Evaluation of tumour-infiltrating immune cells and their prognostic value in feline appendicular osteosarcoma: a pilot study.

This study aimed to evaluate the prognostic relevance of selected immune cell markers in feline appendicular osteosarcoma in relation to clinical outcome. We retrospectively profiled tumour-infiltrating immune cells in 16 primary tumours by immunohistochemistry for CD3, CD8, Foxp3, CD20 and CD204. Immune cell densities were quantified and correlated with clinicopathological features, and the Kaplan-Meier estimator assessed survival. CD204+ macrophages were the most abundant population. Kaplan-Meier analysis revealed that higher intratumoural CD3+ T and CD8+T cell densities were associated with shorter survival, and CD3+, CD8+ and CD204+ densities showed strong positive co-infiltration, consistent with an inflamed yet potentially functionally constrained tumour microenvironment. Foxp3+ cells were rarely detected, and age correlated inversely with CD3+ and CD8+ density. These observations suggest that T-cell quantity may not equate to anti-tumour quality in macrophage-rich feline osteosarcoma. However, the small retrospective cohort, univariable analyses without multiplicity adjustment and the absence of spatial or functional markers limit inference. These findings should be considered a hypothesis-generating, prospective validation integrating spatial profiling and checkpoint/exhaustion markers, and refined macrophage phenotyping is warranted before prognostic application.