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Background Type I conventional DC (cDC1)-mediated antigen cross-presentation is essential for directing tumor-specific CD8 + T cell responses and facilitating therapeutic responses to checkpoint inhibitor immunotherapy. We recently demonstrated that mregDCs rely on SREBP2-dependent mevalonate biosynthesis while driving regulatory T cell development and inhibiting CD8+ T cell activation.1 Indeed, DC-specific silencing of SREBP2 suppresses tumor progression in situ. Proprotein convertase subtilisin/kexin type 9 (PCSK9) is a soluble protein that is transcriptionally upregulated by SREBP2 and has been recently demonstrated to suppress tumor MHC class I expression. Based on these results, we hypothesized that mregDC expression of PCSK9 plays a key role in suppressing DC antigen cross-presentation and the induction of anti-tumor immunity. Clinical studies to assess PCSK9 blockade in augmenting the efficacy of immunotherapy are ongoing.Methods CD11c +MHCIIhiCD63+ mregDCs and CD11c+MHCIIhiXCR1+/SIRPa+ cDC1s/cDC2s were isolated by FACS. mregDC and cDC PCSK9 expression levels were quantified by qrt-PCR and ELISA. mregDCs were co-cultured with OVA-pulsed cDC1s/cDC2s and OT-I CD8+ T cells or OT-II CD4+ T cells in the presence or absence of anti-PCSK9 antibody (ab) or recombinant PCSK9 (rPCSK9). T cell proliferation and activation states were measured by multi-parameter flow cytometry, while cDC transcriptional alterations were characterized by RNAseq. The metabolic profile of DCs following exposure to rPCSK9 was analyzed by the SCENITH assay. PCSK9-mediated effects on various aspects of DC biology, including antigen processing, lipid stores, and MHC class I-peptide surface levels, were analyzed by flow cytometry.Results mregDCs express elevated levels of soluble PCSK9 relative to other cDCs in response to tumor-derived lactate. Blocking PCSK9 in mregDC-cDC1 co-cultures augments cDC1-dependent CD8 + T cell activation. Conversely, rPCSK9 impairs cDC-mediated CD8+ T cell and CD4+ T cell activation and proliferation. While rPCSK9 does not influence DC MHC class I or MHC class II surface levels, it does increase DC lipid stores while also impairing early steps of DC endocytic antigen processing.Conclusions Tumor-derived lactate-induced, SREBP2-dependent production of PCSK9 by mregDCs plays a key role in suppressing cDC antigen cross-presentation and stimulation of CD8 + T cell activation. The use of a paracrine soluble mediator to impair nearby cDC functionality significantly enhances the pro-tolerogenic potency of mregDCs and highlights the importance of PCSK9 as a therapeutic target. Additional studies utilizing DC-specific PCSK9-/- mice and PCSK9-/- tumor models are ongoing to characterize how PCSK9 ab therapy influences anti-tumor immunity and to understand what tumors will be more responsive to these agents.Reference Plebanek M, Xue Y, Wang M, Nguyen Y, DeVito NC, Holtzhausen A, Thievanthiran B, Beasley GM, Hanks BA. A SREBF2-dependent gene program drives an immunotolerant dendritic cell population during cancer progression. bioRxiv doi: 10.1101/2023.04.26.538456. PMID: 37162965. Science Immunology 2024 May 10;9(95):eadi4191. doi: 10.1126/sciimmunol.adi4191. PMID: 38728412.