CD44 signaling through p56 super(lck) involves lateral association with CD4 in human CD4 super(+) T cells

CD44 is a family of mucin-like membrane proteins generated by alternative splicing of several exons, and participate in T cell adhesion and activation. CD44-mediated signaling involves activation of p56 super(lck) and leads to ZAP-70 phosphorylation. The aim of the present study was to identify the signaling pathways that follow CD44-triggered ZAP-70 phosphorylation and the molecular mechanisms underlying the CD44 interaction with p56 super(lck). We found that CD44 cross-linking by mAb in CD4 super(+) peripheral blood T cells promotes formation of a trimeric complex of Grb2, phospholipase (PLC)- gamma 1 and a 36-38 kDa phosphoprotein, and the activation of PLC- gamma 1. The amount of inositol triphosphate and the time kinetics of its generation were comparable to those following CD3 cross-linking. Co-capping, co-immunoprecipitation and fluorescence resonance energy transfer experiments showed that CD44 associates with CD4 and CD3 on the cell surface. This association suggests functional interplay between the CD4-TCR complex and CD44. In line with this possibility, we found that CD4 triggering by gp120, a natural ligand of CD4, potentiates CD44-mediated adhesion to hyaluronic acid. Moreover, Ca super(2+) mobilization induced by CD44 cross-linking by mAb was higher in a subclone of the HUT78 cell line expressing CD4 than in a non-expressing subclone.