Suppression by Allogeneic-Specific Regulatory T Cells Is Dependent on the Degree of HLA Compatibility

Regulatory T cell (Treg) infusion for graft-versus-host disease treatment has been increasingly investigated. However, polyclonal Treg may suppress the desired graft-versus-leukemia effect. Although allogeneic-specific (allo-specific) Treg may provide a more-targeted graft-versus-host disease treatment, there is the need to develop easily translatable expansion protocols and to better characterize their specificity and mechanisms of suppression. In this article, we provide a robust protocol for human allo-specific Treg expansion and characterize their phenotype, potency, and specificity of suppression by testing different expansion conditions and suppression assay milieus. We found that higher concentrations of IL-2 during expansion with allogeneic APC yielded allo-specific Treg that were more-potent suppressors and displayed a more activated phenotype. Although responses to the same APC present during expansion were the most suppressed, responses to third-party APC partially matched to the expansion APC were still significantly more suppressed than responses to fully mismatched APC. Furthermore, suppression of responses to the expansion APC was strictly contact dependent, whereas suppression of responses to mismatched APC was partially independent of contact. Finally, distinct subsets in fresh and expanded Treg could be described using multidimensional visualization techniques. We propose that allo-specific Treg are HLA specific and that the mechanisms of suppression elicited depend on their compatibility with the stimulators.