p33 (gC1q Receptor) Prevents Cell Damage by Blocking the Cytolytic Activity of Antimicrobial Peptides

Abstract

The innate immune system is the first line of defense against invading microbes. Its specificity relies a great deal on host pattern recognition molecules that sense pathogen-associated molecular patterns of the invading pathogen. However, full protection is not always guaranteed, and some early defense mechanisms involved in bacterial killing, such as the complement system, can also exert cytolytic activity against host cells. Although these cascades are tightly regulated, the host has to take additional precautions to prevent its cell destruction. In this study, we describe that p33, a negatively charged surface protein found on endothelial cells also known as gC1q receptor, protects host cells from a cytolytic attack by antimicrobial peptides (AMPs), such as LL37 and β-defensin 3. To this end, we characterized the interaction of p33 with AMPs by biochemical and functional means. Our data show that p33 forms a doughnut-shaped trimer that can bind up to three AMPs, and we identified a segment in p33 forming a β-sheet that mediates the binding to all AMPs. Moreover, our results show that p33 abolishes the lytic activity of AMPs at an equimolar ratio, and it protects endothelial cells and erythrocytes from AMP-induced lysis. Taken together, our data suggest a novel protective mechanism of p33 in modulating innate immune response by neutralizing cytotoxic AMPs at the host cell surface.

LL-37-induced host cell cytotoxicity depends on cellular expression of the globular C1q receptor (p33)

“…Indeed, exogenously applied trimeric p33 binds LL-37 and other HDPs. HDPs show high affinity for the negatively charged beta-sheet (aa 115-135) of p33, thereby neutralizing their antimicrobial activity [27]. p33 has been proposed to prevent HDP-induced cytotoxicity in endothelial cells and erythrocytes through this scavenging mechanism [27].…”

“…HDPs show high affinity for the negatively charged beta-sheet (aa 115-135) of p33, thereby neutralizing their antimicrobial activity [27]. p33 has been proposed to prevent HDP-induced cytotoxicity in endothelial cells and erythrocytes through this scavenging mechanism [27]. Moreover, exogenous p33 (1-10 µM) antagonizes LL-37-induced apoptosis in osteoblasts in a concentration-dependent manner [22].…”

<b><i>Streptococcus pyogenes</i></b> Escapes Killing from Extracellular Histones through Plasminogen Binding and Activation by Streptokinase

“…Extracellular histones are cytotoxic against eukaryotic cells and exert hemolytic activity against human eythrocytes [27]. In the next experiment, we tested whether plasmin could prevent histone-induced hemolysis.…”

Treatment with p33 Curtails Morbidity and Mortality in a Histone-Induced Murine Shock Model

“…We also noted that a significant fraction of the histones was attached to the cell membrane. Recently, we reported that endothelial cells express a negatively charged protein, also known as p33 or globular C1q receptor, which binds to antimicrobial peptides (AMPs) and thereby protects cells from a lytic attack [16]. Like AMPs, histones are positively charged, so we decided to test whether the binding of histone H4 to the endothelial surface is also mediated by p33.…”

“…Recently, we reported that p33 exhibits a high affinity for another group of proteins/peptides, namely AMPs. Our data show that this interaction impairs the bactericidal and cytotoxic features of AMPs and prevents AMP-evoked cell necrosis [16]. It is noteworthy that extracellular histones and AMPs have features in common [32].…”