What is the function of peripheral lymphocytes migrating to the thymus and of B lymphocytes proliferating in the thymus?

Abstract

In studies on the role of other lymphoid organs in lymphocyte production and lymphocyte migration in young pigs and lambs, lymphocytes from these tissues were always found in the thymus. There were no major differences in the entry of labelled cells when the spleen, lymph nodes, bone marrow or Peyer's patches were selectively labelled. The immigrants were both mature small lymphocytes and lymphocytes just produced in the peripheral lymphoid organs. They enter via specialized venules at the outer part of the thymic medulla and do not migrate into the cortex. The lymphocyte homing is markedly reduced by prior incubation with trypsin. The relative numbers of immigrants within the thymus were small but, due to the huge cellular content of the thymus, the absolute number entering per day totalled several million from each peripheral organ. Another unexpected finding was the high mitotic rate in the medulla of the thymus. Moreover, in the adult rat thymus although there were only 0.14% B cells, these proliferated at a higher rate than in peripheral organs. The functional meaning of these data is obscure so far and the studies should stimulate further work on these topics of lymphocyte entry and B lymphocyte production in the thymus.