1653P - Assessment of PD-L1 expression on circulating tumor cells (CTCs) and immune cells in peripheral blood of patients with small cell lung cancer (SCLC)

Background

PD-L1 expression has been reported on tumor cells and tumor-infiltrated immune cells in small cell-lung cancer (SCLC) tissues, however its expression in the blood compartment is largely unexplored. We herein assessed PD-L1 expression on circulating tumor cells (CTCs) and immune cells in the peripheral blood (PB) of SCLC patients.

Methods

PB was obtained from 54 SCLC patients (limited-stage: n=12; extensive-stage: n=42) prior to first-line treatment. Immunofluorescence staining for cytokeratins (CK) and PD-L1 (Clone: E1L3N) was performed on peripheral blood mononuclear cell (PBMC) cytospins. PD-L1 expression was individually assessed and quantified on CK+ circulating tumor cells (CTCs) and PBMCs using the Ariol microscopy system.

Results

CK+ CTCs were identified in 51.1% of patients. PD-L1+ CTCs and PD-L1^high CTCs were detected in 29.8% and 17% of patients, respectively, and represented 48.4% and 22.6% of CTCs. Also, PD-L1+ PBMCs and PD-L1^high PBMCs were detected in 78.9% and 53.7% of patients, respectively. A higher percentage of PD-L1+ PBMCs and PD-L1^high PBMCs was demonstrated in CTC-positive as compared to CTC-negative patients (median: 21.9% vs 6.7%, p=0.017, and 6.5% vs 0%, p=0.026, respectively). The proportion of PD-L1+PBMCs was positively associated with the number of CTCs (p=0.013), PD-L1+ CTCs (p=0.007) and PD-L1^high CTCs (p=0.034). PD-L1^high PBMCs were more frequently detected in patients with extensive-stage SCLC (p=0.046) and those with bone metastases (p=0.039). There was no association between the detection of CTCs or PD-L1^high CTCs and patient outcome. However, a reduced overall survival (OS) was recorded among patients with PD-L1^high PBMCs (median OS: 8.8 vs 15.8, p=0.002, Kaplan Meier analysis) as well as among patients with extensive-stage disease harboring PD-L1^high PBMCs (median OS: 8.7 vs 14.2, p=0.030).

Conclusions

PD-L1 is frequently expressed on CTCs and PBMCs in SCLC. PD-L1 expression on PBMCs is associated with CTC detection and poor patient outcome. These findings indicate that the expression of PD-L1 in the peripheral immune compartment merits further investigation as a prognostic marker in SCLC.

Clinical trial identification

Editorial acknowledgement

Legal entity responsible for the study

The authors.

Funding

Hellenic Society of Medical Oncology (HESMO) and Anticancer Research Support Association (ARSA), Heraklion, Greece.

Disclosure

All authors have declared no conflicts of interest.