Thymus Involution Induced by 5-Azacytidine in the Mouse is by Selected Depletion of Immature Thymocyte Subsets

Abstract

5-Azacytidine, a cytidine analogue well known for its inhibitory effect on DNA methylation of mammalian cells, has been used for chemotherapy of cancers, particularly childhood leukemias, but is also known for its ability to cause thymus involution and lymphoma induction in animal experiments. To understand the acute cytotoxic effects on the thymus, young laboratory mice treated with sublethal doses of 5-azacytidine were examined by immunofluorescent cytometric measurements of thymocyte populations and also by histological and DNA analyses of cell death characteristics. Young FVB/N strain mice, which were injected with 5-azacytidine every other day for three times and examined 48 hours later, showed marked reduction in thymus weight and cellularity. This was primarily due to virtual elimination of the CD4+CD8+ doubly positive cell populations from the thymus. Kinetic changes following a single injection of 5-azacytidine indicated that the thymocytes most susceptible to depletion by 5-azacytidine were those thymocyte subsets that carry low to undetectable levels of the CD3 antigen of the T cell receptor complex and hence represent the precursor and presumably the apoptosis-prone subsets of the CD4+CD8+ population. However, in contrast to the histological feature and nucleosomal DNA fragmentation characteristic of apoptosis observed in the dexamethasonetreated mice, early pyknotic changes of htymocytes in thymic cortex and diffuse chromatin DNA degradation were observed in 5-azacytidine-treated mice. These results suggest that acute cytotoxicity of 5-azacytidine involves necrotic changes of the immature CD4+CD8+ thymocyte subsets in the thymus.