Low Numbers of CD27^- IgD^- 'Double Negative' Senescent B-Cells Early after Reduced Intensity T Cell Depleted Haematopoietic Stem Cell Transplantation Are Predictive of Subsequent Chronic GvHD

Chronic graft versus host disease (cGvHD) is an important complication of allogeneic haematopoietic stem cell transplantation (allo-HSCT). It is now believed that B cells play a major role in the development of cGvHD as evidenced by the efficacy of B-cell directed therapies, and the disturbances in B cell subsets found in patients. This is characterised by a relative lack of naive B cells and increase in CD27+ (antigen experienced) mature B cells that can mediate host directed responses. Despite this the cellular mechanisms underlying the pathogenesis of cGvHD remain unclear.

We investigated the phenotype and function of CD19+ B-cell subsets in a cohort of 46 patients undergoing reduced intensity-conditioned allo-HSCT with alemtuzumab. 76% of patients had myeloid disease and 24% lymphoid disease. All patients received PBMC grafts, fludarabine and melphalan conditioning and 10mg/day alemtuzumab from day -5 for 5 days. Overall survival at 3 years was 65% and relapse free survival was 62%. 16% patients experienced acute GvHD (grade 2+) and 29% developed chronic GvHD.

B cell subsets were examined at 6 weeks post-transplant and CD27^-IgD^- 'double negative' (DN) B-cells were seen to dominate the B cell repertoire (mean 55% of B-cells). DN B-cells are an ill-defined memory B cell subset associated with immune senescence, auto-immune inflammatory conditions and cancer but their role in allo-HSCT has not been described.

The frequency of DN B-cells decreased over time, as the naive B cell compartment regenerated, but still represented 31% of B cells at 1 year. However the mean number of DN B cells remained remarkably stable (0.033x10⁹/L at 6 weeks v 0.037x10⁹/L at 1 year). 14% of DN B-cells early post-transplant were transitional (CD24^high CD38^high) whilst 10% were plasma cells (CD24^- CD38^high). The remaining 76% had increased CD86 expression compared to memory B-cells (16% v 9.6%; *p=0.039) and lower levels of surface immunoglobulin. DN B-cells were less functional than other B cell subsets with decreased proportions of cells producing IL-2 (20%), IL-6 (17%), IFN-γ (6.2%), and IL-10 (6.2%) compared to naive or memory B cells. They also demonstrated reduced proliferation to in vitro stimulation (Ki-67 5.3%).

Importantly, the proportion of DN B cells early post transplant was then studied in relation to transplant outcome and a lower frequency of cells was found to be correlated with subsequent development of cGvHD (38% v 62%; *p=0.04).

These data show that senescent CD27^-IgD^- DN B-cells dominate the B cell repertoire in the early period after lymphocyte-depleted allo-HSCT. Furthermore, they are significantly reduced in patients who subsequently develop cGvHD. These data suggest that early B cell senescence can directly regulate the subsequent development of chronic graft versus host disease and indicate that the composition of the B cell repertoire at an early time-point post-transplant may be used to predict subsequent clinical outcome.