Earlier studies have demonstrated that nitrosoureas such as 1,3-bis (2-chloroethyl)-1-nitrosourea (BCNU) and chlorozotocin (CLZ) can cure almost 100% of C57BL/6 mice bearing syngeneic LSA tumor. In contrast, similiar or higher doses of streptozotocin (STZ) completely failed to cure LSA-bearing mice. Further studies revealed that the efficacy of nitrosoureas may depend on their immunomodulating properties. In the current study, therefore, attempts were made to investigate the effects of these nitrosoureas on the immune system of normal and LSA tumor-bearing mice. Treatment of normal C57BL/6 mice with 5 intraperitoneal injections of 20 mg/kg body weight of BCNU or CLZ caused an increase in the percentage of CD4⁻ CD8⁻ T cells and a decrease in the percentage of CD4⁺CD8⁺ T cells in the thymus. In addition, such treatment also caused an increase in the percentage of CD4⁺ T cells without significantly affecting the CD8⁺ T cells in the thymus. However, when total cellularity of the thymus was studied, BCNU and CLZ were found to decrease the total number of CD4⁺CD8⁺ T cells without significantly affecting the other subsets. In contrast, similiar or higher (100mg/kg body weight) doses of STZ had no significant effect on the total number and percentages of T ceil subsets in the thymus. Also, BCNU and CLZ but not STZ-treatment caused a 50% decrease in the total number of CD4⁺ and CD8⁺ T cells in the spleen. Interestingly in tumor-bearing mice, BCNU treatment was followed by a ten-fold increase in the percentage of CD4⁺ T cells found in the peritoneal cavity. The percentages of CD8⁺ cells increased also, but to a lesser degree. These changes were limited to the peritoneal cavity which is the site of tumor growth. When T cells in the spleens of nitrosourea-treated normal mice were functionally analyzed, it was observed that BCNU and CLZ caused a dramatic decrease in the T cell responsiveness to Con A, anti-CD3, and PMA + calcium ionophore stimulation. In contrast, STZ treatment failed to significantly inhibit the T cell responsiveness to these activation signals. Using the accessory cell-dependent and independent assays, BCNU and CLZ were found to suppress the functions of both T cells and macrophages in normal mice. BCNU and CLZ also suppressed the B cell responsiveness to lipopolysaccharide (LPS). Also, addition of growth factors such as IL-1, IL-2, IL-4 and IL-6 failed to reconstitute the defective responsiveness of BCNU and CLZ-treated T cells and macrophages. Together these data suggest that nitrosoureas have varying immunomodulating properties and this may in turn determine their efficacy in the treatment of cancer. / Master of Science