Dear Editor,

Proliferative vitreoretinopathy (PVR) is one of the main causes of failure after rhegmatogenous retinal detachment (RRD) surgery. Inflammation is a key factor in the development of PVR. This pathology is characterized by proliferation of retinal pigment epithelial (RPE) cells, macrophages, glial cells, fibroblasts, and vitreous cells on the retinal surface, which leads to avascular membranes formation. Growth factors and inflammatory mediators stimulate cell proliferation.

Cyclooxygenases (COX) catalyze the production of prostaglandins from arachidonic acid; this is one of the mechanisms of initiation and maintenance of the inflammatory response. The first and the second types of cyclooxygenase are normally present in the retina. COX-2 expression has been found in human choroidal neovascular membranes [1], in vascularized epiretinal membranes in diabetic retinopathy, and in idiopathic epiretinal membranes [2]. COX-1 was not detected in membranes.

Brain-derived neurotrophic factor (BDNF), nerve growth factor (NGF), and vascular endothelial growth factor (VEGF) regulate the viability of retinal cells and their normal functioning. However, retinal detachment and its ischemia are factors that promote the proliferation of glial and RPE cells [3, 4] which actively take part in the development of membranes during PVR.

The purpose of this research was to study the level of expression of pro-inflammatory COX-1 and COX-2, VEGF, BDNF, and NGF in epiretinal membranes from patients with severe PVR.

Eleven patients suffering from RRD complicated by PVR at late stages were included in the study (Table 1). Patients with other ocular and systemic pathologies were excluded. The localization of retinal breaks was peripheral in all patients.

Table 1 Clinicopathological profile of patients with PVR and mRNA expression of COX-1, COX-2, VEGF, BDNF, and NGF (in standard units)
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All vitreoretinal surgeries were performed with standard 25-gauge instruments by a single surgeon. During the surgery, the epiretinal membrane of all the eyes was made more visible with triamcinolone acetonide (Kenalog®; KRKA, Slovenia). Membranes were immediately placed in liquid nitrogen, after which they were transferred to 500 ml TRI reagent (Sigma-Aldrich, St. Louis, MO, USA) for a period of 12 h.

The standard method of semi-quantitative reverse transcription-polymerase chain reaction (RT-PCR) was used. We used specific primers for β-actin (forward: 5’-AGGCCAACCGCGAGAAGATGAC-3’; reverse: 5’-TCGGCCGTGGTGGTGAAGC-3’), COX-1 (forward: 5’-GCTCCAACCTTATCCCCAGTCCC-3’; reverse: 5’-CATCAACACAGGCGCCTCTTCTAC-3’), COX-2 (forward: 5’-CCTGATGATTGCCCGACTCCC-3’; reverse: 5’-ATACATCATCAGACCAGGCACCAGAC-3’), VEGF (forward: 5’-GGAGGAGGGGGAGGAGGAAGAAGA-3’; reverse: 5’-AGCCCCCGCATCGCATCAG-3’), NGF (forward: 5’-CAGCGTCCGGACCCAATAACAG-3’; reverse: 5’-GTGGAAGATGGGATGGGATGATG-3’), and for the ninth encoding exon of the BDNF gene (forward: 5’-TGGCTGGCGATTCATAAGGATAGAC-3’; reverse: 5’-GGCAACGGCAACAAACCACAA-3’), which were synthesized by CJSC Syntol, Moscow, Russia. The results of PCR were normalized to the RNA content in a sample (Table 1).

Expression of COX-1, COX-2, and trophic and growth factors was observed in all membranes in any of the late stages of the disease and for any duration of membrane existence (Fig. 1).

Fig. 1
figure 1

Examples of gel pictures of RT-PCR results. Numbers refer to the number of the sample; “+” denotes the sample after the reverse transcription, and “−” denotes the negative control sample (without the reverse transcription)

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Dependencies of the expression of pro-inflammatory factors on patients’ gender, their age, frequency of disease recurrences and treatment outcomes have not been identified.

Spearman’s correlation analysis of the data showed that the more the membrane expressed COX-1, the more it produced VEGF (Spearman’s ρ = 0.71; p < 0.05) and BDNF (Spearman’s ρ = 0.74; p < 0.01). Production levels of mRNA VEGF and BDNF were also positively correlated with each other (Spearman ρ = 0.61; p < 0.05).

It is believed that at a later stage of PVR, the intensity of inflammation in the eye decreases. However, we have shown that pro-inflammatory enzymes COX-1 and COX-2 are expressed by membranes at later stages of PVR development. This indicates that the inflammatory processes in membranes remain active in the late stages of the disease.

Membranes also express mRNA of BDNF, VEGF, and in some cases NGF. An increase in the proliferative and secretory capacities of cells forming the membrane under the effects of VEGF, BDNF, and NGF can be seen as a negative effect [3, 4]. These factors also exert neurotrophic effects on neurons and glial cells in the retina. Studies have shown that VEGF reduces the rate of degradation and apoptosis of ganglion cells [5], photoreceptors, and Müller cells [6]. BDNF and NGF are powerful factors that increase the viability of retinal ganglion cells [7, 8] and reduce the degeneration of photoreceptors. These effects are undoubtedly positive.

Our data analysis showed a positive correlation between the expression of VEGF and BDNF in membranes formed in the late stages of PVR. In 2002, Louissaint et al. showed that endothelial cells were able to produce not only VEGF, but BDNF as well [9].

In addition, we found a direct correlation between the expression of VEGF and COX-1 and the expression of BDNF and COX-1 in tested membrane samples. Bryant et al. found that VEGF was able to stimulate the expression of COX-1 in endothelial cells [10]. TGF-β and IL-1β also stimulate the expression of both VEGF and COX-1.

Analysis of our own results and published data allows us to conclude that the presence of the membrane complicating RRD helps to maintain the inflammation in the eye and the growth of the membrane itself.