The best-characterised hidden antigen of Haemonchus contortus, called H11, is a 110 kDa gut membrane glycoprotein (Smith et al., 1993) belonging to the M1 aminopeptidase family. Native H11, isolated from adult H. contortus, had been evaluated in numerous vaccine experiments in sheep of different breeds, including very young lambs, and shown to induce high levels of antibody-mediated protection, reflected in (on average) a >90% reduction in faecal egg counts (FECs) and > 75% reduction in worm burdens (reviewed by Newton and Munn, 1999; Newton and Meeusen, 2003; Knox, 2011). Molecular characterisation showed that H11 is a microsomal aminopeptidase (Graham et al., 1993; Smith et al., 1997), believed to be involved in the degradation of small peptides from dietary proteins (Munn and Munn, 2002). At the time, it was proposed that protection was achieved via antibody-induced disruption of nutrient absorption by the worm.
Originally, three isoforms of H11 (designated H11-1, -2 and -3; GenBank accession nos. AJ249941, AJ249942 and AJ311316, respectively), sharing ~ 61-70% amino acid sequence identity, were reported (Graham et al., 1993; Hederer et al. 2001, direct submission to GenBank). Subsequently, two other isoforms (H11-4 and -5) had been sequenced and deposited in the GenBank database (accession nos. FJ481146 and X94187) (Smith et al., 1997; Zhou et al., 2010). Although vaccination success was achieved using purified, native H11, and cDNA clones were patented