**3.5. Chemokine**

Chemokines are low molecular weight proteins that have many functions, including enhanced immune responses, regulation of homeostasis, and controlling angiogenesis [20, 92, 93]. Chemokines are often referred to as secondary pro-inflammatory mediators, whose activation is induced by pro-inflammatory cytokines or primary pro-inflammatory mediators. Chemokines induce a specific leukocyte type and can bind to chemokine-receptors on target cells [20, 92]. Chemokines are usually categorized into two groups, the CXC group is chemotactic for neutrophils and the CC group is chemotactic for monocytes and lymphocytes [20, 92]. Several studies show an increase of chemokines in vitreous of PDR patients, suggesting that they have roles in mediating angiogenesis and fibrosis in PDR patients [20, 93, 94]. Struyf et al. stated that chemokines have different roles based on disease progression. In the early phase, chemokines can induce leukocyte attraction and in late phase, they can induce neovascularization [93]. Das et al. introduced a new therapy targeting chemokines in patients with DME [94].

*3.5.3. Stromal cell-derived factor-1*

*3.5.4. High-mobility group box-1*

**3.6. Cell adhesion molecules**

sion-dependent immune response [21, 106].

*3.6.1. Intercellular adhesion molecule-1*

Stromal cell-derived factor-1 (SDF-1) is a chemokine with a major role in the ischemic damage repair process. It recruits endothelial progenitor cells (EPCs) from the bone marrow to the site of repair and upregulates expression of VEGF, increasing the angiogenic process. This pro-angiogenic factor is categorized as being hypoxia-responsive and is found to be upregulated in PDR [98–100]. Chen et al. found that vitreous concentrations of SDF-1 and VEGF are correlated in eyes with PDR. They also found that vitreous levels of SDF-1 are significantly higher in PDR patients (306.37 ± 134.25 pg/mL) than in patients with idiopathic macular hole (86.91 ± 55.05 pg/mL) [101]. Butler et al. demonstrate an increase of SDF-1 concentration in the vitreous of patients with PDR and this increase correlates directly with disease severity. They also demonstrated that intravitreal injection of triamcinolone dramatically decreased the concentration of vitreal VEGF and SDF-1, suggesting it as another possible treatment for PDR [98].

Proliferative Diabetic Retinopathy: An Overview of Vitreous Immune and Biomarkers

http://dx.doi.org/10.5772/intechopen.74366

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High-mobility group box-1 (HMGB1) is a nonhistone DNA-binding protein that facilitates transcription. HMGB1 can be released into the extracellular space by active secretion from certain cells such as activated monocytes and macrophages, mature dendritic cells, natural killer cells, and endothelial cells. Necrotic cell death can also cause passive leakage of HMGB1 from the nucleus as the protein is no longer bound to DNA. HMGB1 can bind to the receptor for advanced glycation end products (RAGE) and toll-like receptor 2 (TLR-2), where it acts as a pro-inflammatory cytokine, activating NF-κβ resulting in the overexpression of other proinflammatory molecules such as TNF-α, MCP-1, and ICAM-1 [41, 102, 103]. El-Asrar et al. demonstrated a significant correlation between neovascularization levels in epiretinal membranes of patients with PDR and the expression of HMGB1 and RAGE [41]. Yao Yu et al. also found an increase of HMGB1 concentration in the vitreous of PDR patients. This increase in vitreous happens in the later phases of DR, and differs from other inflammatory cytokines. They also found increases of RAGE protein and decreases of TLR-2 protein in DR rats, suggesting that

Adhesion molecules have many roles in our body, including embryology, immunology, and malignancy [21]. Several studies show increases of these molecules in PDR patients, suggesting that cell to cell interaction plays a major role in the development of PDR [79, 104–106]. These molecules regulate lymphocyte recruitment to vascular endothelium. Well-known adhesion molecules found in PDR patients are intercellular adhesion molecule-1 (ICAM-1) and vascular cell adhesion molecules-1 (VCAM-1), which are required for initiation of adhe-

Intercellular adhesion molecule-1 (ICAM-1), also known as CD54, is a cell surface glycoprotein encoded by the ICAM1 gene. ICAM-1 is usually expressed on the surface of endothelial

the involvement of HMGB-1 is mainly through its binding with RAGE [102].

### *3.5.1. Monocyte chemotactic protein-1*

Monocyte chemotactic protein-1 (MCP-1) is a member of the chemokine group which is responsible for regulating migration and infiltration of monocyte/macrophages to the site of inflammation, making MCP-1 a pro-inflammatory cytokine that plays a central role in CNS inflammation [2]. Hyun et al. stated that MCP-1 is a major cause of vascular complications in diabetes [95]. It is also a potent inducer of angiogenesis and fibrosis. MCP-1 levels were found elevated in the vitreous of diabetic patients and their levels are higher than serum [2, 97]. Ning et al. stated that advanced glycation end product (AGE) stimulation activates retinal neurons to release MCP-1 activating retinal microglial cells. Their study also shows a progressive increase of MCP-1 along with the progression of disease, indicating it may be an important link in diabetic retinopathy pathogenesis [2, 96]. Hyperglycemia also has been shown to increase MCP-1 expression from retinal vascular endothelial cells, RPE cells, and Muller glial cells [2, 97]. Reddy et al. demonstrated significantly higher levels of MCP-1 in PDR patients compared to normal glucose tolerance (NGT) patients. MCP-1 is also steadily increased along with the progression of PDR [97].

#### *3.5.2. Interferon gamma-induced protein-10*

Interferon gamma-induced protein-10 (IP-10), also known as CXCL10, is one of the CXC chemokine members. CXC chemokine has unique properties in which it can act as either an angiogenic or angiostatic factor, depending on the protein configuration of the molecule. IP-10 is inducible directly or through activation of IFN-γ, TNF-α, NFkB, viruses, or microbial products. Boulday et al. reported that VEGF induced the expression of IP-10 [1]. IP-10 binds CXCR3 receptors inducing apoptosis, angiostasis, and chemotaxis. It has been suggested that IP-10 is associated with inflammatory diseases including immune dysfunction and infectious disease. This protein has also been proposed to be involved in the pathophysiology of diabetic retinopathy, especially in the development of neovascularization. Elner et al. found a significant increase in the level of IP-10 in patients with PDR compared to the patients with non-diabetic eye diseases (NDED; 11.7 ± 1.1 ng/mL and 4.6 ± 0.9 ng/mL; p < 0.001, CI 95%). They also assumed that pan-retinal laser photocoagulation (PRP) might influence elevated IP-10 levels. The exact mechanism of the PRP-induced IP-10 involution of PDR remains to be elucidated [79].
