Canine mast cell tumors express stem cell factor receptor

c-kit protooncogene encodes a type III transmembrane receptor kinase, the s tem cell factor receptor, or KIT. The ligand of the KIT, stem cell factor, is a cytokine that stimulates mast cell growth and differentiation. We have studied immunohistochemically KIT expression in 23 canine mast cell tumors (MCTs), 10 histiocytomas, 5 malignant melanomas, and in 2 cell lines deriv ed from mast cells (HMC-1, human and C2, canine). As expected, KIT was dete cted both in the human mast cell leukemia cell line (HMC-1) and in the cani ne mastocytoma cell line C2. In normal canine skin, KIT expression was conf ined to mast cells. All canine MCTs expressed KIT, although the intensity o f the staining reaction varied considerably among the 23 neoplasms. Grade I II tumors showed the highest expression of KIT, whereas grade I tumors show ed the lowest expression of KIT. Two patterns of KIT expression were detect ed in mast cells. In normal canine mast cells and in some neoplastic mast c ells, KIT appeared mainly on the cell membrane. However, in many canine MCT s, KIT is accumulated in the cytoplasm, usually near the cell nucleus. The meaning of these two patterns is not clear. Expression of KIT could not be detected immunohistochemically in any of the other neoplasias investigated. According to our results, it can be concluded that most, if not all, canin e MCT express KIT. Furthermore, there is an inverse correlation between the degree of differentiation and the expression of KIT. Moreover, according t o our results, KIT can be used as a reliable immunohistochemical marker for canine mast cells and undifferentiated mast cell tumors.