The success of immune checkpoint therapy shows tumor-reactive T cells can eliminate cancer cells but are restrained by immunosuppression within the tumor micro-environment (TME). Cancer associated fibroblasts (CAFs) are the dominant stromal cell in the TME and co-localize with T cells in non-small cell lung cancer. We demonstrate the bidirectional nature of CAF/T cell interactions; T cells promote expression of co-inhibitory ligands, MHC molecules and CD73 on CAFs, increasing their production of IL-6 and eliciting production of IL-27. In turn CAFs upregulate co-inhibitory receptors on T cells including the ectonucleotidase CD39 promoting development of an exhausted but highly cytotoxic phenotype. Our results highlight the bidirectional interaction between T cells and CAFs in promoting components of the immunosuppressive CD39, CD73 adenosine pathway and demonstrate IL-27 production can be induced in CAF by activated T cells.

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348 Background: A subset of patients with metastatic renal cell carcinoma (mRCC) follow an indolent disease course. Given the toxicity and non-curative nature associated with systemic anti-cancer therapy (SACT), some patients may benefit from initial active surveillance (AS). However, selecting patients suitable for this approach is challenging. Biomarkers of systemic inflammation predict survival in mRCC, both independently and as part of the International Metastatic Database Consortium (IMDC) risk score. We sought to use these biomarkers to characterise the time to initiation of SACT (tSACT) in mRCC patients on AS. Methods: 126 mRCC patients clinically assessed and commenced on AS prior to any systemic therapy were retrospectively identified from a regional mRCC clinical database. Patients who underwent metastasectomy for oligometastatic disease at any time were excluded. The primary endpoint, tSACT, was defined as the time from radiological diagnosis of mRCC until SACT initiation, or death, or censorship if continuing AS at follow-up date. Inflammatory biomarkers from routine blood tests (haemoglobin, white cell count, neutrophil count, platelets, C-reactive protein (CRP), albumin) and the IMDC score, measured at the time of diagnosis of mRCC, were recorded. The relationship between these and tSACT was examined using Kaplan-Meier and Cox-regression methods. Results: 66 (52%) patients had commenced SACT. 17 (13.5%) had died without commencing SACT (median survival of the 17 was 40.4 months, range 9.1-130.2 months, and comorbidities may have affected fitness for starting therapy or led to all-cause mortality). 43 patients remained on AS, with minimum and median follow-up of 12.6 months and 39.6 months respectively. The median tSACT was 17.2 months (IQR 8.8-34.8 months). On univariate analysis, CRP and albumin were predictive of time on AS ( p= 0.01 and p= 0.049 respectively). On multivariate analysis, only CRP was independently associated with tSACT ( p= 0.035), stratifying tSACT from 9.1 months (CRP > 10) to 20.9 months (CRP≤10) ( p= 0.009). 111 (88.1%) patients were IMDC 0-1, while 12 (9.5%) and 3 (2.4%) were IMDC 2 or 3 and may have had comorbidities that influenced the initial AS decision. In our cohort the IMDC risk score did not predict time on AS. Conclusions: These results highlight that some patients with mRCC may undergo active surveillance for a marked time period before SACT initiation. We identify routine biomarkers of the systemic inflammatory response that predict time to systemic therapy. In particular CRP, a simple measure of inflammation, stratifies the time to initiation of SACT across a clinically significant time period. This simple, widely available test may help to objectively inform clinical decisions about AS in patients in mRCC. Additional experience is necessary to further define the risks and benefits of this approach.