Abstract Body (Do not enter title and authors here): Salt-sensitivity of blood pressure (SSBP) is an independent risk factor for cardiovascular morbidity and mortality. The exact mechanism by which salt intake increases blood pressure is complex and not fully understood. We previously found that sodium entry into antigen-presenting cells (APCs) via the epithelial sodium channel (ENaC) activates proinflammatory cytokines to activate T cells and modulate salt-sensitive hypertension. The activator protein 1 (AP-1) transcriptional factors (FOS/JUN) have been implicated in activating the pro-inflammatory pathway, but its role in SSBP is unknown. We hypothesized that high salt activates the AP-1 signaling pathway and inflammation in APCs and contributes to SSBP. Our bulk RNA-sequencing data in human monocytes, demonstrated that high salt increases the expression of the AP-1 gene family, FOS (2,378.1 ± 480.7 vs 6,494.0 ± 945.5, p= 0.0009), and JUN (7,313.9 ± 984.9 vs11,370.0 ± 1,286.3, p= 0.0015) compared to normal salt-treated monocytes. In additional experiments, we enrolled patients with hypertension and phenotype them for SSBP using an established inpatient protocol of salt-loading/depletion and performed single-cell transcriptomic analyses on peripheral blood mononuclear cells (PBMCs). We observed expression of the e FOS (r= 0.5041, p= 0.0464) and JUN (r= 0.7083, p= 0.0021) genes changes in concert with blood pressure in salt-sensitive (SS) but not in salt-resistant (SR) patients. To confirm whether genes expression translates to protein expression, we cultured total splenocytes from salt-sensitive SV/129 mice for 24 hours in either normal (150 mM) or high salt media (190 mM) and performed flow cytometry. Compared to normal salt, high salt-induced a significant increase in the expression of FOS-JUN genes in monocytes of the spleen. We also adoptively transferred PBMCs from salt-sensitive (SS) and salt-resistant (SR) hypertensive individuals into immunodeficient NSG mice, treated with the high salt diet for three weeks and performed flow cytometry on immune cells of the kidney, aorta, and spleen. We found that immune cells from SS people demonstrated a higher propensity to infiltrate mouse tissues with increased expression of FOS-JUN genes than immune cells from SR people. These preliminary findings disclose the role of the AP-1 gene family in salt-sensitive hypertension and may provide a potential therapeutic target for the treatment and diagnosis of SSBP.