Life-threatening infectious mononucleosis: is it correlated with virus-induced T cell proliferation?

Infectious mononucleosis is a well-established clinical entity characterized by the proliferation of B lymphocytes that are infected with Epstein-Barr virus (EBV). These lymphocytes give rise to an increase in specifically reacting cytotoxic T cells, which leads to self-limitation of the lympho-proliferative process. We describe the case of a 1-year-old boy who developed life-threatening EBV infection in association with liver failure, depletion of bone marrow, and severe encephalitis. The fact that clinical cure was achieved when acyclovir (50 mg/[kg.d]) and prednisolone (1 mg/[kg.d]) were administered indicates a correlation between antiviral therapy and clinical improvement. Hypogammaglobulinemia--which had not been present at the onset of disease--persisted after clinical recovery. During the acute phase of the illness, the patient's blood lymphocytes were predominantly T cells, most of which contained the EBV genome, as shown by in situ hybridization; some of these cells stained positive for EBV-specific latent membrane protein. Examination of peripheral blood mononuclear cells in vitro revealed an exceedingly high histocompatibility antigen-unrestricted cytotoxicity against the K562 cell line, which is normally not an immunogenic target of cytotoxic cells in patients infected with EBV. This anomalous natural killer cell-like T cell function suggests that EBV infection of T cells might cause auto-aggressive activity, which was probably responsible for the severity of the infection in our patient.