4IgB7-H3 is the major isoform expressed on immunocytes as well as malignant cells.

Human B7-H3, a novel member of B7 family, has two isoforms (2IgB7-H3 and 4IgB7-H3). As costimulatory functions of both isoforms are not clarified, there has been much discussion on their expression patterns, T-cell responses, etc. This study generated two specific mouse anti-human 2IgB7-H3 monoclonal antibodies (mAbs) (7D7 and 10F1), whose specificities are quite different from those of the available B7-H3 mAb (21D4) by competition assay. The use of antibodies indicated that B7-H3 was found to have different expression patterns on monocyte-derived dendritic cells, that is the isoform 2IgB7-H3 is tended to express on immature dendritic cells (iDCs), whereas 4IgB7-H3 could be detected in the whole process of maturation of dendritic cells. The isoform 4IgB7-H3 was shown in the immunohistochemistry assay to be more widely expressed than 2IgB7-H3 in human benign and malignant hepatic tissues. Furthermore, flow cytometry and reverse transcription-polymerase chain reaction indicated that 4IgB7-H3 rather than 2IgB7-H3 was the major isoform on many human tumor cell lines. In addition, both 2IgB7-H3- and 4IgB7-H3-transfected cells could promote T-cell proliferation, which could be blocked by 7D7 mAb. These two novel antibodies may shed light on the function of the two B7-H3 isoforms.