In-vitro synthesis and release of immunoglobulins by thymocytes of inbred rats

Thymic and splenic lymphocytes were isolated from inbred female rats, 9-16 weeks of age, and cultured in Basal Medium Eagle's supplemented with 0.4% bovine serum albumin. Cells were cultured for 20 hr at a density of 5 × 10⁷ per ml of medium containing ¹⁴C-labeled amino acids. The culture supernatants were analyzed for labeled immunoglobulins released into the medium by double antibody precipitation. Thymocytes which contained only 0.2% immunoglobulin-bearing lymphocytes (B-cells). released an IgG-like immunoglobulin which constituted 3.7-7.3% of the total labeled protein. Splenic lymphocytes, which contained about 42% B-cells, secreted IgG which amounted to about 20-25% of the total secreted protein. IgM synthesis and secretion were not detected in thymic cultures, but accounted for 4.5-6.2% of the total labeled protein in splenic cultures. The antigenic reactivity and mol. wt of the thymocyte immunoglobulin and of serum IgG chains were similar though the behavior of thymocyte Ig in sodium dodecyl sulfate-polyacrylamide gels indicated that the heavy and light chains may be non-covalently linked. Primary immunization in vivo with low doses of keyhole limpet hemocyanin resulted in increased immunoglobulin and total protein synthesis by thymocytes in vitro. The synthesis and release of this IgG-like immunoglobulin by normal thymic lymphocytes correlates with our earlier finding of a high content of this immunoglobulin in isolated thymocyte plasma membranes and indicates that it is synthesized by thymocytes.