Activation-induced expression of cell surface CD28 on mouse T lymphocytes is inhibited by cyclosporine A

T-cell activation requires both T-cell receptor signaling and a costimulatory signal provided by CD28 which enhances and prolongs interleukin-2 (IL-2) production. To determine the effect of cyclosporine A (CsA) on constitutive and activation-induced CD28 expression, mouse T cells were exposed to CsA (0.1 microM) in the absence or presence of anti-CD3 monoclonal antibody (mAb). CD28 expression was then determined by flow cytometry. CsA treatment prevented activation-induced CD28 expression but did not affect constitutive CD28 expression. Inhibition of inducible CD28 expression by CsA was not rapidly reversible, requiring 48h of restimulation in the absence of CsA for CD28 expression to return to control levels. T cells activated in the presence of combined anti-IL-2 and anti-CD25 mAb (both 10 microg/mL) also exhibited reduced CD28 expression, suggesting that activation-induced CD28 expression is, at least in part, an IL-2-dependent process. However, the inhibitory effect of CsA on activation-induced CD28 expression was maintained in the presence of exogenous IL-2 (250 U/mL). We conclude that CsA, by inhibiting activation-induced expression of costimulatory CD28 molecules by T lymphocytes, may interfere with the ability of CD28 to provide an optimal costimulatory signal for sustained IL-2 production following T-cell activation.