Histidine-rich glycoprotein promotes bacterial entrapment in clots and decreases mortality in a mouse model of sepsis

Abstract

Streptococcus pyogenes is a significant bacterial pathogen in humans. In this study, histidine-rich glycoprotein (HRG), an abundant plasma protein, was found to kill S pyogenes. Furthermore, S pyogenes grew more efficiently in HRG-deficient plasma, and clots formed in this plasma were significantly less effective at bacterial entrapment and killing. HRG-deficient mice were strikingly more susceptible to S pyogenes infection. These animals failed to control the infection at the local subcutaneous site, and abscess formation and inflammation were diminished compared with control animals. As a result, bacterial dissemination occurred more rapidly in HRG-deficient mice, and they died earlier and with a significantly higher mortality rate than control animals.HRG-deficient mice supplemented with purified HRG gave the same phenotype as control animals, demonstrating that the lack of HRG was responsible for the increased susceptibility. The results demonstrate a previously unappreciated role for HRG as a regulator of inflammation and in the defense at the local site of bacterial infection. (Blood. 2010;116(13):2365-2372) IntroductionHistidine-rich glycoprotein (HRG) is an abundant plasma protein synthesized in the liver and also stored in the ␣-granules of platelets, from which it is released on activation. 1 The protein has a diverse array of ligands, including heparin, plasminogen, immunoglobulin, thrombospondin, and fibrinogen. 2 The diversity of ligands probably contributes to the fact that new activities are regularly attributed to HRG in vitro, whereas a definitive biologic function in vivo has been difficult to identify. Based on in vitro studies, HRG has been reported to modulate angiogenesis, 3 phagocytosis, 4 and complement function 5 and to be antibacterial. 6 HRG is negatively charged at physiologic pH but becomes positively charged at low pH or in the presence of zinc. Several the reported biologic effects have also been shown to be dependent on pH or zinc, suggesting that HRG may be a sensitive adaptor molecule to changing environments. 7 The generation of HRG-knockout mice has facilitated the search for the biologic function of HRG. These animals were viable with no apparent abnormalities, but ex vivo studies demonstrated a mild dysregulation of hemostasis with effects on plasma-clotting factors and platelets, and in vivo HRG seemed to have both anticoagulant and antifibrinolytic properties. 8 Recently, it has been shown that HRG-deficient animals have diminished antifungal activity in vivo. 9 In the present study, we have assessed the role of HRG in response to a pathologic situation (ie, bacterial infection). Streptococcus pyogenes is a significant human bacterial pathogen responsible for both superficial and invasive infection. The pathogenesis of S pyogenes infection is a complex multifactorial process, and diverse host systems are involved. [10][11][12] For instance, S pyogenes can counteract phagocytosis, complement activity, antibody recognition, and antimicrobial peptides. S pyogenes infection al...

Histidine-rich Glycoprotein Binds Fibrin(ogen) with High Affinity and Competes with Thrombin for Binding to the γ′-Chain

“…The importance of HRG in modulating the inflammatory response has been confirmed in mouse models. Thus, compared with wild-type mice, HRG-deficient mice given subcutaneous injections of S. pyogenes exhibit attenuated abscess formation and reduced recruitment of neutrophils and macrophages to the site of infection (21), suggesting that HRG modulates the inflammatory response. In support of this concept, a synthetic peptide analog of the HRR of HRG attenuated the secretion of interleukin-8 from lipopolysaccharide-stimulated, CD14-transfected monocytes (53).…”

“…Furthermore, bacteria trapped within fibrin clots are protected from host defenses and the action of antibiotics (51). However, because HRG has antimicrobial properties, the HRG-fibrin interaction promotes bacterial entrapment and killing (21). The capacity of HRG to enhance bacterial killing has been localized to its NH 2 -terminal and HRR domains.…”

“…The antimicrobial activity of HRG has also been demonstrated in vivo and appears to be fibrin-dependent. Thus, compared with wild-type mice, HRG-deficient mice are more susceptible to the lethal effect of Streptococcus pyogenes infection and are rescued with HRG supplementation (21). This phenomenon is fibrindependent because fibrin is essential for HRG-mediated bacterial entrapment and killing, processes that prevent bacterial dissemination.…”

“…In addition, the HRG-fibrin interaction modulates inflammation because HRG-deficient mice exhibit attenuated abscess formation in response to subcutaneous injection of bacteria. Based on these findings, it has been postulated that HRG plays a fibrin-dependent role in both inflammation and innate immunity (21).…”

Genetic Deficiency in Plasma Protein HRG Enhances Tumor Growth and Metastasis by Exacerbating Immune Escape and Vessel Abnormalization

“…HRG moreover enhances the ability of immune complexes to activate complement, allowing faster clearance of necrotic cells (18). HRG has also been implicated in the defense at the local site of bacterial infection (25).…”