Atypical clonal T-cell proliferation in infectious mononucleosis.

An atypical case of infectious mononucleosis characterized by fever, acute tonsillitis, and bilateral cervical adenopathy is reported in a previously healthy young man. Although serology was positive for the Epstein-Barr virus, the patient did not display peripheral blood lymphocytosis or atypical, reactive lymphocytes. The patient's tonsilar tissue revealed an expanded T-zone of diffuse, monomorphous lymphocytes suggestive of lymphoma. Immunophenotypic analysis of the tonsilar tissue demonstrated more than 90% expression of pan-T markers, while pan-B markers were positive in 5-10% of the interfollicular T-zone cells and in 90% of germinal centre cells. In situ hybridization with a probe specific for EBER1 demonstrated positive staining in approximately 1% of the interfollicular tonsilar lymphocytes. Finally, Southern blot analysis of tonsilar tissue demonstrated a clonal rearrangement of the T-cell receptor gene. The patient recovered from his infection and remains in good health years after presenting with his illness. This case illustrates that T-cell clonality must be evaluated with caution in the setting of a viral infection and can occur in association with benign, self-limited infectious mononucleosis.