Downregulation of CD 5 and dysregulated CD 8 + T‐cell activation

CD 5 is a cell surface molecule that is expressed on most circulating T cells and a small population of B cells, and is involved in modulation of antigen‐specific receptor‐mediated activation. Downregulation of CD 5 on CD 8 + T cells is a poorly understood but increasingly recognized phenomenon that may be associated with dysregulated T‐cell activation. An increased subpopulation of activated CD 8 + T cells with downregulation of CD 5 has recently been described in patients with Epstein–Barr virus‐associated hemophagocytic lymphohistiocytosis ( HLH ) and familial HLH caused by perforin deficiency and Munc 13‐4 deficiency. These cells were detectable only in the acute phase of HLH , in which patients exhibited hypercytokinemia, and declined progressively after successful treatment in parallel with improvement of systemic inflammation. It is unknown whether CD 8 + T cells from HLH with other causes have similar profiles. Assessment of CD 5 expression on T cells has the potential to assist in the understanding of the diagnosis and pathogenesis of human inflammatory diseases such as HLH .