Protective effect of pregnancy against transplantation of lymphoma in rats

Pregnancy was considered to aggravate concomitant neoplasia; however, recent observations in humans and experimental animals suggest that on the contrary tumor growth may be unfavorably affected by pregnancy. For this relationship to be investigated, a serially transplantable virus-induced lymphoma was inoculated into female rats of two different strains (WF and SD) in early or in late pregnancy. While all nonpregnant control rats died with lymphoma in 8-10 weeks, 36-75% of rats inoculated with lymphoma cells in the 1st week of pregnancy remained free of tumors, and 20-54% had tumors half the size of those in controls. When lymphoma cells were inoculated in the last week of pregnancy, the protective effect of pregnancy, although less clearly manifested, still resulted in 11% rats free of tumors and 78-91% rats with tumors markedly smaller than those of controls. Nonpregnant rats receiving pregnant rat serum in twice weekly injections grew tumors only 25-60% the size of those in rats receiving sera of nonpregnant rats. In vitro, pregnant rat sera added to the tissue culture media of rat lymphoma (AS line) or human leukemia (MOLT-4) cells resulted in reductions of viable target cells of 67-71%, respectively. The present experiments show that transplanted lymphoma cells that are 100% lethal in nonpregnant rats were totally or partially inhibited in their growth when the recipients were pregnant. Similar inhibiting effects were observed when pregnant rat serum was administered to lymphoma cells in vivo and in vitro. The protective effect against tumor growth was greater during early pregnancy, less accentuated during late pregnancy, and ceased entirely post partum and particularly during lactation, when tumor growth resumed on an accelerated course.