AIMS To determine whether the infiltrating cells in the vitreous and subretinal fluid of patients with proliferative vitreoretinopathy (PVR) express messenger RNA for various cytokines found in this condition.
METHODS The presence of mRNA coding for HPRT, IL-6, IL-1β, IL-8, and TNFα was investigated in 20 vitreous and subretinal fluid (SRF) samples from patients with PVR by reverse transcriptase polymerase chain reaction (RT-PCR). 16 samples from patients with retinal detachment and macular holes were used as controls.
RESULTS HPRT was detected in all samples of PVR and in 11 (69%) control cases. Patients with PVR demonstrated mRNA for the cytokines tested more often than controls. The difference was statistically significant.
CONCLUSION The presence of mRNA encoding for IL-6, IL-1β, IL-8, and TNFα is significantly detected by RT-PCR in vitreous and SRF samples of patients with PVR, indicating local production of these cytokines by vitreous and SRF cells.
- proliferative vitreoretinopathy
- retinal detachment
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Proliferative vitreoretinopathy (PVR) is a process of cellular proliferation and contraction that complicates rhegmatogenous retinal detachment. PVR is the most common cause of failure when the primary break has been appropriately treated.1 2 The exact pathogenic mechanisms involved in the formation of PVR are not completely understood. However, five distinct stages appear to be important in its development including breakdown of the blood-retinal barrier (BRB), chemotaxis and cellular migration, cellular proliferation, membrane formation, and contraction.3-5
It is believed that different cytokines, produced as a consequence of immune or inflammatory reactions, are involved in the pathogenesis of PVR.6-9 Cytokines such as interleukin 1 (IL-1), interleukin 6 (IL-6), tumour necrosis factor α (TNFα), interferon γ (IFNγ), interleukin 8 (IL-8), and monocyte chemotactic protein 1 (MCP-1) have been detected in the vitreous samples as well as epiretinal membranes obtained from PVR patients.10-15
It is not known whether these cytokines are produced locally by the cells infiltrating the vitreous or diffuse into the vitreous cavity following a breakdown of the BRB. We therefore tested the infiltrating cells in vitreous obtained from patients with PVR, for mRNA encoding for HPRT (housekeeping gene), IL-1β, IL-6, TNFα, and IL-8.
Materials and methods
VITREOUS AND SRF SPECIMENS
Twelve vitreous and eight subretinal fluid (SRF) samples were obtained by vitrectomy from 18 patients with PVR (both vitreous and SRF were obtained from two patients). Ten vitreous samples obtained from patients with macular hole and six vitreous samples from patients with retinal detachment (RD), not complicated by PVR, were examined as a control group. The presence of mRNA coding for HPRT, IL-6, IL-1β, TNFα, and IL-8 was investigated by reverse transcriptase polymerase chain reaction (RT-PCR).
The severity of PVR was graded according to the criteria of the Retina Society Terminology Committee.2 Samples were obtained through conventional three port closed vitrectomy technique by manual suction before opening the infusion line. SRF samples were obtained during the drainage procedure with a Charles flute needle connected to a 1 ml syringe with a side port. Samples with vitreous haemorrhage were excluded from the study. The study complied with the declaration of Helsinki.
mRNA EXTRACTION AND cDNA SYNTHESIS
Cellular pellets were obtained from the samples by centrifugation and mRNA extracted using Qiagen RN easy (UK) method following the manufacturer’s procedure. Eluted RNA was made up to 50 μl with DEPC water and cDNA was prepared using Oligo-(dT) priming in ready to go cDNA synthesis tubes (Pharmacia Biotech, UK) following the manufacturer’s procedure. Peripheral blood mononuclear cells (PBMC) stimulated with phytohaemagglutinin (PHA) (Sigma) were prepared by a standard method to serve as our positive control. Cloned PCR products of each cytokine were used as another positive control. These clones (in pNEB 193) have a 5 bp internal deletion generated by PCR mutagenesis for other purposes (P J Tighe, unpublished data). These were used at 1000 single stranded copies/PCR for positive control purposes.
Primers used were designed using the program primer 3 at:http://www.genome.wi.mit.edu/cgi-bin/primer/primer3.cgi. All primers were cross checked against the Genbank database to ensure no cross reactivity with other human sequences (Table 1), and verified to amplify only cDNA.
PCR was performed using standard agents (buffers from Invitrogen UK, dNTPs from Sigma, Poole UK, and Ampli Taq Gold from Perkin Elmer, Warrington) for 35–37 cycles using a Progene instrument from Techne (UK). PCR product was subjected to electrophoresis on 2% agarose gel, stained with ethidium bromide and visualised by ultraviolet illumination (Fig 1).
The χ2 test (statistics program, spss for Windows) was used to test for significance between presence of cytokines in the PVR samples versus the control group.
