Toxin Listeriolysin O (LLO)

A potentially important role that γδ⁺ T cells may play in host defense is responding to bacterium-derived toxins. Since toxins are usually important virulence factors for many bacteria, early recognition by γδ⁺ T cells could represent an important component of host defense.

Introduction of Toxin listeriolysin O (LLO)

Listeriolysin O (LLO) is a member of the cholesterol-dependent cytolysin (CDC) family of pore-forming toxins. CDCs represent the largest family of pore-forming toxins (PFTs) and the subset of PFTs that form the largest pores. To date, more than 50 CDCs have been identified in Firmicutes, Actinobacteria, and most recently in Proteobacteria. Except those produced by Proteobacteria, CDCs are produced by primary and opportunistic Gram-positive pathogens, and many of which have important roles in pathogenesis. Among CDCs that have demonstrated contributions to pathogenesis are perfringolysin (PFO) of Clostridium perfringens, pneumolysin (PLY) of Streptococcus pneumoniae, streptolysin O (SLO) of Streptococcus pyogenes, anthrolysin (ALO) of Bacillus anthracis, and LLO of Listeria monocytogenes.

LLO is distinct in that it is the only CDC produced by an intracellular pathogen and has specialized features that make it suitable for its intracellular localization. Additionally, CDCs also have conserved lectin-binding properties. Using glycan array analysis, it was shown that PLY and SLO had affinities for different glycan structures and that binding these glycans altered the hemolytic activity of these toxins. Like cholesterol, the functional domain responsible for glycan-binding is domain 4. While glycan-binding has not yet been reported for LLO, many of the modeled carbohydrate-binding sites within domain 4 are conserved between LLO and CDCs from extracellular pathogens. Future experiments should investigate the roles of glycosylation for cellular tropism and pathogen lifestyle.

Fig.1 Schematic representation of the regulation and activities of LLO during infection. (Schnupf, 2007)

Listeriolysin O (LLO) and γδ T

• γδ T cell recognition of LLO

LLO-responsive T cells usually contained mixtures of TcRαβ⁺ and TcRγδ⁺ T cells. It has been reported that both TcRαβ⁺ and TcRγδ⁺ T cells can recognize LLO. LLO 470-508 was a commonly recognized peptide and was shown to be recognized by a CD41 TcRαβ⁺ T-cell line and by a TcRγδ⁺ T-cell line. This recognition appeared to be MHC restricted for the TcRαβ⁺ T-cell line. The restriction element necessary for the TcRγδ⁺ T-cell line was not identified. Research data showed that LLO is recognized by PBMC exposed to live listeria. PBMC challenged with fixed listeria gave significant proliferative responses to fixed listeria and was associated with TcRαβ⁺ T cells that did not recognize LLO. These results suggest that human anti-listerial immunity involves specificities to LLO and other non-LLO-derived determinants.

• Significance

LLO is a potentially important antigen in human T-cell recognition of L. monocytogenes. T cells bearing TcRαβ and TcRγδ can recognize this antigen, and T-cell epitopes can be identified by using LLO peptides. These results support the concept that TcRγδ⁺ T cells are important cells in host defense to bacterial infections.

Experimental infection of mice with L. monocytogenes has been widely used for the study of cell-mediated immunity. In mice, LLO has been shown to play a pivotal role in the development of immunity to L. monocytogenes. Moreover, TcRγδ⁺ T cells have also been shown to play an important role in host defense to primary infection with listeria and are significantly expanded in mice challenged with listeria as well as other gram-positive or gram-negative bacteria. Although much is now known about the mouse immune response to listeria and LLO, relatively little is known about human T-cell recognition of listeria or LLO. We need more exploration in this field.

Reference

1. Schnupf, P.; Portnoy, D. A. Listeriolysin O: A phagosome-specific lysin. Microbes Infect. 2007, 9(10): 1176-87.