Profiling tumor immune microenvironment of epithelial ovarian carcinoma

Epithelial ovarian carcinoma (EOC) comprises five main histological subtypes: high-grade serous (HGSOC), low-grade serous (LGSOC), clear cell (CCOC), mucinous (MOC), and endometrioid (ENOC). Each histotype harbors specific genomic alterations and clinical outcome. Few studies systematically compared the tumor immune microenvironment across the five subtypes.

We performed 7-plex (CD45, CD8, CD68, CD163, FoxP3, CD20, and cytokeratin) sequential immunohistochemistry on a clinically annotated tissue microarray including 139 EOC representing the five subtypes and 26 borderline tumors (serous and mucinous). Digital pathology was used to quantify immune cell abundance, their spatial distribution (stroma vs tumor core), and correlation with survival.

Immune cells were dominated by macrophages and more abundant in the stroma than tumor core across the five subtypes, consistent with immune excluded pattern. Compared to HGSOC, CCOC displayed the highest infiltration by CD45⁺ leukocytes and CD68⁺ macrophages, particularly M2-like CD163⁺ cells, suggesting a macrophage-rich, immunosuppressive phenotype. LGSOC exhibited the highest infiltration by intraepithelial FoxP3⁺ regulatory T cells. Comparison of borderline tumors with invasive carcinoma (LGOSC and MOC) revealed that malignant progression is accompanied by loss of CD8⁺ T cells, enrichment in regulatory T cells and increase of CD163⁺/CD68⁺ ratio, consistent with immune evasion during tumorigenesis. There was a trend toward better survival in HGSOC highly infiltrated by lymphocytes, either intraepithelial (CD8⁺ and FoxP3⁺) or stromal (FoxP3⁺ and CD20⁺).

EOC is characterized by histotype-specific immune milieux defined by macrophage dominance, epithelial immune exclusion and dynamic immune remodeling during progression from borderline tumors to invasive carcinomas.