Defective recruitment and activation of ZAP-70 in common variable immunodeficiency patients with T cell defects

We have previously identified a subset of common variable immunodeficiency (CVID) patients with defective T cell function associated with impaired act ivation of the TCR-dependent tyrosine phosphorylation cascade. Here we have assessed the structural and functional integrity of the principal componen ts involved in coupling the TCR/CD3 complex to intracellular tyrosine kinas es in two of these patients. We show that ZAP-70 fails to bind the signalin g-competent CD3 zeta tyrosine phosphorylation isoform and to become activat ed following TCR engagement, suggesting that defective recruitment of ZAP-7 0 might underlie the TCR signaling dysfunction in these patients. Determina tion of the nucleotide sequences encoding the intracellular domains of the CD3/zeta subunits and ZAP-70 did not reveal any mutation. Furthermore, ZAP- 70 from these patients could interact in vitro with recombinant phospho-zet a, ruling out genetic defects at the immunoreceptor tyrosine-based activati on motif/SH2 domain interface responsible for ZAP-70 recruitment to the act ivated TCR. No defect was found in expression, activity or subcellular loca lization of Lck, which is thought to be primarily responsible for CD3 zeta phosphorylation. Hence, while the T cell defect in these CVID patients can be pinpointed to the interaction between ZAP-70 and CD3 zeta, the integrity in the components of the signaling machinery involved in this process sugg ests that additional components might be required for completion of this st ep.