Small Cell Lung Cancer Neuroendocrine Subtypes are Associated with Different Immune Microenvironment and Checkpoint Molecule Distribution

Abstract

Small cell lung cancer (SCLC) has recently been sub-categorized into neuroendocrine (NE)- high and NE-low subtypes showing immune desert and immune oasis phenotypes, respectively. We aimed to characterize the immune cell localization and the microenvironment according to immune checkpoints and NE subtypes in human SCLC tissue samples at the protein level. In this cross-sectional study, we included 32 primary tumors and matched lymph node (LN) metastases of resected early-stage, histologically confirmed SCLC patients, which were previously clustered into NE subtypes using NE-associated key RNA genes. Immunohistochemistry (IHC) was performed on FFPE TMAs with antibodies against CD45, CD3, CD8 and immune checkpoints including poliovirus receptor (PVR) and Indoleamine 2,3-dioxygenase (IDO). According to our results, the stroma was significantly more infiltrated by immune cells both in primary tumors and LN metastases (vs tumor cell nests). Immune (CD45+) cell density was significantly higher in tumor nests (110.6 {+/-} 24.95 vs 42.74 {+/-} 10.30, cell/mm2, p= 0.0048), with increased CD8+ effector T cell infiltration (21.81 {+/-} 5.458 vs 3.16 {+/-} 1.36 cell/mm2, p < 0.001) in NE-low vs NE-high tumors. Furthermore, the expression of IDO was confirmed on stromal and endothelial cells, and it positively correlated (r= 0.755, p<0.01) with higher immune cell density both in primary tumors and LN metastases, regardless of the NE pattern. Expression of IDO in tumor nests was significantly higher in NE-low (vs NE-high) primary tumors. PVR expression was significantly higher in NE-low (vs NE-high) patients both in primary tumors) and LN metastases. To our knowledge, this is the first human study that demonstrates in situ that NE-low tumors are associated with increased immune cell infiltration compared to NE-high tumors. PVR and IDO are potential new targets in SCLC, with increased expression in the NE-low subtype, providing key insight for further prospective studies on potential biomarkers and targets for SCLC immunotherapies.