Science - USA (2021-12-24)

RESEARCH ARTICLE SUMMARY

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IMMUNOLOGY

Fibrin is a critical regulator of neutrophil effector

function at the oral mucosal barrier

Lakmali M. Silva, Andrew D. Doyle, Teresa Greenwell-Wild, Nicolas Dutzan, Collin L. Tran,
Loreto Abusleme, Lih Jiin Juang, Jerry Leung, Elizabeth M. Chun, Andrew G. Lum, Cary S. Agler,
Carlos E. Zuazo, Megan Sibree, Priyam Jani, Vardit Kram, Daniel Martin, Kevin Moss,
Michail S. Lionakis, Francis J. Castellino, Christian J. Kastrup, Matthew J. Flick, Kimon Divaris,
Thomas H. Bugge, Niki M. Moutsopoulos

INTRODUCTION:Mucosal barrier sites are con-
tinuously exposed to diverse environmental
stimuli, and the fine-tuning of mucosal tissue–
specific immune responses is key to main-
taining tissue homeostasis. However, specific
mechanisms of mucosal barrier immunity re-
main to be elucidated. In humans, congenital
deficiency in plasminogen—the proenzyme form
of the abundant plasma protease plasmin—
leads to severe mucosal inflammatory disease
with oral (ligneous periodontitis), ocular (lig-
neous conjunctivitis), lung, vaginal, and gastro-
intestinal tract involvement, which implicates
plasmin in mucosal barrier homeostasis.

RATIONALE:Fibrin is the principal physiologi-
cal target of plasmin, and abnormal inflamma-
tory responses associated with plasminogen
deficiency in humans and in animal models
have been linked to extravascular fibrin dep-
osition. In the oral cavity, excess extravascular

fibrin is hypothesized to underlie severe oral

mucosal (gingival) inflammation, supporting

bone (periodontal) destruction, and adolescent

loss of dentition associated with plasminogen

deficiency in humans. Mucosal inflamma-

tion surrounding dentition and destruction of

underlying bone are also hallmarks of com-

mon forms of periodontitis. In this study, we

sought to determine the mechanistic link

between fibrin deposition and oral mucosal

immunopathology.

RESULTS:We employed an array of genetically

engineered mouse models and pharmacolog-

ical inhibitors, complemented by histologi-

cal and genetic studies in humans, to probe

the association between fibrin deposition and

local immunopathology. Consistent with find-

ings in plasminogen-deficient humans, mice

lacking plasminogen displayed extravascular

fibrin accumulation, gingival inflammation,

and periodontal bone loss. Oral immunopath-

ology in plasminogen deficiency was prevented

by genetic or pharmacological elimination of

the fibrin precursor, fibrinogen, demonstrat-

ing the critical role of fibrin in plasminogen

deficiency–associated mucosal immunopathology.

Notably, oral immunopathology was signifi-

cantly reduced in plasminogen-deficient germ-

free mice compared with specific pathogen–free

mice. This suggests that commensal micro-

bial communities represent a trigger for oral

mucosal fibrin deposition and associated

immunopathology.

Oral lesions in plasminogen-deficient mice

were sites of neutrophil accumulation prox-

imal to fibrin deposits. Notably, plasminogen-

deficient mice expressing a mutant fibrin lacking

the binding site for the myeloid integrinaMb 2

did not develop periodontal disease, despite

featuring unabated gingival fibrin deposi-

tion and neutrophil accumulation. Fibrin

engagement byaMb 2 promoted neutrophil

effector functions, including the production of

reactive oxygen species and neutrophil extra-

cellular trap (NET) formation (NETosis). Ac-

cordingly, genetic or pharmacological ablation

of NETosis reduced periodontal bone destruc-

tion in plasminogen-deficient mice. Thus, neu-

trophil activation, through fibrin-neutrophil

engagement, triggered oral mucosal immu-

nopathology. Abrogation of fibrin-neutrophil

engagement also reduced age-associated peri-

odontal immunopathology under normal

fibrinolytic conditions, which was sugges-

tive of a broader role for this mechanism

in non-Mendelian forms of periodontitis.

Consistent with this notion, variation in the

humanPLGgene, which encodes plasmino-

gen, was associated with increased prevalence

of common forms of severe periodontal dis-

ease in a cohort of Americans of European

ancestry.

CONCLUSION:We identify fibrin as a critical

immune regulator of oral mucosal barrier

homeostasis, mediated by the local engage-

ment and activation of neutrophils. These

findings provide insights into the role of

fibrin in neutrophil activation and oral mu-

cosal immunity under both defective and

normal fibrinolysis. Our findings pave the

way for the investigation of fibrin-neutrophil

engagement as a target for the prevention or

treatment of the prevalent human disease

periodontitis.

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RESEARCH

SCIENCEscience.org 24 DECEMBER 2021•VOL 374 ISSUE 6575 1575

The list of author affiliations is available in the full article online.

*Corresponding author. Email: [email protected] (T.H.B.);

[email protected] (N.M.M.)

Cite this article as L. M. Silvaet al.,Science 374 , eabl5450

(2021). DOI: 10.1126/science.abl5450

READ THE FULL ARTICLE AT

https://doi.org/10.1126/science.abl5450

Fibrin–neutrophil-mediated

immunopathology

Homeostatic inflammation

Commensal microbiome

Neutrophil recruitment

Neutrophil

recruitment

Neutrophil

activation

Plasmin-mediated

fibrinolysis Impaired

fibrinolysis

Fibrin

Plasmin NETs

αMβ 2

Resolution

Neutrophil

activation

Immunopathology

Fibrin-neutrophil engagement mediates oral mucosal immunopathology.In the oral mucosa, commensal
microbiota triggers homeostatic inflammation, fibrin deposition, and associated neutrophil activation,
which physiologically resolves in the setting of efficient plasminogen-mediated fibrinolysis (left). When
fibrinolysis is impaired, fibrin accumulation leads to persistent activation of neutrophils and promotes local
immunopathology (right). Dashed arrows denote mechanisms that are not yet fully defined.