Of the seven species that affect the poultry industry, E. maxima is considered as the most immunogenic (56). Early studies therefore concentrated on the E. maxima model and much work was focused on better understanding the basis of immunity and the lifecycle stages that are predominantly involved in immune responses (56–58). In addition, E. maxima was selected as the model species for initial work on development Decitabine clinical trial of a subunit vaccine as its gametocytes are very large in size and relatively easily visualized and purified (59). The induction

of immunity using the E. maxima model was first demonstrated by Rose (56), who showed that a single low dose of E. maxima oocysts could protect chickens against a challenge with high doses of oocysts of the homologous strain, and that one

cycle of infection was enough to stimulate this protective immunity. It was further demonstrated that sera taken 14 days post-infection with E. maxima can give passive protection Nutlin-3 molecular weight to naive chickens against a challenge (57,58). However, it was first thought that the asexual stages of the lifecycle of Eimeria were predominantly responsible for the immune response invoked in chickens. Analysis of convalescent sera taken 14–20 days after an E. maxima infection, which was shown to passively protect naive birds, demonstrated that the early stages of development have a strong immunogenic effect, and the selleck later sexual stages were poorly immunogenic in E. maxima,

E. necatrix and E. tenella (58,60). Kouwenhoven and Kuil (61) also reported that sera taken from chickens infected with E. tenella showed no reaction with sexual stages or first generation schizonts, indicating that gametocytes had poor immunizing capabilities in chickens. Later studies, however, contradicted these earlier findings (62,63). The antigenicity of sexual stages of Eimeria was first demonstrated when a monoclonal antibody to an E. tenella gametocyte antigen was shown to inhibit fertilization in vitro (64). In 1989, Pugatsch et al. (62) developed a method to isolate and purify gametocytes by enzymatic digestion of the infected mucosa with hyaluronidase, followed by size separation. They showed that whole gametocytes were highly antigenic both in the course of an infection and when injected into rabbits/mice. During the same year, convalescent sera from E. maxima immune chickens were found to recognize two immunodominant macrogametocyte antigens of 56 and 82 kDa in size (63). When these two proteins were administered to a variety of hosts in the form of a crude extract, they were found to be highly immunogenic (63). Following this discovery, it was hypothesized that these macrogametocyte antigens may play a role in conferring protective immunity to the host (63).