Neonatal tolerance induction in the thymus to MHC-class-II-associated antigens. V. Thymus medulla and the site for deletional signaling achievement in Mls tolerance.

Intravenous (i.v.) injection of Mls-1a peritoneal cavity (PerC) cells from (BALB/c x AKR)F1 (Mls-1b/a, H-2d/k) mice into newborn BALB/c (Mls-1b, H-2d) mice induced thymus cell tolerance by one week of age, accompanied by V beta 6+ cell elimination. The tolerant state is associated with intrathymic chimerism with MHC-class II(IA+) cells, confluent in the medulla and scattered in the cortex. To clarify the anatomical site for the deletional signaling, we injected Mls-1a PerC cells directly into the thymus lobe of BALB/c mice on the day of birth. Thus induced tolerant state was limited to the injected lobe and there was no penetration to the contralateral lobe. The tolerant state lasted less than 2 weeks, by which time donor-derived Ia+ cell had disappeared from the thymus. Thus, PerC cells seem to have little self-renewing ability. One week after the intrathymic injection of a small amount (0.3 microliter) of PerC cell suspension in several different sites, the thymus lobes were removed without killing the mice and serial cryostat sections were cut and stained immuno-histochemically for analysis of the donor cell distribution. Four weeks later, the functional activities of peripheral T cells in the spleens of the treated mice were tested. These experiments revealed that inoculated cells lodging in the medulla, but not in the cortex, induced tolerance to the Mls determinants. Target thymocytes for negative signaling are probably located in the medulla/juxta-medullary area in the thymus. Data are discussed in relation to Mls-bearing stroma cells in the thymus.