Augmented Induction of CD8+ Cytotoxic T cell Response and Antitumor Resistance by Thl-inducing peptide

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The effector CD8^+ T cells recognize MHC class I binding altered self-peptides expressed in tumor cells. Although the requirement for CD4^+ Th1 cells in regulating CD8^+ T cells has been documented, their target epitopes and functional impact in antitumor responses remain unclear. We examined whether a potent immunogenic peptide of Mycobacterium (M.) tuberculosis eliciting Thl immunity contributes to the generation of CD8^+ T cells and to protective antitumor immune responses to unrelated tumor-specific antigens. Peptide-25, a major Th epitope of Ag85B from M. tuberculosis preferentially induced CD4^+ Thl cells in C57BL/6 mice and showed an augmenting effect on Thl generation for coimmunized unrelated antigenic peptides. Coimmunization of mice with Peptide-25 and OVA or Peptide-25 and B16 melanoma peptide (TRP-2) for MHC class I led to a profound increase in CD8^+ T cells specific for OVA and TRP-2 peptides, respectively. This heightened response depended on Peptide-25 specific CD4^+ I … More FN-y-producing Th1 cells. In tumor protection assays, immunization with Peptide-25 and OVA resulted in the enhancement of CD8^+ cytotoxic cell generation specific for OVA and the growth inhibition of EL-4 thymoma expressing OVA peptide leading to the tumor rejection, that were not achieved by immunization with OVA alone. Peptide-25 reactive Thl cells counteractivated dendritic cells (DCs) in the presence of Peptide-25 leading them to activate and present OVA peptide to CD8^+ cytotoxic T cells. This result indicates that Peptide-25 not only induces a Thl-response by itself but also induces Thl-inducing activation of DCs through interactions with CD4^+ T cells. Therefore, we generated TCR transgenic mice (P25 TCR-Tg) expressing TCR a-and β-chains of Peptide-25-reactive cloned T cells and analyzed Thl development of CD4^+ T cells from P25 TCR-Tg to elucidate cellular and molecular mechanisms of the induction of Thl differentiation by Peptide-25. Naive CD4^+ T cells from P25 TCR-Tg preferentially develop Thl cells upon Peptide-25 stimulation in the presence of I-A^b splenic antigen-presenting cells under neutral conditions. Peptide-25-induced Thl differentiation is observed even in the presence of anti-IFN-y and anti-IL-12. Furthermore, Peptide-25-loaded I-A^b-transfected Chinese hamster ovary cells (Peptide-25-I-A^b-CHO) induce Thl differentiation of naive CD4^+ T cells from P25 TCR-Tg in the A^bsence of IFN-y or IL-12. Three hours after the TCR stimulation with Peptide-25-I-A^b-CHO, transient T-bet up-regulation and suppression of GATA-3 expression were observed by quantitative RT-PCR, both of which were independent of IFN-y and IL-12. These results imply that interaction between Peptide-25/I-A^b and TCR may primarily influence determination of the fate of naive CD4^+ T cells in their differentiation towards the Th1 subset. Taken together, these results indicate that Peptide-25 exerts potent adjuvant activity and provides efficient help for CTL induction against tumor antigen. Less