Haptoglobin phenotype correlates with development of cardiac transplant vasculopathy

Unlocking the link between haptoglobin polymorphism and noninfectious human diseases: insights and implications

Haptoglobin (Hp) is a polymorphic protein that was initially described as a hemoglobin (Hb)-binding protein. The major functions of Hp are to scavenge Hb, prevent iron loss, and prevent heme-based oxidation. Hp regulates angiogenesis, nitric oxide homeostasis, immune responses, and prostaglandin synthesis. Genetic polymorphisms in the Hp gene give rise to different phenotypes, including Hp 1-1, Hp 2-1, and Hp 2-2. Extensive research has been conducted to investigate the association between Hp polymorphisms and several medical conditions including cardiovascular disease, inflammatory bowel disease, cancer, transplantation, and hemoglobinopathies. Generally, the Hp 2-2 phenotype is associated with increased disease risk and poor outcomes. Over the years, the Hp 2 allele has spread under genetic pressures. Individuals with the Hp 2-2 phenotype generally exhibit lower levels of CD163 expression in macrophages. The decreased expression of CD163 may be associated with the poor antioxidant capacity in the serum of subjects carrying the Hp 2-2 phenotype. However, the Hp 1-1 phenotype may confer protection in some cases. The Hp1 allele has strong antioxidant, anti-inflammatory, and immunomodulatory properties. It is important to note that the benefits of the Hp1 allele may vary depending on genetic and environmental factors as well as the specific disease or condition under consideration. Therefore, the Hp1 allele may not necessarily confer advantages in all situations, and its effects may be context-dependent. This review highlights the current understanding of the role of Hp polymorphisms in cardiovascular disease, inflammatory bowel disease, cancer, transplantation, hemoglobinopathies, and polyuria.

Haptoglobin

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 Hp² 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.

Hp: an inflammatory indicator in cardiovascular disease

Over the past decade significant advancement has occurred in the biological and pathological role that Hp has in cardiovascular disease. Hp is an acute-phase protein with a role in the neutralization and clearance of free heme. Iron has tremendous potential for initiating vascular oxidation, inflammation and exacerbating coronary atherosclerosis. Hp genotype has been linked as a prognostic biomarker of acute myocardial infarction, heart failure, restenosis and cardiac transplant rejection. The increased understanding of Hp as a biomarker has provided new insights into the mechanisms of inflammation after cardiac injury and support the concept that Hp is not only an important antioxidant in vascular inflammation and atherosclerosis, but also an enhancer of inflammation in cardiac transplant.

Role of TLRs and DAMPs in allograft inflammation and transplant outcomes

Graft inflammation impairs the induction of solid organ transplant tolerance and enhances acute and chronic rejection. Elucidating the mechanisms by which inflammation is induced after organ transplantation could lead to novel therapeutics to improve transplant outcomes. In this Review we describe endogenous substances--damage-associated molecular patterns (DAMPs)--that are released after allograft reperfusion and induce inflammation. We also describe innate immune signalling pathways that are activated after solid organ transplantation, with a focus on Toll-like receptors (TLRs) and their signal adaptor, MYD88. Experimental and clinical studies have yielded a large body of evidence that TLRs and MYD88 are instrumental in initiating allograft inflammation and promoting the development of acute and chronic rejection. Ongoing clinical studies are testing TLR inhibition strategies in solid organ transplantation, although avoiding compromising host defence to pathogens is a key challenge. Further elucidation of the mechanisms by which sterile inflammation is induced, maintained and amplified within the allograft has the potential to lead to novel anti-inflammatory treatments that could improve outcomes for solid organ transplant recipients.

Inflammatory triggers of acute rejection of organ allografts

Solid organ transplantation is a vital therapy for end stage diseases. Decades of research have established that components of the adaptive immune system are critical for transplant rejection, but the role of the innate immune system in organ transplantation is just emerging. Accumulating evidence indicates that the innate immune system is activated at the time of organ implantation by the release of endogenous inflammatory triggers. This review discusses the nature of these triggers in organ transplantation and also potential mediators that may enhance inflammation resolution after organ implantation.

