SELECTIVE MITOMYCIN-C AND CYCLOPHOSPHAMIDE INDUCTION OF APOPTOSIS IN DIFFERENTIATING B-LYMPHOCYTES COMPARED TO T-LYMPHOCYTES IN-VIVO

Differentiating B and T lymphocytes differ in sensitivity to a number of environmental toxins and anticancer agents. B lymphocytes are susce ptible and T lymphocytes resistant to killing by cyclophosphamide (Cy) metabolites capable of forming DNA interstrand cross-links. However, the mechanisms responsible for the rapid killing and loss of bursal-re sident B lymphocytes are unknown. Therefore, we investigated the cellu lar mechanisms of selective toxicity of two cross-linking drugs, mitom ycin C (MMC) and Cy, towards differentiating B and T lymphocyte popula tions using the chicken embryo model system. Viability of bursal-resid ent B lymphocytes (bursacytes) decreased starting at 5 h post exposure (PE) to MMC, and was maximally reduced by 71.6% by 10 h PE at the hig hest dose examined (9.0 mu g MMC/g). Dose-dependent increases in the p ercentage of apoptotic bursacytes were observed as early as 5 h PE, an d increased to 72% by 10 h PE. This was accompanied by reductions in b ursacyte numbers. Cy also induced apoptosis in bursacytes. In contrast , thymus-resident lymphocytes (thymocytes) were much more resistant to the toxic effects of MMC and Cy. Viability of thymocytes was reduced by only 10% in the 9.0 mu g/g MMC treatment group. In addition, the pe rcentage of thymocytes engaged in apoptosis was much lower than that f or bursacytes. MMC induced modest cell cycle inhibition in bursacytes and thymocytes. These data strongly suggest that MMC- and Cy-induced d ifferential toxicity involves primarily early and extensive triggering of apoptosis in differentiating B lymphocytes, leading to rapid reduc tion of lymphocyte numbers in the embryonic bursa.