The Graft-vs-Host Hematopoietic Effect Generated after Nonmyeloablative Allogeneic Stem Cell Transplantation (NST) Cures Patients with Severe PNH.

Paroxysmal nocturnal hemoglobinuria (PNH) is a clonal disorder of hematopoietic stem cells characterized by RBC susceptibility to complement-mediated lysis. Infections related to neutropenia, bleeding associated with thrombocytopenia, and thrombosis all contribute to morbidity and mortality. Although allogeneic hematopoietic cell transplantation (HCT) can be curative, the high-risk of treatment-related mortality with myeloablative HCT precludes this approach for most patients with severe disease. We previously reported in vitro and in vivo data showing PNH cells could be killed by allo-reactive donor T-cells recognizing minor histocompatibility antigens expressed on both normal and GPI negative cells. Here we present updated data on a cohort of 11 patients with severe PNH who received a NST at the NHLBI from 5/99 through 6/2004. Eligibility included a diagnosis of PNH associated with one or more of the following :1) Transfusion dependence (n=9) 2) Prior thrombotic episodes (n=4) 3) Recurrent debilitating hemolytic crisis (n=7). Patients received a T-cell replete G-CSF mobilized blood stem cell transplant from an HLA-matched family donor following nonmyeloablative conditioning with cyclophosphamide (120mg/kg) and fludarabine (125mg/m²). Patients with a significant transfusion history had horse ATG (40mg/kg/day x 4) added to the conditioning regimen (n=9). CSA either alone (n=1) or combined with either MMF (n=4) or mini-dose methotrexate (n=6) was used as GVHD prophylaxis. The median % of GPI anchored protein negative neutrophils pre-transplant was 83% (range 13%–99%). Blood samples obtained post-transplant were analyzed by FACS to determine the percentage of persisting GPI negative neutrophils (CD15⁺/CD66b⁻/CD16⁻). Chimerism was also assessed post-transplant in T-cell and myeloid fractions by PCR assay of polymorphic short tandem repeats (STR).

Neutrophil recovery occurred at a median 15 days (range 10–19). STR analysis revealed donor engraftment occurred in both myeloid and T-cell lineages in all patients. Self-limiting febrile hemolytic reactions associated with ATG administration (6/9 patients) and grade II-IV acute GVHD (n=5) were the most common complications associated with transplantation. With a median follow-up of 458 days (range 31–1917), all patients survive either in remission (n=8) or with declining GPI negative populations (n=3); GPI negative neutrophils were detected in all patients at engraftment but gradually declined until no longer detectable (<0.1%) in all 8 patients evaluable more than 100 days after transplantation, while 3 with shorter follow-up (days + 37, +51, +78) have persistent albeit rapidly declining PNH populations. The observation that GPI negative neutrophils populations decrease and ultimately disappear when myeloid chimerism transitions from mixed to full donor chimerism is consistent with PNH cells being eradicated through a graft-vs-host hematopoietic effect. None of the 7 patients with more than 1 year follow-up have had reoccurrence of their PNH clone. Conclusion: Alloreactive donor T-cells mediating graft-vs-host hematopoietic effects can immunologically eradicate PNH following NST. NST should be considered a viable and potentially curative option for patients with severe PNH.