either DC subset, but were indispensable in the production of IL-12 from CD8 alpha(+) DCs and their induction of Th1 cells in vitro. Taken together, in our immunization model of Ag-pulsed DC transfer, CD40-CD154 interactions play an important role in the development of CD8 alpha(+) DC-driven Th1 responses but not CD8 alpha(-) DC-driven Th2 responses to protein Ags.     

CD8alpha+ and CD11b+ dendritic cell-restricted MHC class II controls Th1 CD4+ T cell immunity

The activation, proliferation, differentiation, and trafficking of CD4 T cells is central to the development of type I immune responses. MHC class II (MHCII)-bearing dendritic cells (DCs) initiate CD4(+) T cell priming, but the relative contributions of other MHCII(+) APCs to the complete Th1 immune response is less clear. To address this question, we examined Th1 immunity in a mouse model in which I-A(beta)(b) expression was targeted specifically to the DCs of I-A(beta)b-/- mice. MHCII expression is reconstituted in CD11b(+) and CD8alpha(+) DCs, but other DC subtypes, macrophages, B cells, and parenchymal cells lack of expression of the I-A(beta)(b) chain. Presentation of both peptide and protein Ags by these DC subsets is sufficient for Th1 differentiation of Ag-specific CD4(+) T cells in vivo. Thus, Ag-specific CD4(+) T cells are primed to produce Th1 cytokines IL-2 and IFN-gamma. Additionally, proliferation, migration out of lymphoid organs, and the number of effector CD4(+) T cells are appropriately regulated. However, class II-negative B cells cannot receive help and Ag-specific IgG is not produced, confirming the critical MHCII requirement at this stage. These findings indicate that DCs are not only key initiators of the primary response, but provide all of the necessary cognate interactions to control CD4(+) T cell fate during the primary immune response.