In this study, multiplex microsphere flow cytometric immunoassay was used to detect the levels of inflammatory cytokines (IL-1β, IL-2, IL-4, IL-5, IL-6, IL-8, IL-10, IL-12, IL-17, TNF-α, and IFN-γ) in the aqueous humor of patients with PSS and cataract. By comparing the cytokine expression levels and their correlations with various clinical parameters between the two groups, we aimed to identify novel strategies for the laboratory diagnosis of PSS. Our results demonstrated that the levels of IL-1β, IL-4, IL-5, IL-6, IL-8, IL-10, IL-17, IFN-γ, and TNF-α were significantly elevated in the aqueous humor of PSS patients compared with the cataract control group (P < 0.05).
IL-1β, which can stimulate almost all cell types,[12] is one of the most potent cytokines and a crucial mediator of immune and inflammatory responses in various conditions.[13] Elevated IL-1β expression has been reported in the blood and aqueous humor of patients with ocular diseases such as glaucoma and macular degeneration. IL-1β can also alter the extracellular matrix of trabecular meshwork cells and increase aqueous outflow, thereby affecting IOP.[14–16]Our findings of significantly elevated IL-1β in the aqueous humor of PSS patients suggest its involvement in the inflammatory mechanisms of PSS.
Previous studies have reported significantly increased levels of cytokines such as interleukin IL-4 and IL-8 in the aqueous humor of patients with primary open-angle glaucoma or pseudoexfoliative glaucoma.[17] Li et al. [18] demonstrated that the levels of IL-5, IL-6, IL-8, and TNF-α were higher in the aqueous humor of PSS patients compared with controls, while IL-2 and IL-12 levels were lower in the PSS group. Although the difference in IL-4 levels was not statistically significant, its detection rate was higher in the PSS group than in the control group. Our results are consistent with these previous reports, suggesting that IL-4 and IL-5 may be involved in the pathogenesis of PSS.
In a recent study, Li et al. [18] reported significantly elevated levels of IL-6 and IL-8 in the aqueous humor of PSS patients. Moreover, the concomitant elevation of IL-6 and IL-8 levels could serve as a cytokine indicator for predicting the development of secondary open-angle glaucoma in PSS patients. Another study demonstrated that IL-6 levels in the aqueous humor of patients with corneal endothelial dysfunction-induced graft failure were 179 times higher than those in cataract controls, and the elevated aqueous humor cytokine levels were associated with rapid corneal endothelial cell loss.[19] These findings suggest that the immune response is rapidly activated under these conditions and is closely related to the disease status and tissue damage. Such characteristics make inflammatory factors like IL-6 potential biomarkers for tissue damage, disease progression, and the pathological status. Our results showed that the levels of IL-6 and IL-8 in the aqueous humor of PSS patients were significantly higher than those in the cataract control group (P < 0.05) and exhibited correlations with elevated IOP and corneal endothelial cell loss. Therefore, IL-6 and IL-8 play important roles in the anterior chamber inflammation associated with PSS and could serve as potential biomarkers for monitoring inflammation and as novel targets for future anti-inflammatory therapies in PSS.
IL-10 is secreted by Th2 cell and mainly regulates cell proliferation and differentiation. As an important anti-inflammatory cytokine, it participates in inflammatory responses. Viruses have been detected in the aqueous humor of some PSS patients and play a role in PSS pathogenesis, potentially affecting IL-10 levels.[20] Studies have also reported significantly elevated IL-10 levels in the aqueous humor of patients with Fuchs' syndrome.[21] Similarly, our results showed elevated IL-10 levels in the PSS group, suggesting its clinical significance in the diagnosis and management of PSS. IL-10 is an anti-inflammatory cytokine, but our data suggest that the situation may be more complex and could be related to the sampling time. The eye is an immune-privileged site, and maintaining immune balance to prevent immune responses against viral infections from causing tissue damage is an inherent protective mechanism of the eye. Therefore, while the inflammatory cytokine levels induced by viruses increase, the expression of IL-10, which suppresses inflammation, is also upregulated to achieve immune balance.
IL-17, mainly produced by Th17 cells, is an important pro-inflammatory cytokine involved in the pathogenesis of various diseases. It is associated with inflammation and autoimmune disorders, and its high expression usually leads to aggravated inflammation .[22] Our study found that the IL-17 level in the aqueous humor of PSS patients was significantly higher than that in the cataract control group (P < 0.05), suggesting that IL-17 may be an important cytokine involved in the pathological damage of PSS. Other studies have also reported upregulated IL-17 in the aqueous humor of secondary glaucoma, where it promotes the recruitment of inflammatory cells to the lesion and triggers inflammation.[23] Li et al.[24] demonstrated increased IL-17 expression in an animal model of acute IOP elevation. Furthermore, targeted inhibition of IL-17 can significantly relieve uveitis symptoms.[25]
IFN-γ is a cytokine secreted by Th1 cells that can inhibit T cell and macrophage proliferation and promote the apoptosis of inflammatory cells.[26–29] Chua et al.[30] found that IFN-γ levels were upregulated in the aqueous humor of POAG patients. Another study demonstrated that intravitreal injection of IFN-γ could induce intraocular inflammation.[31] Compared with non-infectious uveitis, IFN-γ levels are more significantly elevated in the aqueous humor of infectious uveitis, indicating that IFN-γ is related to inflammation and infection.[32, 33] In our study, the IFN-γ level in the aqueous humor of PSS patients was significantly higher than that in the control group, suggesting the clinical value of IFN-γ in the diagnosis of PSS.
In an animal experiment, the TNF-α concentration in the aqueous humor increased after traumatic cataract surgery and developed into secondary increased IOP, suggesting that TNF-α may be involved in the development of glaucoma. Studies have found that TNF-α in the aqueous humor of glaucoma patients may damage trabecular meshwork cells by inducing inflammation and apoptosis.[34] It has also been reported that TNF-α can increase IOP by promoting nitric oxide synthesis and damaging trabecular meshwork cells. Elevated IOP can then stimulate TNF-α production, leading to a vicious cycle that exacerbates trabecular meshwork damage and IOP elevation.[35]
Research has shown that TNF-α can contribute to the prolongation of inflammation, and its expression level is significantly increased during inflammatory responses.[36]TNF-α levels are upregulated in various types of uveitis, and this is particularly evident in PSS, which is presumably caused by inflammation-induced TNF-α expression.[37] Kelley et al.[36] found that TNF-α and IL-1 can act synergistically on the trabecular meshwork to upregulate MMP-3, which can degrade the extracellular matrix of trabecular meshwork cells, reducing the resistance to aqueous outflow. Our results showed that the TNF-α level in the aqueous humor of PSS patients was significantly higher than that in the control group, indicating that TNF-α plays a role in the pathogenesis of PSS.
Moreover, we found that the aqueous humor cytokine levels of IL-6 and IL-8 were significantly elevated in PSS patients and were both correlated with clinical manifestations of IOP and RECL, suggesting that inflammatory changes in PSS are highly correlated with elevated IOP and corneal endothelial cell damage.
Our study has several limitations.The incidence of PSS is relatively low, and the disease course is prolonged with recurrent episodes, resulting in a small sample size. Moreover, the strict inclusion criteria, high pre-analytical sample control and screening, and high technical requirements for detection led to a small number of eligible cases and control samples finally included in the study. In terms of detection, more sensitive detection techniques and high-throughput research methods are needed in the future to conduct multicenter prospective studies and investigate eye disease-related biomarkers more systematically.