Inhibition of myeloid dendritic cell accessory cell function and induction of T cell anergy by alcohol correlates with decreased IL-12 production

Alcohol consumption inhibits accessory cell function and Ag-specific T cell responses. Myeloid dendritic cells (DCs) coordinate innate immune responses and T cell activation. In this report, we found that in vivo moderate alcohol intake (0.8 g/kg of body weight) in normal volunteers inhibited DC allostimulatory capacity. Furthermore, in vitro alcohol treatment during DC differentiation significantly reduced allostimulatory activity in a MLR using naive CD4(+) T cells, and inhibited tetanus toxoid Ag presentation by DCs. Alcohol-treated DCs showed reduced IL-12, increased IL-10 production, and a decrease in expression of the costimulatory molecules CD80 and CD86. Addition of exogenous IL-12 and IL-2, but not neutralization of IL-10, during MLR ameliorated the reduced allostimulatory capacity of alcohol-treated DCs. Naive CD4(+) T cells primed with alcohol-treated DCs showed decreased IFN-gamma production that was restored by exogenous IL-12, indicating inhibition of Th1 responses. Furthermore, CD4(+) T cells primed with alcohol-treated DCs were hyporesponsive to subsequent stimulation with the same donor-derived normal DCs, suggesting the ability of alcohol-treated DCs to induce T cell anergy. LPS-induced maturation of alcohol-treated immature DCs partially restored the reduced allostimulatory activity, whereas alcohol given only during DC maturation failed to inhibit DC functions, suggesting that alcohol primarily impairs DC differentiation rather than maturation. NFkappaB activation, a marker of DC maturation was not affected by alcohol. Taken together, alcohol both in vitro and in vivo can impair generation of Th1 immune responses via inhibition of DC differentiation and accessory cell function through mechanisms that involve decreased IL-12 induction.     

Carcinoembryonic antigen-related cell adhesion molecule 1 on murine dendritic cells is a potent regulator of T cell stimulation

Dendritic cells (DC) are important APCs that play a key role in the induction of an immune response. The signaling molecules that govern early events in DC activation are not well understood. We therefore investigated whether DC express carcinoembryonic Ag-related cell adhesion molecule 1 (CEACAM1, also known as BGP or CD66a), a well-characterized signal-regulating cell-cell adhesion molecule that is expressed on granulocytes, monocytes, and activated T cells and B cells. We found that murine DC express in vitro as well as in vivo both major isoforms of CEACAM1, CEACAM1-L (having a long cytoplasmic domain with immunoreceptor tyrosine-based inhibitory motifs) and CEACAM1-S (having a short cytoplasmic domain lacking phosphorylatable tyrosine residues). Ligation of surface-expressed CEACAM1 on DC with the specific mAb AgB10 triggered release of the chemokines macrophage inflammatory protein 1alpha, macrophage inflammatory protein 2, and monocyte chemoattractant protein 1 and induced migration of granulocytes, monocytes, T cells, and immature DC. Furthermore, the surface expression of the costimulatory molecules CD40, CD54, CD80, and CD86 was increased, indicating that CEACAM1-induced signaling regulates early maturation and activation of dendritic cells. In addition, signaling via CEACAM1 induced release of the cytokines IL-6, IL-12 p40, and IL-12 p70 and facilitated priming of naive MHC II-restricted CD4(+) T cells with a Th1-like effector phenotype. Hence, our results show that CEACAM1 is a signal-transducing receptor that can regulate early maturation and activation of DC, thereby facilitating priming and polarization of T cell responses.     

IL-1 beta enhances CD40 ligand-mediated cytokine secretion by human dendritic cells (DC): a mechanism for T cell-independent DC activation.

CD40 ligand (CD40L) is a membrane-bound molecule expressed by activated T cells. CD40L potently induces dendritic cell (DC) maturation and IL-12p70 secretion and plays a critical role during T cell priming in the lymph nodes. IFN-gamma and IL-4 are required for CD40L-mediated cytokine secretion, suggesting that T cells are required for optimal CD40L activity. Because CD40L is rapidly up-regulated by non-T cells during inflammation, CD40 stimulation may also be important at the primary infection site. However, a role for T cells at the earliest stages of infection is unclear. The present study demonstrates that the innate immune cell-derived cytokine, IL-1beta, can increase CD40L-induced cytokine secretion by monocyte-derived DC, CD34(+)-derived DC, and peripheral blood DC independently of T cell-derived cytokines. Furthermore, IL-1beta is constitutively produced by monocyte-derived DC and monocytes, and is increased in response to intact Escherichia coli or CD40L, whereas neither CD34(+)-derived DC nor peripheral blood DC produce IL-1beta. Finally, DC activated with CD40L and IL-1beta induce higher levels of IFN-gamma secretion by T cells compared with DC activated with CD40L alone. Therefore, IL-1beta is the first non-T cell-derived cytokine identified that enhances CD40L-mediated activation of DC. The synergy between CD40L and IL-1beta highlights a potent, T cell-independent mechanism for DC activation during the earliest stages of inflammatory responses.     

