Am Risitano

Long-lasting decrease of marrow and circulating long-term culture initiating cells after allogeneic bone marrow transplant

We investigated bone marrow (BM) and circulating (PB) hematopoietic progenitor cells in 37 normal donors and in 25 patients 1 to 8 years after successful allogeneic bone marrow transplant. At the time of testing, transplanted patients had normal blood counts and bone marrow cellularity. By flow cytometry, BM CD34+ cells were found to be three- to four-fold decreased in transplanted patients compared to normal donors, while the number of PB CD34+cells was the same as in normal donors. Using a methylcellulose colony assay, primary BM colony-forming cells (CFU-GM) were decreased 2.1-fold, whereas PB CFU-GM were only marginally decreased. In a long-term culture initiating cell (LTC-IC) assay, an eight-fold decrease of early progenitor cells was observed in the marrow of transplanted patients compared to normal donors, and a five-fold decrease was documented in peripheral blood. We found that the BM LTC-IC cell number correlated with concurrently determined BM CD34+ cells and committed progenitor cell number (measured as CFU-GM) and with PB LTC-IC number, but not with PB CFU-GM and CD34+ cells. We conclude that marrow and circulating early stem cell compartments, as measured by the LTC-IC assay, are greatly and permanently depressed following bone marrow transplant. The correlation between BM and PB LTC-IC indicates that the enumeration of circulating LTC-IC can be used as a measure of the stem cell compartment in the bone marrow after transplant. It seems that the deficiency of the most immature progenitor cells persists forever after successful bone marrow transplant; this means that a complete hematopoietic reconstitution can be sustained by a reduced stem cell pool.

Achievements and limitations of complement inhibition by eculizumab in paroxysmal nocturnal hemoglobinuria: the role of complement component 3.

Paroxysmal nocturnal hemoglobinuria (PNH) is a hematological disorder characterized by complementmediated hemolytic anemia, thrombophilia and bone marrow failure. The clinical hallmark of PNH is evident chronic hemolysis due to the absence of the complement regulators CD55 and CD59 on PNH erythrocytes. Intravascular hemolysis drives the major clinical features of PNH, including anemia, hemoglobinuria, fatigue and other hemolysisrelated disabling symptoms, such as painful abdominal crises, dysphagia and erectile dysfunction. A peculiar thromboembolic risk has been associated with the hemolysis in PNH, but its pathophysiologic cause remains unclear. The treatment of PNH has remained supportive until a few years ago, when the first complement inhibitor, designated eculizumab, became available. Chronic treatment with eculizumab results in sustained control of intravascular hemolysis, leading to hemoglobin stabilization and transfusion independence in half of the patients. However, residual anemia may persist in a substantial fraction of patients. Recent observations by different groups, including our own, have demonstrated that residual hemolysis may be due to persistent activation of the early phases of the complement cascade, leading to progressive C3-deposition on PNH erythrocytes and possible subsequent extravascular hemolysis through the reticuloendothelial system. Here we critically review the available clinical results of eculizumab treatment for PNH patients, pointing out the recent insights into the pathophysiology of the disease. We discuss the role of the different components of the complement cascade leading to hemolysis, in both the absence and presence of the terminal effector pathway inhibition by eculizumab. Finally, we provide a theoretical rationale for the development of novel strategies of complement inhibition which could in the future further improve on the already substantial efficacy of eculizumab.

Novel Immunosuppressive Strategies for Bone Marrow Failure Syndromes: A Focus on Alemtuzumab

Acquired bone marrow failure syndromes (BMFS) are a heterogeneous group of hematological disorders characterized by impaired bone marrow function and subsequent cytopenia of one or more blood cell lineages [1,2]. The well-accepted pathogenic mechanism of the typical bone marrow failure - aplastic anemia (AA)- is a T cell mediated immune attack targeting the hematopoietic tissue [3]. This pathogenic mechanism is at least partially shared by other bone marrow failure syndromes, such as lineage-restricted aplasias and some myelodysplastic syndromes. Thus, for these disorders immunosuppression (IS) is the pivotal etiologic treatment. While the standard IS regimen include the heterologous anti-thymocyte globulin [4], here we review the recent data on the anti-CD52 monoclonal antibody alemtuzumab as a novel IS agent for marrow failures. Alemtuzumab led to objective responses in aplastic anemia patients in 3 recent prospective studies, with overall response rates ranging between 37% and 72%. Adverse events were irrelevant, ruling out even the concerns about the risk of infectious complications. Alemtuzumab was effective even for the treatment of lineage-restricted marrow failure, with very acceptable toxicity and excellent response rates (as high as 80%). More recently, even patients suffering from myelodysplastic syndromes showed a remarkable hematological response to alemtuzumab-based IS treatment. Thus, alemtuzumab is a novel IS agent representing an excellent alternative to ATG for all immune-mediated marrow failure syndromes. Even if the dose and the schedule may still require further refining, the available data support the need of large prospective trials comparing alemtuzumab to current standard IS regimens.