Increased Effector CD4⁺ T-Cells Early Post Hematopoietic Stem Cell Transplantation Using an Alemtuzumab-Based Reduced Intensity Conditioning Regimen Correlate with Subsequent Development of Graft Versus Host Disease.

Lymphocyte depletion using the anti CD52 monoclonal antibody alemtuzumab reduces the incidence of graft versus host disease (GvHD) after allogeneic hematopoietic stem cell transplantation (allo-HSCT), but some patients still develop this potentially life-threatening complication. We previously reported that patients achieving rapid full donor T cell chimerism after fludarabine, busulphan and alemtuzumab (FBC) conditioned allo-HSCT have a significantly increased risk of GvHD compared to patients with prolonged mixed donor chimerism beyond day 100 (Lim et al. Br. J. Haematology 2007). We performed a prospective study of 29 patients who received allo-HSCT with FBC conditioning (median age: 53 years; range: 34 – 69 presenting with AML or MDS) to examine the kinetics of lymphocyte reconstitution in relation to T-cell chimerism patterns and incidence of GvHD. Naïve, memory, effector and terminally differentiated CD4⁺ and CD8⁺ T-cells, activated T-cells (CD25⁺ HLA-DR⁺); putative regulatory CD4⁺ CD25^high Foxp3⁺ T-cells, B-cells and NK cells were enumerated in whole peripheral blood of patients at days 30, 60, 90, 180, 270 and 360 after HSCT. Chimerism analysis of purified T-cells was performed by genetic profiling of polymorphic short tandem repeat loci. Ten patients developed GvHD (acute or chronic). Although alemtuzumab induced profound depletion of all T-cell subsets, significantly higher numbers of CD4⁺ effector (CD45RO⁺ CD27⁻) T-cells were detected at day 30 post transplant in patients who later developed GvHD (24 cells/μl; range: 1 – 84 cells/μl) compared to patients without GvHD (5 cells/μl; range: 1 – 40 cells/μl) (p = 0.026). In contrast, there were no significant differences in the numbers or rate of reconstitution of CD8⁺ T-cell sub-populations, NK cells or B-cells in patients that developed GvHD and those who did not at any time points. T-cells present at day 30 in patients that subsequently developed GvHD were 100% donor whereas the majority of patients that did not develop GvHD exhibited mixed donor and recipient T cell chimerism. Development of GvHD pathology was associated with expansion of these donor effector CD4⁺ T-cells (at day 60: 35 cells/μl; range: 9 – 154 cells/μl compared to 7 cells/μl; range: 1 – 56 cells/μl for patients without GvHD, p = 0.04). Absolute numbers of CD4⁺ CD25^high Foxp3⁺ T-cells at day 30 were similar in both groups of patients (p = 0.8). However, of note, a significant deficit of these putative regulatory T-cells in the group that developed GvHD was apparent when numbers were considered relative to CD4⁺ effector T cells at day 30 (41 CD4⁺ effector T-cells; range: 28 – 51 /per regulatory CD4⁺ T cell for the GvHD group compared to 12 CD4⁺ effector T-cells; range: 2 – 33 /per regulatory CD4⁺ T-cell for patients without GvHD, p = 0.03). We speculate the higher numbers of effector CD4⁺ T-cells detected in patients at day 30 post HSCT are donor-derived mature T cells that alemtuzumab fails to deplete. In the solid organ transplant setting, alemtuzumab has been shown to be relatively sparing of effector memory CD4⁺ T-cells. Our correlation of donor-derived effector CD4⁺ T-cells with subsequent development of GvHD suggests they are alloreactive and that a deficit of T-regs relative to CD4⁺ effector T-cells early post HSCT contributes to GvHD.