Background: Evidence points towards the presence of intratumoral dendritic cells (DC) as predicting treatment response to checkpoint immunotherapy. How the different DC subsets, particularly plasmacytoid dendritic cells (pDC), the primary cell to produce Type I IFN, may or may not be priming cytotoxic T cell lymphocytes remains unclear. Better understanding this could offer important insight into transforming DC from patients with aggressive, less immune-privileged HNSCC into effective T cell priming agents.
Design: Tissue not needed for clinical purposes was collected from consented patients (using an IRB-approved protocol) with head and neck squamous cell carcinoma (HNSCC), including tumor as well as tumor-uninvolved lymphoid tissue. Following enzymatic digestion, lymphoid cells were stained and run by flow cytometry on a BD LSRFortessa using the following staining panel: anti-HLA-DR (APC-Cy7, Biolegend), lineage (anti-CD3/CD14/CD19/ CD20/CD56) (FITC, Biolegend), anti-CD11c (BUV395, BD), anti-CD123 (PE: BD, BV421: BioLegend), anti-CD303/BDCA-2 (PerCP/Cy5.5, Biolegend), CD80 (APC, Biolegend), CD86 (APC-R700, BD), OX40(L) (BV421, BD), ICOS(L) (BV605, BD), PD-L1 (PE-Cy7, BD) and TGF-β LAP(PE-TR, Biolegend).
Results: The inhibitory ligands ICOS(L), OX40(L) and PD-L1 are all present in higher frequencies on tumor-infiltrating pDC from HPV-negative HNSCC (n= 13) compared to HPV-positive HNSCC (n= 12) or pDC from peripheral blood of healthy blood donors (n= 3) (see Figure 1). We also found that pDC from HPV-positive HNSCC secrete less TGF-β LAP compared to HPV-negative HNSCC pDC (see Figure 2) and are more activated based on increased CD40/CD80/CD86 expression compared to autologous tumor-negative lymph node pDC (see Figure 3).
Conclusions: Our findings indicate that the tumor-infiltrating pDC from HPV-positive HNSCC are phenotypically and functionally distinct from both autologous pDC in lymphoid tissue as well as pDC from HPV-negative HNSCC patients. Unlike pDC from other human tumor models, these pDC are unique based on increased expression of activation markers and decreased production of the TGF- β latency associated protein (LAP). This novel population of intratumoral pDC may represent a potential source of increased anti-tumor Type I IFN and determining this is the future direction of our work. In doing so, we will identify a targetable cell population that is important for future immunotherapeutic applications.