Thymic nurse cells as the site of thymocyte apoptosis and apoptotic cell clearance in the thymus of cyclophosphamide-treated mice.

Structural changes in the thymus of BALB/c mice after the intraperitoneal injection with a single high dose (200 mg/kg) of cyclophosphamide (CY) were examined. A large number of thymic nurse cell (TNC)-like structures that contained apoptotic thymocytes were observed in the thymus between 8 and 20 hours after CY treatment, just before a marked reduction in the number and proportion of PNAhi, CD3-, CD4+ CD8+ thymocytes. The majority of apoptotic thymocyte-containing TNC-like structures had an epithelial nature and were mainly located in the subcapsular and outer cortex, perivascular area of the cortex, corticomedullary junction and, in part, in the medulla. Most thymocytes seemed to undergo apoptosis synchronously in every TNC-like structure, but one or two fully intact mature thymocytes were found in every one. By 24 hours after CY treatment, many vacuoles that contained apoptotic cells in various stages of degeneration were observed within the epithelium, and most of the apoptotic cells disappeared at 24 hours. We herein demonstrated that the epithelium of TNC-like structures had a phagocytic activity in the digestion of apoptotic thymocytes by the presence of phagolysosomes with acid phosphatase activity. The epithelium left very small vacuoles with a small amount of nondigestible dense granules. The minority of apoptotic thymocyte-containing TNC-like structures scattered in the entire cortex between 8 and 20 hours after CY treatment was macrophage (M phi)-derived. Many apoptotic cell-containing M phi were observed as early as 4 hours after the CY injection. Apoptotic thymocytes of CY-treated mice showed the internucleosomal DNA fragmentation. In conclusion, CY accelerates physiologic thymocyte apoptosis, and epithelial TNC are one of the principal sites of thymocyte apoptosis as well as an environment for the nursing of immature thymocytes. Both the epithelium of TNC and M phi participate in digestion of apoptotic cells and have the efficient capacity of clearance of apoptotic thymocytes in the normal thymus at a presumed high rate.