Tryptophan concentration is the main mediator of the capacity of adipose mesenchymal stromal cells to inhibit T-lymphocyte proliferation in vitro.

Mesenchymal stromal cells (MSCs) have immunomodulatory properties that are mediated by cell-to-cell interactions and paracrine effects through soluble factors, among which tryptophan (Trp) conversion into kynurenine (Kyn) through the enzymatic activity of indoleamine 2,3-dioxygenase has been proven to be of special relevance. However, the respective role of Trp depletion and/or Kyn accumulation on the inhibition of T-cell proliferation by MSCs remains unclear. The effect of supplementation with increasing concentrations of Trp on the capacity of MSCs to inhibit T-lymphocyte proliferation in vitro was investigated. We report that Trp supplementation impairs the capacity of adipose mesenchymal stromal cells (ASCs) to inhibit T-cell proliferation, despite the accumulation of very high concentrations of Kyn in the medium (>200 μmol/L). Moreover, Trp supplementation after 72 h of peripheral blood mononuclear cell:ASC co-culture, once the inhibitory effect of ASCs was established, reverted ASC inhibition and restored T-cell proliferation. Addition to stimulated lymphocytes of Kyn inhibited T proliferation in 3 of 10 peripheral blood mononuclear cell donors, but at different concentrations, suggesting that sensitivity of lymphocytes to Kyn might be donor-dependent. Our results confirm the relevance of Trp metabolism as a key mediator of the immunomodulatory properties of ASCs and clarify the respective roles of the Trp/Kyn balance.