The Antimicrobial Peptide, Bactenecin 5, Supports Cell-Mediated but Not Humoral Immunity in the Context of a Mycobacterial Antigen Vaccine Model.

Tulika Munshi,Brendan W Wren,Adam Sparrow,Samuel J Willcocks,Rajko Reljic

doi:10.3390/antibiotics9120926

Abstract

Bactenecin (Bac) 5 is a bovine antimicrobial peptide (AMP) capable of killing some species of bacteria through the inhibition of protein synthesis. Bac5 and other AMPs have also been shown to have chemotactic properties and can induce inflammatory cytokine expression by innate immune cells. Recently, AMPs have begun to be investigated for their potential use as novel vaccine adjuvants. In the current work, we characterise the functionality of Bac5 in vitro using murine macrophage-like cells, ex vivo using human tonsil tissue and in vivo using a murine model of vaccination. We report the effects of the peptide in isolation and in the context of co-presentation with mycobacterial antigen and whole, inert Bacillus subtilis spore antigens. We find that Bac5 can trigger the release of nitric oxide from murine macrophages and upregulate surface marker expression including CD86, MHC-I and MHC-II, in the absence of additional agonists. When coupled with mycobacterial Ag85 and B. subtilis spores, Bac5 also enhanced IFNγ secretion. We provide evidence that B. subtilis spores, but not the Bac5 peptide, act as strong adjuvants in promoting antigen-specific immunoglobulin production in Ag85B-vaccinated mice. Our findings suggest that Bac5 is an important regulator of the early cell-mediated host immune response.

Highlights

Antimicrobial peptides (AMPs) are conserved components of innate immunity that can be found across the taxonomic kingdoms [1]
We demonstrated that Bac5 is capable of activating murine macrophage-like cells in vitro, and had direct antimicrobial activity against some species of mycobacteria
We studied the effect of preincubation with the peptide on the ability of murine alveolar macrophages (MH-S cells) to control M. tuberculosis infection

Summary

Introduction

Antimicrobial peptides (AMPs) are conserved components of innate immunity that can be found across the taxonomic kingdoms [1]. They are comprised of varied sequences and secondary structures, but most are shorter than 100 amino-acids in length and carry a positive charge, facilitating electrostatic interaction with bacterial membranes. Bactenecin (Bac) family peptides are well conserved among ruminants [3], and are unusual in that they possess several repeating proline units in their sequence. Unlike many other AMPs that lyse bacteria through assembly in the inner-membrane [5], proline-rich AMPs can kill bacteria via inhibition of protein synthesis in the cytosol [6]. Transport appears to be mediated by a combination of endocytosis in eukaryotic cells [7], and inner-membrane transporter proteins in prokaryotes [6]

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Conclusion