Intravitreal Injection and Quantitation of Infection Parameters in a Mouse Model of Bacterial Endophthalmitis

Cellular communication network factor 1 promotes retinal leakage in diabetic retinopathy via inducing neutrophil stasis and neutrophil extracellular traps extrusion

BACKGROUND

Diabetic retinopathy (DR) is a major cause of blindness and is characterized by dysfunction of the retinal microvasculature. Neutrophil stasis, resulting in retinal inflammation and the occlusion of retinal microvessels, is a key mechanism driving DR. These plugging neutrophils subsequently release neutrophil extracellular traps (NETs), which further disrupts the retinal vasculature. Nevertheless, the primary catalyst for NETs extrusion in the retinal microenvironment under diabetic conditions remains unidentified. In recent studies, cellular communication network factor 1 (CCN1) has emerged as a central molecule modulating inflammation in pathological settings. Additionally, our previous research has shed light on the pathogenic role of CCN1 in maintaining endothelial integrity. However, the precise role of CCN1 in microvascular occlusion and its potential interaction with neutrophils in diabetic retinopathy have not yet been investigated.

METHODS

We first examined the circulating level of CCN1 and NETs in our study cohort and analyzed related clinical parameters. To further evaluate the effects of CCN1 in vivo, we used recombinant CCN1 protein and CCN1 overexpression for gain-of-function, and CCN1 knockdown for loss-of-function by intravitreal injection in diabetic mice. The underlying mechanisms were further validated on human and mouse primary neutrophils and dHL60 cells.

RESULTS

We detected increases in CCN1 and neutrophil elastase in the plasma of DR patients and the retinas of diabetic mice. CCN1 gain-of-function in the retina resulted in neutrophil stasis, NETs extrusion, capillary degeneration, and retinal leakage. Pre-treatment with DNase I to reduce NETs effectively eliminated CCN1-induced retinal leakage. Notably, both CCN1 knockdown and DNase I treatment rescued the retinal leakage in the context of diabetes. In vitro, CCN1 promoted adherence, migration, and NETs extrusion of neutrophils.

CONCLUSION

In this study, we uncover that CCN1 contributed to retinal inflammation, vessel occlusion and leakage by recruiting neutrophils and triggering NETs extrusion under diabetic conditions. Notably, manipulating CCN1 was able to hold therapeutic promise for the treatment of diabetic retinopathy.

Virulence-related genotypic differences among Bacillus cereus ocular and gastrointestinal isolates and the relationship to endophthalmitis pathogenesis

Background

Bacillus cereus (Bc) can cause self-limiting gastrointestinal infections, but when infecting the eye, can cause rapid and irreversible blindness. This study investigated whether clinical ocular and gastrointestinal Bc isolates differed in terms of virulence-related genotypes and endophthalmitis virulence.

Methods

Twenty-eight Bc ocular, gastrointestinal, and laboratory reference isolates were evaluated. Hemolysis assays were performed to assess potential differences in hemolytic activity. The presence of twenty Bc virulence-related genes was assessed by PCR. A subset of ocular and gastrointestinal isolates differing in PCR positivity for 5 virulence genes was compared to strain ATCC14579 in an experimental murine model of endophthalmitis. At 8 hours post infection, retinal function was evaluated by electroretinography, and intraocular bacterial concentrations were determined by plate counts.

Results

Gastrointestinal Bc isolates were more hemolytic than the Bc ocular isolates and ATCC14579 (p < 0.0001). Bc ocular isolates were more frequently PCR-positive for capK, cytK, hblA, hblC, and plcR compared to the gastrointestinal isolates (p ≤ 0.0002). In the endophthalmitis model, mean A-wave retention did not differ significantly between eyes infected with ATCC14579 and eyes infected with the selected ocular or gastrointestinal isolates (p ≥ 0.3528). Similar results were observed for mean B-wave retention (p ≥ 0.0640). Only one diarrheal isolate showed significantly greater B-wave retention when compared to ATCC14579 (p = 0.0303). No significant differences in mean A-wave (p ≥ 0.1535) or B-wave (p ≥ 0.0727) retention between the selected ocular and gastrointestinal isolates were observed. Intraocular concentrations of ATCC14579 were significantly higher than the selected ocular isolate and 3 of the gastrointestinal isolates (p ≤ 0.0303). Intraocular concentrations of the selected ocular isolate were not significantly different from the gastrointestinal isolates (p ≥ 0.1923).

