Spon1+ inflammatory monocytes promote collagen remodeling and lung cancer metastasis through lipoprotein receptor 8 signaling.

Lung cancer is the leading cause of cancer-related deaths in the world, and non-small cell lung cancer (NSCLC) is the most common subset. Although immune checkpoint inhibitors have improved outcomes in a subset of NSCLC patients, these effects are usually not durable. Thus, a more complete understanding of how immune niches within the tumor microenvironment (TME) promote NSCLC progression will allow us to build upon these advances. We previously found that infiltration of tumor inflammatory monocytes (TIMs) into lung squamous carcinoma (LUSC) tumors is associated with increased metastases and poor survival. To further understand how TIMs promote metastases, we compared RNA-seq profiles of TIMs from several LUSC metastatic models with IMs of non-tumor bearing controls. We identified Spon1, a secreted extracellular matrix glycoprotein, as upregulated in TIMs and that Spon1 expression in LUSC tumors corresponds with poor survival. Bioinformatic analyses of the Cancer Genome Atlas (TCGA) revealed that LUSC tumors with high SPON1 expression significantly enriched for collagen extracellular matrix (ECM) signatures. Unexpectedly, we observed SPON1+ TIMs mediate their effects directly through LRP8 (ApoER2) on NSCLC cells, which results in TGFβ1 activation and robust production of fibrillar collagens. Using several orthogonal approaches, we demonstrate that SPON1+ TIMs are sufficient to promote NSCLC metastases. Additionally, we find that Spon1 loss in the host, or Lrp8 loss in cancer cells, results in a significant decrease of both high-density collagen matrices and metastases. Finally, we confirm the relevance of the SPON1/LRP8/TGFβ1 axis with collagen production and survival in NSCLC patients. Taken together, our study describes how SPON1+ TIMs promotes collagen remodeling and NSCLC metastases through an LRP8-TGFβ1 signaling axis in lung cancer cells.