Background: Pre-existing Tumor Antigen specific T-cells describe the endogenous adaptive immunity before any treatment that may represent a valuable novel predictive biomarker for ICI treatment. We investigate the potential value of pre-existing cancer-antigen specific CD8⁺ T-cells as a circulating predictive biomarker. Additionally, we analyze the major differences of known immune-cell phenotypes between Pre-existing positive (PreI+) and Pre-existing negative immunity (PreI^-) NSCLC patients. Here, we present the preliminary results of this study. Methods: Blood was collected from 24 patients with NSCLC, stage III PD-L1⁺, after completion of radical chemoradiation, before initiation of maintenance durvalumab. PBMCs were isolated with Ficoll density gradient centrifugation from patients and 10 healthy donors (HD). PreI was calculated by detecting endogenous IFNg expressing cells with FACS after in-vivo co-cultures of PBMCs with mixes of hTERT, MAGEA1, NY-ESO-1 και Survivin cancer-associated antigens. Immunophenotyping was performed by multi-color flow cytometry using antibodies against CD3, CD4, CD8, CD45RA, CD45RO, CCR7, PD-1, PD-L1 for T-cells. Results: From 24 patients receiving Durvalumab, 10/24 (41.6%) had peptide specific T-cells (PreI ⁺ patients), 60% (6/10) had detectable peptide specific T cells for all (4/4) antigens tested. Survival analysis revealed better PFS in PreI⁺ than PreI ^– patients (Log-rank = 0.06, median 333.5 and 185 days ) and better OS (p = 0.032, median not reached and 254 days). The levels of CD3CD4PD-1 were higher in patients compared to HD (p = 0.019) but there was not a difference between PreI⁺ and PreI^- patients (p = 0.25). PreI⁺ patients had significantly higher numbers of CD3CD8PD-1 cells compared to PreI^- (p = 0.024) but not CD3CD8PD-L1. Prei⁺ patients had high numbers of Teff (CD3+CD8+CD45RA+CD45RO-CCR7) compared to both HD (p = 0.0039) and PreI^- (p = 0.032). Conclusions: Pre-existing tumor antigen specific immunity before initiation of ICI in NSCLC patients could serve as a good predictive factor of response. The study is ongoing.