High incidence of transiently appearing complement-sensitive bone marrow precursor cells in patients with severe aplastic anemia a possible role of high endogenous IL-2 in their suppression

In a prospective long-term study on the incidence of paroxysmal nocturnal h emoglobinuria (PNH), 115 consecutive patients with severe aplastic anemia ( SAA), 97 treated with antilymphocyte globulin (ALG) and 18 with bone marrow transplantation (BMT), were observed over a period of 4-18 yea rs a nd tes ted for the presence of complement-sensitive hematopoietic precursor cells with the bone marrow (BM) sucrose test. Sixteen (14%) of the ALG-treated pa tients developed clinical signs of PNH between 0.5 and 8 years after treatm ent. Complement-sensitive BM precursors were found in 89% of the SAA patien ts at some time during their disease, but in none of 18 normal donors. At d iagnosis, their proportion was significantly higher in patients who later d eveloped PNH than in patients who later achieved disease-free complete remi ssion (CR). After ALG, the abnormal population was found in both groups, bu t it was gradually replaced by normal precursors in remission patients. Aft er BMT, the complement-sensitive population decreased to very low numbers i n patients with a stable graft, but increased again in 3 patients upon graf t rejection. Mimicking the PNH defect by enzymatic removal of glycosyl-phos phatidylinositol (GPI)-linked proteins from CD34+ cells resulted in their c omplement sensitivity, suggesting that the BM sucrose test identifies precu rsor cells carrying the PNH defect. In 66 patients, white blood cells (WBC) in peripheral blood (PB) were examined for GPI-deficient populations by fl ow cytometry (FACS). Ten patients with signs of clinical or laboratory PNH had over 25% complement-sensitive precursor cells in the BM and a GPI-defic ient WBC population in the PB. Of 56 SAA patients without PNH, 8 had an abn ormal population detectable with both tests, 26 only with the BM sucrose te st, 4 only with PB FAGS analysis, and in 18, no abnormal cells were detecte d with either test. In search for parameters which might explain why in som e patients the abnormal population expands, while it regresses or disappear s in others, we tested the release of IL-2 as a parameter of immune compete nce. At diagnosis, IL-2 release was approximately 50% of normal in patients who later developed PNH, while it was double the normal value in patients who later achieved CR. We conclude that the majority of SAA patients transi ently harbor complement-sensitive precursor cells in the BM. Patients with more than 25% abnormal BM precursors and low endogenous IL-2 release are at risk of progression to clinical PNH.