Introduction: PACT Pharma is a clinical-stage adoptive cell therapy company that develops personalized neoTCR-T cells using a state-of-the-art approach to discover and validate predicted neoepitopes and their cognate T cell receptors (clinical trial NCT03970382). An important mechanism of resistance to adoptive cell therapy targeting tumor-specific neoantigens is the genetic loss of human leukocyte antigen (HLA) alleles or alterations in HLA expression in tumor cells. Therapies targeting neoantigens presented by HLA alleles that have either been deleted, mutated or are strongly downregulated are likely to lack efficacy due to the absence or reduction of epitope presentation. Surveying neoepitope presentation escape mechanisms is key for personalized immunotherapy.

Methods: PACT Pharma has developed PACT-ESCAPE, a method that integrates DNA and RNA sequencing data to quantify allelic imbalance at HLA loci, including the most extreme case of loss of heterozygosity (LOH). Allelic imbalance is first determined at the DNA level by determining the patient’s maternal and paternal haplotypes across chromosome 6p to infer their copy number states. Twenty-six HLA Class I and II loci from chromosome 6 are then genotyped at high resolution, enabling accurate assignment of copy number states to the patient’s alleles, including any alleles that have been deleted in the tumor. Unlike other currently available methods, PACT-ESCAPE also measures the relative RNA expression of HLA alleles to provide an orthogonal estimate of allelic imbalance across chromosome 6 and support loss of heterozygosity calls made at the DNA level. Presentation machinery genes are also surveyed for loss of function mutations. To benchmark PACT-ESCAPE, we compared loss of heterozygosity calls at HLA-A, HLA-B, and HLA-C between our method and a previously published DNA-based HLA LOH classifier for 17 samples for which we had matched tumor/normal whole exome sequencing.

Results: We found 100% concordance between PACT-ESCAPE and the benchmark, with four samples showing LOH at HLA-A, HLA-B, and HLA-C at the DNA level. Next, we evaluated RNA sequencing from tumor samples and confirmed that the relative expression of the lost allele compared to the kept allele was lower when LOH was called. High levels of allelic imbalance in gene expression, however, were not unique to samples with LOH, suggesting that differential expression of HLA alleles might also be an important contributor to immune escape in cancer.

Conclusions: Evaluation of allelic imbalance in DNA copy number and RNA expression in PACT-ESCAPE provides new insights into immune escape in cancer. Adoptive cell therapy targeting neoantigens will benefit by enabling the accurate selection of targets most likely to be presented by the tumor.

Citation Format: Chad C. Smith, Yan Ma, Katie Campbell, Zheng Pan, Eric Stawiski. Uncovering HLA loss of heterozygosity and allelic imbalance in cancer for the improvement of personalized neoTCR immunotherapy with PACT-ESCAPE [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2022; 2022 Apr 8-13. Philadelphia (PA): AACR; Cancer Res 2022;82(12_Suppl):Abstract nr 1213.