CLINICAL IMMUNOLOGY AND IMMUNOPATHOLOGY 2,114-120 (1973) T-Lymphocyte Proliferation in Mononucleosis’ MARTTI VIROLAINEN, LEIF C. ANDERSON, MARTTI LALLA, ANDROBERT VON ESSEN Third Department of Pathology, University of Helsinki, and Aurora Hospital, Helsinki, Finland Received March 30,1973 Lymphocytes from patients with mononucleosis were characterized by their receptors for complement (B-cells) and for sheep red cells (T-cells). As judged by these tests, patients with mononucleosis had higher numbers ofT-cells than did con- trol subjects. Lymphocytes were fractionated according to size by lg velocity sedimentation, and subsequently tested for rosette formation. In other tests, cells forming rosettes with sheep red cells were isolated by the same technique. In both types of experiments the circulating lymphoblasts, as quantitated by labeling of cells in DNA synthesis, were found to be predominantly T-cells. Recent reports suggest that human T-lymphocytes but not B-lymphocytes bind various heterologous erythrocytes (l-6). On the other hand, human B- lymphocytes have been shown to carry receptors for complement (C’3) not present on T-lymphocytes (5,7-g). We have used rosette techniques based on these different properties of T- and B-lymphocytes to study whether the circulating lymphoblasts or “atypical lymphocytes” in mononucleosis are T- or B-cells. MATERIALS AND METHODS Patients Nine patients with clinical signs and symptoms of mononucleosis were studied. Six of the patients were positive for heterophile antibodies as measured by sheep and/or horse red cell agglutination after absorption with guinea pig kidney (Table 1). One of these patients (S. L.) had a complicating hepatitis. In three of the patients heterophile antibodies could not be demon- strated (titer d 64). In one of these patients (H. J.) the disease was obviously caused by cytomegalovirus (CMV) infection (10); a rising antibody titer was observed, and CMV was repeatedly isolated from her urine. Controls were healthy laboratory personnel. ’ This work was supported by the National Research Council for Medical Sciences, and the Paasikivi Foundation, Helsinki, Finland Copyright @ 1973 by Academic Press, Inc. 114 All rights of reproduction in any form reserved.