Surviving Host CD4⁺CD25⁺Foxp3⁺ Cells Following Ablative Conditioning Expand and Comprise the Major Component of the Treg Compartment during the Lymphoid Reconstitution Period Following HCT.

The capacity of CD4⁺CD25⁺Foxp3⁺ (Treg) cells to regulate adaptive and innate immune responses has led to studies investigating their application to regulate allogeneic T cell responses arising during hematopoietic stem cell transplants (HCT). With respect to HCT, a fundamental clinical concern is the reconstitution of the lymphoid compartment in recipients, particularly T cells since this process can be exceedingly 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 HCT. Utilizing varying levels of total body irradiation (5.0 – 14Gy), we observed:

1. recipient CD4⁺CD25⁺Foxp3⁺ cells can survive ablative as well as reduced intensity conditioning,

2. the surviving, i.e. residual Treg cells undergo expansion as assessed by BrdU uptake and cell numbers, and

3. these cells comprise the majority of the Treg compartment in recipients for several months post-HCT during which time donor derived Treg cells gradually arise and cede this compartment.

Residual Tregs also dominated the compartment following allogeneic HCT of MHC-matched bone marrow depleted of T cells. To assess the functional capacity of the residual Treg cell compartment, the development of autoimmune disease following transplant of IL-2Rβ ^−/− (CD122^−/−) bone marrow into syngeneic recipients with and without residual Tregs was examined. Autoimmune disease symptoms and T cell alterations were prevented in B6-wt but not T cell deficient recipients. Interestingly, the failure to transfer autoimmune disease following IL-2Rβ ^−/− HCT into lethally conditioned B6-CD4^−/− recipients was associated with the presence of a peripheral CD8⁺FoxP3⁺ population not detected in B6-wt mice or the B6-wt mice transplanted with IL-2Rβ ^−/− BM. This finding indicates that in the genetic absence of CD4⁺ T cells, a CD8 regulatory population appears to 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 conventional T cell populations - even in aggressively conditioned recipients. Finally, these findings imply that the presence of host regulatory cells may be important to consider with respect to eliciting anti-tumor responses and vaccination in recipients during the early period post -hematopoietic cell transplantation.