IL-7 is a growth and maintenance factor for mature and immature thymocyte subsets.

The specific signals inducing the growth and maturation of thymocytes remain undefined. We show here that recombinant IL-7 induces growth of fetal and adult mouse CD4-8- thymocytes. IL-7 also induces a lower but significant response in CD4+8- and CD4-8+ thymocytes. Day 14 fetal thymic lobes cultured in IL-7 for 6 days show a 2-fold increase in cell number when compared to control cultures. The thymocyte subsets that proliferate in response to IL-7 can be maintained in culture for extended periods of time. CD4-8- thymocytes maintained in IL-7 did not change their phenotype with respect to CD4 and CD8 expression. In addition, we show that the combination of IL-7 plus IL-6 provides a potent growth stimulus for CD4+8- and CD4-8+ thymocytes. A cloned thymic epithelial cell, that can be induced to express MHC class II molecules, transcribes both IL-7 and IL-6 mRNA. A cloned thymic macrophage cell line produces IL-6 but no detectable IL-7 mRNA. The pattern of biological activities present in the supernatants of these cell lines is also presented. These observations suggest that the thymic epithelial and macrophage cell types may be an in vivo source of signals which mediate thymocyte development.