The N-terminal segment of glyceraldehyde-3-phosphate dehydrogenase of Haemonchus contortus interacts with complements C1q and C3.

Haemonchus contortus, an economically important blood-sucking parasite of sheep and goats, survives the harsh host gut environment by secreting a number of proteins referred as excretory/secretory (ES) products. Glyceraldehyde-3-phosphate dehydrogenase (GAPDH), a glycolytic enzyme, is one of the components of H. contortus ES products. The parasite enzyme binds to complement C3 and inhibits its activity. In this study, the C3-binding activity of the parasite GAPDH was mapped to the N-terminal part of the enzyme by generating defined recombinant fragments of the protein. The N-terminal fragment also trapped complement C1q but not C5 and inhibited complement-mediated lysis of sensitized sheep erythrocytes. Competitive binding assay indicates different binding regions for C1q and C3 proteins. GAPDH stimulated proliferation of goat peripheral blood mononuclear cells in vitro and reacted with the sera from H. contortus-infected animals. However, the fragments of GAPDH did not stimulate cell proliferation nor reacted with the infected animal sera. Furthermore, denatured GAPDH failed to react with the infected animal sera in dot blot suggesting conformation-dependent epitope. These results demonstrate an elegant strategy of the parasite to completely shut down complement activation and identify GAPDH as a promising target for future therapeutic intervention.

Glyceraldehyde-3-phosphate dehydrogenase of the parasitic nematode Haemonchus contortus binds to complement C3 and inhibits its activity.

Haemonchus contortus is an economically important gastrointestinal parasite that infects primarily sheep and goats. To survive inside the host, the parasite must overcome the host immune response. In this study, we have identified and characterized a complement-C3-binding protein (H.c-C3BP) from this parasite employing biochemical and molecular biology tools. Initially, a truncated form of the protein was isolated from the excretory-secretory products of the parasite using C3-Sepharose column that facilitated its identification by mass spectroscopy. Subsequently, the parent molecule was generated in E. coli, and sequence analysis confirmed it as glyceraldehyde-3-phosphate dehydrogenase (GAPDH). GAPDH reacted with the antiserum raised against the truncated protein, and the truncated protein reacted with anti-GAPDH antiserum. The protein inhibited complement function as measured by haemolytic assay and membrane attack complex (MAC) formation. Sera from H. contortus-infected animals reacted with GAPDH as well as the truncated form of the protein, which further lend support to protein secretion. Thus, the C3-binding property of H. contortus GAPDH is a new function, and it represents a new entity of complement-binding protein. Identification and characterization of H.c-C3BP should facilitate development of new therapeutics considering a key role of this protein in immune modulation.