Peripheral monocyte transcriptomics associated with immune checkpoint blockade outcomes in metastatic melanoma

Clinical responses to immune checkpoint blockade (ICB) for metastatic melanoma (MM) are variable, with patients frequently developing immune related adverse events (irAEs). The role played by myeloid populations in modulating responses to ICB remains poorly defined. We explored the effect of MM and the response to ICB across a cohort of patients with MM (n=116) and healthy donors (n=45) using bulk and single cell RNA-seq, and flow cytometry. Monocytes from patients with MM exhibit highly dysregulated baseline transcriptional profiles, whilst ICB treatment elicits induction of interferon signaling, MHC class II antigen presentation and CXCR3 ligand expression. Although both combination (cICB - anti-PD-1 and anti-CTLA) and single-agent (sICB - anti-PD1) ICB therapy modulates a shared set of genes, cICB displays a markedly greater magnitude of transcriptional effect. Notably, we find increased baseline monocyte counts correlate with a monocyte proliferation signature and risk of early death, whilst a gene-signature corresponding to a subset of platelet-binding classical monocytes conversely associates with improved outcome. This work demonstrates a central role for monocytes in the modulation of treatment response to ICB, providing insights into inter-individual variation in immune responses to ICB and further highlighting the multifarious immunological consequences of ICB treatment.