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However, their roles in co-infection remain unclear. We tested the hypothesis that C. psittaci enhances H9N2 infection through suppression of host immunity. Thus, 10-day-old SPF chickens were inoculated intra-tracheally with a high or low virulence <jats:italic>C. psittaci<\/jats:italic> strain and were simultaneously vaccinated against <jats:italic>Newcastle disease virus<\/jats:italic> (NDV). Significant decreases in body weight, NDV antibodies and immune organ indices occurred in birds with the virulent <jats:italic>C. psittaci<\/jats:italic> infection, while the ratio of CD4+\/CD8+ T cells increased significantly compared to that of the lower virulence strain. A second group of birds were inoculated with <jats:italic>C. psittaci<\/jats:italic> and H9N2 simultaneously (<jats:italic>C. psittaci<\/jats:italic>+H9N2), <jats:italic>C. psittaci<\/jats:italic> 3 days prior to H9N2 (<jats:italic>C. psittaci\/<\/jats:italic>H9N2), or 3 days after H9N2 (H9N2\/<jats:italic>C. psittaci<\/jats:italic>), <jats:italic>C. psittaci<\/jats:italic> or H9N2 alone. Survival rates were 65%, 80% and 90% in the <jats:italic>C. psittaci<\/jats:italic>\/H9N2, <jats:italic>C. psittaci<\/jats:italic>+H9N2 and H9N2\/<jats:italic>C. psittaci<\/jats:italic> groups, respectively and respiratory clinical signs, lower expression of pro-inflammatory cytokines and higher pathogen loads were found in both <jats:italic>C. psittaci<\/jats:italic>\/H9N2 and <jats:italic>C. psittaci<\/jats:italic>+H9N2 groups. Hence, virulent <jats:italic>C. psittaci<\/jats:italic> infection suppresses immune response by inhibiting humoral responses and altering Th1\/Th2 balance, increasing mortality in H9N2 infected birds.<\/jats:p>","DOI":"10.1038\/srep29421","type":"journal-article","created":{"date-parts":[[2016,7,12]],"date-time":"2016-07-12T12:52:53Z","timestamp":1468327973000},"update-policy":"https:\/\/doi.org\/10.1007\/springer_crossmark_policy","source":"Crossref","is-referenced-by-count":39,"title":["Chlamydia psittaci infection increases mortality of avian influenza virus H9N2 by suppressing host immune response"],"prefix":"10.1038","volume":"6","author":[{"given":"Jun","family":"Chu","sequence":"first","affiliation":[],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Qiang","family":"Zhang","sequence":"additional","affiliation":[],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Tianyuan","family":"Zhang","sequence":"additional","affiliation":[],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Er","family":"Han","sequence":"additional","affiliation":[],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Peng","family":"Zhao","sequence":"additional","affiliation":[],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Ahrar","family":"Khan","sequence":"additional","affiliation":[],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Cheng","family":"He","sequence":"additional","affiliation":[],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Yongzheng","family":"Wu","sequence":"additional","affiliation":[],"role":[{"vocabulary":"crossref","role":"author"}]}],"member":"297","published-online":{"date-parts":[[2016,7,11]]},"reference":[{"key":"BFsrep29421_CR1","doi-asserted-by":"publisher","first-page":"1","DOI":"10.1093\/femspd\/ftu007","volume":"73","author":"MR Knittler","year":"2015","unstructured":"Knittler, M. 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