T-cell recruitment from the thymus to the spleen in tumor bearing mice: phenotypical alteration and recruitment of thymocytes raised in a tumor bearing state.

Intrathymic events in mice bearing methylcholanthrene induced fibrosarcoma (MBS-1) were examined using mainly flow cytometric analysis. Ten days after tumor inoculation, the number of whole thymocytes was remarkably decreased. Surface phenotypical analysis by flow cytometry showed that the proportion of thymocytes with a low density of peanut agglutinin (PNAlow thymocytes), which is about 30% in normal mice, was increased to 70%. Histologically, the greater part of the thymus was occupied by the cortex. Moreover, the ratio of proliferating cells was increased in the PNAhigh cells. These findings in in vivo experiments suggested that a tumor bearing state would alter phenotypical characteristics of cortical thymocytes (PNAhigh, Thy 1high, H-2low) to medullary type (PNAlow, Thy 1low, H-2high). To support this hypothesis, in vitro experiments were performed using cultured thymic lymphoma EL4 cells, which possessed an immature thymus cell phenotype. Addition of serum from MBS-1 bearers to the culture medium of EL4 cells differentiated their phenotypical characteristics to the medullary type. Thus, it is assumed that factors in tumor bearers induce a massive migration of thymocytes by altering of phenotypes.