The Role of Haptoglobin in the Immune Response to Gram-Positive Bacteria

Haptoglobin (HP) is a positive acute-phase serum protein. It is upregulated during infection and is a valuable marker for many inflammatory-related diseases. In serum it is present as a disulphide-linked homodimer, with each subunit being composed of two domains, a complement control protein domain (CCP-domain) and a serine-protease-like domain. During biosynthesis the polypeptides are cleaved into α- (the CCP-domain) and β-chains (the SP domain) and both remain linked together by a disulphide bond. A crucial function of HP is to act as a scavenger of free haemoglobin from plasma, since high quantities of free haemoglobin can be deleterious for the host. More recent work indicates that HP also interacts with lipoteichoic acid (LTA) of Gram-positive bacteria, an important virulence factor, and this is the focus of my thesis. The results of this work demonstrate that HP binds to LTA directly as well as to LTA on a wide range of Gram-positive bacteria (including LTA from S. aureus and S. pneumoniae). The IC50 of the interaction is ~40 nM in competition experiments to immobilised S. aureus. My work has shown that the LTA interaction site of HP is located on the β-chain and that LTA competes for binding with haemoglobin indicating that the binding sites for LTA and haemoglobin overlap. Surprisingly, in vivo studies showed that C57BL/6J HP-/- mice show a significant degree of protection from experimental S. pneumoniae infection. Over the course of the experiments, approximately 90% of HP-/- mice were resistance to the pneumococcal infection compared to 40% in strain, age and sex matched control wild-type mice infected in parallel. In line with the infection study, wild-type mice showed significantly higher levels of bacteraemia than HP-/- mice and the bacterial load in the lung, liver, kidney was significantly higher. These findings demonstrate that HP interacts with bacteria and plays an important role during infection. Surprisingly, the presence of HP appears to give S. pneumoniae a so far unknown advantage during infection.