Pre-Transplant T-Cell Lymphopenia Accelerates Early Donor T-Cell and Myeloid Chimerism but Is Not Required for Full Donor Lymphohematopoietic Engraftment or To Prevent Graft Rejection Following Nonmyeloablative Hematopoietic Cell Transplantation (NST).

To facilitate donor engraftment and prevent graft rejection, some NST regimens incorporate chemotherapy prior to transplant conditioning to induce profound lymphopenia. To characterize the impact of the recipients immune status on donor myeloid and T cell engraftment after NST, we performed a detailed analysis of patient (pt) lymphocyte subsets prior to transplant conditioning. The % and absolute numbers of NK cells, B cells and CD3+/CD4+ or CD3+/CD8+ T-cells expressing HLA-DR, CD25, CD45RO, and CD45RA were quantified by FACS. Factors impacting donor myeloid and T-cell engraftment considered in a multivariate regression analysis included pt age, sex, graft CD34 and CD3 doses, pt pre-transplant lymphocyte subset numbers, absolute lymphocyte counts (ALC), and donor-pt sex mismatch. Forty one pts with a variety of malignancies (n=31) and non malignant hematological disorders (n=10) undergoing a T-cell replete, G-CSF mobilized NST from an HLA-matched donor following cyclophosphamide (Cy )(120mg/kg) and fludarabine (Flu) (125 mg/m²) based NST were analyzed. GVHD prophylaxis consisted of CSA +/− MTX (n=35) or MMF (n=6). Blood was collected on post-transplant days 15,30,45,60 and 100 and sorted into CD14+/CD15+ myeloid and CD3+ T-cell lineages for assessment of chimerism using a short tandem repeat PCR-based chimerism analysis. All pts achieved sustained engraftment of donor myeloid and T-cells with no cases of graft rejection. The median % of donor T-cell chimerism was 94, 100, 100, 98, and 100, at 15, 30, 45, 60 and 100 days post-transplant respectively. In contrast, myeloid engraftment was delayed with the median % of donor myeloid chimerism being 45, 54, 43, 48, and 81 at 15, 30, 45, 60, and 100 days post-transplant respectively. Full donor T-cell chimerism was achieved in all pts at a median 30 (range 15–263) days. Full donor myeloid chimerism was achieved in 33 pts (80%) at a median 100 (range 15–405) days; eight pts died before full donor myeloid chimerism was achieved. Median survival was 499 days (95% C.I. 356–736 days). Univariate analysis showed lower baseline absolute CD3+ cell numbers (p=0.033), older pt age (p=0.046) and lower pt ALC before transplantation (p=0.024) were significantly associated with more rapid donor T-cell engraftment by day 15. Prior chemotherapy exposure (p=0.005), lower baseline numbers of either CD3+cells (p=0.03), CD4+ cells (p=0.03), or CD4+ and CD25+ cells (p=0.01) were all significantly associated with more rapid donor myeloid engraftment by day 15. By multiple regression analysis, lower baseline absolute CD3+ cell numbers (p=0.01) and older pt age (p=0.0002) were significantly associated with more rapid donor T-cell engraftment on day 15 while only prior chemotherapy (p=0.008) exposure facilitated day 15 donor myeloid engraftment. The median time to achieving full donor T-cell chimerism was similar regardless of baseline CD3+ cell numbers.

Conclusion: CD3+ lymphopenia facilitates early donor myeloid and T-cell engraftment but does not shorten the time to achieve full donor T-cell chimerism. Induction of CD3+ lymphopenia with chemotherapy before transplant conditioning is not required to achieve full donor chimerism or prevent graft rejection with Cy/Flu-based conditioning used in this regimen.