PDL241 binds human leukocytes and inhibits human Ig production in huSCID mice. NSG mice were transfused with human PBMC. (A) Double staining of CD319 (green fluorescence) and human leukocyte cell surface markers as indicated (red fluorescence) in the spleen. Cells expressing both markers were co-stained and appear yellow. 600X magnification. (B) Mice were sorted into groups on day 10 post-transfusion, and dosed twice weekly with PDL241 or cIgG1. IgM levels in the serum were measured at day 32 and day 47. In this experiment, PDL241 treatment resulted in significant reduction of human IgM compared to cIgG1 (day 32 P = 0.003 and day 47 P = 0.004 two tailed t-test). Significant reduction in IgM was observed in 6 of 11 separate experiments. huSCID, human-severe combined immunodeficiency; Ig, immunoglobulin; PBMC, peripheral blood mononuclear cells.