The Role of Genetics and Immune Mechanisms in the Pathogenesis of Diabetic Retinopathy

Abstract

Diabetes mellitus affects millions of people worldwide especially in Asia, Africa and South America. It can cause many serious complications such as retinopathy and nephropathy. Diabetic retinopathy is a terrible prospect to these patients which is diagnosed with the onset of microaneurysms, haemorrhages and development of cotton wool spots in the retina. Mechanisms underlying pathogenesis of diabetic retinopathy are not completely understood. Integrin α2β1 is a receptor for collagen on platelet cell membrane. Polymorphism in intron 7 of integrin gene produces change in α subunit and this makes retina vulnerable for platelet attachment during chronic hyperglycaemia in diabetes. Earlier studies have established a relationship between variants of α2β1 gene and diabetic retinopathy in Japanese and Caucassions. Derangements of several cytokines and chemokines have been reported in diabetic retinopathy. There are many studies that evaluate the role of IL-6 in the development of ophthalmic complications but they determined the level of IL-6 in the vitreous fluid and majority of them have emphasized the involvement of this cytokine in the development of eye complications. Interleukin 6 increases vascular permeability and neovascularisation and attracts macrophages. A study was performed in Type-I diabetes mellitus to determine its role in diabetic retinopathy whereas some of them have correlated IL-6 with proliferative diabetic retinopathy. In the literature there are a few studies that tried to determine the level of IL-6 in the serum of diabetic retinopathy patients but some of them could not determine its level in the serum while others found its level much less than in the vitreous fluid. The comparatively newly diagnosed subset of T cells known as Th17 cell secretes IL-17 which is a family of six cytokines (IL-17A-E). It is a pro-inflammatory cytokine and mediates inflammation by attracting neutrophils. It has been documented that Th17 cells have major contribution in different human diseases that are related to inflammation and tissue destruction such as rheumatoid arthritis, psoriasis, Crohn’s disease, and multiple sclerosis. Therefore it has also been suggested that IL-17 has got the potential to be used as a treatment option as well. It has been suggested that some early aspects of pathogenesis of diabetic retinopathy could be due to loss of self-tolerance. At the beginning of retinopathy, anti-pericyte and anti-endothelial cell auto-antibodies have been detected in the circulation of diabetic patients. There were increased vitreous concentrations of IL-6 and IL-8 in the patients of diabetic retinopathy while in the serum there were elevated levels of IL-8, TNF-alpha, and soluble IL-2 receptor. T regulatory (Treg) cells: a subset of CD4+ T cells, down regulates the process of autoimmunity. It has been documented that Treg cells are involved in the development of various autoimmune disorders. Two hundred and twelve (212) subjects were divided into three groups i.e. (Group-III) diabetic patients with retinopathy (152), (Group-II) diabetic patients without retinopathy (30) and (Group-I) healthy control without diabetes (30). Blood was drawn after their consent and integrin gene polymorphism was studied by restriction fragment length polymorphism analysis. Concentration of IL-6 and IL-17 was determined by ELISA technique. CD4+CD25+ (T regulatory cells) were enumerated by flow cytometer. There were 77 males and 135 females and their age distribution was from 20 years to 75 years. 109 patients had history of diabetes between 5 and 10 years whereas 73 patients had diabetes for more than 10 years. The percentage of HbA1c was between 5.5% and 15.4%. The mean age of the studied population was 34.66, 49.46, and 50.88 years in Group-I, Group-II and Group-III respectively. There was statistically significant difference of mean age among the three groups. The mean CD4+CD25+ count was 14.53, 14.68, and 16.47 in Group-I, Group-II and Group-III respectively and on comparison of CD4+CD25+ count among the three groups, there was statistically significant difference. The mean of Treg cells was 2.91, 3.07, and 2.88 in Group-I, Group-II and Group-III respectively and there was no statistically significant difference of Treg cells among the three groups,. The mean level of IL-6 was 133.98, 1341.78, and 718.66 in Group-I, Group-II and Group-III respectively and there was statistically significant difference of IL-6 among the three groups. The mean level of IL-17 was 718.05, 415.01, and 375.95 in Group-I, Group-II and Group-III respectively and there was statistically significant difference of IL-17 among the three groups. Mean duration of diabetes was 7.76 and 10.51 years in Group-II and Group-III respectively. There was statistically significant difference of duration of diabetes between these two groups. Mean percentage of HbA1c was 8.54% and 8.83% in Group-II and Group-III respectively and there was no statistically significant difference in percentages of HbA1c between these two groups. There was statistically significant difference in the gender and age of the subjects in all parameters between Group-I and Group-II. There was statistically significant difference in the gender, age, level of IL-6 and the level of IL-17 among the subjects (p<0.05). By comparing Group-II and Group-III, we could find statistically significant difference in the percentage of Treg cells, the level of IL-6 and the duration of diabetes in the studied subjects. Regarding Bgl II polymorphism, 33 (15.6%), 104 (49.05%), and 75 (34.90%) subjects had + +, + -, and - - phenotypes respectively. On comparing Bgl II polymorphism among the three groups, there was no statistically significant difference. By applying logistic regression model between Group-II and Group-III there was statistically significant difference in the percentage of Treg cells and the level of IL-6 in these groups. When the logistic regression model was applied between Group-I and Group-III, significant difference was found in the age of the subjects, the level of CD4+CD25+ cells and the level of IL-6 in these groups. Therefore, it is suggested that age and gender of the subjects, duration of diabetes, levels of IL-6, IL-17, CD4+CD25+ cells and the percentage of Treg cells can contribute towards the development of diabetic retinopathy.