Characterization of Inflammatory Changes Associated with Canine Oligodendroglioma

Oligodendroglioma is a common glial tumour in the dog. In human neuropathology, the immune cell microenvironment of gliomas has been investigated; however, the nature of the inflammatory cells within canine gliomas is currently unknown. The aim of this study was to determine the nature of the immune cells and determine an association between the inflammatory cells and tumour grade. Thirty-four (18 of grade II and 16 of grade III) formalin-fixed and paraffin wax-embedded samples of canine oligodendroglioma were evaluated by light microscopy and immunohistochemistry for expression of CD3, PAX5, Iba-1, HLA-DR, Mac387 and CD31. Variations in immune cell recruitment and activation were evident in all cases. Infiltrating CD3+ T lymphocytes were common in most cases. PAX5+ B lymphocytes were less common and restricted to perivascular cuffs within or around the tumour. Iba-1+ cells were common within the tumour and formed a dense infiltrate around the tumour in a subset of cases. HLA-DR+ cells were common within the tumour and in a subset of cases formed perivascular cuffs. Iba-1+ cells typically had prominent ramified processes suggestive of activated microglia, while the HLA-DR+ cells had a more rounded morphology typical of amoeboid microglia. Rare Mac387+ macrophages were found in the tumour parenchyma, while increased numbers of Mac387+ monocytes were noted within the vasculature. No association or significance was established between the immune cell infiltrate and the grade of the tumour (all P ≥0.16). This study establishes that there is a robust population of immune cells within canine oligodendrogliomas and indicates that further study is needed to determine the role of these cells in tumour pathogenesis and progression.