Identification and characterization of CD8+ T helper cells based on CD40L expression

CD8+ T cells are primarily regarded as cytotoxic cells. We have identified a substantial subset of CD40L expressing non-cytotoxic T-cells among primary human CD8+ T cells. CD40L+ CD8+ T cells exert diverse characteristic Th-cell functions such as activation of B cells, induction of DC maturation and secretion of cytokines such as IL-2, IFNγ, TNFα or IL-4. Up to 25% of total human central and effector memory CD8+ T cells express CD40L including cells specific for pathogens such as influenza, CMV and EBV, as well as yellow fever virus specific cells after primary vaccination. At next we assessed the functional relevance of CD40L expression on CD8+ T cells during a anti-tumor response. We challenged Rag1-/- mice with SV40 Tag expressing tumor cells and injected in parallel wt or CD40L-/- CD8+ T cells. Only application of CD40L competent wt CD8+ T cells prevented the establishment of tumors, whereas injection of CD40L-/- CD8+ T cells resulted in non-controlled tumor progression similar to non-treated tumors, although tumor-specific T cells were primed in these mice. Our results disclose an essential functional relevance of CD40L expressed by CD8+ T cells. Especially, in situations of reduced CD4+ T-cell help and MHC-II antigen presentation and/or limited danger signals CD40L+ CD8+ T cells may exert essential helper responsibilities for immunity and thus are potent candidate T cells to execute or support effective anti-tumor or anti-pathogen immune therapies.