In murine models of lung cancer, pancreatic cancer, and glioblastoma, TAMs originate from both expanding tissue-resident macrophages and circulating monocytes.11,30,63 Regan et al showed that canine HSA cells secreted high levels of the chemokine CCL2, stimulating monocyte migration and recruitment to metastases.48 However, they did not explore the relative contribution of the recruited monocytes to the total TAM number, nor can this be determined based on our results. Here, CCL2 is linked to lung carcinoma.