Directing the immune system to recognise and eliminate developing tumours has been hypothesised for over a century, yet only recently has this concept translated to the clinic with the discovery of immune checkpoint inhibitors (ICIs) which reactivate anti-tumour T cells. However, their rapid translation into front-line treatment has left a number of the fundamental mechanisms of action of the drugs underexplored. One outstanding question is why a significant fraction of patients don’t respond to ICIs? Here, we begin to address this by investigating the response of non-immune components of the tumour microenvironment (TME) to ICI therapy, focusing on cancer-associated fibroblasts (CAFs). CAFs are an abundant and heterogeneous component of the TME, supporting tumourigenesis by secreting growth factors, cytokines, chemokines, extracellular matrix (ECM) and remodelling the physical environment. Indeed, these disparate functions have been shown to be performed by distinct subpopulations present in the TME.

We observed significant alterations to the composition of the CAF compartment following ICI. ICI disrupted the evolutionary trajectory of CAF subpopulations, as ‘S1’ CAFs with increased immune modulatory capacity accumulated at the expense of ‘S3’ CAFs specialised in contractility. Single cell RNA-sequencing suggested S1 CAFs became more immune licenced following αPD-1 treatment, upregulating genes involved in MHC class I antigen presentation at early time points, and later in tumour development, ‘S2’ CAFs classically associated with ECM deposition also increased expression of antigen presentation genes. Alongside a decrease of immune-inhibitory receptors, we detected an increase in co-stimulatory molecules in CAFs from tumours responding to ICI. Our data indicates that these changes are driven by inflammatory mediators including IFN-γ, released by re-activated tumour infiltrating immune cells. Finally, we identified CAF-derived ECM as a potential barrier to ICI response. Combination of αPD-1 with losartan, a collagen synthesis inhibitor, showed a synergistic effect, increasing T cell infiltration and decreasing tumour growth.

Together, these results show that the non-immune stroma plays an important role in determining ICI therapeutic efficacy, and is a source of novel biomarkers, as well as targets for new therapy combinations.