Traffic of peripheral B and T lymphocytes to hyperplastic, preneoplastic thymuses of AKR mice. AKR mice develop hyperplasia of the thymus before the development of retrovirus-associated lymphoma at that site. This hyperplasia, first detectable in AKR/J mice by 4 weeks of age and in AKR/C mice by 4 to 5 months of age, is characterized by an enlarged thymic medulla that contains T and B lymphocytes. In contrast to the general population of thymocytes, most of these T and B lymphocytes have a mature immunophenotype that includes expression of high levels of the MEL-14-defined (gp90) 'homing receptor' for peripheral lymph node high endothelial venules. In vivo homing studies reveal a marked increase in traffic of peripheral lymphocytes (T more than B) to the hyperplastic thymuses of old AKR mice as compared to histologically normal thymuses of age-matched BALB/c and C57BL/Ka mice or young AKR mice. These changes correlate chronologically with changes in retrovirus antigen expression in AKR thymuses and suggest a role for the traffic of lymphocytes from the periphery to the thymus in response to local antigenic stimulation in the pathogenesis of thymic hyperplasia in AKR mice. 