IDO-mediated tryptophan degradation can act as T-cell inhibitory effector pathway in human MSCs. (A) Cocultures of MSCs and MLRs exhibit IDO activity. Supernatants from 4-day cultures of MSCs alone, MLRs alone, or MLR/MSC coculture were harvested and set up for analysis of IDO activity in triplicates. Background OD was not subtracted to allow for comparison of IDO activity of MLRs and MSCs alone. ODs are shown as averaged means of triplicates ± SEM obtained from a total of 5 experiments using MSCs from 4 different donors. ^*P value of difference as calculated by Student t test. (B) Addition of tryptophan restores T-cell proliferation in MLR/MSC cocultures. MLR cultures were set up in presence (+MSC) or absence of 2 × 10⁴ MSCs. Proliferation is depicted relative to the proliferation of MLR cultures in the absence of MSCs (100%). In parallel experiments, 100 μg/mL tryptophan was added (+Tryp) at the time of initiation and at day 3 of MLR culture or was left out. Relative proliferation is depicted as averaged means of triplicates ± SEM from a series of 5 experiments using MSCs from 4 individual donors. ^*P value of difference as calculated by Student t test.