Human Sn(d1-4)Fc binds specifically to granulocytes, monocytes, and a subset of lymphocytes/NK cells.

(A) Blood leukocytes were either untreated (top panels) or treated with sialidase (bottom panels) and then labeled with Fc proteins followed by FITC-antihuman IgG to detect Fc protein binding. FACS histograms show staining of granulocytes, monocytes, and lymphocytes/NK cells with either hSn(d1-4)Fc (solid lines) or with the control protein, NCAM-Fc (dotted lines). The background labeling with NCAM-Fc is likely to be due to nonspecific Fc receptor binding. The population of granulocytes with lower fluorescence following staining with hSn(d1-4)Fc corresponds to eosinophils, as determined by double labeling for CD16 found at high levels on neutrophils and low levels on eosinophils (not shown). Following sialidase treatment, binding of hSn(d1-4)Fc is reduced to the background levels seen with NCAM-Fc for all cell populations. (B) Double labeling of the lymphocyte/NK cell population with NCAM-Fc or hSn(d1-4)Fc together with the indicated mAbs. The percentages of cells in the upper quadrants are indicated. This analysis shows that Sn stains a subset of CD3⁺ T cells, most of which are CD8^high T cells. The CD8^dim cells correspond to NK cells that are labeled nonspecifically by NCAM-Fc but show additional staining with hSn(d1-4)Fc. Most CD19⁺ B cells and most CD16⁺ NK cells were also labeled. The majority of recently activated CD69⁺ lymphocytes were also stained by hSn(d1-4)Fc. Similar results were obtained in 3 independent experiments.