Necrotic enteritis (NE) is an economically important enteric disease of broiler chicken primarily caused by a-toxin (Cpa) secreted by C. perfringens type A. Mice immunized with recombinant C-terminal domain of Cpa (CpaCD) had transient and fewer localized lesions upon challenge with C. perfringens type A. These results demonstrate the usefulness of CpaCD as an immunogen for vaccine development against NE for chickens. Chicken CD40 (chCD40) is mainly expressed on the surface of chicken antigen-presenting cells (APCs), and the interaction of chCD40 and chCD40L (natural ligand for chCD40) provides crucial activation signals for chicken B-cells. A hypothesis was proposed that in ovo vaccination with an adenovirus-vectored CpaCD vaccine capable of targeting immunogen to APCs through the CD40 pathway will improve protection against NE in chickens. One agonistic monoclonal anti-chCD40 antibody (designated 2C5) was produced and characterized. 2C5 not only detected expression of chCD40 on chicken APCs, but also induced NO synthesis in chicken HD11 macrophages and enhanced proliferation of serum-starved chicken DT40 B-cells. This demonstrated substantial functional equivalence of 2C5 with chCD40L. The potential of 2C5 as an immunological adjuvant was further assessed by targeting a hapten to chicken APCs in hopes of enhancing an effective IgG response. Seven-week old chickens were immunized subcutaneously once with a complex consisting of 2C5 and peptide, and relative quantification of the peptide-specific IgG response showed that this complex was able to elicit a strong IgG response as early as four days post-immunization. This demonstrates that CD40-targeting antigen to chicken APCs can significantly enhance antibody responses and induce immunoglobulin isotype-switching. An agonistic anti-chCD40 single-chain variable fragment (designated DAG1) was combined with an adenoviral delivery system to create a vaccine, Ad-(DAG1-Cp aCD-FLAG), for in ovo administration. The efficacy of in ovo vaccination of broilers with Ad-(DAG1-Cp aCD-FLAG) in controlling NE was evaluated by C. perfringens type A challenge at 18 days post-hatch. Neither statistically significant IgA / IgG response nor protection against C. perfringens type A challenge was found in the vaccinated birds. These preliminary data suggest that a super-optimal dose of Ad-(DAG1-Cp aCD-FLAG) may be the main issue, because Cpa-specific B-cells may undergo apoptosis through the CD40 pathway.
