Modulation of lymphocyte activation by the adaptors ALX and RIBP

T cell activation is critical for the development and function of T cells and for immune responses which T cells orchestrate, yet aspects of this process remain poorly understood. Activation of naive T cells requires synergistic signals produced by the TCR and by CD28. We previously identified the novel adaptor ALX. Upon overexpression in Jurkat T cells, ALX inhibited activation of the IL-2 promoter by TCR/CD28, suggesting that it is a negative regulator of T cell activation. To further understand its physiological role, ALX-deficient mice were generated. Purified T cells from ALX deficient mice demonstrated increased IL-2 production, CD25 expression, and proliferation in response to TCR/CD28 stimulation. Enhanced IL-2 production and proliferation was also observed when ALX-deficient mice were primed in vivo with OVA/CFA and then restimulated ex vivo. Consistent with our initial overexpression studies, these data suggest that ALX is a negative regulator of T cell activation. TCR/CD28-mediated activation of phosphotyrosine induction, and Erk1/2, Jnk, IKKα/β and Akt were unaltered in ALX-deficient mice. In contrast, constitutive activation of p38 map kinase and its upstream regulators MKK3/6 was observed in ALX-deficient splenocytes. The phenotype of ALX-deficient mice was reminiscent of the phenotype of those deficient in the transmembrane adaptor LAX, and an association between ALX and LAX proteins was demonstrated. These results suggest that ALX, in association with LAX, negatively regulates T cell activation through inhibition of p38. Furthermore, to investigate the relationship of ALX to the structurally related adaptor RIBP, we interbred ALX-deficient and RIBP-deficient mice, and showed that, thus far, their roles are unique rather than redundant. These studies yield insight into the complex nature of the modulation of lymphocyte signaling by two members of a novel class of adaptor molecules. This, in turn, contributes to our growing understanding of the molecular events of T cell activation.