Multiplexed Spatial Imaging at the Single-Cell Level Reveals Mutually Exclusive Expression of B7 Family Proteins.

Targeting novel inhibitory ligands beyond anti-PD-1 and PD-L1 and CTLA-4 therapies is essential for the next decade of the immunotherapy era. Agents for the B7 family molecules B7-H3, B7-H4, and B7-H5 are emerging in clinical trial phases; therefore, further accumulation of evidence from both clinical and basic aspects is vital. Here, we applied a 7-color multiplexed imaging technique to analyze the profile of B7 family B7-H3/B7-H4/B7-H5 expression, in addition to PD-L1, and the spatial characteristics of immune cell infiltrates in urothelial carcinoma (UC). The results revealed that B7-H3 and B7-H4 were mainly expressed on tumor cells and B7-H5 on immune cells in UC, and most of the B7-H3/B7-H4/B7-H5-positive cells were mutually exclusive with PD-L1-positive cells. Also, the expression of B7-H4 was elevated in patients with advanced pathologic stages, and high B7-H4 expression was a significant factor affecting overall mortality following surgery in UC. Furthermore, spatial analysis revealed that the distance from the B7-H4 cells to the nearest CD8 cells was markedly far compared with other B7 family-positive tumor cells. Interestingly, the distance from B7-H4 cells to the nearest CD8 cells was significantly farther in patients dying from cancer after surgery or immune checkpoint inhibitors compared with cancer survivors; thus, high B7-H4 expression in tumor cells may inhibit CD8 infiltration into the tumor space and that B7-H4-positive cells form a specific spatial niche. In summary, we performed a comprehensive evaluation of B7 family member expression and found that the spatial distribution of B7-H4 suggests the potentially useful role of combination blockade with both B7-H4 and the current anti-PD-1/PD-L1 axis in the treatment of UC.