Phacoemulsification surgery (PHACO) is the most technically advanced and commonly performed cataract surgery, which includes the fragmentation and removal of the opacified crystalline lens and replacement with an intraocular lens (IOL) [1]. PHACO + IOL is a minimally invasive surgical procedure, often performed under topical or local anesthesia, and the patients can be discharged on the same day [1]. The most common postoperative complications are posterior capsule opacification and elevated intraocular pressure, macular edema, fibrinous reaction, vitreous hemorrhage, posterior synechiae, and recurrent retinal detachment [2]. It has been considered that postoperative complications are correlated with patients’ pre-existing ocular diseases or co-morbidities. For instance, diabetes mellitus (DM) was considered a risk factor for macular edema [2, 3], and it was reported that posterior vitreous detachment (PVD) progressed significantly faster in eyes with high myopia (HM) than in eyes without HM [4].

However, the exact correlation and the underlying mechanism of postoperative complications and ocular or system conditions are still largely unclear. Postsurgical intraocular inflammation plays a central role in the development of postoperative complications, while the exact association between inflammatory factors, pre-existing ocular or systemic conditions and postoperative complications remains unknown. Common complications of ocular inflammation such as glaucoma, keratic precipitates, retinal edema and neovascularization may be mediated by cytokines [5]. For instance, choroidal neovascularization (CNV) was reported to develop more frequently in patients with HM after cataract surgeries [6]. Quality human eye tissue is difficult to obtain for research purposes [7]. Cytokines in the intraocular fluid can indicate the pathogenesis and progression of ocular inflammation [5].

Aqueous humor (AH) is a low viscosity fluid secreted from the ciliary body and circulates into the anterior chamber. It can exit the globe through the trabecular meshwork and move into a typical vortex due to the natural convection [8]. AH is a complex mixture of electrolytes, organic solutes, growth factors, cytokines and additional proteins that provide metabolic nutrients to avascular tissues of the anterior segment [9, 10]. Although AH contains minimal amounts of proteins [11], these proteins may have a significant role in the pathogenesis and prognosis of eye disorders [12, 13, 14]. Several studies have measured cytokine concentration in aqueous humor samples of patients with several co-morbidities, including primary angle-closure glaucoma (PACG), myopia and diabetes. The cytokines have been reported to be involved in immune reactions, inflammation, ischemia, hypoxia and oxidative stress. A major limitation of testing aqueous humor in one specific patient group is the difficulty in identifying the unique cytokine changes to the existing co-morbidity. Moreover, the exact roles of these cytokine changes in the existing co-morbidities are not well understood.

Characterizing the AH cytokines at the beginning of first and second eye cataract surgery in patients with different ocular or systemic conditions could provide a foundation for biomarker discovery in various eye diseases, guide optimal operative time, and provide new insights into factors involved in postsurgical inflammation. By comparing the cytokine changes between the groups, we can evaluate the existence of comorbidity-specific cytokine changes. Based on these considerations, the aims of this study are: (1) to identify cytokine changes in AH of patients with different systemic (DM) or topical conditions (HM, PACG); (2) determine whether PACG, HM and DM might lead to different cytokine response to cataract surgery, and; (3) investigate different cytokine network patterns in PACG, HM and DM before cataract surgery and in response to cataract surgery.

In this study, the cytokine profile of aqueous humor in the first eye of cataract controls and patients with DM, PACG or HM were analyzed at baseline and in the contralateral eye after their first-eye cataract surgery. By comparing the cytokine profile of the first-eye operation and the second-eye operation, we found that all groups with pre-existing ocular or systemic conditions had signs of inflammation at both time points. PHACO + IOL was associated with inflammatory reactions in the AH of the contralateral eye. Strikingly, pre-existing ocular or systemic conditions were also associated with distinguishing between inflammatory reactions and topical stimuli, namely, PHACO + IOL.

At baseline, the HM group had the most similar cytokine profile to the controls and only the level of IL-13 and IL-15 was significantly differentially expressed, whereas the level of 11 cytokines in the DM group was significantly different from the control group. Although the exact roles of IL-13 and IL-15 in HM are unclear, our findings align with a previous report in which the level of IL-13 and IL-15 was increased in AH of patients with myopic choroidal neovascularization [17, 18].

The IL-13, TNF-alpha and IL-1ra level was found to be increased in cataract patients with PACG at baseline. Duvesh et al. [19] reported cytokine changes in chronic PACG patients in the Indian population, such as a reduced TNF-alpha and increased IP-10, while TNF-alpha was increased in acute angle-closure glaucoma patients [20]. These studies indicated that the TNF-alpha level might be associated with the disease course. Another study reported that IL-13 was found to be more abundant in AH of primary open-angle glaucoma (POAG) patients [21]. Although the pathology of PACG and POAG is not the same, IL-13 may play a role in POAG and PACG, such as the progressive loss of retinal ganglion cells and their axons.

