HO-1/Nrf2 activation orchestrates protection in sepsis-induced lung injury by suppressing CCR2hi monocyte recruitment and MAPK-driven inflammation.

Sepsis-induced acute lung injury (ALI) remains a devastatingly lethal clinical syndrome driven by aberrant inflammatory dysregulation, wherein monocytes play critical roles in disease pathogenesis. This study elucidates the mechanistic basis by which the HO-1 inducer Hemin alleviates ALI by activating the HO-1/Nrf2 pathway to target pro-inflammatory monocytes. RNA-seq analysis revealed that the most prominently dysregulated genes in LPS-stimulated human THP-1 monocytes (relative to untreated controls) were predominantly enriched in pathways governing inflammatory responses and oxidative stress. In vitro experiments revealed that Hemin suppressed the p38-MAPK/mTOR pathways in human monocytes, inhibiting inflammatory activation, differentiation, and LPS-induced cell death while preserving phagocytosis. The murine ALI model was established in WT, CCR2-/-, and Nrf2-/- mice via tail vein injection of LPS, with assessments conducted 12 h later. In LPS-challenged mice, Hemin pretreatment selectively inhibited the recruitment of CCR2hi monocytes (but not CCR2lo monocytes or neutrophils) into the lungs, thereby attenuating histopathological injury, reducing TNF-α and IL-6 levels, and diminishing monocyte-derived macrophages and their M1/M2 polarization. CCR2 deficiency not only abrogated the therapeutic efficacy of Hemin in ALI, evidenced by the failure to prevent the LPS-induced increase in the proportion of monocyte-derived macrophages and the elevation of macrophage polarization, but also paradoxically elevated pulmonary TNF-α concentrations. Furthermore, experiments using Nrf2-/- mice revealed that the protective benefits of Hemin are strictly Nrf2-dependent. Nrf2 deficiency prevented Hemin from restoring the redox balance (GSH/GSSG ratio) and abolished its systemic and pulmonary anti-inflammatory effects, along with its suppression of CCR2hi subsets and inhibition of macrophages polarization. Collectively, our findings establish that activation of the HO-1/Nrf2 pathway mitigates ALI by selectively targeting CCR2hi pro-inflammatory monocytes, positioning Hemin as a promising therapeutic candidate for ALI and identifying the proportion of CCR2hi monocyte and Nrf2-mediated redox markers as potential biomarkers to guide precision medicine strategies for ALI management.