| Schistosomiasis is a zoonotic parasitic disease that seriously hazard humanhealth in part of developing world. Development of vaccines to protect both humanand the domestic animals from schistosome infection is an attractive goal. However,the research related to development of vaccines against schistosomiasis japonica isfar behind that of schistosomiasis mansoni and lack of awareness of basicimmunology during Schistosoma japonicum infection greatly limits our choice ofrational strategies for vaccine development. Thus, fundamental immunologicalresearch should be highlighted to develop effective and specific vaccines againstschistosomiasis japonica.With the progress of infection, schistosome antigens of the differentdevelopmental stages regulating the host immune responses are released. Amongthem, schistosome eggs as well as soluble egg antigens (SEA) are mainlyresponsible for the immunopathology of schistosomiasis. Both antibody andCD4+T-cell-mediated, IFN-γ-dependent effector can be induced by vaccination with attenuated Schistosoma mansoni cercariae and showed that antigen-specificCD4+T-cell-mediated immunity and humoral response are fundamental to theacquired resistance against schistosome infection. Obviously, most of current studieshave been focused on the arm of effectness for the acquired immunity. But a littleconcerning the linkage between innate and acquired immunity involved inschistosome infection has been known.Antigen presenting cells, particularly dendritic cells (DCs), expresspattern-recognition receptors (PRP). These receptors ‘sense’ local danger signals orpathogen-associated molecular pattern (PAMP). So far, many studies of the innateimmune response during the infection of Schistosoma mansoni have been focused onegg antigens to illuminate why host developed a predominant Th2response after eggdeposition. All these data suggested that antigens derived from the parasite couldactivate the cascade pathway induced by the interections of PRRs and PAMPs toinitiate and direct the acquired immune responses.We previousely reported that TLR2and TLR4might direct the distinct adaptiveimmune responses in mice model infected with Schistosoma japonicum. In the acutephase of Schistosoma japonicum infection (6weeks after infection), TLR2-/-micemanifested lower egg burden as well as the enhancement of T cell activation andupregulated expression of some cytotoxic genes. Interestingly, the oppositeparasitological and immunological effects were observed in TLR4-/-mice. It wasindicated that TLR2could play a negative regulatory role. T cell activation requiresT-cell receptor (TCR) signal and the costimulatory signals. The maintenance ofnormal lymphocyte function requires positive and negative costimulatory signals.Programmed Death Ligand1(PD-L1) or Programmed Death Ligand2(PD-L2)could be combined with Programmed death1(PD-1) that expressed on activated Tcells. PD-1is a key co-inhibitory molecule that suppresses T-cell activation. Toll-like receptor2(TLR2) was important immune receptors involved inrecognition of components of SEA. In our study, TLR2knock out (TLR2-/-) andwild-type (WT) C57BL/6J mice were infected with Schistosoma japonicum. Thenmice were sacrificed to measure the parasite burdens at6weeks after infection. Toevaluate the immune responses after infection, the levels of schistosomaantigen-specific IgG/IgG1/IgG2a were determined for each mouse by indirectELISA; the proportion of the immune cell subsets in spleen were compared betweenTLR2-/-and WT mice by flow cytometry; and the mRNA levels of cytokines andcytotoxicity associated genes were validated by real-time quantitative PCR(RTQ-PCR). Hereafter, the expression of TLR2on schistosome antigen activatedDCs in experiments in vitro and vivo were determined by flow cytometry; meantimethe mRNA levels of tlr1, tlr2and tlr6were validated by RTQ-PCR. Finally, theexpression of PD-L1and PD-L2on schistosome antigen activated DCs werecompared between TLR2-/-and WT mice by flow cytometry in vitro; the mRNAlevels of pdl1and pdl2were validated by RTQ-PCR; the expression of PD-L1andPD-L2on DCs and the levels of PD-1on T cells were compared between TLR2-/-and WT mice by flow cytometry in vivo.The main results are as follows:1. Reduced egg burden in the liver was observed in TLR2-/-mice, compared toWT mice. At six weeks after challenge with40Schistosoma japonicum cercariae,parasite burdens including the numbers of adult worms and total eggs in livers ofeach group were evaluated. The parasite burdens were decreased in TLR2-/-group when compared to the WT mice. Significant differences were observed intotal eggs but not in total worms.2. The level of antigen-specific IgG/IgG1/IgG2a was no significant differencebetween TLR2-/-and B6mice after infection with Schistosoma japonicum. It was showed that there was no significant difference in antibody responsebetween TLR2-/-and B6mice after infection with Schistosoma japonicum. Inacute phase of infection (6weeks after infection), the level of SWAP-andSEA-specific IgG/IgG1/IgG2a antibody were no significantly difference betweenTLR2-/-and B6mice.3. Enhanced T cell-mediated immune response including the function of Th1cells and CD8+T cells were observed in TLR2-/-mice at6weeks afterinfection with Schistosoma japonicum. It was indicated that TLR2couldplay a negative regulatory role in Schistosoma japonicum infection. Theproportion of the immune cell subsets in spleens of TLR2-/-and B6mice wascompared by flow cytometry and the results showed that the proportion ofCD4+T, CD8+T and Th1cells in TLR2-/-mice was significantly higher than thatin B6mice, however, there were no significant differences in the proportion ofTh2, Tc1and Tc2cells between TLR2-/-and B6mice. The mRNA levels of il12and ifng from CD4+T cells of TLR2-/-mice were significantly higher than that ofB6mice; and the expression of gzma, gzmb and fasl from CD8+T cells of TLR2-/-mice were significantly higher than those of B6mice4. TLR2expression on DCs was upregulated by schistosome egg antigen. Itwas indicated that certain composition of schistosome egg antigen couldupregulate TLR2expression. TLR2mRNA expression was significantlyincreased in B6-BMDCs when it was stimulated with SEA. In the nature courseof S. japonicum infection, we found TLR2expression on DCs from B6mice kepton rising by flow cytometry examination. At9weeks after infection, TLR2expression on DCs from B6mice was significantly increased than that of at3weeks. At6weeks after infection, the level of tlr2mRNA on B6-BMDCs wassignificantly increased than that of at0weeks. 5. SEA induced PD-L2upregulation on DCs was TLR2dependent. Theexpression of PD-1on CD4+T cells from TLR2-/-mice was significantlyreduced after Schistosoma japonicum infection. It was indicated that SEAinduced the upregulation of PD-L2on DCs was dependent on TLR2, and theinteraction between PD-L2with PD-1on T cells might inhibite Tcell-mediated immune response. SEA could not upregulate PD-L2expressionon TLR2-/--BMDCs compared with B6-BMDCs; and the level of pdl2mRNA onTLR2-/--BMDCs was significantly decreased than B6-BMDCs. At6and9weeksafter infection, the level of PD-L2on DCs from TLR2-/-mice was decreased thanthat of B6mice and significant differences were observed at6weeks. At6and9weeks after infection, the level of PD-1on CD4+T cells from TLR2-/-mice wassignificantly reduced than that of B6mice. There were no significant differenceson PD-1expression from CD8+T cells between TLR2-/-and B6mice.In summary, after infection with Schistosoma japonicum, enhanced Tcell-mediated immune response including the function of Th1cells and CD8+Tcells were observed in TLR2-/-mice compared with B6mice. Our results alsoindicated that SEA induced the upregulation of PD-L2on DCs was dependenton TLR2, and the interaction between PD-L2with PD-1on T cells mightinhibite T cell-mediated immune response. In conclusion, TLR2plays a negativeregulatory effect in Schistosoma japonicum infection. |
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