Rapid reprogramming of haemoglobin structure-function exposes multiple dual-antimicrobial potencies

Abstract

The intrinsic cytotoxicity of cell-free haemoglobin (Hb) has hampered the development of reliable Hb-based blood substitutes for over seven decades. Notably, recent evidence shows that the Hb deploys this cytotoxic attack against invading microbes, albeit, through an unknown mechanism. Here, we unraveled a rapid molecular reprogramming of the Hb structure-function triggered by virulent haemolytic pathogens that feed on the haem-iron. On direct contact with the microbe, the Hb unveils its latent antimicrobial potency, where multiple antimicrobial fragments are released, each harbouring coordinated 'dual-action centres': microbe binding and pseudoperoxidase (POX) cycle activity. The activated Hb fragments anchor onto the microbe while the juxtaposed POX instantly unleashes a localized oxidative shock, killing the pathogen-in-proximity. This concurrent action conceivably restricts the diffusion of free radicals. Furthermore, the host astutely protects itself from self-cytotoxicity by simultaneously releasing endogenous antioxidants. We found that this decryption mechanism of antimicrobial potency is conserved in the ancient invertebrate respiratory protein, indicating its fundamental significance. Our definition of dual-antimicrobial centres in the Hb provides vital clues for designing a safer Hb-based oxygen carrier blood substitute.

Red alert: labile heme is an alarmin

“…The extracellular Hb generated through hemolysis exerts anti-microbial effects, via the peroxidase activity of its prosthetic heme b groups, which uses hydrogen peroxide (H 2 O 2 ) to oxidize molecules in microbes [38,39].…”

“…Moreover, binding of extracellular Hb to bacterial lipopolysaccharide (LPS) [40], alters the tertiary structure of Hb [39,41], enhancing its peroxidase and hence microbicidal activity [38,39]. Extracellular Hb enhances the pro-inflammatory activity of LPS [41] and when overtly oxidized can form large aggregates that act per se as a pro-inflammatory agonist in endothelial cells [42].…”

A Rabbit Vaginal Cell-Derived Antimicrobial Peptide, RVFHbαP, Blocks Lipopolysaccharide-Mediated Inflammation in Human Vaginal CellsIn Vitro

“…Levels of significance ( * , P Ͻ 0.05; ** , P Ͻ 0.001; compared with the LPSand LPS-nRVFHb␣P-treated groups) were calculated by an ANOVA test followed by a Bonferroni analysis. Also, there is a growing body of evidence which suggests that the vaginal epithelium is one of the critical interfaces with the external environment and VECs synthesize and release a number of Hb-derived peptides into the lumen which act against invading pathogens (6,19,24). Indeed, Hb-derived peptideregulated pathways have been identified in VECs (20,24), and cell survival (26) and inflammatory responses (6,16) have been shown to be modulated by Hb-derived peptides.…”

“…Also, there is a growing body of evidence which suggests that the vaginal epithelium is one of the critical interfaces with the external environment and VECs synthesize and release a number of Hb-derived peptides into the lumen which act against invading pathogens (6,19,24). Indeed, Hb-derived peptideregulated pathways have been identified in VECs (20,24), and cell survival (26) and inflammatory responses (6,16) have been shown to be modulated by Hb-derived peptides. These studies were further strengthened by recent studies, which showed that virulent bacterial proteins (e.g., hemolysin) lyse RBCs and release Hb, which then get oxidized into methemoglobin (metHb).…”

CD163 and IgG Codefend against Cytotoxic Hemoglobin via Autocrine and Paracrine Mechanisms

“…In an infection, microbial proteases specifically trigger the Hb POX activity, leading to a localized oxidative shock at the site of infection (5,6). The Hb-induced microbicidal reactive oxygen species (ROS) also damages the host itself when it is not rapidly detoxified and removed from circulation.…”