O-Polysaccharide Plays a Major Role on the Virulence and Immunostimulatory Potential of Aggregatibacter actinomycetemcomitans During Periodontal Infection.

Emilio A. Cafferata,Eduardo J. Villablanca,Claudia Terraza-Aguirre,Jessica Astorga,Anilei Hoare,Gustavo Monasterio,Rolando Vernal,Francisca Castillo

doi:10.3389/fimmu.2020.591240

Abstract

Aggregatibacter actinomycetemcomitans is a Gram-negative oral bacterium with high immunostimulatory and pathogenic potential involved in the onset and progression of periodontitis, a chronic disease characterized by aberrant immune responses followed by tooth-supporting bone resorption, which eventually leads to tooth loss. While several studies have provided evidence related to the virulence factors of A. actinomycetemcomitans involved in the host cell death and immune evasion, such as its most studied primate-specific virulence factor, leukotoxin, the role of specific lipopolysaccharide (LPS) domains remain poorly understood. Here, we analyzed the role of the immunodominant domain of the LPS of A. actinomycetemcomitans termed O-polysaccharide (O-PS), which differentiates the distinct bacterial serotypes based on its antigenicity. To determine the role of the O-PS in the immunogenicity and virulence of A. actinomycetemcomitans during periodontitis, we analyzed the in vivo and in vitro effect of an O-PS-defective transposon mutant serotype b strain, characterized by the deletion of the rmlC gene encoding the α-L-rhamnose sugar biosynthetic enzyme. Induction of experimental periodontitis using the O-PS-defective rmlC mutant strain resulted in lower tooth-supporting bone resorption, infiltration of Th1, Th17, and Th22 lymphocytes, and expression of Ahr, Il1b, Il17, Il23, Tlr4, and RANKL (Tnfsf11) in the periodontal lesions as compared with the wild-type A. actinomycetemcomitans strain. In addition, the O-PS-defective rmlC mutant strain led to impaired activation of antigen-presenting cells, with less expression of the co-stimulatory molecules CD40 and CD80 in B lymphocytes and dendritic cells, and downregulated expression of Tnfa and Il1b in splenocytes. In conclusion, these data demonstrate that the O-PS from the serotype b of A. actinomycetemcomitans plays a key role in the capacity of the bacterium to prime oral innate and adaptive immune responses, by triggering the Th1 and Th17-driven tooth-supporting bone resorption during periodontitis.

Highlights

Aggregatibacter actinomycetemcomitans is a small, non-motile, Gram-negative coccobacillus, resident in the oral cavity of humans and non-human primates, that preferentially colonizes the tissues that surround teeth [1]
This higher alveolar bone loss was demonstrated by the increased cementenamel junction (CEJ)-alveolar bone crest (ABC) linear distance in the interdental area between the first and second molar (Figure 1E), the enhanced CEJABC linear distance in the distal surface of the distal root of the first molar (Figures 1F, G), and the decreased percentage of remnant alveolar bone (Figure 1H) at both sides of the maxilla. To corroborate that this higher pathogenic potential was attributed to the A. actinomycetemcomitans serotype b instead of only one strain belonging to this serotype, the alveolar bone resorption was analyzed in mice infected with another serotype b strain, the reference strain A. actinomycetemcomitans ATCC 23718
These results demonstrated that the O-PS moiety component of the LPS is involved in the virulence of A. actinomycetemcomitans and in the tooth-supporting alveolar bone loss induced during experimental periodontitis

Summary

Introduction

Aggregatibacter actinomycetemcomitans is a small, non-motile, Gram-negative coccobacillus, resident in the oral cavity of humans and non-human primates, that preferentially colonizes the tissues that surround teeth [1]. A. actinomycetemcomitans is considered a key player of the pathogenic consortium related to severe periodontitis, which causes accelerated periodontal tissue breakdown, in particular, the Th17-driven tooth-supporting bone resorption, the major pathological sign related with tooth loss [2, 3]. The immune response triggered against the dysbiotic subgingival microbiota during periodontitis disturbs the osteoblast/osteoclast equilibrium by dramatically increasing the RANKL local production and cellular sources, leading to pathological alveolar bone resorption [6, 7]. Initial subversion of the host’s immunity by periodontal keystone pathogens and other oral bacteria promotes the accumulation of immunostimulatory pathobionts, such as A. actinomycetemcomitans, that contribute to immunemediated alveolar bone loss [6,7,8]

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Results

Conclusion