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.     

Cholera toxin promotes the induction of regulatory T cells specific for bystander antigens by modulating dendritic cell activation

It has previously been reported that cholera toxin (CT) is a potent mucosal adjuvant that enhances Th2 or mixed Th1/Th2 type responses to coadministered foreign Ag. Here we demonstrate that CT also promotes the generation of regulatory T (Tr) cells against bystander Ag. Parenteral immunization of mice with Ag in the presence of CT induced T cells that secreted high levels of IL-4 and IL-10 and lower levels of IL-5 and IFN-gamma. Ag-specific CD4(+) T cell lines and clones generated from these mice had cytokine profiles characteristic of Th2 or type 1 Tr cells, and these T cells suppressed IFN-gamma production by Th1 cells. Furthermore, adoptive transfer of bone marrow-derived dendritic cells (DC) incubated with Ag and CT induced T cells that secreted IL-4 and IL-10 and low concentrations of IL-5. It has previously been shown that IL-10 promotes the differentiation or expansion of type 1 Tr cells. Here we found that CT synergized with low doses of LPS to induce IL-10 production by immature DC. CT also enhanced the expression of CD80, CD86, and OX40 (CD134) on DC and induced the secretion of the chemokine, macrophage inflammatory protein-2 (MIP-2), but inhibited LPS-driven induction of CD40 and ICAM-I expression and production of the inflammatory cytokines/chemokines IL-12, TNF-alpha, MIP-1alpha, MIP-1beta, and monocyte chemoattractant protein-1. Our findings suggest that CT induces maturation of DC, but, by inducing IL-10, inhibiting IL-12, and selectively affecting surface marker expression, suppresses the generation of Th1 cells and promotes the induction of T cells with regulatory activity.     

Parasite-induced Th2 polarization is associated with down-regulated dendritic cell responsiveness to Th1 stimuli and a transient delay in T lymphocyte cycling

The nature of the signals that bias Th effector choice is still not completely understood. Using parasite extracts from pathogens known to induce polarized Th1 or Th2 responses and an in vitro experimental model for priming murine CD4(+) cells, we demonstrated that splenic dendritic cells (DC), but not B cells, promote Th1/Th2 differentiation of naive CD4(+) lymphocytes. Th polarization in this system was found not to depend on DC secretion of the polarizing cytokines IL-12/IL-4, but instead correlated with distinct states of DC activation induced by the different parasite preparations. As expected, conditioning of DC for Th1 development was associated with up-regulation of costimulatory molecules and enhanced chemokine production and required intact MyD88 signaling. In contrast, conditioning of DC for Th2 differentiation correlated with down-regulation of many of the same functions and was MyD88 independent. This dampened DC activation was accompanied in the cocultures by a reduction in the frequency of CD4(+) lymphocytes exiting the first division of the cell cycle. When the latter was mimicked by drug-induced arrest of peptide-primed CD4(+) cells after the S phase of the first cycle, a marked Th2 polarization was also observed. Together, these findings suggest that the emergence of IL-4-producing CD4(+) lymphocytes results from a suppression in DC function leading to a temporary delay in initial T cell cycling.     

Dendritic cell-derived IL-12 is not required for the generation of cytotoxic, IFN-gamma-secreting, CD8(+) CTL in vivo.

By using adoptive transfer of Ag-loaded bone marrow-derived dendritic cells (BMDC), we have established an in vivo model of CTL priming. Activation of CTL in these experiments required both CD4(+) T cells and CD154, demonstrating that this model reflects CD4(+) T cell-dependent dendritic cell (DC) licensing. Because IL-12 has been suggested to play an important role in CTL activation by DC, we examined the ability of BMDC to prime CTL in the complete absence of IL-12 using p40-deficient mice. We observed that the absence of IL-12 does not affect the phenotype or allostimulatory function of BMDC after in vitro maturation. Moreover, there was no difference in the ability of Ag-loaded DC to elicit CTL cytotoxicity, whether the Ag was delivered by virus infection or peptide pulsing. Equal frequencies of Ag-specific, IFN-gamma-secreting CD8(+) T cells developed in both wild-type and IL-12-deficient backgrounds. Finally, CTL generated in the IL-12-deficient environment were capable of protecting immunized mice against tumor challenge, demonstrating that these CTL were fully functional, despite the absence of IL-12 during the maturation process in vivo. These results indicate that IL-12 is not critical for the development of IFN-gamma secreting, CD8(+) T cells and that another mechanism must be used by licensed DC to prime and activate CTL.     

