Human thymus cells synthesize immunoglobulin in short-term culture, the nascent immunoglobulin appearing in both cytoplasmic and membrane fractions. Surface immunoglobulin was demonstrated by lactoperoxidase radioiodination of the cells. The demonstration of intracellular and surface immunoglobulin required procedures that minimize proteolytic degradation. Noncovalently linked, monomeric , chains and light chains were found in the cytoplasm and on the surface of the cells by means of acrylamide gel electrophoresis and by specific immunoprecipitation of the isolated chains.Mammalian bone marrow-derived lymphocytes (B cells) have membrane immunoglobulins (Ig) which are considered to be receptors for antigens and are indicative of the capacity of these cells to synthesize Igs (1-4). Thymus-derived lymphocytes (T cells) facilitate antibody production by B cells and are involved in cell-mediated immune reactions by means of specific antigen-binding receptors (5). Efforts to demonstrate membrane Ig on T cells have led to conflicting conclusions.In the present communication we report in evidenc shortterm tissue culture for synthesis by human thymus cells of IgM in the surface of these cells, and describe the immunochemical nature of the nascent cytoplasmic and surface immunoglobulin. Our findings emphasize the necessity for minimizing proteolytic degradation during the preparation of cellular and membrane fractions and suggest that variable proteolysis during solubilization of membrane components may account for conflicting reports on the presence or absence of surface Igs in T-cell membranes (6-11). MATERIALS AND METHODSIn Vitro Biosynthesis. Thymuses were obtained from 5-to 9-year-old children undergoing cardiac surgery. Cells were teased from the thymus, filtered through a stainless steel screen, and washed three times and resuspended in Eagle's medium containing 1:100 the standard amount of amino acids. Cell suspensions (5 X 107 cells per ml) were prein- Fig. 3, below), 50,000 K.I.U. of Trasylol (Bayer, 5 ml, 250,000 K.I.U., Kalikrein inactivation units) per liter and 0.2 M iodoacetamide were added to the STKM buffer. The cells were disrupted by homogenization at 40 and fractionated into "cytoplasmic fraction" and "membrane fraction" (13). (2)
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