Pure Hematopoietic Stem Cells Reconstitute Immunity in Allogeneic Hosts Superior to Bone Marrow: Immunosuppression by Subclinical Graft-Versus-Host Disease.

Antigen specific immune responses are known to be impaired following allogeneic hematopoietic cell transplantation (HCT). Some clinical studies suggest that graft T cell depletion for the prevention of graft-versus-host disease (GVHD) leads to poorer immune recovery, while other correlate GVHD with delayed immune reconstitution. Our studies sought to examine the degree to which the co-transplantation of GVHD-inducing mature cells mediated protective immunity post-HCT. We compared the transplant of FACS purified hematopoietic stem cells (HSC: cKit+Thy1.1loSca+Lin-) with bone marrow (BM) between congenic (BA to B6Ly5.2), minor-antigen mismatched (BA to BALB.B), haploidentical (BAxSWR F1 to BALB/cxSWR F1) and MHC-mismatched (BA to BALB/c) donor and host pairs. We show that grafts composed solely of purified HSC give uniformly superior lymphoid reconstitution across all mismatched pairs, both qualitatively and quantitatively. Although absolute blood lymphocytes counts were increased in recipients of BM compared to HSC, lymphoid reconstitution as measured by lymph node size, counts and architecture was significantly improved in the HSC groups regardless of minor or major mismatches between donor and host. Proliferative responses to the allele specific peptides of the antigen hen egg lysozyme were also significantly increased in the HSC as compared BM recipients (p=0.028), with fully MHC mismatched BM recipient cells showing almost no proliferative response. The use of MHC allele specific antigens also revealed that T cell responses post-HCT are dominated by donor-restricted elements. These data suggest that subclinical GVHD mediated by mature cells in the donor BM result in impaired immune reconstitution. While there may be an increase in absolute lymphocyte numbers, this increase does not correlate with an increase in immune cell function. These findings provide important insight into the benefits of purified HSC for preventing post-HCT infectious complication in addition to its known GVHD prophylaxis benefits. Our data highlights the benefit of transplanting a pure HSC population, as the relatively small number of T cells that are found in mouse bone marrow or that remain in the graft after T cell depletion in humans can still result in immune impairment due to subclinical GVHD.