Requirement for CD2 signaling in the costimulation and induction of murine cytotoxic T lymphocytes

Although signal transduction through CD2 is known to be involved in T cell activation, the costimulatory role of CD2 in cytotoxic T lymphocyte development is not entirely clear. CD2 is a surface molecule found on most thymocytes and all mature T cells which is known to function in adhesion and the regulation of cytokine production and responsiveness. In this study, we show that the interaction of CD2 with its ligand CD48 is critical for the induction of cytotoxic effector function by T lymphocytes since anti-CD3-activated killer T (AK-T) cell induction is inhibited in the presence of anti-CD2 and anti-CD48 monoclonal antibodies (mAb). Inhibition is due to decreased conjugation of effector cells to allogeneic target cells and decreased cytolytic molecule expression. One likely explanation for the observed inhibitory effects is that CD2 blockade downregulates IL-2 and IFN-gamma synthesis. Both IL-2 and IFN-gamma are of critical importance for the development of T cell cytotoxicity. Evidence is provided that blockade of CD2--CD48 interactions resulted in decreased IL-2 and IFN-gamma protein and mRNA expression in anti-CD3 activated T cell cultures. Furthermore, culture in the presence of anti-CD2 mAb downregulated perforin and granzyme B mRNA transcripts in AK-T cells. Granzyme B and perforin are two key molecules in the granule-mediated pathway of killing. Moreover, downregulation of baseline perforin expression in anti-CD2 mAb treated AK-T cell cultures, as well as decreased background perforin expression in resting T cells exposed to anti-CD2 mAb indicates that CD2 may have negative signaling properties. Restoration of granzyme B and perforin expression, as well as AK-T cell effector function upon addition of exogenous IL-2 and IFN-gamma to anti-CD2 mAb treated AK-T cells at initiation of culture is consistent with the cytokine-dependent nature of the granzyme B/perforin cytolytic machinery. Although signaling through CD28 and LFA-1 has been implicated in enhancing the stability of mRNAs coding for IL-2 and IFN-gamma, little is known about the role of CD2 signaling in cytokine mRNA stabilization. Here, we show that CD2 signaling enhances the stability of both IFN-gamma and IL-2 mRNA. These results confirm the costimulatory role of CD2 in T cell induction and suggest that inhibition of T cell activation and consequent effector responses by CD2--CD48 blockade involves changes in IL-2 and IFN-gamma mRNA stability. In conclusion, the results of this study indicate that costimulatory signaling through CD2 is critical for the development of cytotoxic T cell activity.