Immunohistochemical evaluation of PD-1 and PD-L1 expression in feline injection site sarcoma

The goal of this study is to investigate the immunohistochemical expression of programmed cell death 1 (PD-1) and programmed cell death 1 ligand (PD-L1), which could lead to a new approach to immunotherapy for feline injection site sarcomas (FISS). In tumors expressing PD-1/PD-L1, treatment strategies based on pathway suppression are possible. To that end, histological classifications of FISS were made first, followed by grading of sarcomas and lymphocyte infiltration density, and finally a comparative evaluation of PD-1, PD-L1 presentations with different grades. Tumors were immunohistochemically stained with vimentin, S100, smooth muscle actin (SMA), and sarcomeric actin in addition to their histopathological appearance, and 8 cases were diagnosed as fibrosarcoma, 1 case as an undifferentiated sarcoma, 1 case as a liposarcoma, and 1 case as a rhabdomyosarcoma. PD-1 was found mostly in lymphoid infiltrates and macrophage-like cells, whereas PD-L1 was mostly found in tumoral cells and infiltrating macrophage-like cells. When the results were compared using Pearson correlation analysis, it was found that tumor differentiation had a moderate correlation with PD-1 (p < 0.05) and a strong correlation with PD-L1 (p < 0.01). Tumor grade was found to have a low correlation with PD-1 and a moderate correlation with PD-L1 (p > 0.05). Based on this data, it was concluded that increased PD-1 and PD-L1 expression may be associated with poor tumor differentiation and thus poor prognosis in FISS.