Roles of pattern recognition receptors in response to fungal keratitis

Rutin resists Aspergillus fumigatus keratitis by activating Nrf2/HO-1 pathway, inhibiting Dectin-1/p-Syk pathway and affecting fungal structures

Fungal keratitis (FK) is a severe vision-threatening eye disease. The fungal invasiveness and excessive inflammatory response contribute to corneal tissue damage. Rutin (RT) possesses anti-inflammatory, antimicrobial, antioxidant, and improved wound-healing characteristics. This study aimed to evaluate antifungal, anti-inflammatory, and therapeutic effects of RT in FK. The results showed that RT exerted antifungal effects by inhibiting fungal growth, altering hyphal morphology, destroying biofilm, and disrupting fungal cellular structures. RT exhibited anti-inflammatory benefits by suppressing the Dectin-1/p-Syk pathway, activating the Nrf2/HO-1 pathway, and decreasing the expression of inflammatory factors in vivo and in vitro. RT demonstrated therapeutic effects by reducing clinical scores, fungal load, macrophage recruitment, and neutrophil activity. In conclusion, RT exhibited anti-inflammatory, antifungal, and therapeutic effects in Aspergillus fumigatus keratitis, and has the potential to become a novel therapeutic strategy for FK.

Involvement of cGAS/STING Signaling in the Pathogenesis of Candida albicans Keratitis: Insights From Genetic and Pharmacological Approaches

Purpose

Fungal keratitis (FK) is an invasive corneal infection associated with significant risk to vision. Although the cyclic GMP-AMP synthase (cGAS)/stimulator of interferon genes (STING) signaling pathway has been recognized for its role in defending against viral infections, its involvement in FK still remains largely unclear. This study sought to elucidate the contribution of the cGAS/STING signaling pathway to the pathogenesis of FK.

Methods

The expression of cGAS/STING signaling components was assessed in a murine model of Candida albicans keratitis through RNA sequencing, western blot analysis, immunofluorescence staining, and real-time PCR. Both genetic (utilizing Sting1gt/gt mice) and pharmacological (using C176) interventions were employed to inhibit STING activity, allowing for the evaluation of resultant pathogenic alterations in FK using slit-lamp examination, clinical scoring, hematoxylin and eosin (H&E) staining, fungal culture, and RNA sequencing. Subconjunctival administration of the NOD-like receptor protein 3 (NLRP3) inflammasome inhibitor MCC950 was performed to evaluate FK manifestations following STING activity blockade. Furthermore, the impact of the STING agonist diABZI on FK progression was investigated.

Results

Compared to uninfected corneas, those infected with C. albicans exhibited increased expression of cGAS/STING signaling components, as well as its elevated activity. Inhibiting cGAS/STING signaling exacerbated the advancement of FK, as evidenced by elevated clinical scores, augmented fungal load, and heightened inflammatory response, including NLRP3 inflammasome activation and pyroptosis. Pharmacological inhibition of the NLRP3 inflammasome effectively mitigated the exacerbated FK by suppressing STING activity. Conversely, pre-activation of STING exacerbated FK progression compared to the PBS control, characterized by increased fungal burden and reinforced inflammatory infiltration.

Conclusions

This study demonstrates the essential role of the cGAS/STING signaling pathway in FK pathogenesis and highlights the necessity of its proper activation for the host against FK.

Pseudolaric acid B exerts an antifungal effect and targets SIRT1 to ameliorate inflammation by regulating Nrf2/NF-κB pathways in fungal keratitis

Fungal keratitis (FK) is a vision-threatening infection. We aimed to explore the antifungal and anti-inflammatory effects of pseudolaric acid B (PAB) on FK and the underlying mechanisms involved. Network pharmacology utilized to acquire the potential target genes, and silent information regulator 1 (SIRT1) was consistently downregulated in Gene Expression Omnibus dataset and clinical samples. Molecular docking analysis showed that PAB and SIRT1 had good binding activity. No toxicity was observed in vivo and in vitro with a PAB concentration below 0.3 μM. PAB exerted its antifungal activity by destroying the integrity of hyphae, and alleviated the severity of FK in rats by decreasing clinical scores, fungal burden and inhibiting inflammatory cell infiltration. PAB increased SIRT1 to regulate the crosstalk between nuclear factor erythroid 2-related factor 2 (Nrf2) and nuclear factor kappa-B (NF-κB), decreasing the levels of inflammatory cytokines tumor necrosis factor (TNF)-α, interleukin (IL)-1β, and IL-6; and pattern recognition receptors, C-type lectin domain containing 7A (Dectin-1), lectin-like oxidized low-density lipoprotein receptor 1 (LOX-1), toll like receptor (TLR)-2, and TLR4 both in vivo and in vitro. However, this anti-inflammatory effect of PAB was abolished by the SIRT1 inhibitor EX527. This study provides new evidence that PAB has antifungal and anti-inflammatory effects in FK and may provide a novel therapeutic strategy for the treatment of FK.

Schaftoside reduces inflammation in Aspergillus fumigatus keratitis through the inhibition of the TLR4/MyD88 pathway

PURPOSE

The purpose of this research study was to investigate the impact of schaftoside on Aspergillus fumigatus (A. fumigatus) keratitis and elucidate its underlying mechanisms.

