Paracrine- and cell-contact-mediated immunomodulatory effects of human periodontal ligament-derived mesenchymal stromal cells on CD4⁺ T lymphocytes

Mesenchymal stromal cells (MSCs) isolated from the periodontal ligament (hPDL-MSCs) have a high therapeutic potential, presumably due to their immunomodulatory properties. The interaction between hPDL-MSCs and immune cells is reciprocal and executed by diverse cytokine-triggered paracrine and direct cell-to-cell contact mechanisms. For the first time, this study aimed to directly compare the contribution of various mechanisms on this reciprocal interaction using different in vitro co-culture models at different inflammatory milieus.

Three co-culture models were used: indirect with 0.4 μm-pored insert, and direct with or without insert. After five days of co-culturing mitogen-activated CD4⁺ T lymphocytes with untreated, interleukin (IL)-1β, or tumor necrosis factor (TNF)-α- treated hPDL-MSCs, the CD4⁺ T lymphocyte proliferation, viability, and cytokine secretion were investigated. The gene expression of soluble and membrane-bound immunomediators was investigated in the co-cultured hPDL-MSCs.

Untreated hPDL-MSCs decreased the CD4⁺ T lymphocyte proliferation and viability more effectively in the direct co-culture models. The direct co-culture model without inserts showed a strikingly higher CD4⁺ T lymphocyte cell death rate. Adding IL-1β to the co-culture models resulted in substantial CD4⁺ T lymphocyte response alterations, whereas adding TNF resulted in only moderate effects. The most changes in CD4⁺ T lymphocyte parameters upon the addition of IL-1β or TNF-α in a direct co-culture model without insert were qualitatively different from those observed in two other models. Additionally, the co-culture models caused variability in the immunomediator gene expression in untreated and cytokine-triggered hPDL-MSCs.

These results suggest that both paracrine and cell-to-cell contact mechanisms contribute to the reciprocal interaction between hPDL-MSCs and CD4⁺ T lymphocytes. The inflammatory environment affects each of these mechanisms, which depends on the type of cytokines used for the activation of MSCs’ immunomodulatory activities. This fact should be considered by comparing the outcomes of the different models.