Four distinct immune microenvironment subtypes in gastric adenocarcinoma with special reference to microsatellite instability

Introduction: Programmed death-ligand 1 (PD-L1) can be overexpressed in tumours other than Epstein-Barr virus (EBV)-positive (EBV⁺) or microsatellite instability-high (MSI-H) gastric cancer (GC) subtypes. We aimed to determine the tumour immune microenvironment (TME) classification of GC to better understand tumour-immune interactions and help patient selection for future immunotherapy with special reference to MSI-H.

Methods: Immunohistochemistry (IHC) for PD-L1 and CD8⁺ T cells in three distinct subtypes of GC (43 EBV⁺, 79 MSI-H and 125 EBV^-/MSS) were performed and analysed. In 66 MSI-H GC, mutation counts were compared with PD-L1 expression and survival of the patients.

Results: GC TME divided by PD-L1 IHC and tumour-infiltrating lymphocytes (TIL) measured by intratumoural CD8 density showed: (1) about 40% of GC are type I (PD-L1⁺/TIL⁺) consisting ~70% of MSI-H or EBV⁺ GC, and ~15% of EBV^-/microsatellite stable (MSS) GC patients show the best survival in both disease-free (HR 2.044) and overall survival (HR 1.993); this type would respond to a checkpoint blockade therapy; (2) almost 30% of GC are type II (PD-L1^-/TIL^-) with the worst survival; (3) approximately 10% of GC are type III (PD-L1⁺/TIL^-); and (4) up to 20% are type IV (PD-L1^-/TIL⁺) and, unexpectedly, ~25% of EBV⁺ or MSI-H GC are within this subtype. In MSI-H GC, frequent frameshift mutations were observed in ARID1A, RNF43, NF1, MSH6, BRD3, NCOA3, BCORL1, TNKS2 and NPM1 and the numbers of frameshift mutation correlated significantly with PD-L1 expression (P<0.05).

Discussion: GC can be classified into four TME types based on PD-L1 and TIL, and numbers of frameshift mutation correlate well with PD-L1 expression in MSI-H GC.