Neutrophil L-Plastin Controls Ocular Paucibacteriality and Susceptibility to Keratitis.

Xiaoxiao Lu,Jeffrey Lamb,Yusha Ru,Sharad K Mittal,Daniel Saban,Kirsten Smith-Page,Sharon Celeste Morley,Mihaela Gadjeva,Tiffany Lin,Abirami Kugadas,Sejiro Littleton,Raina Fichorova,Sunil K Chauhan

doi:10.3389/fimmu.2020.00547

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. CreposLPLfl/fl) and S100A8 Cre (S100A8.CreposLPLfl/fl) promoters had a similar phenotype to the LPL KOs mice. In contrast, infected CD11c.CreposLPLfl/fl 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.

Highlights

L-plastin (LPL) is a leukocyte-specific member of the plastin family of actin remodeling proteins
To determine the impact of L-plastin deficiency on P. aeruginosa–induced keratitis, L-plastin KO mice and wild type (WT) littermates were infected with P. aeruginosa strain 6294
Elevated bacterial counts were detected in the infected LPL KO mice when compared to the WT littermates at 24 and 48 h post-infection (Figures 1A,B, p = 0.0001, p = 0.004, Student’s t-test)

Summary

Introduction

L-plastin (LPL) is a leukocyte-specific member of the plastin family of actin remodeling proteins. Impaired localization of macrophage precursors to the alveoli is reported in LPL deficient mice [8] While both LPL KO mice and CD11c.Crepos-LPLfl/fl mice have defects in handling of pneumococci, it remains unclear whether the lower numbers of LPL-deficient alveolar macrophages or other myeloid defects are causative for the increased susceptibility to disease. To this end, defective responses are detected in L-plastin deficient polymorphonuclear cells (PMNs). L-plastin-deficient PMNs show impaired killing of Staphylococcus aureus and E. coli [9, 10] These studies provide a solid foundation for further work that should elucidate which L-plastin regulated pathways and which myeloid cell types sensitize to infection.

Methods

Results

Conclusion