CELLULAR IMMUNOLOGY 44, 433-441 (1979) Influence of Human Thymocytes on B-Lymphocyte Differentiation in Man1 SAVITA PAHWA, LIEN SHOU, RAJENDRA PAHWA, ELIZABETH SMITHWICK, ANDROBERT A. GOOD Memorial Sloan-Kettering Cancer Center. New York, New York 10021 Received September 5, 1978 Human thymocytes from children less than 6 years of age were tested for their influence on differentiation of normal B cells. The addition of either thymocytes or a culture super- natant from thymocytes to normal peripheral blood lymphocytes (PBL) enhanced pokeweed mitogen-induced B-cell differentiation as tested in a plaque-forming assay for antibody to sheep red blood cells. The thymocytes, however, could not substitute for T lymphocytes in cultures of PBL which had been previously depleted of T lymphocytes. Further, prior treatment of thymocytes with concanavalin A did not result in generation of suppressor cells for either B-cell differentiation or for the responses of PBL to mitogens. Thus, al- though thymocytes were functionally immature by these assays as compared to mature T lymphocytes they exerted an influence on B-cell differentiation in cultures of normal periph- eral blood lymphocytes. INTRODUCTION The thymus-dependent lymphocytes (T cells) play a broad role in the develop- ment of humoral immunity and exert important regulatory influences on B-cell differentiation which may be helper or suppressor in nature (1-4). Little is known of the function of thymocytes themselves on B-cell maturation. Recently, Sherr et al. (5) have shown by cell transfer experiments in LAF, mice that adult thymus cells, but not fetal or neonatal thymus cells, facilitate functional maturation of of neonatal and fetal B cells. These experiments suggest that the thymus cells may play an important role in ontogeny of B lymphocytes. The role of the human thymus cells on B-cell differentiation has not been studied in depth. It has been clearly shown by several investigators (6-9) that the in vitro differentiation of B cells by the plant lectin pokeweed mitogen (PWM) is dependent on the presence of T cells. In this communication the influence of human thymocytes on functional differentiation of B cells is examined and an as- sessment is made of the role of maturation of cells of the T line on their capacity to facilitate B-cell development. ’ Aided by Grants CA-08748, CA-17404, CA 19267-OlAl, AI 11843 from the National Institutes of Health, and grants from the Zelda R. Weintraub Cancer Fund and the Judith Harris Selig Memorial Fund. 433 0008-8749/79/060433-09$02.00/O Copyright 0 1979 by Academic Press, Inc. AU rights of reproduction in any form reserved.