Development of a novel histidine-rich glycoprotein measurement system as a biomarker for sepsis

The plasma histidine-rich glycoprotein concentration is a marker of sepsis severity. In this study, we generated selective and specific monoclonal antibodies against histidine-rich glycoprotein for use in a prototype enzyme-linked immunosorbent assay-based in vitro diagnostic system. First, we investigated the properties of monoclonal antibodies produced by 21 hybridomas that we developed using immunized mice, and we identified monoclonal antibodies 69-1A and 75-2D to be the most suitable combination for use in the sandwich enzyme-linked immunosorbent assay. Wild-type histidine-rich glycoprotein (Form-1, 75 kDa) with a proline residue at amino acid position 204 is the most common isoform of the protein in humans, followed by its variant (Form-2, 77 kDa), which has a serine residue at position 204.
The epitope mapping was examined for the HRG amino acid sequence with 69-1A and 75-2D mAbs to achieve the identification of respective specific binding domains, though the other kinds of mAbs showed considerably complex domains. The identified epitopes recognized by 69-1A and 75-2D monoclonal antibodies did not span position 204. Furthermore, immunoprecipitation-immunoblotting analysis showed that the 69-1A and 75-2D monoclonal antibodies could bind to both Form-1 and Form-2 in human plasma samples. Thus, these two new antibodies can be used to clearly detect both forms of histidine-rich glycoproteins in human plasma samples. In our analysis of clinical samples by enzyme-linked immunosorbent assays using various combinations of our newly synthesized antibodies, we found that the histidine-rich glycoprotein concentration was significantly lower in plasma samples from septic patients than in those from healthy volunteers (p < 0.01). Thus, our novel analysis system using the new antibodies is expected to be a useful tool for sepsis research, and it may be adapted as an in vitro diagnostic tool for many other kinds of diseases in the future.