The in vitro transformation of thymocytes and lymphocytes from humans, rabbits and guinea-pigs and from thymomas.

The response of human, rabbit and guinea-pig thymocytes in culture to blastogenic stimulants such as phytohaemagglutinin (PHA) and staphylococcal filtrate (SF) was usually considerably less than that seen with equivalent numbers of peripheral blood and lymph node lymphocytes. The growth of thymocytes was improved by growing them at higher cell concentrations but they still responded less well than other lymphocytes. A very high degree of DNA synthesis was usually observed over the first 24-hr period in cultures of unstimulated thymocytes and this was often depressed by the addition of stimulants. Thymocytes did not survive as well as peripheral blood, lymph node or splenic lymphocytes in culture. The source of the serum in the cultures at times improved the response to stimulants; normal thymocytes showing no response in the presence of one serum transformed and synthesized DNA when serum from another source (autologous, homologous or heterologous) was used. In contrast the `thymocytes' from two patients with lymphoepithelial types of thymoma and one with benign hyperplasia of the thymus (thymitis) responded much better to stimulants than the normal thymocytes and almost as well as peripheral lymphocytes.