Abstract NG01: Evolution of neoantigen landscape during immune checkpoint blockade in non-small cell lung cancer

Tumor cells contain nonsynonymous somatic mutations that alter the amino acid sequences of the proteins encoded by the affected genes. Those alterations are foreign to the immune system and may therefore represent tumor-specific neoantigens capable of inducing antitumor immune responses. Somatic mutational and neoantigen density has recently been shown to correlate with long-term benefit from immune checkpoint blockade in non-small cell lung cancer (NSCLC) and melanoma, suggesting that a high density of neoepitopes stemming from somatic mutations may enhance clinical benefit from blockade of immune checkpoints that unleash endogenous responses to these mutation-associated neoantigens (MANAs). Expression of the programmed cell death ligand 1 (PD-L1) in tumors or tumor-infiltrating immune cells has been associated with responses to PD-1 blockade; however, PD-L1 expression or other immune biomarkers have not been sufficient to fully explain therapeutic outcomes. Among the patients that initially respond to PD-1 blockade, some become resistant to the therapy. Up-regulation of alternate immune checkpoints, loss of HLA haplotypes, or somatic mutations in HLA or JAK1/JAK2 genes have been proposed as mechanisms of evasion to immune recognition in some patients, but the mechanisms underlying response and acquired resistance to immune checkpoint blockade have remained elusive. To examine mechanisms of resistance to immunotherapy, we performed genome-wide sequence analysis of protein coding genes and T-cell receptor (TCR) clonotype analysis, followed by functional assays of autologous T-cell activation of patients who demonstrated initial response to immune checkpoint blockade but ultimately developed progressive disease.Of a cohort of 42 NSCLC patients treated with single-agent PD-1 or combined PD-1 and CTLA4 blockade, we identified all consecutive cases that at the time of the analysis developed acquired resistance (two subjects treated with nivolumab and two with ipilimumab and nivolumab) and where paired tumor specimens were available both before and after therapy. To examine the landscape of genomic alterations and associated neoantigens, we performed whole exome sequencing of tumors from these patients. Pretreatment and postprogression specimens were obtained from the same anatomic location or from sites in close anatomic proximity. We examined multiple immune-related parameters of peptides stemming from somatic alterations using a computational multidimensiona