Immunosuppression induced by immature dendritic cells is mediated by TGF-beta/IL-10 double-positive CD4+ regulatory T cells.

Dendritic cells (DC) have important functions in T cell immunity and T cell tolerance. Previously, it was believed that T cell unresponsiveness induced by immature DC (iDC) is caused by the absence of inflammatory signals in steady-state in vivo conditions and by the low expression levels of costimulatory molecules on iDC. However, a growing body of evidence now indicates that iDC can also actively maintain peripheral T cell tolerance by the induction and/or stimulation of regulatory T cell populations. In this study, we investigated the in vitro T cell stimulatory capacity of iDC and mature DC (mDC) and found that both DC types induced a significant increase in the number of transforming growth factor (TGF)-beta and interleukin (IL)-10 double-positive CD4(+) T cells within 1 week of autologous DC/T cell co-cultures. In iDC/T cell cultures, where antigen-specific T cell priming was significantly reduced as compared to mDC/T cell cultures, we demonstrated that the tolerogenic effect of iDC was mediated by soluble TGF-beta and IL-10 secreted by CD4(+)CD25(-)FOXP3(-) T cells. In addition, the suppressive capacity of CD4(+) T cells conditioned by iDC was transferable to already primed antigen-specific CD8(+) T cell cultures. In contrast, addition of CD4(+) T cells conditioned by mDC to primed antigen-specific CD8(+) T cells resulted in enhanced CD8(+) T cell responses, notwithstanding the presence of TGF-beta(+)/IL-10(+) T cells in the transferred fraction. In summary, we hypothesize that DC have an active role in inducing immunosuppressive cytokine-secreting regulatory T cells. We show that iDC-conditioned CD4(+) T cells are globally immunosuppressive, while mDC induce globally immunostimulatory CD4(+) T cells. Furthermore, TGF-beta(+)/IL-10(+) T cells are expanded by DC independent of their maturation status, but their suppressive function is dependent on immaturity of DC.     

A new dynamic model of CD8+ T effector cell responses via CD4+ T helper-antigen-presenting cells

A long-standing paradox in cellular immunology has been the conditional requirement for CD4(+) Th cells in priming of CD8(+) CTL responses. We propose a new dynamic model of CD4(+) Th cells in priming of Th-dependent CD8(+) CTL responses. We demonstrate that OT II CD4(+) T cells activated by OVA-pulsed dendritic cells (DC(OVA)) are Th1 phenotype. They acquire the immune synapse-composed MHC II/OVAII peptide complexes and costimulatory molecules (CD54 and CD80) as well as the bystander MHC class I/OVAI peptide complexes from the DC(OVA) by DC(OVA) stimulation and thus also the potential to act themselves as APCs. These CD4(+) Th-APCs stimulate naive OT I CD8(+) T cell proliferation through signal 1 (MHC I/OVAI/TCR) and signal 2 (e.g., CD54/LFA-1 and CD80/CD28) interactions and IL-2 help. In vivo, they stimulate CD8(+) T cell proliferation and differentiation into CTLs and induce effective OVA-specific antitumor immunity. Taken together, this study demonstrates that CD4(+) Th cells carrying acquired DC Ag-presenting machinery can, by themselves, efficiently stimulate CTL responses. These results have substantial implications for research in antitumor and other aspects of immunity.     

