Modulation of CD1d‐restricted NKT cell responses by CD4

CD4 + and CD4 − NKT cell populations have been shown to be functionally distinct, but the role of CD4 molecules in NKT cell activation is not clear. Here, we have used human CD1d‐restricted NKT cell clones to investigate the contribution of CD4 to NKT cell functional responses. Coligation of CD4 with the TCR/CD3 complex resulted in enhanced cytokine secretion and increased calcium flux by CD4 + NKT cell clones, indicating that CD4 is functionally active in these cells. CD4 blockade specifically inhibited cytokine secretion and proliferation of CD4 + NKT cell clones in response to CD1d + APCs but did not affect cytotoxicity, suggesting that CD4 preferentially modulates some NKT cell functional responses and not others. Anti‐CD4 mAb treatment inhibited NKT cell responses to both MHC class II + and MHC class II − APCs, indicating that its effect was not due to blocking CD4 binding to MHC class II molecules on APCs. The inhibitory effect of the anti‐CD4 mAb also did not require recognition of CD1d by the NKT cell, since calcium flux was reduced in response to anti‐CD3 mAb stimulation. Western blot analysis revealed that anti‐CD4 treatment resulted in increased phosphorylation of an inhibitory site of p56 lck (tyrosine 505). Thus, CD4 blockade interferes with the course of CD3‐mediated signaling events in NKT cells. These results indicate that CD4 can contribute to NKT cell activation independently of the presence of a CD4‐ligand on APCs and suggest that it preferentially modulates cytokine and proliferative responses.