Intracellular Signaling Networks in the Immune Response: Interleukin-4 introduces Competition and Cooperation to Interleukin-2-Induced T Cell Proliferation

Cells sense and respond to stimuli through signal transduction pathways, which mediate proliferation, differentiation, and survival. The cytokines interleukin-2 (IL-2) and interleukin-4 (IL-4) are key regulators of the immune system, influencing the expansion and differentiation of T cells. Both synergistic and antagonistic effects of IL-2 and -4 co-stimulation have been shown; the antagonism may arise from the sharing of a common receptor subunit. We have sought to characterize IL-2 and IL-4 signaling at the level of intracellular pathways activated by these receptors. IL-2 receptors are known to activate the Ras/Erk and phosphoinositide (PI) 3-kinase pathways as well as the STAT5 transcription factor. IL-4 is able to activate PI 3-kinase/Akt as well as STAT6, though not Ras/Erk.We found that IL-4 initially antagonizes, and later synergizes with, IL-2-stimulated HT-2 cell proliferation; a murine T cell line. At a signaling level, IL-4 abates IL-2-stimulated activation levels of Akt, Erk and STAT5 possibly through competition for limiting amounts of common receptor subunit. IL-2/IL-4 co-stimulation provokes prolonged activation only of STAT6. This extended STAT6 activation may be critical in the IL-2/IL-4 induced synergy in T cell growth. Currently, we are investigating the crosstalk between these pathways and their functional roles in IL-2 and IL-4-stimulated T cell responses.