**2.4 T helper cells**

T cells and B cells compose the lymphocytes of the adaptive immune system. T cells are further subdivided into T helper (Th) and T cytotoxic (Tc) groups, identified as CD4<sup>+</sup> and CD8+ , respectively. CD4+ Th cells are abundant lymphocytes with the primary role of secreting cytokines to amplify the immune response by promoting crosstalk amongst cells. Once activated, Th cells proliferate and polarize, which means they selectively secrete specific groups of cytokines and chemokines. Currently, effector Th cells are grouped based on the cytokines released as follows: Th1 cells secrete IFNγ; Th2 cells secrete IL-4, IL-5, and IL-13; Th17 cells secrete IL-17; and Th22 cells secrete IL-22. Compared with effector Th cells, regulatory Th cells produce IL-10 and are termed type 1 T regulatory cells (Tr1). As Th cells have been shown to be highly pathogenic and contribute to inflammatory disease, specifically Th1 and Th17 cells, these subpopulations are under intense research in DED.

CD4+ and CD8+ T cells are less abundant on the healthy ocular surface compared to γδ T cells and it has been noted that CD4<sup>+</sup> T cells outnumber CD8<sup>+</sup> T cells. Induction of DED in rodents and in patient with DED, T cells are found to consistently infiltrate the ocular surface. In early mouse studies of DED, upon desiccantstress-induced DED, increasing numbers of CD4+ T cells were observed in the cornea, conjunctiva, and lacrimal gland tissues [39] (**Table 1**). It was demonstrated that transferring these cells is enough to induce DED in mice lacking T and B cells, showing that CD4+ T cells are largely responsible for inducing DED (**Table 1**) [39]. Additional reports also provided evidence of CD4+ T cells driving autoimmune DED in autoimmune regulator-knockout (Aire KO) mice [49] and being present in

lacrimal glands in a novel autoimmune model of DED in rats [40]. This same population was found to be expanded within draining lymph nodes from DED-induced mice [32]. Similarly, human studies showed that DED patients with an autoimmune origin (Sjögren's syndrome) and with a non-autoimmune origin (non-Sjögren) presented comparable numbers of CD4+ T cells assessed per immunohistochemistry using conjunctival samples [50].

Research has also focused on identifying specific subtypes of CD4<sup>+</sup> T cells involved in DED, where Th1 and Th17 cells have received the most attention since DED induction was found to cause increased transcripts of both IFNγ and IL-17 in the ocular surface [34]. Expansion of IFNγ-secreting CD4+ T cells coexpressing CXCR3 and CCR5 chemokine receptors (Th1 polarized T cells) in the regional lymph nodes (submandibular and cervical) has been reported [32, 51]. Interestingly, IFNγ may be highly relevant at the onset of DED, but its role during chronic DED may be limited [49]. The relevance of Th1 in DED immunopathology can be inferred due to the detrimental effect of IFNγ on the ocular surface, since the presence of IFNγ receptor was demonstrated on the conjunctival and corneal epithelium [33]. Additionally, IFNγ is amongst the cytokines elevated in tears from DED patients and was shown to alter mucin secretion by inducing cell death in conjunctiva-residing goblet cells [52]. A diminished density of goblet cells resulting from IFNγ administration was also reported [41]. IFNγ was also found to be responsible for inducing apoptosis in lacrimal gland cells [53].

Once polarized and activated, Th2 cells secrete IL-4, IL-5, and IL-13 cytokines. Even though DED is a Th1-prone inflammatory condition, it has been reported that tear samples from DED patients contain elevated levels of IL-4, IL-5, and IL-13, suggesting activation of the Th2 pathway [54–56]. Intriguingly, Th2 cytokines were detected elevated in tears from DED-induced experimental animals [57]; however, the contribution of the cytokines to mouse DED appears to be strain-dependent, since DED induction caused different cytokine and chemokine profiles in C57BL/6 compared with BALB/c mice [58]. Although IL-13 has been shown to prevent goblet cell loss, innate lymphocytes (NK and NKT cells) were demonstrated to be the cellular source; thus, we can speculate that Th2 cells collaborate in preserving mucinproducing cells as well. However, this remains to be proven.

In terms of Th17 cells, DED induction creates a Th17-inducing microenvironment as gauged by a rapid increase in IL-6 and IL-23 expression. Th17 cells have been readily detected in draining lymph nodes from acute and chronic DEDinduced mice [34, 35], and their pathogenic role is supported by experimental evidence showing that IL-17 neutralization attenuated corneal damage. Furthermore, Th17 cells emerging in experimental DED were shown to be resistant to suppression exerted by T regulatory cells and, unlike Th1 cells, Th17 cells survived longer periods of time, adding to chronic DED [36]. A wide variety of negative effects on the ocular surface have been attributed to Th17 cells and IL-17 including promotion of MMPs expression, corneal barrier damage, and induction of angiogenesis via vascular endothelial growth factor (VEGF) (**Table 1**) [37].

Another group of CD4+ T cells addressed in the context of DED is the T regulatory (Treg) subtype. Treg cells are identified by the expression of the transcription factor Foxp3 and high levels of CD25 and can suppress cell proliferation. Niederkorn et al*.* demonstrated that the presence of Foxp3+ Tregs prevented DED symptoms induced by the adoptive transfer of ocular-surface-specific T effector cells (**Table 1**) [39]. Thereafter, it was shown that a percentage of lymph node residing Foxp3+ Tregs remains unaltered upon DED induction but their suppressive ability is reduced compared with their counterparts from non-DED animals, when tested *in vitro* [36]. Recently, administration of histone deacetylase inhibitors (HDACi)-containing microspheres that stabilized Foxp3+ expression reduced DED signs [59].

#### *Lymphocytes in Dry Eye Disease DOI: http://dx.doi.org/10.5772/intechopen.98969*

Several CD4+ T subtypes have been identified in DED, and the roles of Th1, Th17 and Tregs have been highlighted. A well-established pathogenic role of Th1 and Th17 in inducing ocular surface damage through the release of IFNγ and IL-17, respectively, has been assigned. In sharp contrast, Treg cells are responsible for restraining exacerbated inflammatory responses in the ocular surface. Therefore, the balance between these CD4+ T cells subpopulations seems to be determinant for the onset and chronicity of DED.
