The Gram-negative bacterium Aggregatibacter actinomycetemcomitans, commonly associated with localized aggressive periodontitis (LAP), secretes an RTX (repeats-in-toxin) protein leukotoxin (LtxA) that targets human white blood cells, an interaction that is driven by its recognition of the lymphocyte function-associated antigen-1 (LFA-1) integrin. In this study, we report on the inhibition of LtxA-LFA-1 binding as an antivirulence strategy to inhibit LtxA-mediated cytotoxicity. Specifically, we designed and synthesized peptides corresponding to the reported LtxA binding domain on LFA-1 and characterized their capability to inhibit LtxA binding to LFA-1 and subsequent cytotoxic activity in human immune cells. We found that several of these peptides, corresponding to sequential β-strands in the LtxA-binding domain of LFA-1, inhibit LtxA activity, demonstrating the effectiveness of this approach. Further investigations into the mechanism by which these peptides inhibit LtxA binding to LFA-1 reveal a correlation between toxin-peptide affinity and LtxA-mediated cytotoxicity, leading to a diminished association between LtxA and LFA-1 on the cell membrane. Our results demonstrate the possibility of using target-based peptides to inhibit LtxA activity, and we expect that a similar approach could be used to hinder the activity of other RTX toxins.

For decades, researchers have been trying to elucidate critical determinants of the pathogenicity of human fungi. Only recently, has an international consortium of scientists succeeded in identifying such a factor-a peptide toxin secreted by the clinically important fungus Candida albicans. This peptide Candidalysin plays a crucial role during fungal infections of human mucosae. The peptide's virulence manifests in the direct damage of epithelial membranes, in the stimulation of a danger response signaling pathway and in the activation of epithelial immunity. We provided the first insights into the direct interaction between Candidalysin and lipid membranes [1]. The peptide's amphiphilic alpha-helical structure is described as a prerequisite for its binding to lipid membranes. In consequence of the initialized binding, the peptide inserts between the lipid head groups and aligns parallel to the bilayer surface. Upon increasing surface accumulation, however, the helix starts to penetrate the bilayer with oblique inclination. As a result, the bilayer is destabilized and transient local collapses occur. This carpet-like disintegration eventually leads to the disruption of the entire membrane. A comparison of host, bacterial and fungal model membranes will be used as an approach for understanding the selectivity of Candidalysin for non-self membranes. The influence of peptide aggregation and of C-terminal truncation on the mode of action of Candidalysin will be discussed.

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