Simple Analysis Using Immunohistochemical Staining for Tumor-Infiltrating Lymphocytes in Brain Metastasis of Small Cell Lung Cancer

Background: We investigated the distribution of tumor-infiltrating lymphocytes (TILs) and the expression of programmed cell death-ligand 1 (PD-L1) in patients with brain metastasis (BM) from small cell lung cancer (SCLC). Methods: A retrospective analysis was performed on 12 surgical specimens of BMs from SCLC at our institute for 5 years. The Immunofluorescence-based Tissue Microenvironment Analysis Panel (MAP) was utilized for the detection of TILs, including CD3, CD8, PD-1, and PD-L1, in pathological archival specimens of BMs. The correlation between the overall survival (OS) and the above-mentioned markers was analyzed in the patients.Results: Positive rates of CD3+ TILs in the tumor parenchyma versus tumor stroma were 0.60±0.94% versus 1.76±2.72% (p=0.010), respectively; positive rates of CD8+ TILs in the tumor parenchyma versus tumor stroma were 0.80±0.78% versus 2.46±3.72% (p=0.016), respectively. There was no co-expression of CD8+ and PD-1+ TILs in the tumor parenchyma of 11 cases, and the infiltration density of co-expressed CD3+ and PD-1+ TILs was more than 10/mm2 in only 1 case. There was no co-expression of CD3+ and PD-1+ TIL in the stroma of 10 cases, and the infiltration density of CD8+ and PD-1+ TILs was more than 10/mm2 in 2 cases. Immunohistochemistry was used to detect the expression of PD-L1 in 12 cases of BMs, and 3 cases (25%) were positive. Survival analysis showed that patients with positive CD3+ TILs had significantly longer OS (p=0.040). Conclusions: The distribution of TILs in BM of SCLC is low and mainly distributed in the stroma, with the low expression of PD-L1 in the tumor tissues.