Apoptosis Susceptibility Prolongs the Lack of Memory B Cells in Acute Leukemic Patients After Allogeneic Hematopoietic Stem Cell Transplantation

Long-term survival after allogeneic hematopoietic stem cell transplantation requires intact immunosurveillance, which is hampered by lymphoid organ damage associated with conditioning therapy, graft-versus-host disease, and immunosuppression. Our study aimed to identify the mechanisms contributing to sustained low memory B cell numbers after transplantation. Peripheral B and T cell subset recovery and functional marker expression were investigated in 35 acute leukemic patients up to 1 year after transplantation. Apoptosis of B cells after CD40/TLR-9, CD40/BCR, and CD40/BCR/TLR-9-dependent stimulation and drug efflux capacity were analyzed. One half of the patients suffered from infections after day 180. All patients had strongly diminished CD27+ memory B cells despite already normalized total B cell numbers and fully recovered CD27−IgD− memory B cells, putatively of extra-follicular origin. Circulating memory follicular helper T cells were reduced in the majority of patients as well. Naïve B cells exhibited a decreased expression of CXCR5, which mediates follicular B cell entry. Additionally, a lower HLA-DR expression was found on naïve B cells, impairing antigen presentation. Upon CD40/TLR-9-dependent activation, B cells underwent significantly increased apoptosis paralleled by an aberrant up-regulation of Fas-L on activated T cells and Fas on resting B cells. Significantly increased B cell apoptosis was also observed after CD40/BCR and CD40/BCR/TLR-9-dependent activation. Drug efflux capacity of naïve B cells was diminished in cyclosporin A-treated patients, additionally contributing to an apoptosis-prone phenotype. We conclude that B cell survival and migration and T cell communication defects are contributing candidates for an impaired germinal center formation of memory B cells after allogeneic hematopoietic stem cell transplantation. Follow-up studies should evaluate effectiveness of revaccinations on the cellular level and should address the long-term sequelae of B cell defects after transplantation.