Abstract Introduction: The use of preoperative CDK4/6 inhibitors combined with endocrine treatment remains investigational in breast cancer (BC), while their effect on tumor-immune microenvironment (TIME) in luminal BC is the subject of few published studies. The aim of this study was to characterize the treatment-induced changes of TIME at a spatial proteomic and bulk tumor transcriptomic level in clinical luminal A BC patients treated with neoadjuvant palbociclib and endocrine treatment. Methods: The PREDIX Luminal A phase II randomized clinical trial (NCT02592083) included patients with stage 2-3 BC and IHC-defined luminal A biology (ER+ and PR+, HER2-, low proliferation) treated with preoperative ET (tamoxifen or aromatase inhibitor) with or without the addition of CDK4/6i palbociclib, based on the early change of Ki67 levels. A core biopsy was obtained at baseline, after 10 weeks of treatment (on-treatment) and at surgery. The study was closed after the inclusion of 10 patients due to slow accrual. By using formalin-fixed paraffin-embedded tissue from all timepoints, we performed the GeoMx® Digital Spatial Profiling (DSP, NanoString, Seattle, WA, USA) for the spatial profiling of 27 immune cell and tumor markers (immune cell profiling core and pan-tumor panels) both at intra-epithelial and stromal tissue segments. Upon data normalization, a linear mixed model was adopted for differential marker expression assessment between the different timepoints and treatment arms, using the DSP Suite® software. RNA was also extracted from fresh-frozen biopsies and the nCounter® Breast Cancer 360™ panel (NanoString) was used for gene expression profiling of 776 targets and data analysis was performed using the nSolver® software. Results: A total of 7 patients received palbociclib + ET and 3 were treated with ET only. All patients experienced an on-treatment radiological partial response while no patient achieved pathological complete response. While ET suppressed the ER axis in both palbociclib-treated and untreated patients, treatment with palbociclib additionally led to cell-cycle arrest as evidenced by a decrease in proliferation and downregulation of cell-cycle gene pathways. Immune-related protein markers were enriched in the stroma segments and pan-tumor markers were enriched in the intra-epithelial segments at all timepoints. In palbociclib-treated patients, immune cell markers (e.g. CD45, CD68, CD20, CD4, HLA-DR) were significantly enriched in on-treatment samples (n=3) both in tumor and stromal segments, while these expression changes were reversed between on-treatment and surgery. In the ET-treated patients immune markers reflecting e.g. T-, B- and/or antigen-presenting-cells were significantly enriched at the operation samples (n=2), in the intra-epithelial but not in the stromal areas. This finding was confirmed also at the gene level, where immune-related pathways (e.g. chemokine and cytokine activity and receptor binding) were upregulated in the palbociclib-treated patients on-treatment but they were partially abrogated at surgery. The ET-only treated patients presented with enhanced immune activation gene sets at both on- and post-treatment timepoints Conclusion: The findings of this small hypothesis-generating study indicate an upregulation of immune function during treatment with palbociclib and ET and an altered tumor microenvironment. The prognostic and potential therapeutic implications of sensitizing tumors to subsequent chemotherapy and immunotherapy need to be further investigated in larger studies. Citation Format: Ioannis Zerdes, Dimitrios Salgkamis, Alexios Matikas, Lars Selander, Kang Wang, Evangelos Tzoras, Emmanouil Sifakis, Susanne Agartz, Xinsong Chen, Mats Hellström, Johan Hartman, Jonas Bergh, Theodoros Foukakis, Thomas Hatschek. Longitudinal tumor-immune microenvironment changes in patients with clinical luminal A early breast cancer treated with neoadjuvant palbociclib and endocrine therapy: results from the Swedish randomized PREDIX Luminal A trial [abstract]. In: Proceedings of the 2022 San Antonio Breast Cancer Symposium; 2022 Dec 6-10; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2023;83(5 Suppl):Abstract nr P2-21-02.