Macrophage colony‐stimulating factor potentially induces recruitment and maturation of macrophages in recurrent pituitary neuroendocrine tumors

<jats:title>Abstract</jats:title><jats:p>Although pituitary neuroendocrine tumors (PitNETs) are usually benign, some are highly invasive and recurrent. Recurrent PitNETs are often treatment‐resistant and there is currently no effective evidence‐based treatment. Tumor‐associated macrophages (TAMs) promote tumor growth in many cancers, but the effect of TAMs on PitNETs remains unclear. This study investigated the role of TAMs in the incidence of recurrent PitNETs. Immunohistochemical analysis revealed that the densities of CD163‐ and CD204‐positive TAMs tended to increase in recurrent PitNETs. Compared with TAMs in primary lesions, those in recurrent lesions were enlarged. To clarify the cell–cell interactions between TAMs and PitNETs, <jats:italic>in vitro</jats:italic> experiments were performed using a mouse PitNET cell line AtT20 and the mouse macrophage cell line J774. Several cytokines related to macrophage chemotaxis and differentiation, such as M‐CSF, were elevated significantly by stimulation with macrophage conditioned medium. When M‐CSF immunohistochemistry analysis was performed using human PitNET samples, M‐CSF expression increased significantly in recurrent lesions compared with primary lesions. Although no M‐CSF receptor (M‐CSFR) expression was observed in tumor cells of primary and recurrent PitNETs, flow cytometric analysis revealed that the mouse PitNET cell line expressed M‐CSFR. Cellular proliferation in mouse PitNETs was inhibited by high concentrations of M‐CSFR inhibitors, suggesting that cell‐to‐cell communication between PitNETs and macrophages induces M‐CSF expression, which in turn enhances TAM chemotaxis and maturation in the tumor microenvironment. Blocking the M‐CSFR signaling pathway might be a novel therapeutic adjuvant in treating recurrent PitNETs.</jats:p>