Analysis of the Specificity of Panton-Valentine Leucocidin and Gamma-Hemolysin F Component Binding

Abstract

In this study, the binding of F components of the staphylococcal bicomponent leukotoxins Panton-Valentine leucocidin (LukF-PV) and gamma-hemolysin (HlgB) on polymorphonuclear neutrophils (PMNs), monocytes, and lymphocytes was determined using labeled mutants and flow cytometry. Leukotoxin activity was evaluated by measuring Ca 2؉ entry or pore formation using spectrofluorometry or flow cytometry. Although HlgB had no affinity for cells in the absence of an S component, LukF-PV had high affinity for PMNs (dissociation constant Staphylococcus aureus secretes water-soluble protein monomers which assemble to form well-characterized two-component pore-forming leukotoxins that define a toxin subfamily (17). Each leukotoxin is formed by a class S protein (LukS-PV, HlgA, HlgC, or LukE) that is associated with a class F protein (LukF-PV, HlgB, or LukD). The Panton-Valentine leucocidin (PVL) (16), composed of LukS-PV and LukF-PV, is secreted by strains isolated from humans suffering from abscesses, furuncles (6, 8), and necrotizing pneumonia (11,12). PVL has been shown to contribute to the severity of acute hematogenous osteomyelitis caused by community-acquired methicillinresistant S. aureus in children (2). Moreover, reports of hospital-acquired and community-acquired methicillin-resistant S. aureus possessing PVL genes are increasing (3). The LukELukD-secreting S. aureus strains have been shown to be associated with postantibiotic diarrhea (10). However, gamma-hemolysin (HlgA-HlgB and HlgC-HlgB) is produced by all human clinical S. aureus strains.[The main target cells of staphylococcal leukotoxins are human polymorphonuclear cells (PMNs), monocytes, and lymphocytes (13). However, PVL is not toxic for lymphocytes, and gamma-hemolysin is hemolytic. After binding to the membrane, leukotoxins assemble as a ring-shaped prepore (15) consisting of heterologous octamers with a molar ratio of 1:1 (14). They induce an increase in the intracellular Ca 2ϩ level by opening Ca 2ϩ channels (19, 1), and the deployment of stems forms beta-barrels that create transmembrane pores in target cells (7, 1). Previous perfusion studies of PMNs showed that the initial binding of S components was a prerequisite for the binding of F components to obtain toxic activity on PMNs (4). Conversely, Yokota and Kamio (22) showed that the binding of the F component LukF (HlgB) on human erythrocytes was a prerequisite for the subsequent binding of Hlg2 (HlgA). Later, Gauduchon et al. (9) determined a dissociation constant (K d ) of 0.07 nM for LukS-PV binding on PMNs and showed that HlgC competed with LukS-PV for binding but HlgA and LukE did not compete. However, no data are available yet for the binding of F components.The present study was performed to analyze the binding of the F components of leukotoxins, particularly the binding of LukF-PV compared with that of HlgB, two F components which exhibit 71% identity. LukF-PV* and HlgB* leukotoxins with cysteine mutations were labeled with fluorescein to follow their binding to human leukocytes by flow cy...

The Bicomponent Pore-Forming Leucocidins of Staphylococcus aureus

“…Thus, it has been proposed that, rather than lipids, the leucocidins recognize distinct proteinaceous receptors on the cell surface to target and kill immune cells. Evidence for leucocidin cellular receptors other than membrane lipids is supported by a number of studies which demonstrate specific and saturable binding of the toxins on target cells (173,174,244). In line with this evidence, LukED was recently found to target the chemokine receptor and HIV coreceptor CCR5 to kill inflammatory macrophages, T cells, and dendritic cells (227).…”

“…In contrast, Morinaga et al demonstrated that their identified "variant" of LukED (later determined to be highly conserved in nearly all sequenced strains of S. aureus) could form active toxins through nonconventional subunit pairings of both PVL and gamma-hemolysin subunits, indicating that the overall diversity of active toxins can become quite large (94). It was further demonstrated by Meyer et al that S subunits efficiently recruit nonconventional F subunits to the surface of host cells (244). For example, HlgB and LukD can be recruited to the surface of cells that have been pretreated with LukS-PV, while LukS-PV or HlgC can recruit LukF-PV to the cell surface (244).…”

“…It was further demonstrated by Meyer et al that S subunits efficiently recruit nonconventional F subunits to the surface of host cells (244). For example, HlgB and LukD can be recruited to the surface of cells that have been pretreated with LukS-PV, while LukS-PV or HlgC can recruit LukF-PV to the cell surface (244). This study did not measure whether these mixed pairings form functional toxins; however, it provides further evidence that within mixed populations, leucocidin subunits are capable of assembling the constituents of active toxins albeit comprised of nonconventional subunit pairs.…”

Heavy chain-only antibodies and tetravalent bispecific antibody neutralizing Staphylococcus aureus leukotoxins

“…Leukotoxins exert their action by (i) binding of S and F proteins to target cell membranes (21,30); (ii) oligomerization and rapidly increasing intracellular calcium concentrations independently of pore formation and cell activation (12,31); and (iii) reconfiguration of S and F proteins to form functional pores permeable to monovalent cations (32). The capacity of antibodies to inhibit these three steps was tested.…”

“…A class S and class F component interact sequentially and synergistically, inducing the activation and permeabilization of target cells and leading to their lysis. LukS-PV and LukF-PV bind to the membrane of human polymorphonuclear cells (PMNs), macrophages, and monocytes to constitute PVL (21).…”