Abstract 4693: Characterization of the tumor immune microenvironment in head and neck squamous cell carcinoma (SCCHN)

Approximately 20% of advanced SCCHNs respond to anti-PD-(L)1. We aimed to inform rational combination therapy development, which might have greater efficacy, by understanding the potential immune effects of the oncogenic human papillomavirus (HPV) in SCCHN, and by characterizing potentially targetable immune checkpoints in the tumor microenvironment (TME). Immunohistochemistry (IHC) was performed on archival SCCHN specimens from 27 treatment-naive patients. HPV testing with p16 IHC was confirmed by HPV genotyping. IHC for PD-L1 assessed percentage of positive tumor cells, and whether PD-L1 was expressed in an adaptive or constitutive pattern (i.e., PD-L1+ tumor cells juxtaposed to TILs or in the absence of TILs, respectively). PD-L1 on infiltrating immune cells was scored separately. IHC for CD3, CD4, CD8, CD20, CD68, FoxP3, PD-1, PD-L2, LAG-3, TIM-3, GITR and IDO was quantified using Halo image analysis for density of positive cells. Among 27 SCCHN specimens, 14 were HPV(-) and 13 were HPV+ (one HPV58, 12 HPV16). In 19 of 27 specimens (70%), ≥5% of tumor cells had PD-L1 cell surface expression by IHC, including 9 HPV+ and 10 HPV(-) cases. Among 23 tumors expressing any level of PD-L1 (≥1% tumor cells+), 12 displayed an adaptive PD-L1 expression pattern, 5 a constitutive pattern, and 6 were mixed. All tumors were infiltrated by immune cells (ICs). Notably, 26 of 27 specimens (96%) contained PD-L1+ immune cells (range 5-80% of ICs expressing PD-L1). The proportion of PD-L1+ ICs exceeded PD-L1+ tumor cells in 21/26 (81%) cases. Neither the proportion of PD-L1+ tumor cells nor infiltrating immune cells correlated with tumor viral status. When compared to HPV(-) tumors, HPV+ tumors contained significantly higher densities of CD3+, CD4+, CD8+, CD20+, and PD-1+ cells (p 500 IDO+ cells/mm 2 in 17/27 specimens), IDO was expressed by tumor cells as well as ICs in 12/27 (44%) cases (range 5-95% tumor cells+). In summary, we found that both HPV+ and (-) SCCHNs abundantly express PD-L1 on tumor and/or stromal cells, although HPV+ tumors are more heavily infiltrated by ICs. Multiple other immune checkpoints are expressed in these tumors, providing options for therapeutic co-targeting. Tumor DNA sequencing is in progress to explore the genetic basis for constitutive vs. adaptive PD-L1 expression. These studies are expected to provide a comprehensive portrait of the SCCHN TME, with implications for future immunotherapy development. Funded by the Bristol-Myers Squibb International Immuno-Oncology Network and NCI R01 CA142779 Citation Format: Farah Succaria, Pia Kvistborg, Elizabeth L. Engle, Tracee L. McMiller, Elizabeth Thompson, Alan E. Berger, John Haanen, Suzanne L. Topalian, Janis M. Taube. Characterization of the tumor immune microenvironment in head and neck squamous cell carcinoma (SCCHN) [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2018; 2018 Apr 14-18; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2018;78(13 Suppl):Abstract nr 4693.