Background: Chlamydia psittaci  is a zoonotic bacteria closely associated with psittacosis/ornithosis. Vaccination has been recognized as the best way to inhibit the spread of C. psittaci  due to the majority ignored of infections. The optimal Chlamydia  vaccine was obstructed by the defect of single immunization route and the lack of availability of nontoxic and valid adjuvants.
Methods: In this study, we developed a novel immunization strategy, simultaneous (SIM) intramuscular (IM) and intranasal (IN) administration of a C. psittaci  antigens (Ags) adjuvanted with chitosan nanoparticles (CNPs). And SIM-CNPs-Ags were used to determine the different types of immune response and the protective role in vivo.
Results: CNPs-Ags with zeta-potential values of 13.12 mV and of 276.1 nm showed excellent stability and optimal size for crossing the mucosal barrier with high 71.7% encapsulation efficiency. SIM-CPN-Ags mediated stronger humoral and mucosal responses by producing meaningfully high levels of IgG and secretory IgA (sIgA) antibodies. The SIM route also led to Ags-specific T-cell responses and increased IFN-γ, IL-2, TNF-α and IL-17A in the splenocyte supernatants. Following respiratory infection with C. psittaci , we found that SIM immunization remarkably reduced bacterial load and the degree of inflammation in the infected lungs and made for a lower level of IFN-γ, TNF-α and IL-6. Furthermore, SIM vaccination with CNPs-Ags had obviously inhibited C. psittaci  disseminating to various organs in vivo.
Conclusion: SIM immunization with CNPs-adjuvanted C. psittaci  Ags may present a novel strategy for the development of a vaccine against the C. psittaci  infection.
Keywords: Chlamydia psittaci , vaccine, chitosan nanoparticles, immunization route, respiratory infection