Murine CD4-CD8- thymocytes are stimulated by interleukin-2 to proliferate in vitro in chemically defined medium

The ability of fetal and young adult CD4-CD8- thymocytes to proliferate in chemically defined (serum-free) medium in the presence and absence of IL-2 was examined. Dissociated thymocytes from day 15 and older fetal mice proliferated in vitro in the presence but not the absence of IL-2. The degree of proliferation was increased by including IL-1 with the IL-2. Inclusion of IL-1 in cultures of fetal thymocytes was associated with an increase in the number of IL-2 receptor positive cells, relative to cultures containing IL-2 alone. Although unfractionated thymocytes failed to proliferate in chemically defined medium, CD4-CD8- cells purified from thymic cell suspensions from young adult mice from several inbred strains proliferated to a limited extent in the absence of added cytokines. Proliferation was augmented 40-100 fold by inclusion of IL-2 in cultures. IL-1 stimulated some proliferation by young adult CD4-CD8- cells, but, unlike the effect of IL-1 and IL-2 on fetal thymocytes, combination of IL-1 with IL-2 did not have a notable additive effect on IL-2 induced proliferation. Proliferation stimulated by both IL-1 and IL-2 was completely abrogated by inclusion of anti-IL-2 receptor antibody in the cultures. Thymocytes from F1 progeny of inbred strains of mice proliferated to a greater extent in the absence of IL-2 than did thymocytes from either parent strain, although the response to IL-2 was not significantly different. The data demonstrate that both fetal and adult CD4-CD8- thymocytes area capable of proliferating in response to IL-2 in vitro, suggesting that, as is the case during antigen specific responses by mature T cells, IL-1 and IL-2 cooperate to stimulate T cell proliferation during development in vivo.