Selectively impaired CD8+ but not CD4+ T cell cycle arrest during priming as a consequence of dendritic cell interaction with plasmodium-infected red cells

Individuals living in malaria-endemic areas show generally low T cell responses to malaria Ags. In this study, we show murine dendritic cell (DC) interaction with parasitized erythrocytes (pRBC) arrested their maturation, resulting in impaired ability to stimulate naive, but not recall T cell responses in vitro and in vivo. Moreover, within the naive T cell population, pRBC-treated DC were selectively deficient in priming CD8(+) but not CD4(+) T cells. Indeed, DC that had taken up pRBC were shown for the first time to efficiently prime CD4(+) T cell responses to a known protective merozoite Ag, MSP4/5. In contrast, impaired priming resulted in decreases in both proliferation and cytokine production by CD8(+) T cells. Deficient priming was observed to both a model and a Plasmodium berghei-specific CD8(+) T cell epitope. The mechanisms underlying the inability of parasite-treated DC to prime CD8(+) T cells were explored. pRBC treatment of DC from wild-type C57BL/6, but not from IL-10 knockout animals, suppressed DC-mediated T cell priming across a Transwell, suggesting active IL-10-dependent suppression. CD8(+) T cells were arrested at the G(0) stage of the cell cycle after two cell divisions post-Ag stimulation. The proliferation arrest was partially reversible by the addition of IL-2 or IL-7 to responder cultures. These results suggest that in malaria-endemic areas, priming of CD8(+) T cell responses may be more difficult to induce via vaccination than the priming of CD4(+) T cells. Moreover, pathogens may selectively target the CD8(+) T cell arm of protective immunity for immune evasion.     

Induction of CTL and nonpolarized Th cell responses by CD8alpha(+) and CD8alpha(-) dendritic cells

Two distinct dendritic cell (DC) subpopulations have been evidenced in mice on the basis of their differential CD8alpha expression and their localization in lymphoid organs. Several reports suggest that CD8alpha(+) and CD8alpha(-) DC subsets could be functionally different. In this study, using a panel of MHC class I- and/or class II-restricted peptides, we analyzed CD4(+) and CD8(+) T cell responses obtained after i.v. injection of freshly purified peptide-pulsed DC subsets. First, we showed that both DC subsets efficiently induce specific CTL responses and Th1 cytokine production in the absence of CD4(+) T cell priming. Second, we showed that in vivo activation of CD4(+) T cells by CD8alpha(+) or CD8alpha(-) DC, injected i.v., leads to a nonpolarized Th response with production of both Th1 and Th2 cytokines. The CD8alpha(-) subset induced a higher production of Th2 cytokines such as IL-4 and IL-10 than the CD8alpha(+) subset. However, IL-5 was produced by CD4(+) T cells activated by both DC subsets. When both CD4(+) and CD8(+) T cells were primed by DC injected i.v., a similar pattern of cytokines was observed, but, under these conditions, Th1 cytokines were mainly produced by CD8(+) T cells, while Th2 cytokines were produced by CD4(+) T cells. Thus, this study clearly shows that CD4(+) T cell responses do not influence the development of specific CD8(+) T cell cytotoxic responses induced either by CD8alpha(+) or CD8alpha(-) DC subsets.     

HIV-1 capture and antigen presentation by dendritic cells: enhanced viral capture does not correlate with better T cell activation

During HIV-1 infection, dendritic cells (DC) facilitate dissemination of HIV-1 while trying to trigger adaptive antiviral immune responses. We examined whether increased HIV-1 capture in DC matured with LPS results in more efficient Ag presentation to HIV-1-specific CD4(+) and CD8(+) T cells. To block the DC-mediated trans-infection of HIV-1 and maximize Ag loading, we also evaluated a noninfectious integrase-deficient HIV-1 isolate, HIV(NL4-3ΔIN). We showed that higher viral capture of DC did not guarantee better Ag presentation or T cell activation. Greater HIV(NL4-3) uptake by fully LPS-matured DC resulted in higher viral transmission to target cells but poorer stimulation of HIV-1-specific CD4(+) and CD8(+) T cells. Conversely, maturation of DC with LPS during, but not before, viral loading enhanced both HLA-I and HLA-II HIV-1-derived Ag presentation. In contrast, DC maturation with the clinical-grade mixture consisting of IL-1β, TNF-α, IL-6, and PGE(2) during viral uptake only stimulated HIV-1-specific CD8(+) T cells. Hence, DC maturation state, activation stimulus, and time lag between DC maturation and Ag loading impact HIV-1 capture and virus Ag presentation. Our results demonstrate a dissociation between the capacity to capture HIV-1 and to present viral Ags. Integrase-deficient HIV(NL4-3ΔIN) was also efficiently captured and presented by DC through the HLA-I and HLA-II pathways but in the absence of viral dissemination. HIV(NL4-3ΔIN) seems to be an attractive candidate to be explored. These results provide new insights into DC biology and have implications in the optimization of DC-based immunotherapy against HIV-1 infection.     

