1. Introduction

Diabetes mellitus (DM) has become a major societal problem. Epidemiological studies demonstrate the increasing incidence of type 1 and 2 diabetes mellitus (T1DM and T2DM, respectively)^[1][2][3][4]. It is assumed that in the coming years the number of patients with diabetes will increase ^[1][5][6]. In 2012, the number of people with DM was 371 million, including 25.8 million in the USA. Globally, as many as 4.7 million patients died due to DM complications for the year 2012^[7].

Lesions observed in the retina with diabetes, referred to as diabetic retinopathy (DR), are caused by vascular abnormalities and are ischemic in nature. Due to its widespread prevalence, diabetic retinopathy is considered the major culprit of blindness in industrialized and middle-income countries^[8]. There exists a clinical division of diabetic retinopathy into non-proliferative and proliferative retinopathy. The time of Lesion progression differs among patients and is determined by diabetes duration, glycemia, genetic predispositions and treatment methods. In the USA, it is calculated that among patients with type 2 diabetes T2DM 40.3% have DR, and 8.2% suffer from vision-threatening retinopathy ^[9]. In patients with type I diabetes T1DM 86% have retinopathy and 42% vision impairment due to DR^[10]. Studies conducted on a group of 22,896 diabetic patients showed that 34.6% had DR, and the rising risk was associated with diabetes duration and improper blood glucose and blood pressure monitoring. Vision-threatening stages of DR involve proliferative DR and diabetic macular edema (DME). The incidence of proliferative DR and DME in the researched group totaled 6.96% and 6.81% respectively. Vision impairment related to DR is a serious global health problem.^[11][12]. 

Although the clinical examination may confirm the retina’s normality, after a few years of DM some important histological and biochemical lesions usually appear, including adhesion of leukocytes, thickening of the basement membrane as well as loss of pericytes. Pericytes are the mural cells of blood microvessels, which have recently come into focus for modulating angiogenesis, regulating blood flow, and maintaining blood–retina barrier (BRB) integrity. Pericytes lying on the capillaries, and are surrounded by the basement membrane, they can prevent ischemia-reperfusion after thrombus clearance by constricting capillaries whereas their relaxation increases blood flow ^[13]. When the time of duration of diabetes increases, substantial vascular lesions are more likely to affect the retina. As the duration of diabetes increases, the probability of remarkable vascular alterations in the retinal tissue rises. Advancing dysfunctional process of endothelial cells plays a key role in the structure and pathophysiology of the retina, such as thickening of the basement membrane, loss of perivascular cells, damage to the BRB and neovascularization ^[14][15]. These alterations are accompanied by important biochemical processes, including formation of advanced glycation end-products, and activation of protein kinase C isoforms and the polyol and hexamine pathways. ^[15]. Subsequently, this contributes to oxidative stress, inflammation and vascular dysfunction. Vascular lesions in diabetes affect small vessels (microangiopathy) and involve precapillary arterioles, capillaries, and small veins. The decreasing number of pericytes, thickening of the basement membrane, and proliferation of endothelial cells are observed ^[16][17].

2. The Retina—Hyperglycaemia and Inflammation in the Course of Diabetes

Many years of research have shown that hyperglycaemia plays a central role in the induction of diabetic retinopathy^[18][19][20] Studies conducted in non-obese diabetic mice (NOD mice) have demonstrated that the first changes on the fundus of the eye blood–retinal barrier breakdown are observed already in the first week of exposure to high glucose levels^[21]. In the environment of high glucose concentration, hyperglycaemia causes cell dysfunction, retinal neurovascular impairment, structural defects and functional disorders which lead to further damage of the retinal cells ^[13][18][19]. The next active stage of pathological changes is associated with the inflammatory process^[22][23][24]. The key factors of the inflammatory process in diabetes, first local, then systemic and chronic, are chemokines, growth factors and cytokines^[25][26]. In the course of inflammation, acute-phase proteins, including C-reactive protein (CRP) are produced in response to cytokine stimulation. Under physiological conditions, the level of CRP synthesis is low, however, the production increases in inflammation and it is observed in many inflammatory diseases, including diabetes^[27][28][29]. Previous studies have demonstrated the elevated blood levels of CRP in patients with T1DM and T2DM suffering from diabetic complications, such as diabetic retinopathy (DR) ^{[18][24][25][26][27]}. In patients with T1DM and retinopathy, a five-fold higher level of CRP protein was detected in the blood serum compared to the group of patients with T1DM and without diabetic retinopathy ^[27]. The authors conclude that the persistently elevated levels of proinflammatory cytokines and CRP in chronic diabetes result from an ongoing inflammatory process in diabetes ^[25][26][27]. CRP is a clinically recognized marker of inflammation, however, other proteins are also proposed as useful markers of diabetic retinopathy. For example, in patients with T2DM, interleukin 34 (IL-34) has been shown to be an additional inflammatory marker in predicting the risk of chronic diabetic complications. The IL-34 parameter was found to have better discriminate values for the risk of chronic diabetic complications than the CRP protein. Based on the order of the discriminate power, defined as the area under the curve, it was found that the AUC_ROC area was greater for IL-34 (AUC = 89.88%) than for CRP protein (AUC = 83.96%) ^[26].

