Functional killer Ig-like receptors on human memory CD4+ T cells specific for cytomegalovirus.

Although very few CD4(+) T cells express killer Ig receptors (KIR), a large proportion of CD4(+) T cells with a late memory phenotype, characterized by the absence of CD28, does express KIR. Here, we show that KIR expression on CD4(+) T cells is also associated with memory T cell function, by showing that the frequency of CMV-specific cells is higher in CD4(+)KIR(+) than CD4(+)KIR(-) T cells. In addition, engagement of an inhibitory KIR inhibited the CMV-specific proliferation of these CD4(+)KIR(+) memory T cells, but had no detectable effect on cytokine production. Our data reveal that, in marked contrast with CD8(+) T cells, the activity of a subset of CMV-specific CD4(+) T cells is modulated by HLA class I-specific KIR. Thus, the CMV-induced down-regulation of HLA class I may in fact enhance memory CMV-specific CD4(+) T cell responses restricted by HLA class II.