Differential Expression of S100A8 in Tumor and Immune Compartments of Endometrial Carcinoma and Its Clinical Relevance

<i>Background and Objectives</i>: S100A8 regulates inflammatory responses and immune cell activation and is overexpressed in several solid tumors. However, its clinicopathological significance in endometrial carcinoma (EC) remains unclear. This study aimed to evaluate the expression patterns of S100A8 in both tumor and immune cells of EC and examine its association with clinicopathological features. <i>Materials and Methods</i>: Fifty-two formalin-fixed, paraffin-embedded EC specimens were analyzed using tissue microarray-based immunohistochemistry. S100A8 expression was assessed in tumor and immune cells. The tumor proportion score (TPS), tumor staining intensity (TI), and immune proportion score (IPS) were dichotomized into low and high categories (TPS/IPS: ≤30% vs. ≥31%; TI: 0-1+ vs. 2-3+). Correlations with clinicopathological parameters were examined using the chi-square and Fisher's exact tests. <i>Results</i>: A low TPS, high TI, and high IPS were observed in 51.9%, 63.5%, and 57.7% of patients, respectively. TPS and TI showed no significant correlation with clinicopathological variables, including age, tumor size, invasion depth, histologic grade, T stage, and N stage (all <i>p</i> > 0.05). By contrast, IPS was significantly associated with patients' age (<i>p</i> = 0.044) and histologic grade (<i>p</i> = 0.012), with older patients and those with higher-grade tumors demonstrating a higher IPS. A positive correlation was observed between TPS and IPS (<i>p</i> = 0.044), whereas TI did not correlate with IPS (<i>p</i> = 0.253). <i>Conclusions</i>: S100A8 expression in immune cells, but not in tumor cells, is associated with age and tumor grade in EC. Therefore, immune-related S100A8 expression may serve as a biomarker of the tumor immune microenvironment, warranting further investigation into its prognostic and therapeutic implications.