Siglec-7 and Siglec-9 ligands, and their concomitant receptors, are expressed by MM and NK cells, respectively. Using recombinant Siglec-7 and Siglec-9 chimeras, primary MM cells isolated from BMAs supplied by MGUS and newly diagnosed (ND) patients (A) and a panel of commonly used MM cell lines (B) were stained for Siglec-7L and Siglec-9L expression. (C) Using a fluorescently labeled anti–Siglec-7 antibody primary NK cells from BMAs of patients with MM were screened for the expression of Siglec-7. (D) Primary NK cells (IL-2 activated and expanded) were stained for Siglec-7 and Siglec-9 expression. (E) To elucidate the identity of Siglec-7L in MM, mass spectrometry was carried out on proteins bound to Siglec-7 Fc chimera-magnetic bead complexes after incubation with MM1S, H929, and JJN3 cell lysates untreated or treated with neuraminidase. (A-D) Combined data represented as bar graphs and an representative dot blot from one individual MM BMA sample or biological repeat. Data in E are represented as a volcano plot. n = 8 independent samples for ND. (A) n = 3 for MGUS. (B) n = 3 biological replicates. (C) n = 11; (D) n = 7; (E) n = 3.