The inflammatory process generated during endophthalmitis is critical for elimination of the infectious agent. Recognition of the microorganism and the subsequent activation of immune cells lead to the secretion of inflammatory factors that are involved in the clearance of the pathogen as well as the modulation of the immune system [
3]. This is the first in vivo study to evaluate the inflammatory response through the measurement of cytokine levels in patients with acute endophthalmitis following cataract extraction. The storage of samples at -80°C for up to two years enables cytokines stability for subsequent dosing. IL-2, IL-4 and IL-12 may remain stable for up to three years [
19]. As endophthalmitis after cataract surgery is a rare event, storage is important to enable sample size.
Experimental studies have already shown that infectious endophthalmitis can induce the expression of cytokines [
9,
16-
18]. Petropoulos et al. [
9] inoculated
Staphylococcus epidermidis strains into the eyes of lewis rats. They observed that high levels of TNF-α, IL-1ßand IFN-γ were associated with intensity of the inflammatory process. The association between the inflammatory process and cytokine levels was also observed in the present study. These cytokines could not be detected in the blood, suggesting that their production is local. Giese et al. [
16] used viable colonies of
Staphylococcus aureus to induce endophthalmitis in the eyes of rats and studied the production of TNF-α, IL-1ß, cytokine-induced neutrophil chemoattractant (CINC) and IFN-γ. CINC is the equivalent of IL-8 in humans. The study showed an increase in the four cytokines studied, showing a relationship with the inflammatory process.
We observed high production of cytokines that are involved in several phases of the inflammatory process: recognition of the microorganism, leukocyte recruitment, pathogen clearance and tissue repair. TNF-α is one of the cytokines that emerge early during the inflammatory process, and it has the ability to stimulate other cytokines [
1,
2]. Fourteen of the 25 measured cytokines (IL-1ra, IL-2, IL-4, IL-6, IL-8, IL-17, eotaxin, G-CSF, GM-CSF, IFN-γ, MIP-1a, MIP-1b, PDGF-BB and TNF-α) had higher levels compared to the control group. With the exceptions of IL-7 (higher in patients with diabetic retinopathy) and IL-5, the cytokines were more abundant in patients with endophthalmitis compared to control patients suffering from maculopathy. Patients with idiopathic macular holes or epiretinal membranes associated with changes in the vitreoretinal interface did not present with significant intraocular inflammatory changes or retinal ischemia [
20].
No statistically significant differences in the IL-5 levels were observed between groups, suggesting that this cytokine does not participate in acute infectious endophthalmitis. IL-5 is produced byTH2 lymphocytes in response to stimulation by allergens,
Mycobacterium tuberculosis and
Toxocara canis, and it stimulates the differentiation and proliferation of eosinophils, which are characterized as a mediator of allergic processes such as asthma [
21]. Because the primary causative agent of acute endophthalmitis is
Staphylococcus epidermidis, it is not likely that IL-5 has any significant involvement in ocular immune responses.
IL-7 is a hematopoietic growth factor that is produced by stromal cells in lymphoid tissue in the thymus and bone marrow and that participates in the expansion of the immature precursors of T and B lymphocytes [
22]. IL-7 participates in the immune response to HIV and neoplastic processes such as lymphoma and acute lymphoblastic leukemia. There was no evidence in this study that IL-7 plays a role in acute infectious endophthalmitis. There was, however, an increase in the vitreous levels of IL-7 in patients with diabetic retinopathy.
G-CSF and GM-CSF are also considered hematopoietic cytokines and act on bone marrow progenitors to stimulate leukocyte production. G-CSF can also be produced at the infection site and acts as an endocrine hormone in the bone marrow [
23]. Activated T-cells, macrophages and endothelial cells can express both these cytokines. There were increases in the G-CSF and GM-CSF levels in patients with acute infectious endophthalmitis, suggesting their involvement in the inflammatory process.
