Abstract. Gastrointestinal melanoma (GM) is a rare but aggressive type of malignant melanoma arising in the gastro- intestinal tract. An anti-programmed cell death protein 1 (PD-1) antibody markedly improves prognosis in patients with melanoma

Gastrointestinal melanoma (GM) is a rare but aggressive type of malignant melanoma arising in the gastrointestinal tract. An anti-programmed cell death protein 1 (PD-1) antibody markedly improves prognosis in patients with melanoma. However, little is known regarding the expression of immune-oncology biomarkers in GM compared with skin melanoma (SM), especially in the Asian population. the present study examined clinicopathological characteristics, PD-L1 and HLA expression, and immune-oncology marker expression in 10 cases of GM and 31 cases of SM. Patients with GM exhibited significantly higher incidences of lymph node and distant metastases than patients with SM (P=0.0448 and P=0.0247, respectively). The infiltration of CD8+ lymphocytes was significantly higher in GM than in SM (P=0.0231). The infiltration of PD‐1+ lymphocytes was higher in GM than in SM, but the difference was not significant (P=0.0975). PD-L1-positive melanoma exhibited a higher proportion of BRAFV600E-positive melanoma than PD-L1-negative melanoma (P=0.0317; 39.4 and 0%, respectively). PD-L1-positive melanoma exhibited significantly higher rates of CD8+ and FOXp3+ lymphocyte infiltration than PD‐L1‐negative melanoma (P=0.0221 and P=0.0463, respectively). By contrast, PD-1+ lymphocytes did not differ between PD-L1-positive and -negative cases. Furthermore, HLA-positive melanoma exhibited higher proportions of PD-1 (P=0.0101; 53.7 and 15.4%) and CD8 than HLA-negative melanoma (P=0.0818; 66.7 and 38.2%). These results provided useful information regarding tumor immunity in GM and SM and may contribute to the development of treatment strategies for GM.