CCL2 and neoplasm: Indeed, increased CD18+ monocytes numbers were found in HSA metastases compared with metastases from other tumor types, and this result, in association with an increased expression of the monocyte chemokine CCL2 by tumor cells, would support the hypothesis that overexpression of CCL2 and recruitment of large numbers of monocytes could explain the aggressive metastatic nature of canine HSA [106,107,108].