XmAb5574 binds to cynomolgus monkey CD19 and FcγRs. (A) Binding to a cynomolgus monkey B-cell line was measured with a competitive binding assay. XmAb5574, anti-CD19 IgG1, or XmAb isotype control was used to cross-block binding of 1 μg/mL fluorescently conjugated antihuman CD19 monoclonal antibody (clone J3-119). MFI indicates mean fluorescence intensity (in arbitrary units). Results are mean (± range) for duplicate binding curves from a single run and are representative of 4 independent experiments. XmAb5574 and anti-CD19 IgG1 bind equally to cynomolgus monkey CD19. (B) Binding to cynomolgus monkey peripheral blood lymphocytes was assessed using whole blood samples from 5 individual animals. Blood samples were treated with fluorescently conjugated versions of XmAb5574, anti-CD19 IgG1, and anti-CD20 and analyzed by flow cytometry. XmAb5574 and anti-CD19 IgG1 bind CD20⁺ lymphocytes. (C) Binding to cynomolgus monkey FcγRs was determined using Biacore analysis, and affinities for each FcγR were plotted as the log of the affinity binding constant K_a. Results are the mean (± range) of duplicate experiments and demonstrate that XmAb5574 has increased affinity to all FcγRs relative to anti-CD19 IgG1; enhancements ranged from 3- to 60-fold, with the largest increase seen for FcγRIIIa.