In other studies, the authors attempted to demonstrate whether selected inflammatory proteins may be associated with microvascular complications in adult T1DM patients ^[29]. In a group of 100 subjects with T1DM, the following parameters were determined: epidermal growth factor (EGF), metalloproteinase 2 (MMP-2), growth/differentiation factor 15 (GDF-15) and interleukin 29 (IL-29). Screening was performed for microvascular complications, such as autonomic and peripheral neuropathy, diabetic nephropathy, and diabetic retinopathy. The results of multivariate logistic regression showed that an increase in the EGF concentration was a statistically significant predictor of microangiopathy (p < 0.0001). Moreover, higher levels of GDF-15 have been associated with diabetic nephropathy, peripheral neuropathy and proliferative retinopathy rather than with non-proliferative retinopathy in patients with T1DM ^[30].

On the other hand, in children and adolescents suffering from T1DM and diabetic retinopathy, a higher level of IL-6 was demonstrated in the blood serum^[31]. The authors showed a significant gradual increase of the IL-6 serum level. This was demonstrated by comparing the values in healthy children, children with T1DM without retinal changes the organ of sight and a group of children with the symptoms of non-proliferative diabetic retinopathy ^[31]. Higher serum levels of IL-6 were also shown in patients with T2DM and proliferative diabetic retinopathy rather than in the group of patients with T2DM without complications^[32][33]. The presence of chronic inflammatory environment in the course of diabetes increases the expression of inflammatory factors, also in the aqueous humour of the eye^[22][34]. The eight following factors have been recently found in the aqueous humour of DR patients: interleukin IL-6, IL-8, IL-10, vascular endothelial growth factor (VEGF), transforming growth factor-β (TGF- β), vascular cell adhesion molecule-1 (VCAM-1), intercellular adhesion molecule-1 (ICAM-1), and monocyte chemoattractant protein-1 (MCP-1)^[13]. The study showed that TGF-β, ICAM-1, IL-10, VEGF, and VCAM-1 may play a role in the progression of diabetic retinopathy. The authors suggest that the cytokines could be potentially used as biomarkers for predicting the progression of diabetic retinopathy and help to choose a therapeutic option and/or monitor response to treatment ^[22][23].

Tumour necrosis factor alpha (TNFα) exerts a significantly strong effect on the development and progression of diabetic retinopathy ^{[35][36][37][38]}. Tumour necrosis factor alpha (TNFα), also known as TNF, cachectin, or differentiation inducing factor (DIF), is a pleiotropic proinflammatory cytokine, as well as one of 22 proteins belonging to TNFα superfamily, regulating cell growth and differentiation. Apart from its participation in inflammatory processes, it plays an essential role in angiogenesis. TNFα may have an inhibiting or stimulating effect on the formation of new vessels. The resulting effect of TNFα is most probably dependent on cell exposure time and its local concentration. Using a non-obese diabetic mice (NOD mice) model, it was demonstrated that the administration of TNFα into the vitreous body of the eye causes endothelial ischemia and retinal necrosis. This finding proves TNFα role in the pathogenesis of diabetic complications^[37]. On the other hand, clinical studies in the group of type 1 diabetic children showed that TNFα turned out to be the paramount tested factor ^[35]. Studies conducted have demonstrated a significantly higher level of serum TNFα in 76% of children with T1DM and with NPDR compared to the group of children without DR (35%), as well as compared to healthy control group, in which no serum TNFα was detected. Moreover, findings indicated that from within the proinflammatory factors tested, serum TNFα level may be an independent indicator in the prediction of NPDR development in children^[35]. Other authors have also detected a high blood serum TNF level in adult patients suffering from T1DM. Authors, using a multifactorial analysis of logistic regression, have proven that TNFα was an autonomous determinant of the PDR inflammatory state marker^[36]. The last study tested the level of cytokines in the vitreous body and their correlation with the inflammatory cell density in the fibrovascular membranes (FVM) in patients with proliferative diabetic retinopathy (PDR) in order to assess intraocular inflammatory states in relation to the disease activity ^[38]. The authors’ statistical analysis demonstrated that PDR-affected patients had significantly higher levels of monocyte chemoattractant protein-1 (MCP-1) (p = 0.003), VEGF (p = 0.009) and interleukin 8 (IL-8) (p = 0.02) in the vitreous body compared to patients with inactive PDR. Moreover, statistical methods confirmed a significantly greater number of T lymphocytes (CD3+, CD4+ and CD8+) in PDR patients compared to PDR ones. The authors suggest that a relationship between the level of cytokines (MCP-1 and IL-8) in the vitreous body and the inflammatory cell density in FVM, and differences in cytokine levels in the vitreous body between PDR and without PDR groups of patients indicate the importance of local intraocular inflammation in PDR patients ^[39].