METHODS

In order to establish safe experimental concentrations of schaftoside in human corneal epithelial cells (HCECs), RAW264.7 cells, and mouse models, various techniques were employed including cytotoxicity assay (CCK-8) assay, cell scratch assay, and Draize test. The therapeutic effect of schaftoside was assessed using slit-lamp biomicroscopy, clinical scores, as well as determination of neutrophil infiltration through hematoxylin and eosin (HE) staining, immunofluorescence (IF) staining, and myeloperoxidase (MPO) assay. The levels of Toll-like receptor 4 (TLR4), myeloid differentiation primary response 88 (MyD88), pro-inflammatory mediators interleukin (IL)-1β, tumor necrosis factor (TNF)-α, and IL-6 were determined using quantitative real-time polymerase chain reaction (qRT-PCR), western blotting, and IF techniques.

RESULTS

Schaftoside at a concentration of 160 μM displayed no harmful side effects on HCECs, RAW cells, and mouse corneas, rendering it suitable for further experiments. In a murine fungal keratitis model, schaftoside mitigated the severity of fungal keratitis by inhibiting neutrophil infiltration and reducing MPO activity. Both in vitro and in vivo experiments demonstrated that schaftoside treatment suppressed the upregulation of IL-1β, TNF-α, and IL-6 expression, while also downregulating the expressions of TLR4 as well as MyD88 at both mRNA and protein levels.

CONCLUSIONS

Schaftoside demonstrated a protective effect against A. fumigatus keratitis by reducing corneal damage through inhibition of neutrophil recruitment and downstream inflammatory cytokines. The anti-inflammatory properties of schaftoside in A. fumigatus keratitis may involve modulation of the TLR4/MyD88 pathway.

A20 ameliorates Aspergillus fumigatus keratitis by promoting autophagy and inhibiting NF-κB signaling

Fungal keratitis (FK) is a serious, potentially sight-threatening corneal infection, which is associated with poor prognosis. A20, also called TNFAIP3, plays significant roles in the negative regulation of inflammation and immunity. However, the function of A20 in Aspergillus fumigatus (A. fumigatus) keratitis remains obscure. Herein, we found that the level of A20 is increased in human corneal epithelial cells (HCECs) and in mouse corneas with A. fumigatus infection, and that nuclear factor-κB (NF-κB) signaling is required for A20 upregulation. A20 overexpression inhibits A. fumigatus-mediated inflammatory responses, while A20 knockdown results in opposite effect. Mechanically, we showed that A20 inhibits NF-κB signaling and activates autophagy in infected HCECs. We also showed that inhibition of NF-κB signaling reverses the increased inflammatory responses in infected HCECs with A20 knockdown. Furthermore, autophagy blockage impedes the anti-inflammatory effect of A20 in A. fumigatus infected HCECs. Moreover, A20 ameliorates the corneal damage and inflammation in A. fumigatus infected mouse corneas. In conclusion, this study reveals that A20 alleviates A. fumigatus keratitis by activating autophagy and inhibiting NF-κB signaling. This suggests that exogenous use of A20 protein may be a potentially promising therapeutic strategy for FK treatment.

The preparation and therapeutic effects of β-glucan-specific nanobodies and nanobody-natamycin conjugates in fungal keratitis

Fungal keratitis (FK) is a severe infectious corneal disease. Since traditional eye drops exhibit poor dissolution and high corneal toxicity, the efficacy of current treatments for FK remains limited. It is needed to develop new approaches to control the cornea damage from FK. In this study, a nanobody (Nb) specific to β-glucan in the fungal cell wall was prepared. The conjugate of the Nb with natamycin (NAT), a traditional antifungal drug, was synthesized. Firstly, we found the Nb specific to β-glucan inhibited fungal growth by disrupting cell wall and biofilm formation.. In addition, the content of β-glucan in the fungal cell wall decreased after Nb treatment. The Nb also reduced the adhesion ability of fungal conidia to human corneal epithelial cells (HCECs). Further, we examined the difference between NAT and Nb-NAT in antifungal growth. Nb-NAT showed better antifungal effects than NAT which was caused by the interaction between Nb and β-glucan. Moreover, Nb concentration below 0.5 mg/mL did not affect the viability of HCECs. Nb-NAT had less cytotoxicity and ocular surface irritation than NAT. Nb specific to β-glucan attenuated Aspergillus fumigatus (A. fumigatus) virulence and relieved inflammatory responses in FK. Nb-NAT treatment of the cornea improved therapeutic effects compared with NAT. It decreased clinical scores and expression level of inflammatory factors. To our knowledge, this study is the first to report a Nb specific to β-glucan and Nb-NAT for the treatment of FK. These unique functions of the Nb specific to β-glucan and Nb-NAT would render it as an alternative molecule to control fungal infections including FK. STATEMENT OF SIGNIFICANCE: Fungal keratitis is a corneal disease with a high rate of blindness. Due to the poor dissolution and high corneal toxicity exhibited by traditional eye drops, the efficacy of current therapeutic treatments for fungal keratitis (FK) remains limited. To enhance the therapeutic effect of natamycin in treating fungal keratitis, this study developed an innovative approach by preparing a β-glucan-specific nanobody and loading it with the antifungal drug natamycin. The β-glucan-specific nanobody has the ability to control both fungal pathogen invasion and inflammation, which can cause damage to the cornea in FK. The conjugation with the β-glucan-specific nanobody significantly increased the antifungal capacity of natamycin and reduced its toxicity. The further application of natamycin conjugated with the β-glucan-specific nanobody could be expanded to other diseases caused by fungal pathogen infections.