NKT cells act as helper T cells to cytotoxic T cells by direct recognition of CD1d on T cells (IRC8P.487)

Abstract Cytotoxic T lymphocytes (CTLs), known as CD8+ T cells, recognize their targets by binding to antigens associated with MHC class I. NKT cells recognize foreign glycolipid antigens and endogenous glycolipid antigens presented by CD1d which is an MHC class I- like molecule. NKT cells have been shown to be essential for the optimal activation of CTLs. In this study, we investigated the mechanism how NKT cells provide help for the CTL responses. We observed significantly higher production of IFN-γ by antigen-stimulated OT-1 CD8+ T cells in the presence of NKT cells. This enhanced CTL activation by NKT cells was observed even in the antigen presenting cell (APC)-free conditions. We also found that CD8+ T cells start to up-regulate surface expression of CD1d reaching highest level at around 18 hours upon antigen recognition. These results suggest that not only APC, but also activated CTLs receive direct help from NKT cells by increasing CD1d expression on their cell surface when they recognize cognate antigens.