The human alloreactive CD4(+) T-cell repertoire is biased to a Th17 response and the frequency is inversely related to the number of HLA class II mismatches

Estimates of precursor frequency and assessment of functional characteristics of alloreactive CD4(+) T cells are all biased by the need for long-term culture. In this study, direct visualization of human alloreactive CD4(+) T cells on the single-cell level was achieved using cell surface expression of CD154 as a tool for identification. The average frequency of alloreactive CD154(+)CD4(+) T cells among peripheral blood CD4(+) T cells was 0.1%, with half of the cells displaying a naive phenotype. The proliferation capacity and expression of cytokines after allogeneic stimulation resided in these CD154(+)CD4(+) T cells. The repertoire of alloreactive CD4(+) T cells was biased to a Th17 response, and on average 24% of alloreactive CD154(+)CD4(+) memory T cells produced interleukin-17 (IL-17) after polyclonal stimulation. Unexpectedly, mixed cell cultures from human leukocyte antigen (HLA)-identical donors also generated alloreactive CD154(+)CD4(+) T cells and yielded the highest frequency compared with HLA-nonidentical combinations. Therefore, reactivity to minor histocompatibility antigens between HLA-identical subjects appears to be relatively common. Alloreactive HLA-identical T cells did not proliferate or express cytokines, but were driven to proliferation in the presence of exogenous IL-2. (Blood. 2009;114:3947-3955)