Neonatal thymectomy and tumor induction with methylcholanthrene in mice.

Methylcholanthrene was administered intradermally to adult normal and neonatally thymectomized mice. The incidence, latency period, growth rate, and histological type of the developing tumors were not significantly different for thymectomized mice as compared with those for controls. The antigenicity of the tumors varied in both groups, but the percentage of strongly antigenic tumors was unequivocally higher in thymectomized mice. Other investigators demonstrated a clearly increased susceptibility of thymectomized rodents to the carcinogenic effect of benzopyrene and to certain tumor viruses. They postulated that an enhanced incidence of the mostly antigenic tumors is related to the impairment of immune reactivity after early thymectomy. To test this hypothesis, we determined the immune reactivity of each individual thymectomized mouse by grafting homologous skin before administering the carcinogen. Thymectomized mice with normal homograft reactivity were thus distinguishable from those with depressed reactivity. Surprisingly, tumor incidence in the latter group was, if anything, lower than in the immunologically competent animals. It is plausible that immunological depression as a consequence of thymectomy promotes the establishment of antigenic tumors; on the other hand, the growth conditions for tumors, particularly for the less antigenic variety, may not be ideal in thymectomized mice.