There was a positive correlation between the initial visual acuity and the vitreous levels of TNF-α. In one patient with infectious endophthalmitis, the visual acuity may reflect the intensity of the inflammatory process. As the inflammatory reaction in the vitreous cavity and the anterior chamber increases, the media opacity also increases, hindering the passage of light to the retina and decreasing visual acuity. In one experimental study, a relationship between the levels of TNF-α in the vitreous humor and the intensity of the inflammatory process was observed [
9].
The initial visual acuity is a determining factor in the selection of treatment for these patients. The Endophthalmitis Vitrectomy Study (EVS), developed in the 1990s and still used today, concluded that patients whose visual acuity was hand motions or better should be treated with the injection of intravitreal antibiotics. Patients with light perception only should be treated with vitrectomy combined with the injection of intravitreal antibiotics [
24]. If the TNF-α levels are related to the initial visual acuity and the intensity of the inflammatory process, it may eventually be used as a marker to help to decide between treatment with posterior vitrectomy or intravitreal injection.
The inflammatory process can damage the corneal endothelium through the formation of a severe opacity, which may require corneal transplantation, a fibrin pupillary block and posterior synechiae, including the development of glaucoma, and severe vitreoretinal proliferation in patients with retinal detachment. It is possible to pharmacologically control the inflammatory process. Intravitreal corticosteroids are frequently used in cases of bacterial acute infectious endophthalmitis because the immune reaction itself may damage the ocular structures [
5].
Medications that block TNF-α, which consist of monoclonal antibodies including infliximab, adalimumab and certolizumab, are also available. These medications are used in the treatment of autoimmune diseases such as rheumatoid arthritis, and their benefits have already been demonstrated in several studies [
25]. Although these medications can reduce the inflammation associated with certain pathologies, their benefits in acute infectious endophthalmitis are not well defined.
In an experimental study, Ramadan et al. [
17] evaluated the role of TNF-α in the pathogenesis of
Bacillus cereus-induced endophthalmitis. In that study, the authors used a TNF-α non-producing mouse strain and compared it to control mice. The authors concluded that the TNF-α non-producing mice had decreased migration of polymorphonuclear leukocytes, despite the increased synthesis of IL-6, keratinocyte chemokine (KC), MIP-1a and IL-1ß, resulting in reduced ocular inflammation. However, a histological evaluation showed a significant loss and atrophy of the retinal architecture compared to TNF-α-producing mice. These data suggest that TNF-α-non-producing mice may have a more severe disease course, including greater vision loss due to the increased retinal damage, despite the reduced inflammation. However, modulation of the inflammatory response through the use of TNF-α blockers or blockers for other cytokines, combined with the use of antibiotics, has not yet been evaluated.
In an experimental model, Geiger et al. [
18] reported the destruction of photoreceptors and retinal degeneration in mice that received IFN-γ by an intravitreal route. IFN-γ induced glial cells to express class I and II major histocompatibility complex (MHC) antigens and induced retinal pigment epithelial cells to express class II MHC antigens. The activation of retinal pigment epithelial cells could increase the phagocytosis and subsequent loss of photoreceptors.
There are a number of known prognostic factors for endophthalmitis, such as patient’s age, the virulence of the microorganism, diabetes, posterior capsule rupture, changes in intraocular pressure, corneal infiltrate and especially the initial VA of light perception [
24]. However, there was no correlation between TNF-α, IFN-γ or any other cytokine and final visual acuity. In this study, the cytokine levels could not be used as prognostic markers for infectious endophthalmitis.
There was an increase in the concentration of IL-8 in patients with positive cultures. IL-8 has been identified as a marker for the early detection of bacterial infections [
13]. A diagnosis of infectious endophthalmitis following cataract extraction is based on clinical criteria; thus, there’s a possibility that some patients who are culture negative, have a sterile endophthalmitis. There was also an increase in the concentrations of IL-17 in the patient with
Staphylococcus aureus infection. An alpha-toxin produced by strains of
S. aureus has already been shown to induce IL-17 production [
26]. Additional studies with a larger sample size are needed to assess IL-8 as a marker for bacterial infections and IL-17 as a marker for
S. aureus infections.
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