The lymphocytosis manifested in infectious mononucleosis (IM) during acute phase is ascribed to a reactive expansion of CD8+ T lymphocytes caused by Epstein-Barr virus (EBV)-infected B lymphocytes. Expression of HLA-DR antigen on IM lymphocytes suggests that these T lymphocytes are somehow activated in vivo. In the present study, we analyzed the interleukin-2 (IL-2) receptor expression on lymphocytes from six patients with acute IM. Radiolabeled IL-2 binding assay revealed that IM lymphocytes from all patients examined had a considerable number of IL-2 binding sites with an intermediate affinity, although they did not express the IL-2 receptor recognized by anti-Tac antibody (p55). The number of binding sites (1,070 to 4,600 sites per cell) was larger than that of a normal, resting T lymphocyte-enriched population (650 sites per cell). Furthermore, IM lymphocytes showed marked proliferative responses to higher concentrations of IL-2, which were almost completely blocked by an anti-p70 IL-2 receptor antibody, indicating that their IL-2 receptor is a functional receptor. The results of an affinity cross-linking study seem to indicate that the IL-2 receptor expressed on IM lymphocytes is p70, the second chain of the IL-2 receptor distinct from p55. Flow cytometric analysis following immunofluorescent staining with anti-p70 IL-2 receptor antibody confirmed p70 expression on CD8+ HLA-DR+ lymphocytes. These data suggest that p70 IL-2 receptor expression is involved in the immune response triggered by EBV infection.

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