Galectin-9 induces maturation of human monocyte-derived dendritic cells

Maturation of dendritic cells (DCs) is critical for initiation of immune responses and is regulated by various stimulatory signals. We assessed the role of galectin (Gal)-9 in DC maturation. Culture of immature DCs with exogenous Gal-9 markedly increased the surface expression of CD40, CD54, CD80, CD83, CD86, and HLA-DR in a dose-dependent manner, although Gal-9 had no or little effect on differentiation of human monocytes into immature DCs. Gal-9-treated DCs secreted IL-12 but not IL-10, and they elicited the production of Th1 cytokines (IFN-gamma and IL-2) but not that of the Th2 cytokines (IL-4 and IL-5) by allogeneic CD4+ T cells. These effects of Gal-9 on immature DCs were not essentially dependent on its lectin properties, given that they were inhibited only slightly by lactose. We further found that a Gal-9 mutant that lacks beta-galactoside binding activity reproduced the above activities and that an anti-Gal-9 mAb suppressed them. Gal-9 induced phosphorylation of the MAPK p38 and ERK1/2 in DCs, and an inhibitor of p38 signaling, but not inhibitors of signaling by either ERK1/2 or PI3K, blocked Gal-9-induced up-regulation of costimulatory molecule expression and IL-12 production. These findings suggest that Gal-9 plays a role not only in innate immunity but also in acquired immunity by inducing DC maturation and promoting Th1 immune responses.     

A new mechanism for inhalational priming: IL-4 bypasses innate immune signals

Signaling via innate immune mechanisms is considered pivotal for T cell-mediated responses to inhaled Ags. Furthermore, Th2 cells specific for one inhaled Ag can facilitate priming of naive T cells to unrelated new inhaled Ags, a process we call "Th2 collateral priming". Interestingly, our previous studies showed that collateral priming is independent of signals via the innate immune system but depends on IL-4 secretion by CD4(+) T cells. We thus hypothesized that IL-4 can bypass the need for signals via the innate immune system, considered essential for pulmonary priming. Indeed, we were able to show that IL-4 bypasses the requirement for TLR4- and MyD88-mediated signaling for responses to new allergens. Furthermore, we characterized the mechanisms by which IL-4 primes for new inhaled allergens: "IL-4-dependent pulmonary priming" relies on IL-4 receptor expression on hematopoietic cells and structural cells. Transfer experiments indicate that within the hematopoietic compartment both T cells and dendritic cells need to express the IL-4 receptor. Finally, we were able to show that IL-4 induces recruitment and maturation of myeloid dendritic cells in vivo and increases T cell recruitment to the draining lymph nodes. Our findings bring new mechanistic knowledge to the phenomenon of polysensitization and primary sensitization in asthma.     

Cutting edge: human CD4+CD25+ T cells restrain the maturation and antigen-presenting function of dendritic cells

The characteristics and functions of CD4(+)CD25(+) regulatory cells have been well defined in murine and human systems. However, the interaction between CD4(+)CD25(+) T cells and dendritic cells (DC) remains unclear. In this study, we examined the effect of human CD4(+)CD25(+) T cells on maturation and function of monocyte-derived DC. We show that regulatory T cells render the DC inefficient as APCs despite prestimulation with CD40 ligand. This effect was marginally reverted by neutralizing Abs to TGF-beta. There was an increased IL-10 secretion and reduced expression of costimulatory molecules in DC. Thus, in addition to direct suppressor effect on CD4(+) T cells, regulatory T cells may modulate the immune response through DC.     

Early IL-2 production by mouse dendritic cells is the result of microbial-induced priming

Dendritic cells (DCs) are professional APCs able to initiate innate and adaptive immune responses against invading pathogens. Different properties such as the efficient Ag processing machinery, the high levels of expression of costimulatory molecules and peptide-MHC complexes, and the production of cytokines contribute in making DCs potent stimulators of naive T cell responses. Recently we have observed that DCs are able to produce IL-2 following bacterial stimulation, and we have demonstrated that this particular cytokine is a key molecule conferring to early bacterial activated DCs unique T cell priming capacity. In the present study we show that many different microbial stimuli, but not inflammatory cytokines, are able to stimulate DCs to produce IL-2, indicating that DCs can distinguish a cytokine-mediated inflammatory process from the actual presence of an infection. The capacity to produce IL-2 following a microbial stimuli encounter is a feature shared by diverse DC subtypes in vivo, such as CD8 alpha(+) and CD8 alpha(-) splenic DCs and epidermal Langerhans cells. When early activated DCs interact with T cells, IL-2 produced by DCs is enriched at the site of cell-cell contact, confirming the importance of DCs-derived IL-2 in T cell activation.     

