Foundational concepts in the biology of bacterial keratitis

Abstract

Bacterial infections of the cornea, or bacterial keratitis (BK), are notorious for causing rapidly fulminant disease and permanent vision loss, even among treated patients. In the last sixty years, dramatic upward trajectories in the frequency of BK have been observed internationally, driven in large part by the commercialization of hydrogel contact lenses in the late 1960s. Despite this worsening burden of disease, current evidence-based therapies for BK - including broad-spectrum topical antibiotics and, if indicated, topical corticosteroids - fail to salvage vision in a substantial proportion of affected patients. Amid growing concerns of rapidly diminishing antibiotic utility, there has been renewed interest in urgently needed novel treatments that may improve clinical outcomes on an individual and public health level. Bridging the translational gap in the care of BK requires the identification of new therapeutic targets and rational treatment design, but neither of these aims can be achieved without understanding the complex biological processes that determine how bacterial corneal infections arise, progress, and resolve. In this chapter, we synthesize the current wealth of human and animal experimental data that now inform our understanding of basic BK pathophysiology, in context with modern concepts in ocular immunology and microbiology. By identifying the key molecular determinants of clinical disease, we explore how novel treatments can be developed and translated into routine patient care.

Keywords: Adaptive immunity; Bacterial keratitis; Corneal infections; Immunology; Innate immunity; Microbial keratitis; Pseudomonas aeruginosa; Staphylococcus aureus; Streptococcus pneumoniae.

FIGURE 1.. The ocular surface in health.

(A) Cross sectional view of the cornea and pre-corneal tear film. The trilaminar tear film consists of outermost lipid, middle aqueous, and innermost mucous layers that trap foreign particles and are highly concentrated with protective antimicrobial compounds. (B) Summary of antimicrobial tear compounds, divided into classical and nonclassical proteins. (C) Summary of major resistance mechanisms against infection in corneal cells. Note that Langerhans cell (LC) populations in the cornea are highly diverse; two major populations include mature LCs that express MHC Class II⁺ reside in the peripheral corneal epithelium, and another population of MHC Class II⁻ LCs that reside in the central cornea, and which require stimulation (e.g., with PAMPs) to mature. Key – IgA: immunoglobulin A; MHC: major histocompatibility complex; SLPI: secretory leucocyte protease inhibitor; sPLA₂: secretory phospholipase A; TLR: toll-like receptor. Created with Biorender.com under a standard academic license.

FIGURE 2:. The putative role of toll-like receptors (TLRs) in the cornea.

TLRs are evolutionarily conserved membrane-bound pattern recognition receptors (PRRs) that initiate host innate immune responses to pathogen-associated molecular patterns (PAMPs). TLRs are differentially localized within cells, either on the cell surface (TLR1, 2, 4, 5, 6, 10) or within intracellular endosomes (TLR 3, 7, and 9). TLR8 is not known to exist in the cornea. With the exception of TLR3, TLRs activate the myeloid differentiation primary response protein 88 (MyD88), a master adaptor protein that upregulates NF-κB and MAPK-dependent pathways, leading to the expression of pro-inflammatory cytokines, chemokines, adhesion molecules, and co-stimulatory molecules that are essential for the recruitment of innate effector cells. On the other hand, TLR3 (and also TLR4) stimulate alternative adaptor proteins, such as toll-interleukin receptor domain-containing adapter-inducing interferon-β TRIF), leadin to the upre ulation of interferon regulatory factor (IRF) pathways that lead to the production of type I interferons IFN-α and IFN-β While IFNs have been historically associated with viral infections, recent evidence suggests that IFNs are also potent inducers of innate immune effectors in the setting of bacterial infection (Monroe et al., 2010). For further details regarding PAMP recognition among TLRs, refer to Table 1. Created with Biorender.com under a standard academic license.

FIGURE 3.. Clinical slit-lamp photography of sight-threatening bacterial keratitis caused by common pathogens.

(A) A large, inferior P. aeruginosa corneal ulcer in a soft contact lens user, requiring therapeutic penetrating keratoplasty following perforation; (B) severe methicillin-sensitive S. aureus ulcer arising in the setting of previous herpetic eye disease; (C) large, diffuse, and necrotic S. pneumoniae ulcer arising in an old penetrating keratoplasty, crossing the graft-host junction; (D) M. catarrhalis ulcer causing graft failure in a penetrating keratoplasty; (E) S. marcescens ulcer in a patient with possible underlying herpetic eye disease; and (F) polymicrobial infection (caused by Streptococcus mitis and Enterococcus faecalis).

FIGURE 4:. Schematic overview of host innate responses to bacterial corneal pathogens, which involve the loss of corneal immune privilege.

Pattern recognition receptors (PRRs), predominantly toll-like receptors (TLRs), ligate with pathogen-associated molecular patterns (PAMPs) including bacterial lipoteichoic acid and lipopolysaccharides. PRR-binding leads to the upregulation of pro-inflammatory transcription factors such as NF-κB, with subsequent cellular release of pro-inflammatory cytokines (TNF-α, IL-1, IL-6, IL-8, IL12, IL-18, IFNs and MIF), chemokines (CXCL-1, CXCL-2, CXCL-5, CCL-4, CCL-5, IL-18, MCP-1), endothelial adhesion molecules (ICAM-1, MAC-1, PECAM-1, VCAM-1, E-selectin, and P-selectin). Several cell types (e.g., corneal epithelial cells) also inappropriately upregulate the production and release of digestive MMPs, including MMP-1, MMP-2, MMP-3, and MMP-9. This pro-inflammatory storm of cytokines induces massive neutrophil recruitment; migration of MHC Class II⁺ LCs from the corneal limbus, conjunctiva, and surrounding vascular beds into the stroma; maturation of resident stromal MHC Class II⁻ LCs; recruitment of macrophages and monocytes; activation of complement; and release of neuropeptides such as substance P. The activation of LCs serves as a bridge to the adaptive immune response, which is characterized by recruitment of Th1-predominant helper-T cells that preside over the persistence of PMNs, and are associated with severe disease phenotypes (e.g., corneal perforation).