CD 2 Regulates Responsiveness of Activated T Cells to Interleukin 12

Interleukin (IL) 12 is a 70-kD heterodimeric cytokine produced by antigen-presenting cells (APCs) such as macrophages in response to infectious pathogens and interferon (IFN) 3'. The varied immunomodulatory effects of I1-12 include the stimulation of proliferation and IFN-3, production by T cells, and it also has a central role in the development of the T hdper cell type 1 immune phenotype. We undertook the production of antibodies capable of modulating the response of T ceils to II.-12, and in the process we discovered two antibodies that inhibited the ability of II-12 to stimulate T cell proliferation. In this report, we demonstrate that these antibodies recognize CD2, and we show how antibodies directed toward either the adhesion domain of CD2 or its ligand, CD58, specifically inhibit I1-12-induced proliferation and IFN-'y production by phytohemagglutinin-activated T cells, leaving the response to I1-2 unaffected. A threeto fourfold reduction in proliferation and IFN-3~ production was observed at I1-12 concentrations as high as 1 nM, with complete inhibition occurring at ~<1 pM. This novel effect is not directly mediated at the level of the I1-12 receptor, as shown by the inability of these antibodies to block I1-12 binding to activated T cells. Furthermore, by using activating pairs of CD2 antibodies, we show that CD2 stimulation strongly synergizes with I1-12, even at 0.1 pM, in inducing both T cell proliferation and IFN-~, production. Cytolytic T lymphocyte-associated antigen 4-immunoglobulin-mediated inhibition of the B7/CD28 interaction did not affect the T cell response to either I1-12 or I1-2, but the removal of APCs selectively diminished the proliferative response to I1-12. Based on this data, we hypothesize that CD2 has a central role in an I1-12/IFN-3, positive feedback loop between T cell and APC, providing the key functional link via a CD2/CD58 interaction that controls T cell responsiveness to I1-12. This model provides a basis for future investigations aimed at defining the signaling mechanisms that mediate this cytokine-specific regulatory effect of CD2, and it offers insight into how a cytokine receptor and distinct adhesion molecule can interact to modulate responsiveness to that cytokine. In addition, it underscores the possibility that the clinical potential of an immunomodulatory drug like I1-12 may be governed by the presence or absence of specific costimulation.