Monocyte activation by necrotic cells is promoted by mitochondrial proteins and formyl peptide receptors.

OBJECTIVE: Necrotic cells evoke potent innate immune responses through unclear mechanisms. The mitochondrial fraction of the cell retains constituents of its bacterial ancestors, including N-formyl peptides, which are potentially immunogenic. Thus, we hypothesized that the mitochondrial fraction of the cell, particularly N-formyl peptides, contributes significantly to the activation of monocytes by necrotic cells.
DESIGN: Human peripheral blood monocytes were incubated with necrotic cell fractions and mitochondrial proteins to investigate their potential for immune cell activation.
SETTING: University Medical Center Research Laboratory.
SUBJECTS: Healthy human adults served as blood donors.
MEASUREMENTS AND MAIN RESULTS: Human blood monocyte activation was measured after treatment with cytosolic, nuclear and mitochondrial fractions of necrotic HepG2 cells or necrotic HepG2 cells depleted of N-formyl peptides [Rho(0) cells]. The specific role of the high affinity formyl peptide receptor (FPR) was then tested using specific pharmacologic inhibitors and RNA silencing. The capacity of mitochondrial N-formyl peptides to activate monocytes was confirmed using a synthetic peptide conforming to the N-terminus of mitochondrial nicotinamide adenine dinucleotide subunit 6. The results demonstrated that mitochondrial cell fractions most potently activated monocytes, and interleukin (IL)-8 was selectively released at low-protein concentrations. Mitochondria from Rho(0) cells induced minimal monocyte IL-8 release, and specific pharmacologic inhibitors and RNA-silencing confirmed that FPR contributes significantly to monocyte IL-8 responses to both necrotic cells and mitochondrial proteins. N-formyl peptides alone did not induce monocyte IL-8 release; whereas, the combination of mitochondrial N-formyl peptides and mitochondrial transcription factor A (TFAM) dramatically increased IL-8 release from monocytes. Likewise, high mobility group box 1, the nuclear homolog of TFAM, did not induce monocyte IL-8 release unless combined with mitochondrial N-formyl peptides.
CONCLUSIONS: Interactions between mitochondrial N-formyl peptides and FPR in the presence of other mitochondrial antigens (e.g., TFAM) contributes significantly to the activation of monocytes by necrotic cells.