Immunohistochemical Expression of Ionized Calcium Binding Adapter Molecule 1 in Cutaneous Histiocytic Proliferative, Neoplastic and Inflammatory Disorders of Dogs and Cats

Ionized calcium-binding adapter molecule 1 (Iba1) has been used widely as a marker for microglial cells and, recently, was also recognized as a ‘pan-macrophage marker’, as it is expressed by all subpopulations of cells of the monocyte/macrophage lineage. To determine the specificity of Iba1 as an immunohistochemical marker for canine and feline histiocytic proliferative, neoplastic and inflammatory disorders of the skin, we evaluated its expression in two types of histiocytic tumours, two non-neoplastic histiocytic proliferative conditions, one case of granulomatous dermatitis and four non-histiocytic tumours. Cells of the monocyte/macrophage lineage in all cases of canine cutaneous histiocytoma (9/9), reactive histiocytosis (9/9), histiocytic sarcomas (5/5), feline progressive dendritic cell histiocytosis (3/3) and macrophages in cutaneous mycobacteriosis (7/7) showed strong cytoplasmic expression of Iba1. Neoplastic cells of melanomas (10/10), lymphomas (7/7), mast cell tumours (7/7) and plasmacytomas (4/4) did not express Iba1. Iba1 is therefore a useful marker of cells of the monocyte/macrophage lineage in canine and feline inflammatory, proliferative and neoplastic conditions and can be used to identify these cells in formalin-fixed, paraffin wax-embedded tissues. Iba1 is not able to differentiate between macrophages and dendritic antigen presenting cells and expression does not allow classification of these histiocytic disorders.