P59: Basal cell carcinoma exhibits novel cancer immune evasion via ectodomain shed CD200

Elise Rees, Huw Morgan, Simone Lanfredini, Kate Powell, Jasmine Gore, Alex Gibbs, Carlotta Olivero, Charlotte Lovatt, Gemma Davies, Alex Tonks, Richard Darley, Eddie Wang and Girish Patel

Cardiff University, Cardiff, UK

Cancer immune responses are largely T-lymphocyte and natural killer (NK) cell driven, espoused by the therapeutic effectiveness of T-cell immune checkpoint inhibitors. A hallmark of cancer, immune evasion can differ significantly even within cancers with identical cells of origin resulting in differing clinical outcomes. The aim was to evaluate immune evasion mechanisms in the skin keratinocyte carcinomas, basal (BCC) and squamous (SCC) cell carcinoma. Immunofluorescence was used to compare immune infiltrates in BCC and SCC. CD200 and vector transduced together with CD200R⁺ and CD200R^– NK cells in co-culture experiments were used to demonstrate immune evasion using multiple assays, which included cell viability, apoptosis and NK cell activation. In vivo CD200⁺ and CD200^–, as well as SCC and BCC xenografting experiments, were used to validate the mechanism. Rescue experiments were undertaken with anti-CD200 antibody in vitro. The mechanism underpinning CD200-mediated NK cell apoptosis included transcriptomic and Western blot analysis. Extrapolation of the findings were undertaken by analysing Prediction of Clinical Outcomes from Genomic Profiles (PRECOG) and Cell type Identification By Estimating Relative Subsets Of known RNA Transcripts (CIBERSORT) datasets. Having previously identified cancer stem cells, we herein identified a near absence of NK cells in the BCC tumour immune infiltrate. NK cells were sufficient to kill BCC cells in co-culture. Yet in situ, CD200 undergoes ectodomain shedding by metalloproteinases (MMP-3 and MMP-11) releasing biologically active soluble CD200 into the BCC microenvironment. CD200 binds its cognate receptor on NK cells to block tumour cell killing. Moreover, inactivation of the mitogen-activated protein kinase pathway simultaneously leads to peroxisome proliferator-activated receptor (PPAR)-γ dependent transcription of the death receptor pathway members. NK cell membrane accumulation of the Fas/Fas-associated protein with death domain death receptor and its ligand, FasL, led to activation-induced apoptosis. Blocking CD200 signalling or PPAR-γ was sufficient to prevent NK cell apoptosis. Analysis of 20 cancer types in the PRECOG dataset identified CD200-mediated NK cell depletion as a shared mechanism in many cancer types. CD200-mediated NK cell depletion appears to be associated with poor outcome for many cancers and thus represents a novel targetable NK cell-specific immune checkpoint.