Conclusions

Among the subset of virulence-related genes assessed, 5 were significantly enriched among the ocular isolates compared to gastrointestinal isolates. While hemolytic activity was higher among gastrointestinal isolates, retinal function retention and intraocular growth was not significantly different between the selected ocular and gastrointestinal isolates. These results suggest that Bc strains causing gastrointestinal infections, while differing from ocular isolates in hemolytic activity and virulence-related gene profile, are similarly virulent in endophthalmitis.

Therapeutic potential of Bacillus phage lysin PlyB in ocular infections

Bacteriophage lytic enzymes (i.e., phage lysins) are a trending alternative for general antibiotics to combat growing antimicrobial resistance. Gram-positive Bacillus cereus causes one of the most severe forms of intraocular infection, often resulting in complete vision loss. It is an inherently β-lactamase-resistant organism that is highly inflammogenic in the eye, and antibiotics are not often beneficial as the sole therapeutic option for these blinding infections. The use of phage lysins as a treatment for B. cereus ocular infection has never been tested or reported. In this study, the phage lysin PlyB was tested in vitro, demonstrating rapid killing of vegetative B. cereus but not its spores. PlyB was also highly group specific and effectively killed the bacteria in various bacterial growth conditions, including ex vivo rabbit vitreous (Vit). Furthermore, PlyB demonstrated no cytotoxic or hemolytic activity toward human retinal cells or erythrocytes and did not trigger innate activation. In in vivo therapeutic experiments, PlyB was effective in killing B. cereus when administered intravitreally in an experimental endophthalmitis model and topically in an experimental keratitis model. In both models of ocular infection, the effective bactericidal property of PlyB prevented pathological damage to ocular tissues. Thus, PlyB was found to be safe and effective in killing B. cereus in the eye, greatly improving an otherwise devastating outcome. Overall, this study demonstrates that PlyB is a promising therapeutic option for B. cereus eye infections.IMPORTANCEEye infections from antibiotic-resistant Bacillus cereus are devastating and can result in blindness with few available treatment options. Bacteriophage lysins are an alternative to conventional antibiotics with the potential to control antibiotic-resistant bacteria. This study demonstrates that a lysin called PlyB can effectively kill B. cereus in two models of B. cereus eye infections, thus treating and preventing the blinding effects of these infections.

Roles of CCL2 and CCL3 in intraocular inflammation during Bacillus endophthalmitis

Bacillus cereus (B. cereus) endophthalmitis is a vision-threatening bacterial infection. Uncontrolled inflammatory responses are the hallmark of this disease which cause irreversible damage to the retina. We recently reported C-X-C chemokines as a vital modulators which impacted the pathogenesis of this disease. Here, we investigated the impact of two highly upregulated C-C chemokines, CCL2 and CCL3, on intraocular inflammation this disease. B. cereus was injected into the eyes of C57BL/6J (WT), CCL2-/-, and CCL3-/- mice to induce endophthalmitis. Infected eyes were examined for bacterial growth, retinal function, and inflammation. Bacterial growth in CCL2-/- and CCL3-/- mice were similar, but retained retinal function was greater in CCL2-/- and CCL3-/- eyes compared to that of C57BL/6J eyes. The retinal architecture of infected eyes of CCL2-/- mice were conserved for a longer period of time than in infected CCL3-/- eyes. Infected CCL2-/- and CCL3-/- eyes had less inflammation than did infected C57BL/6J eyes. Based on these results, we assessed the efficacies of intravitreal anti-CCL2 or anti-CCL3 with or without the antibiotic gatifloxacin. Compared to infected untreated eyes, there was significantly less inflammation and greater retention of retinal function in eyes treated with anti-CCL2 or anti-CCL3 with gatifloxacin. This study showed that B. cereus endophthalmitis in CCL2-/- mice had a better clinical outcome than in CCL3-/- mice. Intravitreal administration of anti-CCL2 and anti-CCL3 with gatifloxacin significantly reduced inflammation and provided protection of retinal function. These results suggest that CCL2 and CCL3 are prospective anti-inflammatory targets that should be tested along with other antibiotics for treating Bacillus and perhaps other forms of endophthalmitis.