In DM patients, 11 cytokines were significantly increased in their AH despite their lack of ocular symptoms. Similar to our results, the aqueous levels of IL-8, MCP-1 and TNF-alpha were higher in diabetic macular edema patients [22, 23], and lower levels of IL-5 and IL-8 were associated with improved diabetic macular edema [23].

In response to cataract surgery, IL-2 was increased in all groups, and was correlated with intraocular inflammation. Inflamed uveal and retinal tissues were detected in patients with certain types of uveitis [24] despite an increase in the level of VEGF and PDGF-BB in the PACG, HM and DM groups compared with the control group. VEGF has been associated with pathological angiogenesis and increased vascular permeability in eye diseases such as diabetic retinopathy and age-related macular degeneration [25]. Retinal vascular function damage is well known in both glaucoma and diabetic retinopathy, while the role of PDGF-BB in the maintenance of retinal vasculature was just recognized [26, 27]. In mice and rabbits, it was shown that the retina-specific expression of PDGF-B was associated with severe neovascularization and retinal detachment [28, 29].

In response to cataract surgery, pre-existing ocular disease conditions were associated with more significant and distinguishing inflammatory reactions. Four cytokines were significantly altered in the control group, 3 in the DM group, 9 in the PACG group, and 8 in the HM group. Patients with PACG and HM had a stronger reaction than the control and DM groups. Surprisingly, DM patients did not show a stronger inflammation in reaction to surgical stimuli.

After the first-eye cataract surgery, only bFGF was significantly upregulated in the HM group compared with the controls, while 3 cytokines in the PACG group and 6 cytokines in the DM group were significantly different from the control group. The level of bFGF was significantly higher in the PACG and HM groups. The level of bFGF was also increased in the irises of patients with neovascular glaucoma and in the aqueous humor of patients with exfoliation syndrome or exfoliative glaucoma [30, 31]. In an animal model of photic-induced retinopathy, bFGF up-regulation was associated with the protection of retinal functions [32]. Nonetheless, the exact source of bFGF and its role in the eye remains to be studied.

In the PACG group, the level of IP-10 and MCP-1 was significantly lower after the first-eye surgery. In acute angle-closure glaucoma patients, the level of IP-10 was found to be significantly higher [20]. It was also reported that IP-10 in AH was positively correlated with IOP reduction after trabeculectomy [33]. Taken together, these findings indicate that IP-10 might be correlated with the IOP of glaucoma patients. Furthermore, MCP-1 was also reported to play a role in IOP regulation. The enhanced expression of MCP-1 was correlated with trabecular meshwork cell contractile activity, potentially implicated in the pathobiology of abrupt IOP elevation [34].

To our knowledge, this is the first study comparing the cytokine profiles of patients with different ocular and systemic disease conditions to identify the comorbidity-specific cytokines in each group. However, due to the nature of human-related studies, the underlying molecular mechanisms responsible for the findings reported here are largely unknown. Therefore, the precise functions of those cytokines and their downstream targets should be further elucidated in the future. Furthermore, from the data reported it is not evident a dramatic surge of inflammatory cytokines after the first surgery, and their clustering was not so pronounced to allow a characterization of the different conditions. Last but not least, it is unlikely that all the proteins responsible for intraocular inflammation related to the comorbidities are identified using cytokine assay. Mass-spectrometry-assisted proteomics approaches on a larger sample size could have returned results describing better the different conditions at the different time points. As a propaedeutic study, the findings are envisioned to provide a useful reference point for future studies. Further studies with larger samples are warranted to establish more solid patterns of variations in cytokine expression associated with different comorbidities.

In summary, our data show that pre-existing ocular or systemic conditions could lead to intraocular inflammation even before surgery, particularly in patients with type 2 DM. Strikingly, pre-existing ocular or systemic disease conditions led to distinguishing inflammatory reactions in the second eye after the first eye surgery. PACG and HM patients showed a change in some cytokine expression to topical stimuli compared with controls, while fewer cytokines appear to be altered in DM patients. These findings suggest that ophthalmologists should pay close attention to the inflammatory response in the second eye, especially in patients with pre-existing ocular conditions.

JT designed the project, performed the analysis and wrote the manuscript. HL processed the experimental data, performed the analysis, wrote the manuscript and designed the figures. MS checked the experimental data, performed the analysis and revised the manuscript. JT, QL, HC and HpC helped recruiting the patients and collected samples. XZ collected the patients data. VP supervised the project and revised the manuscript. HpC performed the surgery, supervised the project and acquired funding. All authors contributed to editorial changes in the manuscript. All authors read and approved the final manuscript.

The study was performed according to local ethical approval protocol no. EC. D (BG) .016.03.1. Informed consent was obtained from all subjects enrolled in the study. The study was in accordance with the guidelines of the ethical commission of Shanghai East Hospital of Tongji University.

Not applicable.

This research was funded by National Natural Science Foundation of China (81870634).

The authors declare no conflict of interest.