44: The CD4 +CD25 +Foxp3 + Compartment Following Conditioning and Transplant: Host Treg Cells Expand and Comprise the Predominant Component for Several Months During Reconstitution post-HCT

Abstract

The capacity of CD4+CD25+Foxp3+ (Treg) cells to regulate adaptive and innate immune responses has led to studies investigating their use in novel strategies to regulate allogeneic T cell responses during hematopoietic stem cell transplants (HCT). A fundamental clinical concern post-HCT is the reconstitution of the lymphoid compartment, particularly T cells which can be exceptionally delayed. We have previously found that host Treg cells can regulate resistance to engraftment following HCT, demonstrating that such cells survive and function at least transiently in recipients. The present studies investigated the residual host Treg compartment following varying levels of conditioning (3.0 – 14Gy TBI), and transplant. We found that recipient CD4+CD25+Foxp3+ cells: 1) can survive ablative as well as reduced intensity conditioning, 2) undergo expansion (BrdU uptake/cell numbers) and 3) contribute greatly to the Treg compartment for several months post-HCT during which time donor derived Treg cells gradually arise and cede this compartment. Within the first 3 weeks post-lethal conditioning and HCT, 95% of the splenic CD4+FoxP3+ cells are positive for BrdU, vs. ∼40% in normal mice. Using Thy1.1 congenic mice, the vast majority of these cells were found to be resistant (host) Tregs. Two months post-HCT, almost 30% of the compartment was still of host origin. To assess the functional capacity of the residual Treg cell compartment, we examined development of autoimmune disease following transplant of IL-2Rβ−/− BM into syngeneic recipients. Autoimmune disease was prevented in B6-wt but not T cell deficient recipients. Interestingly, the failure to transfer autoimmune disease following IL-2Rβ−/− HCT into B6-CD4-/- recipients was associated with the presence of a peripheral CD8+FoxP3+ population not detected in B6-wt mice. This finding suggests that in the genetic absence of CD4+ T cells, a CD8 regulatory population may emerge. In total, our observations support the notion that functioning host Tregs initially occupy a niche in the transplant recipient permitting lymphopenic expansion and an extended period of contribution to this compartment. Notably, this contribution reflected much greater levels than that by other lymphoid cell populations. Overall, the findings imply that host Treg cells may be important to consider with respect to eliciting anti-tumor and vaccination responses in recipients during the early period post-hematopoietic cell transplantation. The capacity of CD4+CD25+Foxp3+ (Treg) cells to regulate adaptive and innate immune responses has led to studies investigating their use in novel strategies to regulate allogeneic T cell responses during hematopoietic stem cell transplants (HCT). A fundamental clinical concern post-HCT is the reconstitution of the lymphoid compartment, particularly T cells which can be exceptionally delayed. We have previously found that host Treg cells can regulate resistance to engraftment following HCT, demonstrating that such cells survive and function at least transiently in recipients. The present studies investigated the residual host Treg compartment following varying levels of conditioning (3.0 – 14Gy TBI), and transplant. We found that recipient CD4+CD25+Foxp3+ cells: 1) can survive ablative as well as reduced intensity conditioning, 2) undergo expansion (BrdU uptake/cell numbers) and 3) contribute greatly to the Treg compartment for several months post-HCT during which time donor derived Treg cells gradually arise and cede this compartment. Within the first 3 weeks post-lethal conditioning and HCT, 95% of the splenic CD4+FoxP3+ cells are positive for BrdU, vs. ∼40% in normal mice. Using Thy1.1 congenic mice, the vast majority of these cells were found to be resistant (host) Tregs. Two months post-HCT, almost 30% of the compartment was still of host origin. To assess the functional capacity of the residual Treg cell compartment, we examined development of autoimmune disease following transplant of IL-2Rβ−/− BM into syngeneic recipients. Autoimmune disease was prevented in B6-wt but not T cell deficient recipients. Interestingly, the failure to transfer autoimmune disease following IL-2Rβ−/− HCT into B6-CD4-/- recipients was associated with the presence of a peripheral CD8+FoxP3+ population not detected in B6-wt mice. This finding suggests that in the genetic absence of CD4+ T cells, a CD8 regulatory population may emerge. In total, our observations support the notion that functioning host Tregs initially occupy a niche in the transplant recipient permitting lymphopenic expansion and an extended period of contribution to this compartment. Notably, this contribution reflected much greater levels than that by other lymphoid cell populations. Overall, the findings imply that host Treg cells may be important to consider with respect to eliciting anti-tumor and vaccination responses in recipients during the early period post-hematopoietic cell transplantation.