The effect of platelet lysate fibrinogen on the functionality of MSCs in immunotherapy

Human platelet lysate (PL) represents an attractive alternative to fetal bovine serum (FBS) for the ex vivo expansion of human mesenchymal stromal cells (MSCs). However, there is controversy whether MSCs propagated in unfractionated PL retain their immunosuppressive properties. Since fibrinogen can be a major component of PL, we hypothesized that the fibrinogen content in PL negatively affects the suppressor function of MSCs. Pools of outdated plateletpheresis products underwent a double freeze–thaw centrifugation and filtration to produce unfractionated platelet lysates (uPL), followed by a temperature controlled clotting procedure to produce a fibrinogen depleted platelet lysate (fdPL). Fibrinogen depletion affected neither the mitogenic properties of PL or growth factor content, however fdPL was less prone to develop precipitate over time. Functionally, fibrinogen interacted directly with MSCs, dose dependently increased IL-6, IL-8 and MCP-1 protein production, and compromised the ability of MSCs to up-regulate indoleamine dioxygenase (IDO), as well as, mitigate T-cell proliferation. Similarly uPL expanded MSCs showed a reduced capability of inducing IDO and suppressing T-cell proliferation compared to FBS expanded MSCs. Replacing uPL with fdPL largely restored the immune modulating effects of MSCs. Together these data suggest that fibrinogen negatively affects the immunomodulatory functions of MSCs and fdPL can serve as non-xenogenic mitogenic supplement for expansion of clinical grade MSCs for immune modulation.