Listeriolysin O as a strong immunogenic molecule for the development of new anti-tumor vaccines

Abstract

The pore-forming toxin listeriolysin O (LLO), which is produced by Listeria monocytogenes, mediates bacterial phagosomal escape and facilitates bacterial multiplication during infection. This toxin has recently gained attention because of its confirmed role in the controlled and specific modulation of the immune response. Currently, cancer immunotherapies are focused on conquering the immune tolerance induced by poorly immunogenic tumor antigens and eliciting strong, lasting immunological memory. An effective way to achieve these goals is the co-administration of potent immunomodulatory adjuvant components with vaccine vectors. LLO, a toxin that belongs to the family of cholesterol-dependent cytolysins (CDCs), exhibits potent cell type-non-specific toxicity and is a source of dominant CD4(+) and CD8(+) T cell epitopes. According to recent research, in addition to its effective cytotoxicity as a cancer immunotherapeutic drug, the non-specific adjuvant property of LLO makes it promising for the development of efficacious anti-tumor vaccines.

Keywords: LLO; Listeria; anti-tumor vaccine; cancer immunotherapy; cytotoxicity; immunogenicity; pore-forming toxin.

Figure 1. Structural information of the cholesterol-dependent pore-forming cytolysin listeriolysin O (LLO). (A) Putative three-dimensional model of LLO monomer based on suilysin crystal structure generated by SWISS-MODEL. Suilysin shares a sequence similarity of 44% to LLO in PDB database. The monomer of LLO contains four domains (D1–4), and the conserved undecapeptide (Undeca) and three short loops are located on the tip of Domain 4. Two transmembrane helices of TMH1,2 are made up of the two sets of α-helices in Domain 3. (B) The analyzed primary structure of LLO. The gray number above the amino acid sequence roughly represents the position of a single amino acid. SS, the signal peptide sequence of LLO showed in a red straight line and the cleavage site (residues 24–25) indicated with a red arrow indicates. PEST, a putative PEST-like motif identified in LLO showed by a dark green box. CTL(91–99), an immunodominant CTL epitope consisting of amino acids from number 91 to number 99 indicated in a black box. +, the two clusters of positively charged residues flanking the CTL epitope indicated in blue. CD4⁺(189–201), a characteristic immunodominant CD4⁺ T cell epitope consisting of amino acids from number 189 to number 201 indicated in a black box. CD4⁺(215–226), an immunodominant CD4⁺ T cell epitope contained in TMH1 region indicated with red box, consisting of amino acids from number 215 to number 226. TMH1,2, two sets of transmembrane α-helices showed in two pink boxes. Undeca, the conserved region belonging to a cytolysin family consisting of 11 amino acids.