Characterization of lymphocyte subsets in spontaneous mouse mammary tumors and host lymphoid organs.

The subsets of small lymphocytes (surface Igm-positive or B, Thy-I-positive or T and "double-negative" or null) appearing within spontaneous mammary tumors of C3H retired breeder female mice and in various lymphoid organs of the host at different stages of tumor development were characterized directly using a radioautographic method. Tumor-bearing mice showed early transient splenomegaly and progressive atrophy of the thymus. The proportion of T small lymphocytes increased with tumor age within the tumor and in the spleen, blood and thymus; then, except in the spleen, these proportions declined again by 8 weeks of tumor growth. The incidence of B small lymphocytes showed no change in the tumor or thymus; there was an increase in the blood after 3 weeks, and a small decrease in the spleen after 7 weeks. At all the sites examined, the proportion of null small lymphocytes declined from an initial high level observed in the elderly tumor-free control mice, and then recovered to control levels after 7 weeks. The absolute numbers of T and null cells in the spleen changed in parallel to their proportions, while splenic B-cell numbers increased at 1-3 weeks. In the thymus the absolute numbers of all subsets decreased, with a late recovery of null cell numbers. Age-matched control mice showed higher proportions of null cells in the spleen, blood and thymus, and lower proportions of T cells in the blood and spleen, than did young normal mice. This spontaneous tumor appears to be characterized by an increase in T cells, rather than in null cells, as observed for rapidly growing transplanted tumors. The null cell rise in the present case takes place before the clinical appearance of tumors. In both cases, however, tumor-infiltrating B cells exhibited low levels of surface Ig, possibly related to a low level of maturation. The functional significance of these findings remains to be determined.