Toll‐like receptor 2 (TLR2) engages endoplasmic reticulum stress sensor IRE1α to regulate retinal innate responses in Staphylococcus aureus endophthalmitis

Abstract

Endoplasmic reticulum (ER) stress response has been implicated in a variety of pathophysiological conditions, including infectious and inflammatory diseases. However, its contribution in ocular bacterial infections, such as endophthalmitis, which often cause blindness is not known. Here, using a mouse model of Staphylococcus (S.) aureus endophthalmitis, our study demonstrates the induction of inositol‐requiring enzyme 1α (IRE1α) and splicing of X‐box binding protein‐1 (Xbp1) branch of the ER‐stress pathway, but not the other classical ER stress sensors. Interestingly, S aureus‐induced ER stress response was found to be dependent on Toll‐like receptor 2 (TLR2), as evident by reduced expression of IRE1α and Xbp1 mRNA splicing in TLR2 knockout mouse retina. Pharmacological inhibition of IRE1α using 4µ8C or experiments utilizing IRE1α−/− macrophages revealed that IRE1α positively regulates S aureus‐induced inflammatory responses. Moreover, IRE1α inhibition attenuated S aureus‐triggered NF‐κB, p38, and ERK pathways activation and cells treated with these pathway‐specific inhibitors reduced Xbp1 splicing, suggesting a positive feedback inhibition. In vivo, inhibition of IRE1α diminished the intraocular inflammation and reduced PMN infiltration in mouse eyes, but, increased the bacterial burden and caused more retinal tissue damage. These results revealed a critical role of the IRE1α/XBP1 pathway as a regulator of TLR2‐mediated protective innate immune responses in S aureus‐induced endophthalmitis.

Integrative metabolomics and transcriptomics identifies itaconate as an adjunct therapy to treat ocular bacterial infection

“…However, in Nrf2 −/− BMDMs, although, reduced by OI or DMI treatments, NLRP3/Casp1 levels did not change by S. aureus alone, indicating Nrf2- independent regulation of NLRP3 inflammasome. Some potential mechanisms by which S. aureus could activate NLRP3/Casp-1 in an Nrf2 -independent manner include (1) induction of oxidative (i.e., ROS generation; Figure S3 A) and endoplasmic reticulum (ER) stress, 61 which can directly activate NLRP3/Casp1/IL-1β cascade; 62 , 63 and (2) the production of saturated fatty acids, such as palmitate, under inflammatory conditions in the retina, 64 , 65 which could also trigger NLRP3 activation, leading to IL-1β and IL-18 production. 66 We acknowledge that further studies using Irg1 and Nrf2; Nlrp3 and Nrf2 double knockout mice and cells are needed to determine the role of endogenous itaconate and to dissect Irg1-Nrf2-Nlrp3 crosstalk in ocular infection.…”

Bacterial Burden Declines But Neutrophil Infiltration and Ocular Tissue Damage Persist in Experimental Staphylococcus epidermidis Endophthalmitis

“…Because different pathogens cause experimental endophthalmitis at certain infectious doses ( Gupta et al., 2019 ; Francis et al., 2020 ; Kumar et al., 2020 ; Livingston et al., 2021 ), we performed a dose-response study beginning with an intravitreal injection of 5,000 cfu of S. epidermidis , a dose required for S. aureus endophthalmitis ( Singh et al., 2021 ). However, this inoculum was rapidly cleared from the mouse eyes resulting in transient inflammation (data not shown).…”

“…One of the hallmarks of bacterial infection is the induction of inflammatory response ( Kumar et al., 2016 ; Kumar et al., 2020 ; Francis et al., 2020 ). To determine innate immune response during S. epidermidis endophthalmitis, we assessed the expression of key inflammatory mediators in infected eyes.…”

“…Neutrophils are the primary infiltrating cells of the vitreous during endophthalmitis ( Miller et al., 2019 ; Kumar et al., 2020 ; Singh et al., 2021 ). Although neutrophil deficiency reduces ocular inflammation, it comes with the price of increased bacterial proliferation in the eye ( Talreja et al., 2014 ; Talreja et al., 2015 ).…”

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“…Because different pathogens cause experimental endophthalmitis at certain infectious doses (Gupta et al, 2019;Francis et al, 2020;Kumar et al, 2020;Livingston et al, 2021), we performed a doseresponse study beginning with an intravitreal injection of 5,000 cfu of S. epidermidis, a dose required for S. aureus endophthalmitis (Singh et al, 2021). However, this inoculum was rapidly cleared from the mouse eyes resulting in transient inflammation (data not shown).…”