IL-17 is a cytokine that has a role in tissue inflammation by inducing the release of other proinflammatory and neutrophil mobilizing cytokines [10, 14]. Until a few years ago, KC was defined as a noninflammatory degenerative disease. Yet, recent studies have shown that the altered balance between inflammatory cytokines, proteases, and protease inhibitors, as well as free radicals and oxidants, have a crucial role in the pathogenesis of this disease [6, 19–21].
KC affects both genders in all ethnicities, manifesting at puberty and progressing until the third or fourth decade of life, when it becomes stable, with approximately 20 % of cases progressing to the stage of corneal transplantation or keratoplasty [5, 20]. The fact that KC initiates at puberty may explain the lower mean age observed in the group of patients who developed the disease. Additionally, the higher number of females among the patients who did not develop KC may not actually represent a protective factor for the development of the disease, given that no significant gender pattern has been established for the development of KC [3]. Moreover, the higher frequency of women may refer to prevention habits, as men seek health services less often than women [22–24].
This study investigated the role of IL17A and IL17F polymorphisms in the immunopathogenic mechanism of KC in a population from the northwestern part of the State of São Paulo and found that the TT genotype of IL17F polymorphism suggests a higher risk of developing KC, while influential correlations were not found between the disease and the IL17A polymorphism.
IL-17F and IL-17A interleukins seem to function in a similar way, but the latter seems to have greater potency [25]. Nowadays, it is acknowledged that the production of some cytokines is under genetic control and that polymorphisms in several cytokine genes, mainly SNPs or microsatellites, located in regulatory regions, may affect gene transcription and cause inter-individual variations. Some of these polymorphisms have been identified and influence the level of cytokine production, which may confer flexibility in the immune response [26–31].
Regarding the IL17 gene, genetic polymorphisms of IL17A G197A and IL17F T7488C affect IL-17A and F production, respectively. The presence of the A allele of IL-17A rs2275913 polymorphism is related to higher IL-17 secretion [16, 32]. The correlation between T and C alleles and genotypes of IL17F rs763780 polymorphism with the risk of developing inflammatory or inflammation-related diseases is diverse in the literature. Colorectal cancer, which has its risk increased by a proinflammatory diet, occurs more frequently in individuals carrying the C allele [33]. Osteoarthritis, another disease with a probable inflammatory influence, has the TT genotype as a protector of hip osteoarthritis [34, 35]. On the other hand, in psoriasis, a chronic inflammatory skin disease, the TT and TC genotypes were associated with a higher risk of developing the disease [36]. Furthermore, in asthma, the variant form (C allele) of the IL-17F protein (His121Arg) appears to suppress the expression and activity of the wild type (T allele) and is associated with protection against this inflammatory disease [37].
An important role in the immune system is played by T helper (Th) cells. These cells can be categorized into Th1, Th2, and Th17. Th1 and Th17 cells are responsible for the secretion of proinflammatory cytokines such as Interleukin 2 (IL-2) and Interferon-gamma (INF- γ) – which have an important role in the activation of macrophages and cytotoxic T cells – and IL-17 – which induces cell infiltration and the production of other proinflammatory cytokines – respectively [38, 39]. As for Th2 cells, they are responsible for the secretion of anti-inflammatory cytokines that induces the humoral immune response, such as IL-4, IL-5, and IL-10. Especially in KC, the increase in proinflammatory cytokines may generate a complex imbalance between Th1 and Th2 response cytokines, and together with an exacerbated Th17 response seems to cause alterations in epithelial and stromal functions [6, 40–42].
Jun et al. (2011) observed elevated levels of IL-17 in tear samples from keratoconus patients [10]. Inflammatory molecules such as cytokines and chemokines immunologically alter the corneal microenvironment and seem to act on several inflammatory pathways in the pathophysiology of KC [12]. IL-17 is hypothesized to be related to the corneal inflammatory process by stimulating stromal cells to produce other proinflammatory interleukins, such as IL-6, which mediate the inflammation process [43, 44]. In addition, the IL-17 receptor is constitutively expressed on corneal resident fibroblasts, and stimulation of these cells by IL-17 leads to the synthesis of several matrix metalloproteinases, which ultimately cause corneal structural damage, which are present in KC. Thus, increased IL-17 expression may lead to structural damage on keratoconus corneas and be related to disease severity [10, 12].