Active CD4+ helper T cells directly stimulate CD8+ cytotoxic T lymphocyte responses in wild-type and MHC II gene knockout C57BL/6 mice and transgenic RIP-mOVA mice expressing islet beta-cell ovalbumin antigen leading to diabetes.

CD4+ helper T (Th) cells play crucial role in priming, expansion and survival of CD8+ cytotoxic T lymphocytes (CTLs). However, how CD4+ Th cell's help is delivered to CD8+ T cells in vivo is still unclear. We previously demonstrated that CD4+ Th cells can acquire ovalbumin (OVA) peptide/major histocompatibility complex (pMHC I) and costimulatory CD80 by OVA-pulsed DC (DC(OVA)) stimulation, and then stimulate OVA-specific CD8+ CTL responses in C57BL/6 mice. In this study, we further investigated CD4+ Th cell's effect on stimulation of CD8 CTL responses in major histocompatibility complex (MHC II) gene knockout (KO) mice and transgenic rat insulin promoter (RIP)-mOVA mice with moderate expression of self OVA by using CD4+ Th cells or Th cells with various gene deficiency. We demonstrated that the in vitro DC(OVA)-activated CD4+ Th cells (3 x 10(6) cells/mouse) can directly stimulate OVA-specific CD8+ T-cell responses in wild-type C57BL/6 mice and MHC II gene KO mice lacking CD4+ T cells. A large amount of CD4+ Th cells (12 x 10(6) cells/mouse) can even overcome OVA-specific immune tolerance in transgenic RIP-mOVA mice, leading to CD8+ CTL-mediated mouse pancreatic islet destruction and diabetes. The stimulatory effect of CD4+ Th cells is mediated by its IL-2 secretion and CD40L and CD80 costimulations, and is specifically delivered to OVA-specific CD8+ T cells in vivo via its acquired pMHC I complexes. Therefore, the above elucidated principles for CD4+ Th cells will have substantial implications in autoimmunity and antitumor immunity, and regulatory T-cell-dependent immune suppression.