Haptoglobin polymorphism: a key factor in the proatherogenic role of B cells?

B cells play a role in atherosclerosis. B lymphocytes may reduce the progression of vascular disease. Antibody production against modified auto-antigens is an element in the atheroprotective involvement of B lymphocytes. Paradoxical evidence is emerging from animal studies that suggest a proatherogenic B-cell behaviour independently of autoantibody production. One aspect that has received limited consideration is the role of genetic susceptibility modulated by extracellular matrix proteins. Haptoglobin is a polymorphic glycoprotein that binds to CD22 on B lymphocytes. Hp phenotypes show an important molecular heterogeneity. Hp 2-2 has been linked to an increased susceptibility for atherosclerosis. Haptoglobin and its polymorphism play a role in B-cell migration and function. Hp phenotypes may influence B-T cell dialogue and T cell activation. Haptoglobin is involved in the interplay of lymphocytes, neutrophils, and monocytes. Haptoglobin binds to the CD11b/CD18 receptor and to mast cells. HDL particles can become pro-inflammatory through interactions of Hp-Hb complexes with apolipoprotein A1. Haptoglobin is a chemoattractant to pre-B lymphocytes and monocytes. Beyond the conventional view of haptoglobin as a marker of hemolysis, several findings point towards an immunomodulatory effect of haptoglobin in B-cell mediated progression of atherosclerosis. The balance between proatherogenic and protective immunological properties of the different Hp phenotypes determines if lesions progress or regress. Clinical studies indicate a strong association between the Hp 2-2 phenotype and a more frequent onset of diabetic complications and cardiovascular disease. Findings in animal models (where no haptoglobin polymorphism is present) cannot always be extrapolated to humans.