Haptoglobin activates innate immunity to enhance acute transplant rejection in mice

Immune tolerance to transplanted organs is impaired when the innate immune system is activated in response to the tissue necrosis that occurs during harvesting and implantation procedures. A key molecule in this immune pathway is the intracellular TLR signal adaptor known as myeloid differentiation primary response gene 88 (MyD88). After transplantation, MyD88 induces DC maturation as well as the production of inflammatory mediators, such as IL-6 and TNF-α. However, upstream activators of MyD88 function in response to transplantation have not been identified. Here, we show that haptoglobin, an acute phase protein, is an initiator of this MyD88-dependent inflammatory process in a mouse model of skin transplantation. Necrotic lysates from transplanted skin elicited higher inflammatory responses in DCs than did nontransplanted lysates, suggesting DC-mediated responses are triggered by factors released during transplantation. Analysis of transplanted lysates identified haptoglobin as one of the proteins upregulated during transplantation. Expression of donor haptoglobin enhanced the onset of acute skin transplant rejection, whereas haptoglobin-deficient skin grafts showed delayed acute rejection and antidonor T cell priming in a MyD88-dependent graft rejection model. Thus, our results show that haptoglobin release following skin necrosis contributes to accelerated transplant rejection, with potential implications for the development of localized immunosuppressive therapies.

Immunochemical property of human haptoglobin phenotypes: determination of plasma haptoglobin using type-matched standards

OBJECTIVES

Haptoglobin (Hp) phenotypes 1-1, 2-1, and 2-2 are associated with inflammatory diseases. Since their biochemical structures are rather heterogeneous, it is necessary to accurately determine the plasma Hp levels.

DESIGN AND METHODS

Immunodiffusion, immunoturbidimetric, and noncompetitive ELISA were conducted to determine the differences in immunoreactivity among Hp phenotypes and to verify that such difference may significantly affect the outcome of Hp determinations. A novel ELISA using phenotype-matched calibrators was performed to compared with a commercial GenWay ELISA kit using a single calibrator in normal healthy males.

RESULTS

In immunodiffusion and immunoturbidimetric assays, the immunoreactivity of Hp 1-1 was markedly higher than 2-1 and 2-2, while an opposite result was observed using an ELISA. The latter was primarily due to the repeated antigenic epitopes in polymeric 2-1 and 2-2. Thus, Hp levels could be significantly over- or underestimated depending on the method. An accurate ELISA could be achieved when using each type-specific Hp calibrator matched to each type subject. We show the mean levels of Hp 1-1 subjects (n=16; 184+/-42 mg/dL) to be significantly and differentially greater than 2-1 (n=28; 153+/-55 mg/dL) (p<0.05) and 2-2 (n=24; 93+/-54 mg/dL) (p<0.01) subjects.

CONCLUSIONS

Due to the diverse immunochemical structure among the Hp types, phenotyping should be performed in all the patients and a type-matched Hp calibrator should be used in clinical Hp determination.

Haptoglobin expression and activity during coronary collateralization

Coronary collateral development relies on the coordinated secretion of growth factors. However, alone they are insufficient for permanent collateral growth. We utilized proteomics to identify other important proteins in the extracellular environment that could facilitate collateralization. Chronically instrumented dogs developed coronary collaterals by the repetitive occlusion method. Subendocardial (0.19 ± 0.04, 0.27 ± 0.06, 0.48 ± 0.10, and 0.81 ± 0.11 ml·min−1·g−1 on days 1, 7, 14, and 21, respectively) and subepicardial (0.14 ± 0.01, 0.36 ± 0.06, 0.51 ± 0.07, and 0.71 ± 0.08 ml·min−1·g−1 on days 1, 7, 14, and 21, respectively) blood flow increased in animals subjected to repetitive occlusion. Sham animals exhibited no changes in blood flow. Myocardial interstitial fluid (MIF) from both groups was analyzed by two-dimensional electrophoresis with matrix-assisted laser desorption/ionization time-of-flight identification. The acute-phase protein haptoglobin was identified in the group subjected to repetitive occlusion. ELISA of MIF showed haptoglobin to be elevated at all time points of collateral development compared with sham, with maximal production on day 7. Purified haptoglobin dose dependently stimulated endothelial cells to form tubes and vascular smooth muscle cells to migrate. Purified haptoglobin did not stimulate proliferation of either cell type. The relative contribution of haptoglobin to the chemotactic properties of MIF was tested using a neutralizing antibody. Neutralized MIF could not stimulate smooth muscle cells to migrate at any time during collateral development. Endothelial cell tube formation was inhibited after the midpoint of collateralization. Therefore, the acute-phase protein haptoglobin plays a critical role during coronary collateralization.