Dendritic cells a critical link to alveolar bone loss and systemic disease risk in periodontitis: Immunotherapeutic implications

Abstract

Extensive research in humans and animal models has begun to unravel the complex mechanisms that drive the immunopathogenesis of periodontitis. Neutrophils mount an early and rapid response to the subgingival oral microbiome, producing destructive enzymes to kill microbes. Chemokines and cytokines are released that attract macrophages, dendritic cells, and T cells to the site. Dendritic cells, the focus of this review, are professional antigen-presenting cells on the front line of immune surveillance. Dendritic cells consist of multiple subsets that reside in the epithelium, connective tissues, and major organs. Our work in humans and mice established that myeloid dendritic cells are mobilized in periodontitis. This occurs in lymphoid and nonlymphoid oral tissues, in the bloodstream, and in response to Porphyromonas gingivalis. Moreover, the dendritic cells mature in situ in gingival lamina propria, forming immune conjugates with cluster of differentiation (CD) 4+ T cells, called oral lymphoid foci. At such foci, the decisions are made as to whether to promote bone destructive T helper 17 or bone-sparing regulatory T cell responses. Interestingly, dendritic cells lack potent enzymes and reactive oxygen species needed to kill and degrade endocytosed microbes. The keystone pathogen P. gingivalis exploits this vulnerability by invading dendritic cells in the tissues and peripheral blood using its distinct fimbrial adhesins. This promotes pathogen dissemination and inflammatory disease at distant sites, such as atherosclerotic plaques. Interestingly, our recent studies indicate that such P. gingivalis-infected dendritic cells release nanosized extracellular vesicles called exosomes, in higher numbers than uninfected dendritic cells do. Secreted exosomes and inflammasome-related cytokines are a key feature of the senescence-associated secretory phenotype. Exosomes communicate in paracrine with neighboring stromal cells and immune cells to promote and amplify cellular senescence. We have shown that dendritic cell-derived exosomes can be custom tailored to target and reprogram specific immune cells responsible for inflammatory bone loss in mice. The long-term goal of these immunotherapeutic approaches, ongoing in our laboratory and others, is to promote human health and longevity.

Keywords: Porphyromonas gingivalis; dendritic cells; dysbiosis; exosomes, atherosclerosis; periodontitis; senescence-associated secretory phenotype.

Figure 1.. Dendritic cell-T cell Conjugates: Tale of 3 signals.

Immature dendritic cells (DCs) receive microbial instructions through their pattern recognition receptors (PRR), including a large array of C-type lectin receptors and Toll-like receptors. Bacterial antigens taken up by DCs are processed and presented to T cells in the context of MHCII (Signal 1). DCs undergo maturation, upregulating costimulatory molecules CD80/CD86/CD40 for engagement with and activation of T cells (Signal 2) . Depending on microbial signal received by PRRs, DCs release polarizing cytokines (Signal 1) including TNFα/IL-1β or TGFβ/IL-10, which promote Th17/Th1 effector responses associated with bone loss, or Treg responses that are bone protective, respectively. All three signals are required for “productive’ antigen presentation, leading to clonal expansion of naïve T cells bearing cognate T cell receptor (TCR).