Initial Lesion (2-4 days)

If a biofilm is allowed to form on the tooth surface, a vast number of bacterial cell products are produced. Many of these bacterial products and structures are referred to as Pathogen-Associated Molecular Patterns (PAMPs) and can be recognized by cell membrane receptors called Toll-like Receptors (TLRs). TLRs are part of the innate immune system and are expressed by several different cell types including epithelial cells, endothelial cells, fibroblasts, cementoblasts, osteoblasts, osteoclasts, dendritic cells, PMNs, macrophages and lymphocytes. There are 11 TLRs identified in humans of which TLR-2 and TLR-4 appear to be the most important.

When PAMPs (Table 1) bind to TLRs on the cell membrane, a molecular signal is produced which launches one of several different enzymatic pathways within the cell’s cytoplasm. One of these pathways eventually will eventually signal the nucleus of the cell to begin production of certain types of molecules called cytokines. The result is an inflammatory response that is initiated by the release of these pro-inflammatory cytokines, and other soluble mediators of inflammation from the cell. The major pro-inflammatory cytokine is Interleukin-1 (Il-1α, Il-1β, Il-1Ra), which is released by several different cell types including sulcular and junctional epithelium, fibroblasts, macrophages and PMNs. Some functions of Il-1 are listed in Table 2.

The release of Il-1 initiates a series of events that are usually associated with an acute inflammatory phase. Most of the events described below occur in an orchestrated manner to eliminate the pathogens.

• Chemotactic cytokines or chemokines, such as Il-8, will be released which attract circulating PMNs; “the first line of defense.”

• PMNs then migrate through the vascular wall (diapedesis) and follow a concentration gradient of chemical molecules (chemotaxis) to the site of infection.

• PMNs not only release enzymes that are capable of destroying pathogens but also attach to antigen-antibody complexes for phagocytosis. PMNs will also migrate between junctional epithelial cells to reach the sulcus, where they literally burst and release their enzymes.

• Histamine and kinins released from tissue Mast cells promote vasodilation and increase vascular permeability, enhancing the influx of cells and protein molecules such as antibodies and complement.

• Plasma proteins called Complement will be activated through a cascade of enzymatic reactions. There are three pathways for the activation of complement: The Classical (figure below); Alternative; and Mannose-binding lectin.

Complement has several biologic functions

• C3a activates basophils and mast cells causing release of vasoactive substances including histamine.

• In concert with antibodies, C3b and C4b opsonize, or clump, the antigens together for easier phagocytosis by PMNs and macrophages

• C5a and C5a des Arginine enhance PMN activation and chemotaxis

• C5b, C6, C7, C8 form a Membrane Attack Complex (MAC) that can destroy bacteria by punching holes in their cell wall.

The antibodies entering the tissue come from the circulation. Some may be specific for certain pathogen-associated antigens and come from the IgG and IgA classes and subclasses. Others that are less specific or of weaker affinity will be of the IgM class.

PMNs and gingival fibroblasts may release enzymes called matrix metalloproteinases, or MMPs (Table 3). These include collagenase and elastase that are capable of breaking down extracellular matrix like collagen fibers.

In addition, several cell types will be induced by Il-1 to produce Prostaglandin (PG), a by-product of the enzymatic breakdown of arachidonic acid found in the lipid layer of a cell membrane. All nucleated cells except lymphocytes produce PG. The prostaglandin of significance in periodontal disease is PGE2. It has been demonstrated to increase in concentration as the severity of the lesion increases. PG functions include platelet aggregation, vasodilation, vasoconstriction, chemotaxis of PMNs, increased vascularity and bone resorption.

Clinically, the manifestation of these events results in the early stages of gingivitis. There may be bleeding on probing, slight gingival swelling along with mild erythema (reddening) of the tissue, and increased flow of gingival crevicular fluid. Microscopically, the inflammatory infiltrate is predominated by PMNs located primarily around vessels within the connective tissue, just below the junctional epithelium (JE). Loss of some perivascular collagen will also be seen. Within the lamina propria (connective tissue layer beneath epithelium) of the sulcular epithelium, the number of dendritic cells called Langerhan’s cells will increase. These cells can internalize and process pathogen-associated antigens and migrate, via lymphatic channels, to the lymph node where they become Antigen Presenting Cells (APCs) to naïve (immature) T Cells. Free antigen may also travel to the lymph node, where macrophages or specialized dendritic cells internalize, process and present the antigen to T Cells. Newly activated T Cells will in turn stimulate B Cells to switch immunoglobulin from IgM to IgG, IgA or IgE. The B Cell may also be stimulated to proliferate and differentiate into Plasma Cells which then produce the antigen specific antibody presented to it earlier by the T Cell.

If the immune response is effective in eliminating the pathogens in the early phase of the acute inflammation, lipoxins from the enzymatic breakdown of arachidonic acid are generated. The lipoxins (A and B) may act to inhibit PMN chemotaxis, inhibit secretion of proinflammatory mediators, induce apoptosis (cell death) of PMNs, and, recruit macrophages to the site for removal of cell debris. Thus, the inflammation resolves and tissue repairs. Other molecular compounds, called Resolvins and Protectins, are produced from the metabolism of dietary Omega 3, and act in a similar fashion to the lipoxins.