Introduction/Background The prognostic relevance of tumour infiltrating lymphocytes (TILs) in high-grade serous ovarian carcinoma (HGSOC) has been established almost 20 years ago. Currently, immune checkpoint inhibitors (ICI) have been tested in ovarian cancer, but response rates of ICI monotherapy remain disappointingly low.

Methodology In this retrospective, exploratory analysis, ovarian tumour tissue from 252 patients and, in 187 cases, corresponding metastatic peritoneal and/or greater omentum metastatic tissue were assessed by immunohistochemistry. Expression of CD3, CD8, FOXP3, and PD-L1 on TILs as well as PD-L1 expression on tumour cells was comparatively evaluated, and the prognostic impact determined.

Results Altogether, expression was higher in metastatic tissue than in ovarian tumour tissue. For all analysed markers we found weak to moderate positive correlations between the expression in ovarian tumour and metastatic tissue. For the expression of PD-L1 the strongest correlation was found between peritoneal and omental metastatic tissue in tumour cells (ρ=0,66; p<0.05) and in immune cells (ρ=0,6; p<0.05). In univariate survival analysis, we confirmed the positive prognostic effect of CD3⁺ and CD8⁺ cells mainly in metastatic tissue. FOXP3 expression was not clearly correlated with survival. The expression of PD-L1, which was observed much more frequently in TILs compared with tumour cells, showed a positive association with survival in this collective in ovarian tumour tissue and only a tendency in metastatic tissue.

Conclusion In summary, we observed that in HGSOC immune infiltration tended to be more prominent in metastatic tissue than in ovarian tumour tissue, and despite a general positive correlation, there were differences between localizations in individual cases. This heterogeneity should be taken into account for the establishment as prognostic biomarkers or biomarkers predictive for ICI response.