Contribution of Soluble Forms of Programmed Death 1 and Programmed Death Ligand 2 to Disease Severity and Progression in Systemic Sclerosis

Objective: To determine the function and serum levels of soluble forms of programmed death 1 (sPD-1) and one of its ligands, soluble PD ligand 2 (sPD-L2), in patients with systemic sclerosis (SSc) and in a mouse model of topoisomerase I (topo I)–induced SSc. Methods: Serum levels of sPD-1 and sPD-L2 in 91 patients with SSc were examined by enzyme-linked immunosorbent assay (ELISA). Expression of PD-1 and PD-L2 on T cells, B cells, and macrophages was quantified by flow cytometry. The effects of blockade of PD-1 and PD-L2 were analyzed by microfluidic ELISA (micro-ELISA), a technique that can measure very low amounts of cytokines. In addition, the effects of sPD-1 and sPD-L2 on disease progression were assessed in mice with topo I–induced SSc. Results: Serum levels of sPD-1 and sPD-L2 were elevated in patients with SSc and correlated with the extent of fibrosis and immunologic abnormalities. Expression levels of PD-1 and PD-L2 were significantly elevated on SSc T cells, B cells, and macrophages. Micro-ELISA analysis of serum samples from patients with SSc showed that PD-L2^high B cells had higher levels of interleukin-10 (IL-10) production compared with PD-L2^low B cells, indicating that PD-L2 acts as a regulator of T cell cytokine production via cognate interactions with T cells and B cells. In mice with topo I–induced SSc, production of IL-10 by topo I–specific B cells in cultures with T cells and topo I protein was significantly higher than that by conventional B cells, and intraperitoneal injection of recombinant chimeric PD-1-Fc and PD-L2-Fc canceled these enhanced effects. Conclusion: These results suggest that sPD-1 and sPD-L2 contribute to disease development in SSc via the regulation of cognate interactions with T cells and B cells.