Cellular and molecular mechanisms of CTLs responses mediated by type I and type II NKT cells. Activated type I NKT cells have two distinct effects that regulate CTL function. CTLs are activated by type I NKT cells via cytokines (IL-2, IL-12, IFN-, and TNF-) and costimulatory molecules (CD40L and CD28). In contrast, increased expression of Th2 cytokines (IL-4) and other costimulatory molecules (CTLA-4 and PD-L) inhibits CTL production or activation. Activated type II NKT cells suppress CTL activity via cytokines (IL-4, IL-13, and TGF-).