Effect of methicillin-resistant Staphylococcus aureus (MRSA) infection in human microglial cells during endophthalmitis

Abstract

Background: Infection caused by methicillin-resistant Staphylococcus aureus, or MRSA, is a growing concern in medicine, and ophthalmology has not been spared. MRSA is the predominant cause of postoperative endophthalmitis which occurs as a sight-threatening complication of cataract surgery. Toll-like receptors (TLRs) and immune mediators are important segments of the host response to infection and are expressed and secreted by a variety of cells including human microglial cells. To understand how MRSA modulates the innate immune response in comparison to methicillin-susceptible Staphylococcus aureus (MSSA), causing a dysregulated host-induced pathophysiology in retinal microglial cells during endophthalmitis. Methods and materials: Clinical isolates of MSSA and MRSA strains from patients with endophthalmitis were used for infecting human microglial cells (CHME-3) at an MOI 10:1, following which the mRNA expression analysis for various TLRs, pro-inflammatory cytokines and anti-inflammatory cytokines, along with MMPs were assessed at regular time intervals until 24 hrs post-infection (p.i). The expression of IL-10, IL-6, IL-8, M-CSFs was further confirmed by ELISA and of TLR-4, IFN-γ, and IL-1β by Immunofluorescence assay. Results: MRSA had a higher growth rate compared to the susceptible strain from every time point p.i (p< 0.05). mRNA analysis showed that the MRSA strain had increased expression of IL-6, IL-8, IL-10, IL-1α, IL-1β, TNF-α, IFN-γ, MMP-2, MMP-9, TLR-1, TLR-2, TLR-6, TLR-9 in a time-dependent manner compared to susceptible strain. Increased protein level expression of IL-10, IL-6, IL-8, and GM-CSF were further confirmed in the culture supernatant, by ELISA. The expression of immune mediators (TLR-4, IL-1β, and IFN-γ) was assessed by immunostaining, which showed differences in immunofluorescence intensity in resistant stimulated versus susceptible stimulated human microglia cells. Conclusion: Our work demonstrates that microglial cells incite differential innate responses when infected with an MRSA strain, compared to the susceptible strain. It is also associated with the potential for substantial immunopathology, which underscores the need for tight control of inflammatory responses. These immune system interactions may result in enhanced bacterial proliferation but also provoke systemic cytokine responses associated with endophthalmitis.