Approaching canine inflammatory myopathy by cluster analysis: an immunohistopathological observational study

Introduction Immune-mediated inflammatory myopathies (IMs) in dogs represent a heterogeneous group of disorders with incompletely characterized histopathological and immunophenotypic characteristics. This study aimed to identify multiparametric histological and immunophenotypic patterns in canine IMs to support accurate diagnosis from formalin-fixed paraffin-embedded (FFPE) muscle biopsies. Methods A cohort of 162 dogs was retrospectively analyzed using a standardized 91-variable histopathological grading scheme, immunohistochemistry for CD3, CD8, CD20, IBA-1, and MHC II, and hierarchical k-means cluster analysis. Multimodel inference was applied to identify associations between immune markers and infiltrating cells with the presence of myonecrosis, fibrosis, myoregeneration, and muscle atrophy within the study population. Results Four distinct histopathological subgroups were identified. Cluster 1 (56%) was characterized by fiber-invasive, endomysial infiltration with CD3 + T-cell predominance over CD20 + B-cells and extensive myonecrosis, encompassing cases of polymyositis, infectious myositis, and breed-specific myositides. Cluster 2 (28%) demonstrated broad, compartment-spanning infiltration with CD20 + B-cell predominance over T-cells, lower rates of myonecrosis, and included masticatory muscle myositis cases alongside limb muscle polymyositis of similar immunophenotype. Cluster 3 (10%) exhibited diffuse, severe inflammatory infiltration with a balanced B/T-cell composition and marked fibrosis, representing end-stage disease. Cluster 4 (6%) was defined by vasculocentric inflammatory infiltration, B-cell predominance over T-cells, and minimal myonecrosis, consistent with immune-mediated vasculitis. Multimodel inference further revealed that B-cell infiltration was negatively associated with myonecrosis across the total cohort, while macrophages and mast cells were positively associated with fiber destruction. B-cells and plasma cells were independently associated with fibrosis across multiple clusters. Discussion These findings establish a histopathology- and immunophenotype-based classification framework for canine IMs, highlighting the need for subtype-specific diagnostic and therapeutic strategies.