Functional inactivation of immature dendritic cells by the intracellular parasite Toxoplasma gondii

Despite its noted ability to induce strong cellular immunity, and its known susceptibility to IFN-gamma-dependent immune effector mechanisms, the protozoan Toxoplasma gondii is a highly successful parasite, able to replicate, disseminate, and either kill the host or, more commonly, establish resistant encysted life forms before the emergence of protective immune responses. We sought to understand how the parasite gains the advantage. Using transgenic clonal parasite lines engineered to express fluorescent markers in combination with dendritic cells (DC) grown from the bone marrow of wild-type mice or transgenic mice expressing fluorescent protein-tagged MHC class II molecules, we used flow cytometry and fluorescence microscopy to analyze the responses of infected DC to both invasion by the parasite and subsequent DC maturation signals. We found that T. gondii preferentially invades immature dendritic cells but fails to activate them in the process, and renders them resistant to subsequent activation by TLR ligands or the immune-system-intrinsic maturation signal CD40L. The functional consequences of T. gondii-mediated suppression of DC activation are manifested in a relative inability of infected immature DC to activate naive CD4(+) Th lymphocytes, or to secrete cytokines, such IL-12 and TNF-alpha, that play important roles in innate and/or adaptive immunity. The findings reveal that T. gondii suppresses the ability of immature DC to participate in innate immunity and to induce adaptive immune responses. The ability of T. gondii to temporarily evade recognition could provide a selective advantage that permits dissemination and establishment before adaptive immune response initiation.     

On CD28/CD40 ligand costimulation, common gamma-chain signals, and the alloimmune response

Activation and robust expansion of naive T cells often require T cell costimulatory signals and T cell growth factors. However, the precise growth and costimulation requirements for activation and expansion of CD4(+) and CD8(+) T cells in vivo in allograft response are still not clearly defined. In the present study, we critically examined the role of CD28/CD40 ligand (CD40L) costimulation and the common gamma-chain (gamma(c)) signals, a shared signaling component by receptors for all known T cell growth factors (i.e., IL-2, IL-4, IL-7, IL-9, IL-15, IL-21), in activation and expansion of CD4(+) and CD8(+) T cells in the allogeneic hosts. We found that CD28/CD40L costimulation and the gamma(c) signals are differentially involved in proliferation and clonal expansion of CD4(+) and CD8(+) T cells in response to alloantigen stimulation. CD8(+) T cells are highly dependent on the gamma(c) signals for survival, expansion, and functional maturation, whereas in vivo expansion of alloreactive CD4(+) T cells is largely gamma(c) independent. T cell costimulation via CD28 and CD40L, however, is necessary and sufficient for activation and expansion of CD4(+) T cells in vivo. In a skin transplant model, blocking both CD28/CD40L and the gamma(c) pathways induced prolonged skin allograft survival. Our study provides critical insights that the CD4 and CD8 compartments are most likely governed by distinct mechanisms in vivo, and targeting both costimulatory and gamma(c) signals may be highly effective in certain cytopathic conditions involving activation of both CD4(+) and CD8(+) T cells.     

Human CD4+CD25high regulatory T cells modulate myeloid but not plasmacytoid dendritic cells activation

Human CD4(+)CD25(+) regulatory T cells (Treg) play an essential role in the prevention of autoimmune diseases. However, the mechanisms of immune suppression and the spectrum of cells they target in vivo remain incompletely defined. In particular, although Treg directly suppress conventional T cells in vitro, they have been shown to inhibit the Ag-presenting functions of macrophage- and monocyte-derived dendritic cells (DC). We have now studied the maturation of human blood-derived myeloid DC and plasmacytoid DC activated with TLR ligands in the presence of Treg. Preactivated Treg suppressed strongly TLR-triggered myeloid DC maturation, as judged by the blocking of costimulatory molecule up-regulation and the inhibition of proinflammatory cytokines secretion that resulted in poor Ag presentation capacity. Although IL-10 played a prominent role in inhibiting cytokines secretion, suppression of phenotypic maturation required cell-cell contact and was independent of TGF-beta and CTLA-4. In contrast, the acquisition of maturation markers and production of cytokines by plasmacytoid DC triggered with TLR ligands were insensitive to regulatory T cells. Therefore, human Treg may enlist myeloid, but not plasmacytoid DC for the initiation and the amplification of tolerance in vivo by restraining their maturation after TLR stimulation.