Infiltrating Myeloid Cells Drive Osteosarcoma Progression via GRM4 Regulation of IL23

Abstract

The glutamate metabotropic receptor 4 (<i>GRM4</i>) locus is linked to susceptibility to human osteosarcoma, through unknown mechanisms. We show that <i>Grm4^-/-</i> gene-targeted mice demonstrate accelerated radiation-induced tumor development to an extent comparable with <i>Rb1^+/-</i> mice. GRM4 is expressed in myeloid cells, selectively regulating expression of IL23 and the related cytokine IL12. Osteosarcoma-conditioned media induce myeloid cell <i>Il23</i> expression in a GRM4-dependent fashion, while suppressing the related cytokine <i>Il12</i>. Both human and mouse osteosarcomas express an increased IL23:IL12 ratio, whereas higher IL23 expression is associated with worse survival in humans. Consistent with an oncogenic role, <i>Il23</i> ^-/- mice are strikingly resistant to osteosarcoma development. Agonists of GRM4 or a neutralizing antibody to IL23 suppressed osteosarcoma growth in mice. These findings identify a novel, druggable myeloid suppressor pathway linking GRM4 to the proinflammatory IL23/IL12 axis. SIGNIFICANCE: Few novel systemic therapies targeting osteosarcoma have emerged in the last four decades. Using insights gained from a genome-wide association study and mouse modeling, we show that GRM4 plays a role in driving osteosarcoma via a non-cell-autonomous mechanism regulating IL23, opening new avenues for therapeutic intervention.<i>See related commentary by Jones, p. 1484</i>.<i>This article is highlighted in the In This Issue feature, p. 1469</i>.