The 2Ig isoform contains one IgV-IgC domain pair and is the predominant form in mice. The 4Ig isoform contains a tandem duplication (two IgV-IgC pairs) and is the dominant form in humans (~100 kDa vs ~45 kDa). Most therapeutic anti-B7-H3 antibodies in clinical trials are raised against the 4Ig human isoform. When selecting reagents for preclinical work, ensure the protein isoform matches your experimental species — use 4Ig for human studies and 2Ig for mouse syngeneic tumor models.

B7-H3 has several properties that favor ADC development: (1) high and homogeneous tumor surface expression across many solid tumor types, (2) limited expression on normal tissues (reducing off-target toxicity), (3) efficient receptor-mediated internalization upon antibody binding (essential for intracellular payload delivery), and (4) expression on tumor vasculature in addition to tumor cells (dual targeting effect). PD-L1, while broadly expressed on tumors, is also expressed on normal immune cells and tissues, increasing the risk of on-target off-tumor toxicity for ADC approaches.

The cognate receptor for B7-H3 on T cells has not been definitively identified, which is unusual for a B7 family member. TREM-like transcript 2 (TLT-2 / TREML2) was initially proposed as a receptor but this finding has been disputed. Several other candidate receptors have been suggested, including interleukin-20 receptor subunit alpha (IL-20Rα). This unresolved receptor biology means that most therapeutic approaches to B7-H3 (ADCs, CAR-T) exploit its tumor expression pattern rather than its signaling function.

B7-H3 is overexpressed in a wide range of solid tumors, with particularly high expression reported in prostate cancer, NSCLC, ovarian cancer, neuroblastoma, glioblastoma, triple-negative breast cancer, and pancreatic ductal adenocarcinoma. In pediatric cancers, B7-H3 expression is especially prominent — Omburtamab (anti-B7-H3 8H9 antibody) was developed for CNS neuroblastoma. B7-H3 is also expressed on tumor-associated vasculature, which contributes to the therapeutic window.

For FFPE sections, use an anti-B7-H3 monoclonal antibody with heat-induced epitope retrieval (citrate pH 6.0 or Tris-EDTA pH 9.0 — optimize for your tissue type). B7-H3 staining is typically membranous and cytoplasmic. Include a known B7-H3-positive tumor section (prostate or neuroblastoma) as positive control. For dual staining with immune cell markers, combine with CD8 or CD68 to correlate B7-H3 expression with T-cell or macrophage infiltration.