Phenotypic and functional analysis of lymphocytes in paroxysmal nocturnal hemoglobinuria

The hematologic disorder paroxysmal nocturnal hemoglobinuria (PNH) arises from a somatic mutation within the Piga gene important for the biosynthesis of glycosylphosphatidylinositol (GPI) anchors. The PNH defect has been identified in all cells of the myeloerythroid lineage, but involvement of the lymphoid lineage in PNH is more controversial. We therefore analyzed lymphocytes from 22 patients with PNH to characterize phenotypically the GPI-deficient population, and to investigate the functional consequences of GPI deficiency. GPI-deficient T lymphocytes, B lymphocytes, and NK cells were identified, but at a lower percentage than granulocytes and erythrocytes. CD8+ lymphocytes were significantly more affected than CD4+ T cells, and CD45RA+ lymphocytes were significantly more affected than CD45RO+ cells. Proliferation assays demonstrated that lymphocytes from PNH patients, either unfractionated or purified GPI-deficient cells, responded normally to in vitro stimuli. When stimulated with phytohemagglutinin (PHA), naive CD45RA+ GPI-deficient T lymphocytes acquired the memory CD45RO+ phenotype. In addition, GPI-deficient T lymphocytes had a relative growth advantage as compared to normal T cells. The results demonstrate that PNH involves the lymphoid as well as the myeloerythroid lineage, and therefore arises from a totipotent bone marrow stem cell. The in vitro growth advantage of GPI-deficient lymphocytes in PNH may have important implications for the pathogenesis of some puzzling clinical aspects of PNH, including predominance of the PNH clone, defective hematopoiesis, and leukemogenesis.