Characterization of the tumor immune microenvironment in human papillomavirus-positive and -negative head and neck squamous cell carcinomas

Approximately 15% of advanced head and neck squamous cell carcinomas (HNSCC) respond to anti-PD-(L)1 monotherapies. Tumor PD-L1 expression and human papillomavirus (HPV) status have been proposed as biomarkers to identify patients likely to benefit from these treatments. We aimed to understand the potential immune effects of HPV in HNSCC and to characterize additional potentially targetable immune-regulatory pathways in primary, treatment-naïve tumors. CD3, CD4, CD8, CD20, CD68, FoxP3, PD-1, PD-L2, LAG-3, IDO-1, and GITR cell densities were determined in 27 HNSCC specimens. IHC for PD-L1 assessed percentage of positive tumor cells and immune cells separately or as a combined positive score (CPS), 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). HPV testing with p16 IHC was confirmed by HPV genotyping. When compared to HPV(−) tumors (n = 14), HPV+ tumors (n = 13) contained significantly higher densities of CD3+, CD4+, CD8+, CD20+, and PD-1+ cells (P < 0.02), and there was a trend towards increased density of FoxP3 + cells. PD-L1 expression patterns did not vary by tumor viral status, suggesting possible heterogeneous mechanisms driving constitutive vs adaptive PD-L1 expression patterns in HNSCC. IDO-1 expression was abundant (> 500 IDO-1+ cells/mm² in 17/27 specimens) and was found on tumor cells as well as immune cells in 12/27 (44%) cases (range 5–80% tumor cells+). Notably, the studied markers varied on a per-patient basis and were not always related to the degree of T cell infiltration. These findings may inform therapeutic co-targeting strategies and raise consideration for a personalized treatment approach.