Phenotypic analysis of CTLA-4 and CD28 expression during transient peptide-induced T cell activation in vivo

The T cell co-stimulatory receptors CD28 and CTLA-4 appear to have opposite effects on T cell activation, mediating augmentation and inhibition of T cell responses respectively. Since these two receptors use the same ligands, CD80 (B7-1) and CD86 (B7-2), the co-ordinate timing of CD28 and CTLA-4 expression has a major impact on the regulation of immune responses. While the kinetics of co-stimulatory molecules have been established for T cell stimulation in vitro, little is known about CD28 and CTLA-4 expression in response to T cell activation in vivo. In this study we have investigated the kinetics of CD28 and CTLA-4 expression upon CD4(+) T cell activation in response to soluble peptide in vivo. Using mice transgenic for a T cell receptor specific for the I-Au-restricted N-terminal peptide of myelin basic protein MBP Ac1-9, we show maximal up-regulation of both CD28 and CTLA-4 2 days after peptide administration. CTLA-4 expression correlated positively with early activation markers on the same cells and was high on blast cells. Administration of peptide analogs with higher affinity for I-Au MHC class II revealed a higher increase in CTLA-4 than in CD28 expression in response to improved TCR ligation. Further, a small population of CD4(+) T cells expressing CTLA-4, CD25 and CD45RBlow was identified in mice that had not been treated with specific peptide. The implications of these observations for immune regulation are discussed.