Effect of cyclophosphamide on spleen cell suppressor activity and tumour growth in mice.

Following a single injection of cyclophosphamide (Cy) a biphasic spleen reaction occurred. During the first week the spleen size as well as the proliferative activity of the spleen cells were markedly reduced. However, during the next week a compensatory reaction developed with spleen enlargement and cellular proliferation far above normal. In the period of increasing spleen size, days 7-12 after Cy injection, the mitogenic response of the spleen cells was reduced and simultaneously they developed a suppressor activity which was able to inhibit the mitogen response of normal spleen cells. This suppressor activity could be reduced if the mice were reconstituted by an injection of normal spleen cells after Cy injection. Injection of Cy one day before tumour transplantation resulted in reduced tumour growth in syngeneic recipients compared with non-treated controls. However, this inhibition of tumour growth was abolished when normal spleen cells were injected after Cy treatment.