Immune responses induced by a subunit vaccine against Mycoplasma gallisepticum

Mycoplasma gallisepticum (MG) is an avian respiratory pathogen responsible for global economic loss to the poultry industry. We previously developed an efficacious subunit vaccine to address the drawbacks of current bacterin and live-attenuated vaccine designs. To improve our understanding of how the subunit vaccine works, we sought to understand if the vaccine influences the biology of the pathogen, whether the subunit vaccine has correlates of protection when analyzing antibody responses or tracheal-associated cells, and whether each component of the subunit vaccine contributes to protection. We performed five experiments to answer these questions. In the first experiment, we found that the subunit vaccine induces changes in the in vivo vlhA expression patterns of the infecting MG. We also found that on day 1 post-challenge, anti-VlhA 3.06 systemic IgY correlated negatively with the thickest tracheal mucosal section. In the second experiment, we found that on day 1 post-challenge, anti-MG lysate systemic IgY, anti-VlhA 3.03 systemic IgY, and anti-VlhA 5.05 systemic IgY negatively correlated with bacterial recovery. We also found that when looking at tracheal-associated cell frequencies, γδ T cells, with specific regard to CD8^- γδ T cells, and monocytes/macrophages, with specific regard to the monocyte/macrophage:granulocyte ratio, were negatively correlated with bacterial recovery, average tracheal mucosal thickness, or thickest tracheal mucosal section. In experiments three through five, we found that VlhAs 3.03, 3.06, and 5.05 contributed to reducing bacterial recovery, whereas VlhA 4.07 was dispensable, and we found that GapA and CrmA contributed to reducing bacterial recovery and average tracheal mucosal thickness.