CD4⁺CD25⁺Foxp3⁺ Regulatory T Cell Function Outside the Immune System: Differential Regulation of Hematopoietic Progenitor Cell Populations.

Naturally occurring CD4⁺CD25⁺Foxp3⁺ regulatory T (Treg) cells are responsible for a physiologically essential peripheral mechanism to maintain self tolerance. The suppressor capacity of Treg cells has been found to extend to various compartments within the host and includes the ability to modulate adaptive and innate immune responses. Interestingly, several Treg cytokines which mediate immune regulation concomitantly possess hematopoietic modulating activity. For example, transforming growth factor beta 1 (TGF-beta1) and interleukin 9 (IL-9) have been found to have opposing effects during early and late stages of myelopoiesis. We have hypothesized that the inhibition of CFU-GM observed after a co-culture period of 1–3 days with activated Tregs occurs in a contact dependent manner through a mechanism involving TGF-beta1 on the surface of the regulatory cells. Additionally, we found that Treg cells fail to inhibit CFU-GM activity from MHC class II deficient bone marrow (BM) in vitro as well as in vivo following bone marrow transplantation further supporting the notion that cognate interaction is required for this negative regulation to take place. Studies then examined Treg modulation of erythroid progenitor cells (PC). IL-9 is a T cell growth factor known to promote both growth and survival signals possessing synergistic activity together with SCF and Epo which induces enhancement of colony forming activity by erythroid PC. Anti-CD3/CD28+IL-2 activated Treg cells co-cultured with syngeneic unfractionated BM as well as lineage depleted populations resulted in a 2–3X augmentation of CFU-E vs. control cultures lacking Tegs. To examine a role for IL-9 in these observations, anti-IL-9 specific mab was added to Treg/BMC co-cultures. This antibody but not isotype control Ig abolished the enhancement of CFU-E activity. Moreover, although Tregs obtained from IL-9 deficient mice inhibited CFU-GM activity, these cells failed to enhance CFU-E. These findings indicated that IL-9 produced by Tregs is crucial for such activity and supernatants from activated Treg cultures were found to contain IL-9. Finally, Treg augmentation of CFU-E in transwell cultures using B6-wt and B6-Class II^−/− lineage depleted populations showed that this enhancing activity:

• was not contact dependent and

• did not require MHC class II expression on the targeted populations.

These findings reveal two distinct pathways of regulation, i.e. inhibition and enhancement as defined by cytokines, contact dependency and an MHC class II requirement. Current studies examining apoptosis and the blockade /induction of differentiation have found that Treg cells simultaneously lacking perforin and fasl are capable of mediating differential regulation indicating neither of these lytic pathways is required. In total, the present results demonstrate for the first time that Tregs can modulate responses outside the adaptive and innate immune systems.