Targeted Lymphocyte Depletion Prior to Reduced Intensity Allogeneic Stem Cell Transplantation: A Personalized Approach to Facilitate Engraftment.

Abstract 865

Reduced intensity allogeneic stem cell transplantation (RIST) is associated with decreased transplant-related mortality (TRM), broadening the pool of patients who could potentially benefit from allogeneic cellular therapy. However, RIST is typically associated with higher rates of mixed chimerism and graft rejection compared to myeloablative conditioning. Data from clinical studies have shown that the number of therapies prior to transplant, which inversely correlates with host T-cell immunity, is a statistically important predictor of graft rejection and mixed chimerism. In order to compensate for variability in host immune status and facilitate early full-donor chimerism (>95%), we developed a strategy of targeted lymphocyte depletion (TLD) which uses repetitive cycles of disease-specific conventional-dose chemotherapy to provide both tumor cytoreduction and lymphocyte depletion prior to RIST. The number of TLD cycles (0-3 maximum) was based on reaching a target CD4⁺ count <100 cells/μl; patients with higher pre-TLD CD4⁺ count required more cycles. We employed the TLD approach in 111 patients (mean age = 49 years, (19-71) with advanced hematologic malignancies. Median CD3⁺, CD4⁺, and CD8⁺counts at enrollment were: 673 cells/μl (5-3953), 286 cells/μl (5-3888), and 277 cells/μl (1-1763) respectively. Following TLD chemotherapy, median CD3⁺, CD4⁺, and CD8⁺ counts were: 164 cells/μl (1-1496), 82 cells/μl (0-508), and 52 cells/μl (1-1195) respectively. All patients then received an identical reduced intensity conditioning regimen (fludarabine/cyclophosphamide) followed by HLA-matched sibling peripheral blood stem cell allografts. All patients received cyclosporine as graft versus host disease (GVHD) prophylaxis (1) alone, (2) with methotrexate, (3) with TH2 cells, (4) with sirolimus or (5) with TH2 cells and sirolimus. Immediately prior to stem cell infusion (Day 0) median host CD3⁺, CD4^+, and CD8+ counts were 4 cells/μl (0-69), 3 cells/μl (0-65) , and 1 cell/μl (0-44) respectively. 109 evaluable patients demonstrated 100% engraftment; there were no graft failures. At Day +14, median lymphocyte chimerism was 99% and median myeloid and whole blood chimerism were 100%. Patients were able to maintain chimerism as evidenced by median 100% chimerism in the myeloid, lymphoid and whole blood compartments at Day +28 and median 100% whole blood chimerism at Day +100. Full donor lymphocyte chimerism at Day +14 was associated with lower post-TLD CD4⁺ counts (p=0.012) and Day 0 CD3⁺, CD4⁺, and CD8⁺ counts (p<0.0005). Full donor myeloid chimerism at Day+14 and Day+28 was associated with lower Day 0 CD3⁺, CD4⁺, CD8⁺ counts (p<0.05). Full donor whole blood chimerism at Day +14, Day +28 and Day+100 was associated with lower Day 0 CD3⁺, CD4⁺, and CD8⁺ counts (p<0.05). The CD3+ and CD34+ dose contained in the allograft was associated with Day +14 lymphoid chimerism (p=0.04) and with Day +28 myeloid chimerism (p=0.03) respectively. CMV status was associated with both lymphoid and whole blood chimerism at Day +14 (p<0.05). Only lymphoid chimerism at Day+28 was associated with GVHD prophylaxis regimen (p<0.05). Patients with acute GVHD II-IV had significantly lower CD4⁺ counts post-TLD (p=0.01). Patients with acute GVHD III-IV had significantly lower CD4⁺ and CD3⁺ counts at Day 0 (p<0.05). These data demonstrate the tremendous variability in pre-transplant host T-cell numbers. By using the strategy of TLD, we were able to compensate for this variability while achieving levels of T-cell depletion comparable to myeloablative conditioning. We conclude that TLD provides a personalized approach to pre-transplant host immune status resulting in absence of graft rejection and rapid and full donor chimerism following RIST.