Macrophage Colony Stimulating Factor (M‐<scp>CSF</scp>) and Interleukin‐34 (<scp>IL</scp>‐34) Expression in Canine Osteosarcoma in the Context of the Tumour Immune Microenvironment

Canine osteosarcoma (OSA) is a malignancy that has been shown to modulate the host immune system. Macrophage colony-stimulating factor (M-CSF; CSF1) and interleukin-34 (IL-34; IL34) are both ligands of colony stimulating factor 1 receptor (CSF-1R), and may play a role in the pathogenesis of a variety of human cancers, including OSA. This study aimed to, (1) assess M-CSF and IL-34 expression in canine OSA cell lines and tissue samples, and (2) determine any correlations between M-CSF and IL-34 expression and immune cell infiltrates within canine OSA tissues. Four canine OSA cell lines and canine osteoblasts were treated with control media, TNFα (10 ng/mL) or IL-1β (10 ng/mL) and analysed with RT-qPCR and ELISA. IL-34 and M-CSF mRNA and protein were detectable in all cell lines, however upregulation following TNFα or IL-1β exposure was only consistently observed for transcript expression. Baseline expression of CSF1 and IL34 mRNA in OSA cell lines was equal to or higher than that of canine osteoblasts. All 10 OSA tissue samples expressed IL34 and CSF1 transcripts to varying degrees. Furthermore, CSF1 and IL34 expression both showed a moderate to high degree of correlation with M1 macrophage lineage-associated transcripts (CD80 and IL15RA). There was a moderate degree of correlation between CSF1 and CD163, but no correlation between IL34 and either M2 macrophage-associated transcripts (CD163 and CCL24). In summary, IL-34 and M-CSF are expressed in canine OSA cell lines and tissues, and expression positively correlates with a wide range of immune-related transcripts.