Introduction: FoxP3+ regulatory T cells (Tregs) fine tune the immune system by controlling the initial immune response to inflammation and preventing autoimmunity. It has been shown that Tregs enter and accumulate in tissue, likely due to the need to regulate the interface between the tissue immune environment and commensal microbiota. Identifying Tregs by the intracellular marker, FoxP3, requires permeabilization and fixation, precluding them from functional studies. It has been found that CD127-CD25hi cells were enriched for a FoxP3-expressing immunosuppressive subset in human blood (Liu et al., 2006). In mice, IL-7 was identified to play a role in Treg maintenance in tissue, and CD127 (IL-7 receptor) expression on FoxP3+ cells was observed (Gratz et al., 2013). However, CD127+ was not found on FoxP3+ cells in human skin tissue (Sanchez Rodriguez et al., 2014). This is inconsistent with the hypothesis that human Tregs maintain long-term residency. Therefore, we interrogated the residency phenotype of FoxP3+ Tregs in human tissue.
Methods: We designed a high parameter flow cytometry panel to overcome non-specific cleavage of surface proteins that occurs during cell liberation from tissue via enzymatic digestions. We applied this panel to human genital skin and type II tissues to characterise Tregs across genital mucosa.
Results: We found that the commonly used CD127 clone hIL-7R-M21 is susceptible to enzymatic cleavage, in contrast to the R34.34 clone, where we were able to detect CD127. As such, we found that FoxP3+ CD4+ T cells liberated from human tissues do express CD127 in a dichotomous pattern with TIGIT. Phenotypically, tissue CD127+ Tregs express a signature similar to resident T cells, whereas TIGIT+ Tregs were activated and resembled non-resident cells.
Conclusion: We identified two Treg populations. CD127+ Tregs in tissue phenotypically resemble a tissue resident cell, and contrast in protein signature to the TIGIT+ Tregs, which resemble non-resident cells.