Immunomodulatory effects of transforming growth factor-β on T lymphocytes: Induction of CD8 expression in the CTLL-2 cell line and in normal thymocytes

We investigated the role of transforming growth factor-β₁ (TGF-β) in regulation of T cell growth and differentiation. Treatment of CTLL-2 cells with TGF-β inhibited IL-2-dependent proliferation and caused morphologic changes as well as increased adherence. A major change of phenotype in TGF- β-treated cells was the de novo expression of CD8α chain in 35% of cells, which required the continuous presence of TGF-β. Of the CD8α⁺ cells, 20 to 30% coexpressed CD8β chain. Increased CD8 expression occurred even in the total absence of cell growth, was not a consequence of growth inhibition, and was not a result of selective growth or survival of CD8⁺ cells. New RNA synthesis was required for TGFβ-induced CD8α surface expression, inasmuch as this was prevented by treatment with actinomycin D. Northern blot analysis demonstrated that cells treated with IL-2 + TGF-β rapidly accumulated mRNA encoding both chains of the CD8 dimer, to a level fourfold greater than control by 6 to 12 h. In contrast, the IL-2-dependent increases in IL-2Rα, IL-2Rβ, and Granzyme B mRNA levels in these cultures were profoundly inhibited by TGF-β. When unfractionated murine thymocytes were stimulated with phorbol dibutyrate plus ionomycin and cultured with IL-2 + TGF-β, an increase in CD8α mRNA was seen and greater numbers of CD8⁺ cells with higher levels of CD8α and CD8β surface expression resulted, as compared to controls treated with IL-2 alone. Furthermore, similar treatment of CD4^- CD8^- (double negative) thymocytes with TGF-β induced de novo CD8α expression by a substantial number of cells, and the majority of these CD8⁺ cells lacked TCR/CD3. These data suggest that TGF-β has both positive and negative regulatory effects on the expression of gene products important for T lymphocyte differentiation and function.