A novel regulation of PD-1 ligands on mesenchymal stromal cells through MMP-mediated proteolytic cleavage

ABSTRACTWhether fibroblasts regulate immune response is a crucial issue in the modulation of inflammatory responses. Herein, we demonstrate that foreskin fibroblasts (FFs) potently inhibit CD3+ T cell proliferation through a mechanism involving early apoptosis of activated T cells. Using blocking antibodies, we demonstrate that the inhibition of T cell proliferation occurs through cell-to-cell interactions implicating PD-1 receptor expressed on T cells and its ligands, PD-L1 and PD-L2, on fibroblasts. Dual PD-1 ligand neutralization is required to abrogate (i) binding of the PD-1-Fc fusion protein, (ii) early apoptosis of T cells, and (iii) inhibition of T cell proliferation. Of utmost importance, we provide the first evidence that PD-1 ligand expression is regulated through proteolytic cleavage by endogenous matrix metalloproteinases (MMPs) without transcriptional alteration during culture-time. Using (i) different purified enzymatic activities, (ii) MMP-specific inhibitors, and (iii) recombinant human M...