328MO Residual tumour immune microenvironment shapes relapse risk in early TNBC following immunotherapy

Background: Adoptive cell therapy with TILs (TIL-ACT) can induce objective responses in a considerable proportion of patients with anti-PD-1-resistant melanoma, but over half of initial responders develop acquired resistance (AR).We conducted multiomic profiling of paired tumors to study the genomic and functional features of AR. Methods:We analyzed matched baseline and progression tumors from 19 patients with initial clinical benefit across four TIL trials (NCT00937625, NCT02379195, NCT02354690, NCT02278887).11/19 patients were anti-PD-1 resistant, with 18/19 resistant to anti-PD-1 and/or anti-CTLA-4.DNA-seq (n=15) was performed in combination with spatial transcriptomics (n=6), RNASeq (n=18), and longitudinal in-vitro T-cell reactivity (n=5 at baseline, n=9 at progression).Results: 13/15 tumors carried known alterations associated with genomic immune escape (GIE) at baseline, as expected in a cohort with resistance to previous immunotherapy.In progressing tumors, we found de novo alterations in genes linked to known melanoma malignant transformation (i.e affecting proliferation) in 4/15 cases, and GIE in 3/15 cases, totaling 7/15 patients developing previously described resistanceassociated genomic alterations (RGA).While tumor reactive TILs could still be recovered from 6/9 progressing biopsies (of which 1 with GIE), a trend towards decreased immunogenicity was observed in patients with RGA (mean log2FC = -0.15;p = 0.18; n = 6).In 3/5 patients, decreased CD8 TIL reactivity was observed at progression (mean log2FC = -0.587;p = 0.28; n = 1 with GIE).RNASeq analysis (n=18) revealed heterogeneous tumor states at progression with downregulation of interferon signaling and antigen presentation pathways (FDR < 0.01).Through spatial integration, in a patient with GIE we observed the co-occurrence of phenotypic reprogramming of tumor cells into a MITF -VEGF + neural-crest-like phenotype, associated with T cell exclusion. Conclusions:In patients with melanoma developing acquired resistance to TIL-ACT, GIE and T-cell immune evasion phenotypes are recurrent, alongside alterations linked to melanoma progression and malignancy.Tumor-reactive TILs in progressing biopsies demonstrate a potentially ongoing, but incompletely effective immune surveillance.