Abnormalities in the expression of CD55 and CD59 surface molecules on peripheral blood cells are not specific to paroxysmal nocturnal hemoglobinuria.

The regulatory proteins CD55 and CD59 are glycolsylphosphatidylinositol-anchored, type I cell surface proteins, which inhibit formation of the C3 convertases and prevent the terminal polymerization of the membrane attack complexes, respectively. Paroxysmal nocturnal hemoglobinuria (PNH) is a genetic disorder due to the impaired conformation of the glycolsylphosphatidylinositol anchor, which results in the deficient expression of CD55 and CD59 leading to excessive destruction of red cells and leukocytes. We have studied the expression of these two molecules in red blood cells, granulocytes and platelets in patients with PNH (two patients), autoimmune hemolytic anemia (AIHA) (seven patients), autoimmune thrombocytopenia (ATP) (22 patients), systemic lupus erythematosus (SLE) (19 patients), aplastic anemia (AA) (eight patients), and Evans syndrome (ES) (two patients). A diminished expression of CD55 and CD59 was found in the three cell lines of the two patients with PNH. In the seven patients with AIHA two were found with CD59 diminished expression in red blood cells and one with CD59 diminished expression in granulocytes. In the patients with ATP one was found with CD55 diminished expression in red blood cells, one with CD59 diminished expression granulocytes and one with a CD59 diminished expression in the platelets. In the subset of patients with SLE only one was found with a CD55 diminished expression in the red blood cells. In the patients with AA, a diminished expression in red blood cells was not found; however, one patient was found with a diminished expression of CD59 in granulocytes, and one patient with a diminished expression of CD55 in the platelets. In the two patients with ES we did not found changes in the expression of CD55 and CD59. We conclude that the diminished expression of the glycolsylphosphatidylinositol-anchored type I cell surface proteins CD55 and CD59 is not specific to PNH and that it can be found in patients with a variety of autoimmune disorders. Additional studies are needed to define the role of the deficiencies of CD55 and CD59 in the pathogenesis of autoimmune hemocytopenias.