HPRT mRNA was detected in all samples from PVR (100%) and in 11 of 16 control samples (69%). Of the 20 samples of PVR, IL-6 mRNA was detected in 16 samples (80%), (10/12 vitreous (83%) and 6/8 SRF (75%)). IL-1β mRNA was detected in 12 samples (60%) (6/12 vitreous (50%) and 6/8 SRF (75%)). TNFα mRNA was detected in 10 samples (50%) (6/12 vitreous (50%) and 4/8 SRF (50%)). IL-8 mRNA was detected in 15 samples (75%) (9/12 vitreous (75%) and 6/8 SRF (75%)). In all, seven samples were positive for all cytokines tested. Six of the seven patients who tested positive for all cytokines had PVR graded between C2 and D1.
Only a small proportion of samples from the control group tested positive for cytokine mRNA. Four out of 16 (25%) were positive for IL-6, one (6.25%) for IL-1β, one (6.25%) for TNFα, and three (18.75%) for IL-8 (Table 1). This difference in the presence of cytokine mRNA between samples from the PVR group and the control group, for each cytokine tested, was highly significant (p = 0.00097, 0.00085, 0.00080, and 0.00463, for IL-6, IL-1β, IL-8, and TNFα respectively).
The results obtained from RT-PCR analysis of vitreous and SRF samples from patients undergoing vitrectomy for PVR provide evidence for the expression of mRNA for IL-6, IL-1β, TNFα, and IL-8.
Previous studies10-15 have been based on the detection of protein secretion and quantitation using bioassay and enzyme labelled immunosorbent assay (ELISA) techniques. Bioassays which measure functional properties, depending on the responsiveness of indicator cells lines, are technically restrictive.16 These techniques require in vitro stimulation, which could result in distortion of the true in vivo response.17 Techniques based on ELISA are highly specific but do not distinguish between biologically active or inactive substances. Furthermore, these assays determine the extracellular presence of a secreted interleukin, reflecting the net outcome of produced, absorbed, and degraded cytokine.18
Our approach to the role of cytokines in PVR pathogenesis was to directly detect cytokine mRNA from cells within the vitreous and SRF. This approach provides information on the possibility of local production of the cytokines in question, within the vitreous cavity, by the infiltrating cells. In addition, analysis of mRNA can detect the presence of a cytokine which may be absent in culture supernatants due to uptake and utilisation by cells bearing receptors. Thus, mRNA profiles can provide fairly accurate account of potential cytokine production.16 On the downside, however, the presence of cytokine mRNA does not guarantee that it will be translated. Potential regulation at transcriptional, post-transcriptional, translational, and post- translational levels may occur.19
To our knowledge the only work to detect the presence of cells expressing mRNA coding for IL-6, IL-1β, TNFα in epiretinal membranes from patients with PVR was carried out by Limbet al.20 They used an in situ hybridisation technique which, although sensitive, is technically very demanding to quantify. Owing to the different distribution of cytokines between the vitreous and epiretinal membranes in patients with PVR,11 the above study data did not provide information on cytokine production in the vitreous. This information was obtained by our approach. Both studies, however, have a similar drawback in that the demonstration of mRNA does not necessarily imply the presence of the protein product. RT-PCR was used because of its sensitivity, specificity, and its particular value where a limited amount of tissue is available and small amounts of cells express the gene of interest.
The majority of cells present in the vitreous samples of patients with PVR are capable of producing cytokines in vitro. RPE cells, upon activation, release IL-8, IL-6, and granulocyte macrophage colony stimulating factor.21-23 Glial cells produce IL-1.8 Macrophages and T lymphocytes produce many different cytokines including IL-1, IL-6, TNFα, and IFNγ.24 The actual role of cytokines in the pathogenesis of PVR is not clear, but their involvement in regulating immune and inflammatory responses is well described.19 In the retinal microenvironment, they could contribute substantially to the occurrence of PVR.
This study, like that of Kauffmann et al 12 and Elner et al 13 demonstrated a consistent presence of IL-6 and IL-8 in samples obtained from PVR patients. The definitive role of IL-8 in the pathogenesis of PVR is not clear, but owing to its chemotactic activity for leucocytes, it was postulated that TNFα and IL-1β, which were not directly chemotactic for leucocytes, exert their chemotactic effect through induction and release of secondary cytokines including IL-8 and MCP-1.13 Both IL-1 and IL-6 mediate inflammatory cell functions such as cell growth, migration, and differentiation. They overlap in their functions, and IL-1 is a potent inducer of IL-6 expression in many cell types.25 26
Although some patients with macular holes had epiretinal membrane (ERM), the macular hole group did not show the same cytokine profiles as did the PVR patients. This may be due to difference in the composition of ERM in macular holes. ERM is primarily a glial reaction with few inflammatory cells and lymphocytes.27
In conclusion, our data indicate that mRNA encoding for IL-6, IL-1β, TNFα, and IL-8 is significantly detected by RT-PCR in vitreous and SRF samples of patients with PVR, indicating probable local production of these cytokines by vitreous and SRF cells and their possible role in pathogenesis of the disease. Further work is needed to correlate the level of mRNA to respective cytokines in the vitreous and to severity of the disease and prognostic outcome of surgery.
The authors wish to acknowledge the help of Ms Sue Stevens, Mrs Jennie Phillips, and Dr A Azuara-Blanco. Supported by a grant from the Uveitis Foundation, Wills Eye Hospital, Philadelphia and the government of Egypt (IA El-Ghraby).
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