2.3.1. T Lymphocytes

The characterisation of T lymphocytes in healthy gingiva has shown a dominance of CD4+ helper T cells [6]. These cells play fundamental roles in the adaptive immune responses, and their cytokine production in response to specific immunological challenges led to the classical framework of distinct Th cell subsets [29]. CD8+ T cells were the second most abundant T lymphocyte in healthy gingiva, followed by a small percentage of γδT cells. Gingival CD8+ T cells seem to have regulatory/suppressor properties important to the maintenance of gingival tissue integrity by downregulating inflammation under homeostatic conditions. These cells can produce IL-10 and TGF-β, which then suppress osteoclastogenesis [30]. Tissue-resident epithelial γδ T cells have been reported to be the major T cell population in the epithelial tissues and are important in carrying out barrier surveillance and helping to keep tissue homeostasis, and to some extent, epithelial repair [31]. Gingival γδ T cells accumulate after birth in response to barrier damage and are crucial for immune homeostasis. These cells produce amphiregulin, a wound healing-associated cytokine, which limits the development of periodontitis [32]. γδ T cells are also the major source of IL-17 in homeostasis. Interestingly, ablation of γδT cells resulted in increased gingival inflammation and alterations in the microbial diversity [33]. Within the CD4 compartment, about 15% are presumed to be Treg cells, which are crucial for periodontal homeostasis. Increased numbers of Tregs are associated with bone homeostasis, even in the presence of local inflammation [34] and may be related to the non-progression of gingivitis lesions in some patients, even after a long period of oral biofilm exposure. The new roles of γδT cells and Tregs in periodontal tissue homeostasis are crucial to the understanding of periodontal disease initiation and progression.

Dutzan et al. [6] characterised memory and naive T-cell subsets in the gingiva, showing that approximately 80% of CD4+ and 50% of CD8+ T cells had a CD45RO+ (activated T cell) memory phenotype. The CD4+ cell compartment in gingiva had a minimal CD45RA+ (naive T cell) population, but the CD8+ T cell compartment had a substantial population of CD45RA+/CCR7− cells (terminally differentiated effector T cells, TEMRA) alongside a smaller population of naive CD45RA+CCR7+ cells [6,35]. CCR7 is a chemokine receptor that divides human memory T cells into two functionally distinct subsets. CCR7− memory cells express receptors for migration to inflamed tissues and display immediate effector function; CCR7+ memory cells lack immediate effector function, but efficiently stimulate dendritic cells and differentiate into CCR7− effector cells upon secondary stimulation [36].

The combination of CD45RO and CCR7 (memory subset markers) with CD69 (a stimulatory receptor expressed at sites of chronic inflammation) was performed by Dutzan et al. [6] to analyse circulating and tissue-resident memory CD4+ and CD8+ T cell subsets. Their results showed that the majority of CD4 memory T cells in gingiva were CCR7−CD69<sup>+</sup> (resident effector memory, rTEM cells), followed almost equally by resident memory, effector memory (TEM), and central memory (rTCM; CCR7+CD69+) cells. Regarding CD8, memory CD45RO+ cells were also rTEM in their majority, followed by a large population of TEM and a small population of central memory cells (TCM). Increased proportions of resident memory T cells are common at barrier sites, where they have been reported to support early/immediate defence mechanisms, providing site-specific protection from pathogen challenges [37]. Resident memory T cells would then have special importance to protect the connective tissue form the bacterial products released in the periodontal sulcus.

Besides the T-cell characterisation, it is important to understand how these cells behave under physiological conditions. Having clinically healthy gingiva does not ensure that T cells are not being activated. Dutzan et al. [38], demonstrated in mouse gingiva that gingiva-resident Th17 cells developed via a commensal colonisation-independent mechanism. Th17 cells might accumulate at the gingiva in response to the physiological barrier damage that occurs, for instance, during mastication. The authors showed that physiological mechanical damage could induce the expression of IL-6 from epithelial cells, promoting an increase in gingival Th17 cell numbers.
