Characterization of the expanded T cell population in infectious mononucleosis: apoptosis, expression of apoptosis-related genes, and Epstein-Barr virus (EBV) status

Infectious mononucleosis (IM), a manifestation of primary infection with EB V, is characterized by a massive expansion of the T cell population. In thi s study we examined this expanded T cell population regarding its EBV statu s, its proliferative and apoptotic activity, and its expression of apoptosi s-related genes. Whereas previous studies were performed on ex vivo culture s or on peripheral blood, our investigations included in vivo analysis of I M tonsillectomy specimens (14 cases) by in situ hybridization for viral RNA (EBERs) combined with immunohistochemistry (IHC; CD3, CD45RO, CD20, CD79a, Ki-67, Bcl-2, Bax, Fas, FasL) and the TUNEL method. Of the EBER+ cells 50- 70% showed expression of the B cell markers CD20/CD79a. The remainder of th e EBER+ cells expressed neither B nor T cell antigens. No co-expression of EBERs and T cell antigens was detected in any of the specimens. In accordan ce with a high rate of apoptosis (up to 2.37%) within the expanded T cell p opulation, Bcl-2 expression was drastically reduced and FasL expression rem arkably increased. The levels of Bax and Fas expression showed no or modera te up-regulation. In conclusion, the massive expansion of IM T cells is not caused by EBV infection of these cells but merely represents an intense im mune reaction. Through altered expression of Bcl-2/Bax and Fas/FasL, the ac tivated T cells are subject to enhanced apoptosis while residing within the lymphoid tissue, which eventually allows the efficient silencing of this p otentially damaging T cell response.