Abstract
Although immune checkpoint molecules regulate the progression of certain cancers, their significance in malignant development of Waldenstrom macroglobulinemia (WM), an incurable low-grade B-cell lymphoma, remains unknown. Recently, cytokines in the bone marrow (BM) microenvironment are shown to contribute to the pathobiology of WM. Here, we investigated the impact of cytokines, including interleukin-6 (IL-6) and IL-21, on immune regulation and particularly on the programmed death-1 (PD-1) and its ligands PD-L1 and PD-L2. We showed that IL-21, interferon γ, and IL-6 significantly induced PD-L1 and PD-L2 gene expression in WM cell lines. Increased PD-L1 and PD-L2 messenger RNA was also detected in patients' BM cells. Patients' nonmalignant BM cells, including T cells and monocytes, showed increased PD-L1, but minimal or undetectable PD-L2 surface expression. There was also very modest PD-L1 and PD-L2 surface expression by malignant WM cells, suggesting that ligands are cleaved from the cell surface. Levels of soluble ligands were higher in patients' BM plasma and blood serum than controls. Furthermore, IL-21 and IL-6 increased secreted PD-L1 in the culture media of WM cell lines, implying that elevated levels of soluble PD-1 ligands are cytokine mediated. Soluble PD-1 ligands reduced T-cell proliferation, phosphorylated extracellular signal-regulated kinase and cyclin A levels, mitochondrial adenosine triphosphate production, and spare respiratory capacity. In conclusion, we identify that soluble PD-1 ligands are elevated in WM patients and, in addition to surface-bound ligands in WM BM, could regulate T-cell function. Given the capability of secreted forms to be bioactive at distant sites, soluble PD-1 ligands have the potential to promote disease progression in WM.