Prognostic Significance of the Tumour Microenvironment in Surgically Resected Canine Pulmonary Adenocarcinomas

The prognostic relevance of the tumour microenvironment in surgically resected canine pulmonary adenocarcinomas (cPACs) remains uncertain. This retrospective single-centre cohort study evaluated associations between intratumoural immune features and postoperative outcomes. Dogs treated between 2005 and 2021 with histologically confirmed cPACs that had undergone surgical excision and had an evaluable tumour microenvironment were included. Immunohistochemistry for CD8, Foxp3, CD3 and CD20 was performed. CD8+ and Foxp3+ cell densities were quantified in five intratumoural hotspots of 0.237 mm² each. Lymphoid aggregates (LAs) were defined as intratumoural aggregates located within tumour nests or intratumoural stroma and composed of at least 50 CD20+ B cells with intercellular spacing ≤ 10 μm and admixed CD3+ T cells. Primary outcomes were progression-free interval (PFI) and overall survival time (OST). A total of 71 dogs were enrolled. LAs were identified in 49 dogs (69.0%). The median densities of CD8+, CD20+ and Foxp3+ cells were 130.0, 398.3 and 79.3 cells/mm², respectively. Tumours with LAs showed higher CD20+ and Foxp3+ cell densities than tumours without LAs. Median PFI and OST were 754 days (95% CI, 375-not reached) and 716 days (95% CI, 399-936), respectively. In multivariable analysis, tumour size classification, lymph node metastasis, surgical margin status and the presence of LAs were independently associated with shorter PFI. Age, lymph node metastasis and the CD8/Foxp3 ratio were independently associated with OST. These findings indicate that immunohistochemical features may provide additional prognostic information for postoperative risk stratification in cPACs and complement established clinicopathological factors.