Cell-mediated immunity to Epstein-Barr-virus-transformed lymphoblastoid cells in acute infectious mononucleosis.

Mononuclear peripheral blood leukocytes from 21 patients with infectious mononucleosis and 16 healthy controls were tested in a 51Cr-release assay for cytotoxicity against two human lymphoblastoid cell lines derived from the same donor. One line contained the Epstein-Barr virus (EBV); the other did not. Acute-phase leukocytes were significantly more cytotoxic against the EBV-infected cell line than were control leukocytes. Mean (+/- S.E.) lysis at a leukocyte-target-cell ratio of 100:1 was 10.6 +/- 1.6 per cent for patients and 3.4 +/- 0.6 per cent for controls (P less than 0.0005). Cytotoxicity correlated with the percentage of atypical lymphocytes. Cells of three patients with acute mononucleosis-like illnesses failed to show killing activity above those of normal controls. Cytotoxicity against the EBV-negative line was not significantly different for each group. The finding of cytotoxic cells in infectious-mononucleosis patients with atypical lymphocytes suggests that these cells operate in vivo to limit the proliferation of altered EBV-transformed B lymphoblasts.