Does the B7-H3 Immune Checkpoint Have High Potential as a Therapeutic Target?

B7-H3 (CD276), a member of the B7 family of proteins, is known to play a key role in the progression of a number of cancers. This protein is selectively expressed in both tumor cells and immune cells within the tumor microenvironment. Various investigations, including a number of clinical trials, have reported high levels of expression of this protein in cancerous tissues compared to their healthy counterparts. This difference in expression attracted various research efforts to establish whether such a difference can be linked to the therapeutic potential of this molecule. It is worth noting that B7-H3 is not the only immune checkpoint expressed at different levels in cancerous and healthy cells. Therapeutic strategies, based on different levels of expression, have been tested with other checkpoints. To inhibit the expression of some checkpoints, immune checkpoint inhibitors (ICIs) were developed. The introduction of these inhibitors for the treatment of some forms of advanced-stage tumors has been justly described as an important milestone in the landscape of immune therapy. Years after the launch of these inhibitors, numerous clinical trials revealed that these inhibitors benefit a narrow subset of patients suffering from advanced-stage tumors, while the majority of patients treated with these inhibitors either did not respond positively or simply did not respond at all (refractory patients). Other clinical trials showed that this form of treatment can provoke serious immune-related adverse events (irAEs). It is fair to state that changes in the expression level of a given protein in diseased tissue is an important parameter to take into account in the assessment of such a protein as a therapeutic target. However, the last ten years have demonstrated that taking the level of expression of a given checkpoint within a cancerous tissue is not sufficient to consider such expression a reliable predictive biomarker for the investigated disease. On the other hand, to establish a solid base for a given therapeutic strategy, these varying levels of expression have to be combined with a deep understanding of the biology of the molecule under investigation, as well as the identification and thorough analysis of the relevant signaling pathways, particularly those communicating with both the investigated molecule and the immune system. Recently, a number of pharmaceutical and biotechnology firms have suggested that B7-H3 is a highly promising therapeutic target for the development of immune therapeutics. In this review, we ask why hopes of better therapeutic performance are attached to this immune checkpoint. A partial answer to this question is provided through the careful consideration of the available data generated by various clinical trials. The contribution of mass spectrometry-based proteomics to this area of research is highlighted.