The biological role of Langerin+ dendritic cells (DCs) such as Langerhans cells and a subset of dermal DCs (dDCs) in adaptive immunity against cutaneous pathogens remains enigmatic. Thus, we analyzed the impact of Langerin+ DCs in adaptive T cell-mediated immunity toward Leishmania major parasites in a Lang-DTR mouse model that allows conditional diphtheria toxin (DT)-induced ablation of Langerin+ DCs in vivo. For the first time, infection experiments with DT-treated Lang-DTR mice revealed that proliferation of L. major-specific CD8+ T cells is significantly reduced during the early phase of the immune response following depletion of Langerin+ DCs. Consequently, the total number of activated CD8+ T cells within the draining lymph node and at the site of infection is diminished. Furthermore, we show that the impaired CD8+ T cell response is due to the absence of Langerin+ dDCs and not Langerhans cells. Nevertheless, the CD4+ T cell response is not altered and the infection is cleared as effectively in DT-treated Lang-DTR mice as in control mice. This clearly demonstrates that Langerin+ DCs are, in general, dispensable for an efficient adaptive immune response against L. major parasites. Thus, we propose a novel concept that, in the experimental model of leishmaniasis, priming of CD4+ T cells is mediated by Langerin− dDCs, whereas Langerin+ dDCs are involved in early priming of CD8+ T cells.

IntroductionHelminth parasites have evolved immune evasion strategies necessary for their continued transmission. This immune evasion is achieved at the expense of both antigen-presenting cells (APCs) and T cells. Filarial parasites have been shown to induce dysfunction in both dendritic cells (DCs) and Langerhans cells, resulting in diminished capacity of these cells to activate CD4 ϩ T cells. 1,2 In addition, Toll-like receptor (TLR) expression and function appear to play an important role in filaria-induced immune dysregulation, 3,4 as patent filarial infection has been associated with diminished expression of TLR1, TLR2, and TLR4 and diminished responses to TLR2 ligands in both B and T cells.We have previously shown that monocytes from filaria-infected patients have a diminished capacity to produce IL-12, IL-10, MIP-1␣, IL-1␣, IL-8, and MIP-1␤ in response to Staphylococcus aureus Cowan I bacteria (SAC), a ligand that works through TLR2/TLR4. 5 This phenomenon extends to other helminth infections, as children with schistosomiasis have also been shown to have diminished responses to TLR ligands compared with those of uninfected children from the same endemic area. 6 TLRs are important initiators of innate immune responses through their ability to recognize a variety of microbial products bearing pathogen-associated molecular patterns (PAMPs). 7 Although there have been many studies examining TLR signaling in response to intracellular pathogens (including the parasitic protozoa [reviewed in Gazzinelli and Denkers 8 ; Yarovinsky and Sher 9 ; and Miyake 10 ]), many fewer have examined interaction of the multicellular helminth parasites and the TLR system. Indeed, the majority of these have focused on the glycans of schistosomes and TLR2 and the wolbachial endosymbiont of the filariae and TLR2 and TLR4. [11][12][13][14] The filarial nematode phosphorylcholine-containing secreted product, ES-62, has been shown to affect on IL-12 and TNF-␣ production by macrophages and DCs through a TLR4 MyD88-dependent pathway. 13 TLR-dependent proinflammatory cascades triggered by infections with protozoan parasites and other microbial agents must be tightly regulated to avoid severe pathology or even mortality. Once activated by microbial PAMPs, TLRs transduce signals through 2 pathways involving distinct adaptor proteins containing Toll/IL-1R (TIR) domains. 15,16 MyD88 is one of the adaptors used by each of the TLRs except TLR3, which signals mainly through the TIR domain-containing adaptor-inducing IFN-␤ (TRIF). 17 TLR4, the receptor for LPS, is the only TLR that can use either of the 2 adaptors. 18 The end result of TLR signaling is activation of NF-B, resulting in induction of proinflammatory cytokines or interferon regulatory factor (IRF)-dependent induction of type I interferons.Previously, we have demonstrated that monocyte-derived human DCs (mhDCs) exposed to live microfilariae (mf) of B malayi become less responsive to activation with SAC/IFN-␥ to produce IL-12p40 or IL-12p70. In the present study, we have extended these...

TNF and TNF receptor type 2 (TNFR2) have been shown to be important for generation of myeloid-derived suppressor cells (MDSC). In order to analyze whether and how TNFR2 passes the effect of TNF on, myeloid cells from TNFR2-deficient mice were compared to respective cells from wild-type mice. Primary TNFR2-deficient myeloid cells showed reduced production of NO and IL-6 which was attributable to CD11b+ CD11c− Ly6C+ Ly6G− immature monocytic MDSC. TNFR2-deficient MDSC isolated from bone marrow were less suppressive for T cell proliferation compared to WT-derived MDSC. These differences on myeloid cells between the two mouse lines were still observed after co-culture of bone marrow cells from the two mouse lines together during myeloid cell differentiation, which demonstrated that the impaired functional capacity of TNFR2-deficient cells was independent of soluble factors but required membrane expression of TNFR2. Similarly, adoptive transfer of TNFR2-deficient bone marrow cells into wild-type hosts did not rescue the TNFR2-specific phenotype of bone marrow-derived myeloid cells. Therefore, membrane TNFR2 expression determines generation and function of monocytic MDSC.

Immunosuppression, impaired cytokine production and high susceptibility to secondary infections are characteristic for septic patients, and for mice after induction of polymicrobial septic peritonitis by sublethal cecal ligation and puncture (CLP). Here, we demonstrate that CLP markedly altered subsequent B-cell responses. Total IgG and IgM levels, as well as the memory B-cell response, were increased in septic mice, but antigenspecific primary antibody production was strongly impaired. We found that two days after CLP, CD11b 1 splenocytes were activated as demonstrated by the increased expression of activation markers, expression of arginase and production of NO by immature myeloid cells. The in vivo clearance of a bacterial infection was not impaired. DCs demonstrated reduced IL-12 production and altered antigen presentation, resulting in decreased proliferation but enhanced IFN-c production by CD4 1 cells. CD4 1 T cells from mice immunized on day 2 after CLP showed reduced Th1 and Th2 cytokine production. In addition, there was an increase in Treg cells. Interestingly, levels of immature B cells decreased but levels of mature B cells increased two days after CLP. However, adoptive transfer of naïve CD4 1 T cells, naïve B cells, or naïve DCs did not rescue the antigen-specific antibody response.