The pathogenic impact of tumor-infiltrating B cells is unresolved at present, however, some studies suggest that they may have immune regulatory potential. Here, we report that the microenvironment of various solid tumors includes B cells that express granzyme B (GrB, GZMB), where these B cells can be found adjacent to interleukin (IL)-21-secreting regulatory T cells (Treg) that contribute to immune tolerance of tumor antigens. Because Tregs and plasmacytoid dendritic cells are known to modulate T-effector cells by a GrB-dependent mechanism, we hypothesized that a similar process may operate to modulate regulatory B cells (Breg). IL-21 induced outgrowth of B cells expressing high levels of GrB, which thereby limited T-cell proliferation by a GrB-dependent degradation of the T-cell receptor z-chain. Mechanistic investigations into how IL-21 induced GrB expression in B cells to confer Breg function revealed a CD19

Background aimsThe clinical use of human mesenchymal stromal cells (MSC) requires ex vivo expansion in media containing supplements such as fetal bovine serum or, alternatively, human platelet lysate (PL).MethodsPlatelet concentrates were frozen, quarantine stored, thawed and sterile filtered to obtain PL. PL content and its effect on fibroblast-colony-forming unit (CFU-F) formation, MSC proliferation and large-scale expansion were studied.ResultsPL contained high levels of basic fibroblast growth factor (bFGF), soluble CD40L (sCD40L), vascular cell adhesion molecule-1 (VCAM-1), intercellular adhesion molecule-1 (ICAM-1), platelet-derived growth factor AA (PDGF-AA), platelet-derived growth factor AB/BB (PDGF-AB/BB), chemokine (C-C) ligand 5 (CCL5; RANTES) transforming growth factor-β1 (TGF-β1) and chemokine (C-X-C) ligand 1/2/3 (GRO), with low batch-to-batch variability, and most were stable for up to 14 days. Inhibition of PDGF-BB and bFGF decreased MSC proliferation by about 20% and 50%, respectively. The strongest inhibition (about 75%) was observed with a combination of anti-bFGF + anti-PDGF-BB and anti-bFGF + anti-TGF-β1 + anti-PDGF-BB. Interestingly, various combinations of recombinant PDGF-BB, bFGF and TGF-β1 were not sufficient to promote cell proliferation. PL from whole blood-derived pooled platelet concentrates and apheresis platelet concentrates did not differ significantly in their growth-promoting activity on MSC.ConclusionsPL enhances MSC proliferation and can be regarded as a safe tool for MSC expansion for clinical purposes. \in particular, PDGF-BB and bFGF are essential components for the growth-promoting effect of PL, but are not sufficient for MSC proliferation.