Defective TCR signaling events in glycosylphosphatidylinositol-deficient Tcells derived from paroxysmal nocturnal hemoglobinuria patients

Paroxysmal nocturnal hemoglobinuria (PNH) is an acquired hemolytic disorder characterized by the presence of abnormal cells of various hematopoietic c ell lineages deficient in surface expression of glycosylphosphatidylinosito l (GPI)-anchored molecules. By analyzing T cells isolated from patients aff ected with PNH, it was found that ex vivo GPI-deficient CD4(+) and CD8(+) p eripheral T cells display a more naive phenotype as compared to wild-type c ells. In addition, in vitro proliferative responses to allogeneic antigen-p resenting cells were shown to be reduced in mutant T cells. To investigate the molecular basis responsible for defective T cell activation in GPI-defi cient T cells, T cell lines and T cell clones were generated from patients affected with PNH. When stimulated with anti-CD3 epsilon mAb, mutant cells displayed a significantly decreased activation of protein tyrosine kinase p 56(lck). The decreased kinase activity was accompanied by a delayed TCR cap ping and internalization. Interestingly, protein tyrosine phosphorylation i s not only quantitatively but also qualitatively affected, with one substra te being more intensively phosphorylated in mutant than in wild-type cells. These observations suggest that a defective activation of p56(lck) contrib utes to the depressed immune responses observed in GPI-deficient T cells de rived from PNH patients.