The effect of azathioprine on host cell infiltration and growth of a murine fibrosarcoma.

Azathioprine (AZ) was injected into C57 Black mice before or after IM implantation of the syngeneic fibrosarcoma FS6, and observations were made on tumour growth and on the cellular composition of the fibrosarcomas. When multiple injections were given to the tumour-bearing host the tumours regressed. The percentage of tumour-associated macrophages remained essentially stable as regression occurred whereas the percentage of theta-antigen-positive cells increased. In the case of single IP injections of AZ, changes in the cellular composition of tumours were seen only when AZ was given 1-72 h before implantation but not when it was given after implantation. Compared with controls there was a 2- to 3-week delay in the appearance of large numbers of tumour-associated macrophages. The percentage of theta-antigen-positive cells within the tumours mass was elevated 7 days post AZ treatment but the level declined to control values between days 10 and 13. The significance of the results is discussed in relation to the dependency of the macrophage content of this tumour upon the development of an immune response towards the tumour.