Haptoglobin and CD163 in Aspects of Disease

Extracellular hemoglobin (Hb) is rapidly captured by haptoglobin (Hp) in the circulation to preventoxidative stress and tissue damage. This tight complex is recognized and endocytosed by themacrophage-specific scavenger receptor CD163 leading to lysosomal degradation of the Hp-Hbcomplex and reuse of the components maintaining iron homeostasis. The aim of this thesis is toexplore the proteins within this Hb scavenging system in relation to aspects of pathogenesis and theirutility in diagnosis of intravascular hemolysis and non-alcoholic fatty liver disease (NAFLD).

During excessive hemolysis, Hp is virtually undetectable in plasma because the Hp-Hb complexes are degraded and the Hb scavenging capacity is exhausted. Thereby low Hp levels are used in the clinic as a biomarker of hemolysis. The clinical analytical methods rely on epitoperecognition which is challenged by the presence of Hp-related protein (Hpr) with the same epitopes
and by complex structural differences between Hp phenotypes. Therefore, specific assays that differentiated between Hp and Hpr were generated, and phenotype-specific calibrators were used to
overcome the oligomerization challenge. This uncovered a significant bias using unmatched calibrators especially in patients with the Hp1-1 phenotype (Paper I), but also revealed that the
presence of Hpr has negligible influence on the measurements. Furthermore, the data showed that the concentration of Hpr correlated significantly with Hp phenotype and plasma concentrations. In fact, Hpr was also reduced during hemolysis concurrently with Hp (Paper II). Finally, the data provided evidence for a model explaining why Hp1-1 individuals generally have a lower Hp concentration compared to Hp2-1 and Hp2-2 individuals.

A soluble form of CD163 (sCD163) is released to the circulation during macrophage activation which has been associated with various inflammatory conditions such as non-alcoholic steatohepatitis (NASH), which is the progressive form of NAFLD. Increased plasma levels of sCD163 confirmed sCD163 as a potential biomarker of NASH in a unique cohort of obese patients
before bariatric surgery. Matched liver biopsies revealed a concurrent reduction in CD163 expression in more severe NAFLD (Paper III). This negative association was further investigated in western diet-fed mice. As steatosis intensified the expression of CD163 decreased in the liver and CD163 deficiency exacerbated hepatic steatosis in early stages of steatosis suggesting a protective role of CD163 in fat deposition in the liver. Overall, this indicates a pathological importance of macrophages in the development of NAFLD that potentially can be used in therapy (Paper IV).