Abstract
No established method currently exists for evaluating tumor-infiltrating lymphocytes (TILs) in gastric cancer (GC), and their clinical significance based on infiltration site in GC remains unclear. In this study, we developed a method to evaluate TILs according to their infiltration site as a prognostic marker for GC. We retrospectively analyzed 103 patients with advanced GC who underwent curative resection. TILs located at the invasive margin (TIL(IM)) and the center of tumors (TIL(CT)) were scored semi-quantitatively using immunohistochemical staining of CD8+ T cells. The sum of the TIL(IM) and TIL(CT) scores was defined as the TILs score. Based on this score, patients were classified into low and high TILs groups. Quantitative TILs were also assessed to validate the semi-quantitative scoring method. Furthermore, we confirmed a tumor suppressive effect due to CD8+ T cells co-cultured in GC cell lines in vitro. In the univariate analysis, patients with low TIL(IM) were significantly more likely to be female, younger, and have undifferentiated histological types and deeper tumor invasion compared to those with high TIL(IM). Similarly, patients with low TIL(CT) had significantly more positive lymph node metastases than those with high TIL(CT). In the multivariate analysis, deeper tumor invasion and positive lymph node metastasis were identified as independent risk factors for patients with low TIL(IM) and low TIL(CT), respectively. According to our semi-quantitative TILs scoring method, the low TILs group had significantly poorer prognoses compared to the high TILs group. This group had significantly larger tumor diameters, deeper tumor invasion, and more positive lymph node metastases. Additionally, deeper tumor invasion was an independent risk factor for the low TILs group. Quantitative TILs analysis revealed that the low TILs group had significantly lower TIL levels compared to the high TILs group. In vitro, CD8+ T cells induced apoptosis in GC cells in a concentration-dependent manner. Furthermore, these cells significantly suppressed the proliferative, migratory, and invasive capacities of GC cells. Our simple and versatile semi-quantitative scoring method for CD8+ TILs indicates that CD8+ TILs are sensitive prognostic markers. The low TILs group accurately reflects the low quantitative TIL levels and is associated with poor oncological prognosis.