Circulating Biomarkers of Inflammation and Endothelial Activation in Diabetic Retinopathy

Abstract

Inflammation and endothelial activation play a pivotal role in development and progression of diabetic retinopathy (DR), a vision-threatening complication of diabetes mellitus (DM) and the leading cause of blindness in the working age population. Easily accessible and validated biomarkers for DR early diagnosis and progression are required for use in clinical trials: here, we reviewed the available literature to understand the association of circulating levels of selected markers of inflammation and endothelial activation with the presence of nonproliferative and proliferative DR (NPDR and PDR) and investigate the relationship between their systemic and ocular levels. We additionally provide data synthesis and perform statistical analysis for interpretation of the collected evidence. CRP, IL-1β, IL-6, TNFα, sICAM1, and sVCAM1 circulating levels were increased in subjects with DM compared to healthy individuals. Moreover, TNFα and sVCAM1 showed increasing systemic levels with DR presence and severity; circulating CRP increased with the transition from no DR to NPDR, whereas IL-6 was increased in PDR compared to NDPR stages. The relationship between ocular and systemic concentrations of these proteins remained unclear due to the low number of studies with matched sampling. In conclusion, the available data supports the use of systemic biomarkers of inflammation and endothelial activation to identify DM status and DR severity. These systemic biomarkers are likely reflecting an overall state of inflammation and vascular activation in different tissues of the body, including the eyes. Prospective, longitudinal datasets are required to validate these biomarkers as predictors of early DR presence, of DR progression, or for disease monitoring.

Figure 1.

Statistical analysis summary results. Summary results of the statistical analysis (meta-analysis approach) are shown as standardized mean differences (SMDs) and 95% confidence interval (CI) for the following comparisons. (A) Subject with DM (pooled data regardless of DR status) versus healthy controls (Cntrl) in black and subjects with DM without DR (no DR) versus healthy controls (Cntrl) in blue. (B) Subjects with DM affected by NPDR versus subjects with DM without DR (no DR) in black and subjects with DM affected by PDR versus subjects with DM without DR (no DR) in red. (C) Subjects with DM affected by PDR versus subjects with DM affected by NPDR. When the CI does not cross the line of null effect (0), the result is significant (p < 0.05). The number (N) of studies and subjects included in each group comparisons are shown on the right side. The heterogeneity measure I2 and corresponding p value are shown on the left; statistically significant I2 measures are highlighted in bold.

Figure 2.

Distribution of the included studies into the considered subgroups. Distribution of the studies included in this review and statistical analysis (for the comparison DM versus Cntrl) into the considered subgroups. (A) DM type, (B) study size (median and range of number of participating subjects per study, divided by DM type), (C) region (according to the country where the study was conducted), and (D) sample matrix used for measurement of proteins of interest. Below each pie chart A, C, and D or box plot B the total number of considered studies is shown; note that this number is not always equal for the same analyte because some studies did not report the necessary characteristics to be categorized in all subgroups. The total number of subjects included in all the considered studies per analyte and DM type is shown above the boxes in B. Papers investigating “type 1 or 2” DM (pooled data or unspecified type) were excluded from the region, matrix and sample size subgroup analysis B, C, and D.

Figure 3.

Ocular and circulating concentrations of IL-1β, IL-6, TNFα, sICAM1 , and sVCAM1 in healthy control and subjects with PDR. Mean concentrations of vitreous humor (VH, red circles) and blood (plasma or serum, black circles) inflammatory (A) and endothelial activation (B) markers in matched Cntrl and PDR population (references in the Table). Mean and SDs of biomarker levels from each study are shown separately for Cntrl and PDR groups.

Figure 4.

Systemic and local inflammation and endothelial activation during development and progression of DR as a consequence of systemic DM. Schematic representation of systemic and local inflammatory events and endothelial activation during development and progression of DR as a consequence of systemic DM. With increasing age and time from DM diagnosis a number of systemic factors significantly increase in subjects with diabetes, including HbA1c, blood pressure, and circulating markers of inflammation and endothelial activation (CRP, IL-1β, IL-6, TNFα, sICAM1, and sVCAM1). Before DR can be detected in fundus images of patients with diabetes (DM / no DR), the presence of a systemic pro-inflammatory environment is indicated by the significant upregulation of CRP, IL-1β, IL-6, and TNFα as well as an endothelial cell activation is indicated by the significant upregulation of sICAM1 and sVCAM1. This is consistent with broader systemic inflammation and vascular impairment in DM. As vascular changes are not yet manifest and overall immune homeostasis is likely being maintained in the eye in this stage of the disease, a very low-grade local inflammation in the retina cannot be excluded. After onset of NPDR and manifestation of vascular fundus abnormalities, the retinal blood barrier becomes compromised and circulating immune cells, already pre-exposed to the systemic pro-inflammatory environment, might show increase adhesion to the vessel walls (leukostasis). At this disease stage, CRP, TNFα, and sVCAM1 are additionally increased in the systemic circulation compared to the DM / no DR stage. As disease progresses, the worsening of the vascular phenotype in the retina (pronounced ischemia, leakage and, eventually, neovascularization in the PDR stage) is accompanied by enhanced immune cell infiltration / activation and more pronounced local inflammation with ocular production of IL-6, sICAM1, and sVCAM1. Resident retinal cells as well as infiltrating immune cells could be sources of cytokine production. In order to better understand the interplay and contributions of local versus systemic biomarkers to DR progression, it is important to collect matched ocular and blood samples in future clinical studies, as shown on the right side of the figure. Note that only changes that showed statistical significance in our analysis are shown in the figure and that only data from papers with matched blood and vitreous sampling were included in this analysis, which generally led to a low number of data sets being included here. For NPDR, no data on ocular cytokine levels were available. In addition, matched data were available for vitreous, but not aqueous humor in the reviewed evidence. This figure was created with BioRender.com.