Cell-surface expression of the RNA binding protein U5 snRNP200 on acute myeloid leukemia and other tumor cells is independent of FcγR-mediated interactions

The development of novel immunotherapeutics for acute myeloid leukemia (AML) is hampered by the limited knowledge of tumor-expressed targets absent on non-malignant cells. We recently identified the RNA binding protein U5 small nuclear ribonucleoprotein 200 kDa (U5 snRNP200) as a novel antigen that is overexpressed on AML blasts in 50% of cases. As a non-canonical membrane protein lacking a transmembrane domain, a cofactor is required for surface expression. Previous studies suggested the Fc gamma receptor (FcγR) IIa (CD32a) as a cofactor. Here we show that solid tumor cell lines, especially SKOV3 (ovarian carcinoma), express U5 snRNP200 in the absence of FcγRIIa. Moreover, CRISPR-Cas9 induced deletion of FcγRIIa from AML-derived THP-1 cells had no impact on U5 snRNP200 expression. However, binding of the U5 snRNP200 antibodies was enhanced when FcγRII or FcγRI were expressed, suggesting that U5 snRNP200 antibody binding is stabilized by co-binding to FcγR. Antibody binding was markedly reduced using Fab’ or F(ab’) 2 formats, Fc-silent antibodies, FcγR deficient cells, or FcγR-blocking antibodies. Other cell surface expressed RNA-binding proteins are tethered to the membrane via heparan sulfate, and reducing heparan sulfate expression decreased U5 snRNP200 expression. Together, these data demonstrate that cell surface expression of the U5 snRNP200 complex by tumor cells is independent of FcγRIIa co-expression, and suggest that cell surface expressed U5 snRNP200 may be tethered to the tumor membrane by heparan sulfate.