Background INF-gamma secreted by CD8+ lymphocytes upregulates PD-L1 expression in cancer cells. We recently identified STAT3 and YAP1 as compensatory mechanisms of resistance to EGFR tyrosine kinase inhibition in EGFR mutant cells. STAT3 and YAP1 up-regulate CCL5 (Rantes) and CXCL5, respectively, with both chemokines attracting the myeloid-derived suppressor cell. STAT3 stimulates DNMT1 by repressing STAT1 and retinoic acid-inducible gene-I (RIG-I) expression. STAT1 and RIG-I are key mediators in INF-gamma signaling. We assume that alterations in the INF-gamma signaling pathway could be present in EGFR mutant NSCLC.

Method Total RNA from 53 EGFR mutant NSCLC patients was reversed transcribed and analyzed by qRT-PCR. STAT3, YAP1, RIG-I, STAT1, PD-L1, DNMT1 and CXCL5 mRNA were examined with specific primers/probes in triplicates. Progression-free survival (PFS) and overall survival (OS) were estimated.

Result Fifty-three EGFR mutant NSCLC patients treated with erlotinib were analyzed, 72% were female, 62% never-smoked, 70% had exon 19 deletion and 36% brain metastases. A positive correlation was found between RIG-1 and STAT1 (r=0.42, p=0.003). An anti-correlation trend was noted between STAT3 and PD-L1, YAP1 and PD-L1 and DNMT1 and STAT1. Median PFS was 22, 12.9 and 8.6 months for patients with high, intermediate and low PD-L1 mRNA, respectively (P=0.04). Median PFS was numerically longer for patients with low levels of DNMT1, RIG1 STAT1 and CXCL5, although the differences were not statistically significant. A similar trend was observed for OS.

Conclusion PD-L1 mRNA could be a prognostic marker in EGFR mutant NSCLC patients. Down-modulation of PD-L1 indicates alterations in pattern-recognition receptors (PRRs), like RIG-1 or downstream interferon signaling factors. The dysregulation of the pathway is multifactorial, and the role of STAT3 and YAP1 hyperactivation merits further research. DNMT1 overexpression ablates STAT1. Since the cyclin-dependent kinase 4 (CDK4) interacts with DNMT1, therapies with CDK4 inhibitors can directly neutralize the main defects in the INF-gamma signaling pathway.

Keywords PD-L1, EGFR mutant NSCLC, INF-gamma signaling pathway