Antigen-dependent and antigen-independent thymocyte migrations are described. Rats injected i.v. with a large dose of ovalbumin (100 mg) showed a 3-fold increase in the numbers of large, low density cells in thymus 2 days later. When subpopulations of different density were labeled with 3H-adenosine and transferred to normal recipients, they migrated preferentially to spleen, unlike cells from saline-injected donors, which equilibrated among all lymphoid organs. Data from autoradiographs and dissection of spleens showed that the increased radioactivity in spleen resulted from a 3-fold increase in the number of large cells, predominantly in the marginal zone and, secondarily, in the red pulp. This increased migration to marginal zone with antigen was superimposed upon a normal pathway taken by large, low density thymocytes to this site. When adrenalectomized rats, similarly treated, received intrathymic 125IUdR, radioactivity in spleens of the ovalbumin group after 24 hr was 2- to 3-fold greater than controls and, again, associated only with marginal zone-red pulp. When only cortical thymocytes were labeled by topical application of 125IUdR to the thymus capsule, the same enhanced localization in spleen was seen at 24 hr. Migration of small, dense, medullary thymocytes to the periarteriolar lymphoid sheath (classical thymus-dependent area) was unaffected by antigen.

Only large, low density thymocytes migrated to Peyer's patches. This migration was unaffected by antigen. Only dense thymocytes localized in the thymus. However, if thymocytes were transferred from antigentreated donors, no subpopulation homed to thymus.

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