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Neutrophil L-Plastin Controls Ocular Paucibacteriality and Susceptibility to Keratitis
Xiaoxiao Lu1, Abirami Kugadas1, Kirsten Smith-Page1
1Division of Infectious Diseases, Department of Medicine, Brigham and Women's Hospital and Harvard Medical School, Boston, MA, United States.
Abstract:
Why ocular mucosa is paucibacterial is unknown. Many different mechanisms have been suggested but the comprehensive experimental studies are sparse. We found that a deficiency in L-plastin (LCP1), an actin bundling protein, resulted in an ocular commensal overgrowth, characterized with increased presence of conjunctival Streptococcal spp. The commensal overgrowth correlated with susceptibility to P. aeruginosa-induced keratitis. L-plastin knock-out (KO) mice displayed elevated bacterial burden in the P. aeruginosa-infected corneas, altered inflammatory responses, and compromised bactericidal activity. Mice with ablation of LPL under the LysM Cre (LysM. CreLPL ) and S100A8 Cre (S100A8.CreLPL ) promoters had a similar phenotype to the LPL KOs mice. In contrast, infected CD11c.CreLPL mice did not display elevated susceptibility to infection, implicating the myeloid L-plastin-sufficient cells (e.g., macrophages and neutrophils) in maintaining ocular homeostasis. Mechanistically, the elevated commensal burden and the susceptibility to infection were linked to defects in neutrophil frequencies at steady state and during infection and compromised bactericidal activities upon priming. Macrophage exposure to commensal organisms primed neutrophil responses to P. aeruginosa, augmenting PMN bactericidal capacity in an L-plastin dependent manner. Cumulatively, our data highlight the importance of neutrophils in controlling ocular paucibacteriality, reveal molecular and cellular events involved in the process, and suggest a link between commensal exposure and resistance to infection.
Insights
L-plastin deficiency in immune cells allows eye bacteria to overgrow, increasing infection risk. Neutrophils, crucial for eye health, require L-plastin for effective bacterial control.
Area of Science:
- Immunology
- Microbiology
- Ocular Surface Disease
Background:
- The ocular surface is typically sparsely populated with bacteria, a state known as paucibacteriality.
- The mechanisms maintaining this balance are not fully understood, with limited experimental data available.
Purpose of the Study:
- To investigate the role of L-plastin (LCP1), an actin-bundling protein, in maintaining ocular mucosal homeostasis.
- To determine the impact of L-plastin deficiency on commensal bacterial load and susceptibility to bacterial keratitis.
Main Methods:
- Generation and analysis of L-plastin knockout (KO) mouse models using various Cre-lox systems (LysM Cre, S100A8 Cre, CD11c Cre).
- Assessment of bacterial burden (conjunctival and corneal), inflammatory responses, and bactericidal activity in response to *Streptococcal* spp. and *Pseudomonas aeruginosa* infection.
- Evaluation of immune cell populations, particularly neutrophils and macrophages, and their functional responses.
Main Results:
- L-plastin deficiency in mice led to ocular commensal overgrowth (*Streptococcal* spp.) and increased susceptibility to *P. aeruginosa*-induced keratitis.
- L-plastin KO mice exhibited higher bacterial burden, altered inflammation, and reduced bactericidal capacity.
- Myeloid cells expressing L-plastin (macrophages, neutrophils) are critical for ocular homeostasis, as L-plastin sufficient myeloid cells maintained resistance to infection.
- Defects in neutrophil frequency and compromised neutrophil bactericidal activity were linked to L-plastin deficiency.
- Macrophages primed neutrophil responses to *P. aeruginosa* in an L-plastin-dependent manner, enhancing bacterial killing.
Conclusions:
- Neutrophils play a vital role in controlling ocular paucibacteriality.
- L-plastin is essential for neutrophil function, including bacterial clearance and maintaining ocular homeostasis.
- Commensal bacterial exposure, modulated by L-plastin, influences resistance to ocular infections.
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