Abstract 5188: Defining PD-1/PD-L1 receptor-ligand interactions in the head and neck cancer tumor microenvironment identifies unique ‘immune cell rivers’ of cellular interactions

Abstract Head and neck squamous cell carcinoma (HNSCC) is the 7th most common cancer globally with a poor 5-year survival rate of ~50%. With the emerging successes with immune checkpoint blockade (ICB) in the metastatic and recurrent setting, there is a need for predictive biomarkers to identify patients likely to achieve clinical benefit. Whilst PD-L1 expression on tumour cells and PD-1 on immune cells have been investigated, these remain inconclusive biomarkers. To better understand and map the PD-L1/PD-1 receptor-ligand interactions, we evaluated a novel in situ proximity ligation assay (is PLA) across whole tissue sections of pre-treatment HNSCC patients to received ICB therapy. Pre-treatment FFPE whole tissue samples were collected from n=25 advanced stage HNSCC patients across two major Queensland Hospitals (Royal Brisbane and Women’s Hospital and the Princess Alexandra Hospital). Tissues were stained with multiplex immunofluorescence and the Navinci Diagnostics fluorescent is PLA assay for PD-1/PD-L1. Tissues were incubated with the PD1/PD-L1 antibodies and subsequent incubation with oligonucleotide-tagged secondary probes enabled PLA, which highlights PD1 and PD-L1 interactions. Following the application of secondary probes, the sections underwent a ligation step, then a post-blocking, and an amplification process to enhance the detection of protein interactions. Next, the sections were treated with a detection solution to visualize the amplification product. The PLA assay findings were measured against clinical endpoints (PFS/OS criteria). In n=5/25 HNSCC tissues, positive PD-1/PD-L1 interactions were measured across the tumour microenvironment (TME), specifically in the immune rivers on the immediate tumour periphery. These cells showed highly specific binding of the PLA product. These areas positive by the PLA assay were in stark contrast to the tumour bulk/nests which were absent in PD-1/PD-L1 interactions. Notably, the PLA PD-1/PD-L1 do not correlate with tumour PD-L1 expression or tumour proportion scores (TPS). Rather, our study highlighted the difference in spatially mapping PD-1/PD-L1 interactions which may better recapitulate the tumour-immune recognition and activity in the TME. Taken together, these data provide a novel approach for determining clinically relevant receptor/ligand interactions in the HNSCC TME. Citation Format: Habib Sadeghirad, Vahid Yaghoubi Naei, James Monkman, Subham Basu, Agata Wicher, Rahul Ladwa, Brett GM Hughes, Arutha Kulasinghe. Defining PD-1/PD-L1 receptor-ligand interactions in the head and neck cancer tumor microenvironment identifies unique ‘immune cell rivers’ of cellular interactions abstract. In: Proceedings of the American Association for Cancer Research Annual Meeting 2024; Part 1 (Regular Abstracts) ; 2024 Apr 5-10; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2024;84 (6Suppl): Abstract nr 5188.