Common expression of an unusual CD45 isoform on T cells from patients with large granular lymphocyte leukaemiaand autoimmune lymphoproliferative syndrome

Summary. Patients with T‐cell large granular lymphocyte (T‐LGL) leukaemia and autoimmune lymphoproliferative syndrome (ALPS) share many features, including autoimmunity and an expansion of (cytotoxic) T cells, which in ALPS patients express an unusual (B220) isoform of CD45, corresponding to an altered O ‐glycosylation profile. Here we showed that T‐LGL leukaemia cells also expressed this B220 isoform. We hypothesize that B220 + T cells constitute proliferating T cells that have become competent to undergo apoptosis, but that constitutive (ALPS) or functional (T‐LGL) defects prevent this process. Altered O ‐glycosylation of the extracellular domains of CD45 may have consequences for this tyrosine phosphatase as a regulator of cell proliferation and survival.