PPAR-δ senses and orchestrates clearance of apoptotic cells to promote tolerance

Macrophages coordinate the disposal of apoptotic cells. Ajay Chawla and his colleagues show that PPAR-δ, a sensor of fatty acids, is involved in this process. Ingestion of apoptotic cells by macrophages prompts the upregulation of PPAR-δ, which then responds by enhancing the expression of opsonins. Lack of PPAR-δ reduces apoptotic cell clearance and predisposes to autoimmunity ( pages 1246–1248 ). Macrophages rapidly engulf apoptotic cells to limit the release of noxious cellular contents and to restrict autoimmune responses against self antigens. Although factors participating in recognition and engulfment of apoptotic cells have been identified, the transcriptional basis for the sensing and the silent disposal of apoptotic cells is unknown. Here we show that peroxisome proliferator–activated receptor-δ (PPAR-δ) is induced when macrophages engulf apoptotic cells and functions as a transcriptional sensor of dying cells. Genetic deletion of PPAR-δ decreases expression of opsonins such as complement component-1qb (C1qb), resulting in impairment of apoptotic cell clearance and reduction in anti-inflammatory cytokine production. This increases autoantibody production and predisposes global and macrophage-specific Ppard −/− mice to autoimmune kidney disease, a phenotype resembling the human disease systemic lupus erythematosus. Thus, PPAR-δ has a pivotal role in orchestrating the timely disposal of apoptotic cells by macrophages, ensuring that tolerance to self is maintained.