Background: Consolidation tumor ratio (CTR) is associated with cancer progression and histological invasiveness in lung adenocarcinoma (LAD). However, little is known about the association between CTR and immune-related factors, including tumor-infiltrating lymphocytes (TILs) density or tumor expression of programmed death ligand 1 (PD-L1) and indoleamine 2,3-dioxygenase 1 (IDO1) in small-sized LAD. Methods: This study included 258 patients with LAD (<3 cm) who underwent surgery. Patients were assigned to four groups: CTR = 0; 0 < CTR <0.5; 0.5 ≤ CTR <1 (ground-glass opacity [GGO] group); and CTR = 1 (pure-solid group). CD4+, CD8+, and FoxP3+ TIL density and PD-L1 and IDO1 tumor expression were assessed by immunohistochemistry. Results: Among the GGO group, CD8+ and FoxP3+ TIL density increased significantly with increasing CTR (p < 0.001 and p < 0.001, respectively). Moreover, PD-L1 and IDO1 expression was significantly higher in the pure-solid group than in the GGO group (p < 0.001 and p < 0.001, respectively). Conclusions: CTR was correlated with the abundance of CD8+ and FoxP3+ TILs in the GGO group. PD-L1 and IDO1 positivity rates were significantly higher in the pure-solid group than in the GGO group. Increased CTR may be correlated with immunosuppressive condition.
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