Response to primary chemoradiotherapy of locally advanced oropharyngeal carcinoma is determined by the degree of cytotoxic T cell infiltration within tumor cell aggregates

Background: Effective anti-tumor immune responses are mediated by T cells
and require organized, spatially coordinated interactions within the tumor
microenvironment (TME). Understanding coordinated T-cell behavior and
deciphering mechanisms of radiotherapy resistance mediated by tumor stem
cells will advance risk stratification of oropharyngeal cancer (OPSCC)
patients treated with primary chemoradiotherapy (RCTx).  Methods:
To determine the role of CD8 T cells (CTL) and tumor stem cells in
response to RCTx, we employed multiplex immunofluorescence stains on
pre-treatment biopsy specimens from 86 advanced OPSCC patients and
correlated these quantitative data with clinical parameters. Multiplex
stains were analyzed at the single-cell level using QuPath and spatial
coordination of immune cells within the TME was explored using the
R-package Spatstat. Results: Our observations demonstrate that a strong
CTL-infiltration into the epithelial tumor compartment (HR for overall
survival, OS: 0.35; p<0.001) and the expression of PD-L1 on CTL
(HR: 0.36; p<0.001) were both associated with a significantly
better response and survival upon RCTx. As expected, p16 expression was a
strong predictor of improved OS (HR: 0.38; p=0.002) and correlated with
overall CTL infiltration (r: 0.358, p<0.001). By contrast, tumor
cell proliferative activity, expression of the tumor stem cell marker
CD271 and overall CTL infiltration, regardless of the affected
compartment, were not associated with response or survival. Conclusion: In
this study, we could demonstrate the clinical relevance of the spatial
organization and the phenotype of CD8 T cells within the TME. In
particular, we found that the infiltration of CD8 T cells specifically
into the tumor cell compartment was an independent predictive marker for
response to chemoradiotherapy, which was strongly associated with p16
expression. Meanwhile, tumor cell proliferation and the expression of stem
cell markers showed no independent predictive effect in response to RCTx
and require further study.