ABSTRACT
CTLA-4 is a CD28 homologue believed to be a negative regulator of T cell function. However, the mechanism of this downregulatory activity is not well understood. The present study was designed to examine the effect of CTLA-4 ligation on cytokine production, cell survival, and cell cycle progression. The results demonstrate that the primary effect of CTLA-4 ligation is not the induction of apoptosis. Instead, CTLA-4 signaling blocks IL-2 production, IL-2 receptor expression, and cell cycle progression of activated T cells. Moreover, the effect of CTLA-4 signaling was manifested after initial T cell activation. Inhibition of IL-2 receptor expression and cell cycle progression was more pronounced at late (72 h) time points after initial activation. The effects of anti-CTLA-4 mAbs were most apparent in the presence of optimal CD28-mediated costimulation consistent with the finding that CTLA-4 upregulation was CD28-dependent. Finally, the addition of exogenous IL-2 to the cultures restored IL-2 receptor expression and T cell proliferation. These results suggest that CTLA-4 signaling does not regulate cell survival or responsiveness to IL-2, but does inhibit CD28-dependent IL-2 production.
Subject(s)
Antibodies, Monoclonal/pharmacology , Antigens, Differentiation/physiology , CD28 Antigens/physiology , Immunoconjugates , Lymphocyte Activation , T-Lymphocytes/immunology , Abatacept , Animals , Antigens, CD , Antigens, Differentiation/immunology , CTLA-4 Antigen , Cell Cycle , Cell Line , Cell Survival , Cells, Cultured , Cricetinae , Cytokines/biosynthesis , Interleukin-2/pharmacology , Kinetics , Mice , Mice, Inbred C57BL , Mice, Transgenic , Receptors, Antigen, T-Cell/biosynthesis , Receptors, Antigen, T-Cell/immunology , T-Lymphocytes/cytology , T-Lymphocytes/drug effectsABSTRACT
CD28 and CTLA-4 are related glycoproteins found on T cells. Ligation of CD28 following antigen receptor engagement provides a costimulatory signal required for T cell activation. Anti-CTLA-4 antibodies were generated to examine the role of the CTLA-4 receptor on murine T cells. Expression of CTLA-4 as a homodimer is up-regulated 2-3 days following T cell activation. Anti-CTLA-4 antibodies and Fab fragments augmented T cell proliferation in an allogeneic MLR. However, when optimal costimulation and Fc cross-linking were present, anti-CTLA-4 Mabs inhibited T cell proliferation. Together, these results suggest that the MAb may obstruct the interaction of CTLA-4 with its natural ligand and block a negative signal, or directly signal T cells to down-regulate immune function.
