Comparative responses to phytohaemagglutinin of appendiceal, thymic and splenic lymphocytes of rabbits.

Lymphocytes from the appendix, thymus and spleen of young rabbits were cultured in the presence of varying concentrations of phytohaemagglutinin and their proliferative response during a 72-hour culture period measured with [H]thymidine uptake. At the time of culture initiation, appendiceal lymphocyte populations showed the highest spontaneous rate of DNA synthesis followed by thymic and splenic lymphocytes in that order. In the rabbit appendix the presence of a small nondividing population of phytohaemagglutinin-responsive small lymphocytes was demonstrated. Additional evidence suggests that low concentrations of PHA accelerated or initiated DNA synthesis during the first 24-hour culture period in a proportion of appendiceal lymphocytes which were in the mitotic cycle at the time of culture initiation. In the thymus, as in the appendix, only a small fraction of the lymphocyte population responded to phytohaemagglutinin during the 72-hour culture period. Splenic lymphocytes showed the greatest response to PHA with a peak of DNA synthesis between 48 and 60 hours. The results indicate that varying concentrations of PHA should be employed to detect optimum responses by lymphocyte populations from various lymphoid organs and that different lymphoid organs have different proportions of and PHA-responsive non-PHA-responsive cells.