Small cell lung cancer: The immune microenvironment and prognostic impact of checkpoint expression.

8569 Background: To date, immunotherapy has had limited success in small cell lung cancer (SCLC), despite the tumor’s high mutation load. Little is understood of the immune tumor microenvironment in SCLC due to a paucity of resected tumor. We present a SCLC cohort and describe the prognostic impact of checkpoint expression. Methods: SCLC tissue microarrays with triplicate cores from 105 SCLC specimens underwent IHC assessment for PD-L1, PD-L2, LAG3, TIM3, FoxP3, CD4 and CD8 on tumor and/or tumor infiltrating lymphocytes (TILs). Checkpoint positivity was defined > 5% tumor expression or TIL expression in > 5% of the total core area. Associations with clinicopathologic characteristics and survival were assessed. A Cox model was used for univariate and multivariate survival analysis. Results: Tumor expression of PD-L1 was positive (+) in 17/95 (18%), PD-L2+ in 2/96 (2%), and TIM3+ or LAG3+ in no cases. TILs expressed PD-L1+ in 64/95 (67%), PD-L2+ in 22/96 (22%), TIM3+ in 57/96 (59%) and LAG3+ in 43/96 (45%). FoxP3+ lymphocytes were found in all samples (range 0.02 – 2.98% of total core). TIL expression of PD-L1, PD-L2, TIM3 and LAG3 were all significantly correlated (p value ≤0.001 for all comparisons), and were associated with high FoxP3+ expression. All four checkpoints were expressed on TILs in 20/105 (19%) patients. PD-L1+ and PD-L2+, but not TIM3 or LAG3, on TILs were significantly higher in limited stage compared with extensive stage SCLC (76% v 52%, p 0.045 and 28% v 7%, p 0.02 respectively). There was no association between stage and tumor expression. TIL expression of PD-L1, PD-L2, TIM3 and LAG3 were all associated with improved prognosis. PD-L1+ median OS: 17.2 v 7.9 months (HR 0.36; 95%CI 0.22 – 0.6; p < 0.001). Univariate analysis showed stage and TIL expression of PD-L1, PD-L2, TIM3 and LAG3 were associated with improved survival, but only stage and PD-L1+ TILs remained significant on multivariate analysis (p < 0.01). Conclusions: Immune checkpoint molecules are frequently expressed in SCLC-associated TILs, but not the tumor itself. TIL expression of checkpoint molecules is associated with improved survival. Limited tumor expression of PD-L1 and an exhausted immune cell phenotype may contribute to immunotherapy failure.