Histomorphological features and PD-L1 expression in canine gastric carcinoma using full thickness biopsy

Abstract

Canine gastric carcinoma (CGC) is a rare and aggressive malignancy in dogs with poor clinical outcomes. The biological relevance of microscopic tumor architecture and immune microenvironmental features in CGC is poorly defined. This study assessed key histomorphological features, including mitotic count, tumor budding, depth of infiltration, intravascular invasion, total stromal percentage (TSP), and tumor‑infiltrating lymphocytes (TILs), along with expression of the immune checkpoint marker programmed death‑ligand 1 (PD‑L1) by immunohistochemistry, on full‑thickness biopsy specimens (n = 40). CGC was highly invasive, with most tumors extending deeply into the muscularis and serosa. Depth of tumor infiltration was positively associated with intravascular invasion, which was detected in over half of cases. Tumor budding (TB) at the invasive front was frequent. TILs were identified at the invasive front, within the intratumoral stroma, and in peritumoral regions, with a strong positive correlation between TSP and TILs, suggesting close interactions between stromal architecture and immune cell distribution. PD‑L1 expression was predominantly cytoplasmic in neoplastic cells, and positively associated with mitotic activity. While most histomorphological parameters were not independently associated with survival outcome, current exploratory analysis indicated that only PD‑L1 score was associated with shorter survival. These findings highlight the invasive nature of CGC and the complex interplay between tumor budding, stromal composition and TILs in shaping tumor biology. The anti-tumor environment, partly influenced by PD-L1 expression, may also contribute to poor clinical outcomes.