The VTCN1 (B7-H4) gene located in chromosome 1p13.1, consists of six exons and five introns and the coding region spans 849 bp. The mature protein is coded by the exons 3, 4, and part 5 while exons 1 and 2 encodes a signal peptide. The IgV-IgC domain, comprised of the extracellular region, is coded by exons 3, 4 and parts of 5 (Chen L. et al. 2003).

B7-H4 mRNA can be detected in many tissues including placenta, kidney, liver, lung, ovary, testis and spleen. There are two transcripts of B7-H4 and both transcripts share complete homology with exons 1 to 5 in the full length B7-H4 gene. The smaller transcript of the two, generated by alternative splicing, lacks part of exon 6 (Chen L. et al. 2003).

A possible B7-H4 pseudogene has a single exon with 94% similar nucleotide sequence identity to the cDNA of B-7H4 and is located in chromosome 20p11.1 (Chen L. et al. 2003).

The predicted 282-amino acid B7-H4 protein contains a 2-amino acid intracellular domain, a large hydrophobic type 1 transmembrane domain and an extracellular domain (Prasad et al. 2003).

Prasad et al. (2003) showed that B7-H4 is expressed in professional antigen presenting cells. Although B7-H4 is overexpressed in several human cancers including ovary, endometrium, lung and kidney, its expression is limited in normal tissues. Shroyer et al. (2005) showed that there is a limited focal expression of B7-H4 by immunohistochemistry in several normal human tissues including fallopian tubes, endometrial glands, pancreas, larynx, lung, kidney and urinary bladder.

B7-H4 is localized to the cell surface and cytoplasm of epithelial cells and macrophages. Expression in benign glandular cells (ductal epithelium in breast and pancreas) is localized to the apical cell surface but there is circumferential membranous localization in B7-H4 positive tumor cells.

Published data shows that B7-H4 functions as a negative regulator of T cell responses and it negatively regulates the T cell immunity by the inhibition of T cell proliferation, cytokine production and cell cycle progression. Prasad et al. (2003) reported that B7S1/B7-H4 is expressed on professional antigen presenting cells, binds to its putative receptor on activated T cells, and inhibits T cell activation and IL2 production. Sica G.L..et al (2003) also reported that B7-H4 inhibits T cell activation and the production of both IL2 and IL10. They further showed that B7-H4 inhibits the induction of Cytolytic T Lymphocytes (CTL) in vitro and it also arrests cell cycle of T cells in G0/G1 phase.
B7-H4 may also play a role in tumor biology by providing tumors with a protective mechanism to escape from immune surveillance. Several human cancers such ovary, endometrium, breast, kidney and lung (non small cell) are known to overexpress B-7H4 and the level of B7-H4 expression in these tumors has been correlated to the number of tumor-associated and tumor-infiltrating T cells. Papkoff J. et al. (2005) found that overexpressed B7-H4 promotes epithelial cell transformation by protecting cells from apoptosis and a siRNA knockout of B7-H4 in tumor cell lines lead to an increased apoptosis. Kryczek et al. (2006) reported that primary ovarian tumor cells express exclusively intracellular B7-H4 protein, whereas the majority of ovarian tumor macrophages, but not tumor T cells or blood macrophages, express surface B7-H4, possibly by stimulation with tumor-associated IL6 and IL10. They also showed that B7-H4 expressing tumor macrophages suppressed HER2 specific T-cell proliferation and cytotoxicity. Further, the blocking of B7-H4 expression with specific oligonucleotides improved the tumor-associated antigen T-cell responses. They concluded that B7-H4 expressing tumor macrophages are a suppressive cell population in ovarian cancer and might prove to be a good therapeutic target.

B7-H4 shares a 24%-31% homology with other members of the B7 family and has the highest homology with B7H3 with 31% homology (Chen L. et al. 2003).