Bacterial DNA delays human eosinophil apoptosis

Area of Science:

• Immunology
• Molecular Biology
• Respiratory Medicine

Background:

• Asthma is exacerbated by bacterial infections, partly due to increased eosinophil survival.
• Eosinophil apoptosis is crucial for resolving airway inflammation.
• Current CpG oligodeoxynucleotide (ODN) therapies for asthma have shown limited success in patients.

Purpose of the Study:

• To investigate the impact of bacterial DNA and CpG ODNs on human eosinophil apoptosis in vitro.
• To elucidate the signaling pathways involved in bacterial DNA and CpG ODN-mediated effects on eosinophils.
• To explore potential novel mechanisms of bacterial DNA in exacerbating eosinophilic lung inflammation.

Main Methods:

• Human eosinophils isolated from peripheral blood.
• Apoptosis assessed via flow cytometry (DNA content, Annexin-V staining) and morphology.
• Toll-like receptor 9 (TLR9) expression analyzed by western blotting and intracellular flow cytometry.
• Inhibitor studies used to determine involvement of PI3K, NF-kappaB, and MAPKs.

Main Results:

• Bacterial DNA and CpG ODNs delayed spontaneous eosinophil apoptosis; vertebrate DNA did not.
• CpG ODN effects were TLR9-dependent, requiring endosomal acidification.
• TLR9 was expressed in eosinophils from both atopic and healthy donors.
• CpG ODN-induced anti-apoptosis involved PI3K and NF-kappaB pathways.
• Bacterial DNA's anti-apoptotic effect was independent of CpG motifs, suggesting a novel recognition structure.

Conclusions:

• CpG ODNs, via TLR9, can inhibit eosinophil apoptosis, but bacterial DNA utilizes a distinct, CpG-independent mechanism.
• Bacterial DNA's ability to suppress eosinophil apoptosis may contribute to persistent eosinophilic airway inflammation during respiratory infections.
• This study reveals a novel pathway for bacterial DNA in exacerbating asthma, distinct from CpG-mediated effects.