Haemonchus contortus is an economically important gastric parasite infecting sheep and goats. The parasite survives the host immune attack by releasing protective molecules. In the present study, a 55 kDa secretory glycoprotein (gp55) was identified that inhibited host neutrophils as judged by reduced H2O2 production by these cells. The binding of gp55 to neutrophils was confirmed by flow cytometry. This binding was mediated by cellular CD11b/CD18 integrin. The glycoprotein gp55 also bound to goat monocytes and lymphocytes and inhibited monocyte function. Using light fluorescence microscopy, gp55 was localized at the surface of adult worms. The absence of gp55 in the infective L3 larvae and its expression in the blood-feeding stage support a role for gp55 in the parasitic stage of the organism.

Calreticulin (CalR), a Ca(2+) binding multifunctional protein, is secreted by the parasitic nematode Haemonchus contortus. We have earlier observed binding of this protein to a 24-kDa polypeptide (p24) present in an enriched preparation of prothrombin. In the present study, the identity of p24 was established as a C-reactive protein (CRP) by several criteria. CalR binding to CRP is an elegant strategy devised by the parasite to survive in the host. The secreted CalR may achieve this either by limiting the free concentration of CRP, which has antiparasite activity or inhibit the activation of the classical complement pathway triggered on binding of CRP to C1q protein. CalR binding to CRP would also ensure a check on the procoagulant activity of the CRP enabling parasite to feed on the host blood. Thus, targeting CalR could be a novel strategy to tackle this parasite, which has developed resistance to many anthelmintics.