Tissue-Specific Immunity at the Oral Mucosal Barrier

Abstract

The oral mucosal barrier is constantly exposed to a plethora of triggers requiring immune control, including a diverse commensal microbiome, ongoing damage from mastication, and dietary and airborne antigens. However, how these tissue-specific cues participate in the training of immune responsiveness at this site is minimally understood. Moreover, the mechanisms mediating homeostatic immunity at this interface are not yet fully defined. Here we present basic aspects of the oral mucosal barrier and discuss local cues that may modulate and train local immune responsiveness. We particularly focus on the immune cell network mediating immune surveillance at a specific oral barrier, the gingiva - a constantly stimulated and dynamic environment where homeostasis is often disrupted, resulting in the common inflammatory disease periodontitis.

Keywords: gingival barrier; oral immunity; oral mucosa; periodontal immunity.

Figure 1. The oral and gingival barrier

The oral mucosa is lined by stratified squamous epithelia of varying thickness and level of keratinization. Most the oral mucosa is covered by lining, non-keratinized epithelium (the floor of mouth being particularly thin and vascular). The areas related to mastication (hard palate, outer surface of gingiva) are partially keratinized epithelia and the tongue is covered with a specialized epithelium that incorporates the taste buds. The gingival crevice is a particularly open and vulnerable site. It is lined by the sulcular epithelium that is non-keratinized and becomes progressively thinner transitioning to the junctional epithelium, which connects to the tooth surface and is constantly exposed to the microbial biofilm and experiences trauma, leading to constant immune activation.

Figure 2. The immunological network of the gingiva

The gingiva houses a rich immunological network. Neutrophils continuously transmigrate through the junctional epithelium. Resident lymphocytes are predominately T cells, with some B cells and ILCs. Diverse mononuclear phagocytes are also present at this site.

Figure 3. Tissue-specific cues tailoring immunity at the oral barrier

Diverse microbial commensal communities and ongoing damage are established to educate immune function at the gingiva, while dietary elements and airborne allergens/particles are speculated to play a role in the training of local immunity.