MHC Class II Antigen Expression was Different from MHC Class I Antigen Expression in Irradiated and Recovering Rat Thymus

Abstract

We exposed rats to a heavy, but sublethal, dose of X-radiation that was calculated as the maximum that left them alive. Immediately after irradiation the rats lost body and thymus weight. The number of thymocytes decreased and the structure of the thymus was destroyed. The thymus weights then increased rapidly for a few days from day 7 followed by a slower increase. Immunohistochemically, the recovery of major histocompatibility complex (MHC) class II-positive cells in the thymus was slower than that of MHC class I-positive cells. Recovery of MHC Class II-positive and ED1-positive cells in the thymus was slow, especially in the medulla. Thymus structure appeared similar to normal control animals on day 14, although the number of MHC class II- and ED1-positive cells in the medulla was lower than that of controls even 21 days after radiation exposure. The delay in recovery of MHC class II-positive stromal cells might affect the regeneration of thymocyte subpopulations after irradiation.