Haptoglobin (Hp) is an abundant human plasma protein that tightly captures hemoglobin (Hb) during hemolysis. The Hb-Hp complex formation reduces the oxidative properties of heme/Hb and promotes recognition by the macrophage scavenger receptor CD163. This leads to Hb-Hp breakdown and heme catabolism by heme oxygenase and biliverdin reductase. Gene duplications of a part of or the entire Hp gene in the primate evolution have led to variant Hp gene products that collectively may be designated "the haptoglobins (Hps)" as they all bind Hb. These variant products include the human-specific multimeric Hp phenotypes in individuals, which are hetero- or homozygous for an Hp gene allele. The Hp-related protein (Hpr) is another Hp duplication product in humans and other primates. Alternative functions of the variant Hps are indicated by numerous reports on association between Hp phenotypes and disease as well as the elucidation of a specific role of Hpr in the innate immune defense. Recent Advances: Recent functional and structural information on Hp and receptor systems for Hb removal now provides insight on how Hp carries out essential functions such as the Hb detoxification/removal, and how Hpr, by acting as an Hp-lookalike, can sneak a lethal toxin into trypanosome parasites that cause mammalian sleeping sickness. Critical Issues and Future Directions: The new structural insight may facilitate ongoing attempts of developing Hp derivatives for prevention of Hb toxicity in hemolytic diseases such as sickle cell disease and other hemoglobinopathies. Furthermore, the new structural knowledge may help identifying yet unknown functions based on other disease-relevant biological interactions involving Hps. Antioxid. Redox Signal. 26, 814-831.

Edited by F. Peter GuengerichHemolysis is a complication in septic infections with Staphylococcus aureus, which utilizes the released Hb as an iron source. S. aureus can acquire heme in vitro from hemoglobin (Hb) by a heme-sequestering mechanism that involves proteins from the S. aureus iron-regulated surface determinant (Isd) system. However, the host has its own mechanism to recapture the free Hb via haptoglobin (Hp) binding and uptake of Hb-Hp by the CD163 receptor in macrophages. It has so far remained unclear how the Isd system competes with this host iron recycling system in situ to obtain the important nutrient.

Staphylococcus aureus is able to disseminate from vascular device biofilms to the blood and organs, resulting in life-threatening infections such as endocarditis. The mechanisms behind spreading are largely unknown, especially how the bacterium escapes immune effectors and antibiotics in the process. Using an in vitro catheter infection model, we studied S. aureus biofilm growth, late-stage dispersal, and reattachment to downstream endothelial cell layers. The ability of the released biofilm material to resist host response and disseminate in vivo was furthermore studied in whole blood and phagocyte survival assays and in a short-term murine infection model. We found that S. aureus biofilms formed in flow of human plasma release biofilm thromboemboli with embedded bacteria and bacteria-secreted polysaccharides. The emboli disseminate as antibiotic and immune resistant vehicles that hold the ability to adhere to and initiate colonisation of endothelial cell layers under flow. In vivo experiments showed that the released biofilm material reached the heart similarly as ordinary broth-grown bacteria but also that clumps to some extend were trapped in the lungs. The clumping dispersal of S. aureus from in vivo-like vascular biofilms and their specific properties demonstrated here help explain the pathophysiology associated with S. aureus bloodstream infections.

Background Intravascular hemolysis and in vitro hemolysis are prevalent contributors to failed blood sample analysis in the routine hospital laboratory. Interferences by hemoglobin in spectrophotometric and certain enzyme activity assays is the major causative factor. Methods By exploiting the hemoglobin-binding properties of the iron-regulated surface determinant H (IsdH) protein from Staphylococcus aureus we have developed a new method to instantly remove hemoglobin and hemoglobin-haptoglobin complexes from plasma in vitro thereby enabling the measurement of hemoglobin-sensitive analytes in hemolyzed plasma. In the present study we used an engineered IsdH mutant form conjugated to Sepharose for the efficient removal of plasma hemoglobin in concentrations up to 15 mg/mL. The high abundance of haptoglobin, which forms a tight complex with hemoglobin in plasma, did not affect the hemoglobin removal by IsdH Sepharose. Results Applying the method on plasma samples that beforehand were spiked with blood hemolysate re-enabled measurement of the hemolysis sensitive parameters: alkaline phosphatase, conjugated bilirubin, iron, ferritin, γ-glutamyltransferase, total thyroxine and troponin T. IsdH Sepharose-mediated hemoglobin removal also enabled measurement of hemolysis sensitive parameters in hemolyzed samples from anonymized patients. Conclusions In conclusion, IsdH Sepharose is a simple cost-effective pretreatment of hemolyzed samples correcting and enabling the measurement of several important hemoglobin-sensitive parameters in a way compatible with standard procedures in routine laboratories.