Macrophages mediate acute kidney allograft rejection via a toll-like receptor 4-dependent mechanism.

INTRODUCTION Macrophages play an important role in acute renal allograft rejection (RAR). Here we report that macrophages mediate acute RAR via a toll-like receptor 4 (TLR4)-dependent mechanism. METHODS Acute RAR was induced in myeloid cell-specific TLR4 knockout mice. Rejection severity was assessed by Banff scores, kidney immune cell infiltration/activation, and kidney function. Mechanisms were investigated using single cell RNA sequencing (scRNA-seq) and pharmacological hypoxia-inducible factor (HIF)1α inhibition. Immune profiling evaluated macrophage polarization, antigen presentation and T cell subsets. RESULTS Disruption of myeloid TLR4 largely suppressed acute RAR, evidenced by lower Banff scores, inhibited kidney immune cell infiltration and activation, and preserved kidney function. scRNA-seq identified HIF1Α as a key downstream target of myeloid TLR4. Importantly, both genetic TLR4 ablation and pharmacological HIF1α inhibition attenuated acute RAR via reducing proinflammatory macrophages (F4/80+ iNOS+ and F4/80+ HIF1α+), suppressing antigen presenting by F4/80+MHCII+ macrophages and modulating T cell immunity by suppressing CD8+ T cells and Th1 cells, while expanding regulatory T cell and Th2 populations. Notably, targeting TLR4/HIF1α signaling also ameliorated fibrosis in long-term allograft rejection. CONCLUSIONS Macrophages mediate acute RAR via a TLR4/HIF1α-dependent mechanism. Targeting TLR4/HIF1α signaling may be a novel therapeutic strategy for acute RAR, with additional benefits mitigating chronic allograft injury.