Dietary carboxymethylcellulose metabolite promotes heart allograft rejection through induction of proinflammatory macrophages via lysophosphatidic acid.

Carboxymethylcellulose (CMC), a common food emulsifier, induces microbiota dysbiosis and systemic inflammation; however, its impact on transplant immunity remains unclear. Allogenic heart rejection was observed in CMC-fed recipient mice, with increased abundance of lysophosphatidic acid (LPA)-producing bacteria and increased serum LPA concentration. CMC-induced transplant rejection was caused by the gut microbiota, as confirmed by fecal microbiota transplantation and gut microbiota depletion. Furthermore, LPA-treated macrophages demonstrated a proinflammatory ability to accelerate allograft rejection in cytotoxic T lymphocyte-associated protein 4 immunoglobulin-induced allograft survival by upregulating glycolysis. Conversely, the administration of a glycolysis inhibitor resulted in allograft survival and abrogated the detrimental effect of LPA. Mass spectrometry and single-cell RNA sequencing confirmed that transplant patients with rejection showed significantly elevated serum LPA levels and LPA receptor 6 (LPAR6) expression in graft-infiltrate macrophages. Mechanistically, LPA preferentially promoted LPAR6 expression, which interacted with Rho-associated protein kinase 2 to activate the mammalian target of rapamycin/hypoxia-inducible factor 1-alpha pathway, thereby enhancing glycolysis and inducing proinflammatory macrophage polarization. Treatment with Ki16425, an LPAR antagonist, prolonged allograft survival in CMC-fed recipients. Our findings reveal a major detrimental effect of CMC on macrophage physiology and suggest that controlling LPAR6 expression or glycolysis in macrophages may improve allograft survival in transplant recipients.