Single-cell sequencing of tumor-associated macrophages in a Drosophila model

Tumor-associated macrophages may act to either limit or promote tumor growth, yet the molecular basis for either path is poorly characterized. Using a Drosophila model for dysplastic growth during tumor early progression in combination with single-cell RNA sequencing of macrophage-like hemocytes, we identified 5 cell clusters. We included a second tumor model in our analysis where we inhibited the activation of effector caspases, partially mimicking apoptosis-resistant tumor cells. Circulating hemocytes from both tumor models differ qualitatively from control wild-type cells – they display an enrichment for genes involved in cell division, which was confirmed using proliferation assays. Split analysis of the tumor models further reveals that proliferation is strongest in the caspase-deficient setting. Similarly, depending on the tumor model, hemocytes that attach to tumors activate different sets of immune effectors – antimicrobial peptides dominate the response against the tumor alone, while caspase inhibition induces a shift toward members of proteolytic cascades. Finally, we find evidence for transcript transfer between hemocytes and possibly other tissues. Here, to identify genes that are differentially expressed in hemocytes from tumor larvae, we profiled the transcriptome of circulating single-cell hemocytes from wild-type and tumor larvae, as well as from tumor-associated hemocytes (TAHs), which were extracted manually from tumorous salivary glands (Fig 1A). Since we had previously observed strong activation of caspases in RasV12-expressing SGs (20), and in light of the bi-edged nature of caspase activation and apoptosis in cancer (see above), we included larvae where effector caspases were inhibited by the expression of the specific inhibitor p35