Novel effects of interleukin-2 on signals generated through the abTcRCD3 complex

CD4 is a critical signalling molecule for T cell activation. Independent ligation of CD4 prior to anti-TcRbeta versus anti-CD3epsilon induced activation, precludes alphabetaTcR induced, but not CD3epsilon induced signalling in primary CD4+ T cells. Thus, CD4 pretreatment Prior to activation with these antibodies correlates with an apparently uncoupled antigen receptor complex in primary CD4+ T cells. Moreover, CD4 pretreatment prior to activation with anti-TcRbeta can also activate alphabetaTcR induced programmed cell death in primary CD4+ T cells. These results indicate that CD4 can exert a negative effect on alphabetaTcR induced signalling.We recapitulate these findings in a CD4+, interleukin 2 (IL-2)-dependent T cell clonal system, and we observe that one plausible mechanism for the negative effect associated with the expression of CD4, is the ability of CD4 to sequester its associated Ick. In physiological circumstances, this would ensure that CD4+ T cell activation requires MHC restricted antigen presentation.Moreover, non-productive anti-TcRbeta induced signalling in the CD4 + clones is alleviated by the withdrawal of exogenous IL-2 with concomitant accumulation of cells in G0/g1. In subcellular fractionation studies, we demonstrate that the two plasma membrane-bound pools of Ick (ie. that pool which is associated with CD4 and that pool which is non-CD4 associated), are both kinase active in quiescent CD4+ cells, while both pools are kinase inactive in CD4+ IL-2 blasts. Thus, the rescue of anti-TcRbeta induced growth in quiescent CD4+ cells correlates with the presence of kinase active Ick on the plasma membrane.The results indicate that of the two membrane-bound pools of Ick, antigen induced activation likely utilizes the CD4 associated pool, and anti-TcRbeta likely utilizes only the non-CD4 associated pool. Since anti-CD3epsilon induced signalling is intact in CD4+ IL-2 blasts, we present the findings that IL-2 can functionally uncouple alphabetaTcR/CD3 complex signalling, by altering critical signals provided by p56Ick