Leukocidins: staphylococcal bi-component pore-forming toxins find their receptors

Abstract

Staphylococcus aureus is a major bacterial pathogen that causes disease worldwide. The emergence of strains that are resistant to commonly used antibiotics and the failure of vaccine development have resulted in a renewed interest in the pathophysiology of this bacterium. Staphylococcal leukocidins are a family of bi-component pore-forming toxins that are important virulence factors. During the past five years, cellular receptors have been identified for all of the bi-component leukocidins. The identification of the leukocidin receptors explains the cellular tropism and species specificity that is exhibited by these toxins, which has important biological consequences. In this Review, we summarize the recent discoveries that have reignited interest in these toxins and provide an outlook for future research.

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Staphylococcal leukocidins and receptors.

Figure 1. Pore formation by staphylococcal leukocidins

a| Individual crystal structures of single leukocidin protein components and multimer beta-barrel leukocidin pores show high structural similarity. In soluble form, hydrophobic residues in the beta-barrel stem of both S- and F-component are covered by the cap. The rim domain of the S-component, responsible for initial binding to the host target cell, is involved in receptor recognition. Hetero-oligo-merization of the S- with the F-components induces a conformational change resulting in insertion of the hydrophobic stem into the membrane of the target cell. The resulting octameric beta-barrel pore consists of alternating four S- and four F-components. Red: HlgA; Blue: HlgB. Structural information was acquired from the Protein Data Bank, with accession numbers 2QK7 (unbound HlgA), 1LKF (unbound HlgB), and 3B07 (single HlgA and HlgB from HlgAB octamer). The major structural domains were colored using PyMOL software. Courtesy of Dr. B.W. Bardoel, University Medical Center Utrecht, The Netherlands. b| Sequences of binding and pore formation of different leukocidins to their respective receptor targets. Differences in the sequences between leukocidins targeting chemokine receptors (PVL, LukED, HlgAB, HlgCB, on the left) versus the leukocidin targeting CD11b (LukAB, on the right) are highlighted. For PVL, LukED, HlgAB, and HlgCB the initial binding of the respective S-component to its specific receptor allows secondary binding of the polymerizing F-component, hetero-oligomerization, and pore formation. In the rim domain of the S-component (labeled green), the divergent region (DR) 1 of LukE determines receptor recoginition of CCR5, while DR4 of LukE determines recognition of CXCR1 and CXCR2. The bottom loops in the rim domain of LukS-PV are essential for targeting C5aR1. The interaction of C5aR1 and C5aR2 with LukS-PV and HlgC is multi-factorial and involves the N-termini and extracellular loops of the receptors. Sulfated tyrosines in the N-termini of the receptors C5aR1 and DARC (labeled with a yellow star) are essential for interaction of the receptor with PVL and HlgAB and LukED, respectively. Uniquely, LukAB is secreted as a pre-assembled dimer. Dimerization results in high affinity for the I-domain of its receptor CD11b. Receptor recognition of LukAB is mediated by a divergent C-terminal extension of LukA (labeled with a black spike). The actual number of receptors per pore is unkown.

Figure 2. Leukocidins in pathophysiology

a| Cell death by leukocidins. Disturbance of cell homeostasis by the leakage of cations through pores and by the mobilizaiton of ions through ion channels results in osmotic disbalance and inflammatory cell death, as NFkB stimulation and inflammasome activation lead to the release of pro-inflammatory cytokines and the assembly of endogenous N-terminal Gasdermin-D pores. Cas1: Caspase-1; IL1b: Interleukin-1b; GasD: Gasdermin D. b| Host cell signaling modulation by leukocidins. Depending on the receptor targeted, single leukocidin S-components in the absence of an F-component can functionally antagonize G-protein coupled receptors by preventing singnaling induced by the endogenous receptor ligand. The significance of functional antagonism by single S-components during infection is unkown. At sublytic concentrations, leukocidins prime neutrophils in a receptor-specific manner resulting in increased production of reactive oxygen species, enhanced degranulation and phagocytosis, and enhanced bactericidal activity of phagocytes. c| Leukocidins promote S. aureus growth. Hemoglobin is the most abundant source of iron in mammals, and heme iron is the preferred source of iron for S. aureus to grow. DARC is the erythroid receptor for HlgAB and LukED. Targeting of DARC by HlgAB and LukED results in hemolysis, which promotes S. aureus growth in a hemoglobin acquisition-dependent manner. d| Antagonism of leukocidins by the formation of inactive hybrid complexes as a result of sequestration of the S-component from its cognate F-component. Antagonism of cytotoxicity by non-cognate paring has been described for LukED and PVL.

Figure 3. Strategies for the employment of leukocidins in anti-staphylococcal therapies and vaccines

Antagonists of the receptors employed by the leukocidins can protect target cells against cytotoxicity. Bi-specific mAbs can neutralize both S- and F-components. Broadly neutralizing mAbs target multiple PFTs. A LukAB neutralizing mAb targets the LukAB dimer in solution. Fusion mAbs combine opsonophagocytic activity with neutralization of leukocidins, and potentially act intracellularly. Toxoids derived from mutant proteins induce a broadly neutralizing antibody response.