A novel histological examination with dynamic three‐dimensional reconstruction from multiple immunohistochemically stained sections of a PD‐L1‐positive colon cancer

Aims Programmed cell death‐ligand 1 ( PD ‐L1) expression is observed in patients with microsatellite instability‐high ( MSI ‐H) colon cancer, which is susceptible to immune checkpoint blockade. The aim of this study was to investigate the interrelationship between PD ‐L1‐positive cells and cytotoxic T cells, lymphatic vessels and vascular endothelium by using histological examination with the three‐dimensional (3D) reconstruction of a PD ‐L1‐positive colon cancer. Methods and results Serial sections of MSI ‐H colon cancer tissue were stained with haematoxylin and eosin (H&E) and Masson trichrome stains; immunohistochemical analysis of PD ‐L1, CD 8, D2‐40 and CD 31 was performed. Several 3D models of MSI ‐H colon cancer were reconstructed with a 3D data visualisation system. Moreover, 18 serial sections were stained with PD ‐L1, cytokeratin AE 1/ AE 3, CD 45, CD 31, CD 68 and H&E in the same case to confirm that PD ‐L1 was expressed on tumour cells, CD 31‐positive cells and macrophages in the invasive frontal region. Notably, there was a peak in the expression of PD ‐L1 and CD 31 in the invasive frontal region. D2‐40‐positive cells were abundant in the overall tumour stroma, and CD 8‐positive cells infiltrated the tumour parenchyma. PD ‐L1 was expressed on tumour cells in the parenchyma and other cells in the stroma. Additional staining of 18 consecutive sections revealed that the other cells were CD 68‐positive and CD 45‐positive macrophages and CD 31‐positive proliferating vascular endothelial cells. Conclusions We confirmed that PD ‐L1 was highly expressed in the invasive frontal region in 3D models of MSI ‐H colon cancer tissue. This method can be useful for accurately evaluating the localisation of immune checkpoint molecules.