Presence Of CD55- and/Or CD59-Deficient Erythrocytes In Patients With Rheumatic Diseases: An Immune-Mediated Phenomenon?

Paroxysmal nocturnal hemoglobinuria (PNH) is an acquired clonal hematopoietic stem cell disorder characterized by the somatic mutation of X-linked gene PIG-A, required for glycosylphosphatidylinositol (GPI)-anchor biosynthesis. This results in absent or decreased expression of all membrane proteins normally anchored by GPI - including CD55 and CD59 - in all circulating cells, leading to an unusual sensitivity of red blood cells (RBCs) to complement lysis and subsequently intravascular hemolysis and hemoglobinuria. According to the “dual pathogenesis” model, there is an immunoregulatory selection in favor of PNH clones to proliferate preferentially over normal hemopoiesis on a microenvironment of bone marrow failure. The incidence of “PNH-like” defect has been also demonstrated in many hematological diseases and on peripheral blood cells (PBC) of normal individuals.

Complement system is recognized as having the potential to provoke severe impairment to host tissues. This is extensively demonstrated in autoimmune disease setting. Multiple regulatory and inhibitory enzymes, such as CD55 and CD59, known as complement regulatory proteins, adjust the progression of complement cascade at all levels, protecting the autologous cells. Complement activation and cytopenias have been associated with diminished CD55 and/or CD59 expression on PBC membranes. The aim of this study was to evaluate the presence of “PNH-like” red-cell populations in patients with rheumatic diseases and investigate possible correlations with clinical or laboratory parameters.

CD55 and CD59 expression was evaluated in erythrocytes of 113 patients (94 females, 19 males, median age: 64 years) with rheumatic diseases: 38 with rheumatoid arthritis, 25 with systemic lupus erythematosus, 17 with Sjögren’s syndrome, 7 with systemic sclerosis, 12 with vasculitis, 2 with dermatomyositis, 1 with ankylosing spondylitis and 11 with mixed connective tissue diseas, using the sephacryl-gel microtyping system, a semi-quantitative, inexpensive and simple method useful in screening “PNH-like” red-cell defect, with sensitivity comparable with that of flow cytometry. One hundred and twenty-one (121) healthy blood donors of similar age and gender and 10 patients with PNH were also studied, as control groups. In all samples with CD55- and/or CD59- negative RBCs, Ham and sucrose tests were also performed.

Interestingly, the majority of patients (104/113, 92%) demonstrated “PNH-like” erythrocytic populations: 47 (41.6%) with concomitant deficiency of CD55 and CD59, 50 (44.2%) with isolated deficiency of CD55 and 6 (6.2%) with isolated deficiency of CD59. In healthy donors, only 2 (1%) had red cells with concomitant CD55/CD59 negativity and 3 (2%) with isolated CD55 or CD59 deficiency. “PNH-like” erythrocytic clones never surpassed 25% of the total red-cell population, while the most common proportion of deficiency for both antigens was 10%. All PNH patients exhibited simultaneous CD55/CD59 deficiency. Moreover, it should be high-lightened that we found an unprecedented relation between patients' hemoglobin (Hb) and CD55 expression on RBCs (r_s= -0.205, p=0.029), while there was a significant difference (δ) when the mean concentration of Hb was compared between patients with normal expression of CD55 and those with deficiency of this protein (δ=-1.4534 g/dl, p=0.0151). There was no clinical or laboratory evidence of hemolysis in our patients. There was no association between the presence of “PNH-like” red-cell populations and cytopenias or specific treatment for the autoimmune disorder. Positive Ham and sucrose tests were found only in PNH patients.

In conclusion, this study provides evidence supporting the presence of erythrocytes with CD55- and/or CD59- deficiency in patients with rheumatic diseases. The pre-existence of small PNH clones in the bone marrow of these patients, that acquire a survival advantage to proliferate against normal hemopoietic tissue and become detectable with our methodology, may be the underlying cause for this phenomenon. Moreover, it was demonstrated that CD55- deficiency on RBCs influences the levels of Hb, in these patients. Further studies, using molecular techniques, will be required, to clarify the exact pathophysiologic mechanisms for this deficiency.