Interaction of the Class A Scavenger Receptor with Multiple Myeloma Cells

Abstract

Tumor‐associated macrophages (TAMs) play a central role in regulating the immune response within the cancer microenvironment. TAMs express Class A Scavenger Receptors (SR‐A), which are pattern recognition receptors that bind a wide array of ligands. SR‐A expression by TAMs is often associated with an immune‐suppressive macrophage phenotype. Multiple myeloma (MM) is a malignant disease of the bone marrow characterized by the proliferation of differentiated plasma cells. The disease progresses from asymptomatic monoclonal gammopathy of undetermined significance (MGUS) to smoldering myeloma (SMM) and finally to symptomatic MM. In many solid tumors, the number of SR‐A‐expressing TAMs correlates with tumor aggressiveness and poor prognosis. Although SR‐A‐expressing macrophages are abundant in the bone marrow, the importance of SR‐A in the progression of MM is unknown. All patient samples for the study were collected under IRB approved protocol 260665. To determine if SR‐A may be important in MM, we utilized a multiplexed immunofluorescence (IF) approach to localize SR‐A‐expressing macrophages within the MM bone marrow microenvironment. Macrophages were detected using CD163 and SR‐A antibodies and plasma cells were identified using CD138 antibody. Multiplexed‐IF staining showed that SR‐A‐positive macrophages were localized near MM cells. This was especially evident in tumor‐rich regions where SR‐A‐expressing TAMs displayed a distinct branched morphology. SR‐A binding to ligand induces macrophage attachment and spreading. Thus, we hypothesized that the localization of SR‐A‐expressing TAMs near MM cells results from SR‐A binding to a ligand expressed on these cells. We tested this possibility using a flow cytometry‐based assay to measure the binding of a soluble SR‐A (sSR‐A) protein with MM cell lines and plasma cells obtained from bone marrow aspirates of MM patients. sSR‐A was able to bind to all six different MM cell lines that were examined. In bone marrow aspirates (n=15), sSR‐A bound to neoplastic plasma cells (CD138+CD45‐) but not to non‐plasma cells (CD45+). sSR‐A binding to neoplastic plasma cells was confirmed using enriched plasma cells isolated from bone marrow aspirates using anti‐CD138 magnetic beads. sSR‐A binding to plasma cells was abolished by the SR‐A ligand, fucoidan, demonstrating specificity. Together, our results demonstrate that SR‐A is expressed on TAMs within the MM microenvironment and binds to MM cells. SR‐A binding to MM cells may be a mechanism for regulating TAM localization and function in the tumor microenvironment.