Assessment of Mycoplasma gallisepticum vaccine efficacy in a co-infection challenge model with QX-like infectious bronchitis virus

Mycoplasma gallisepticum (MG) is the primary cause of chronic respiratory disease in poultry. We investigated the protective efficacy of the live-attenuated ts-11 and 6/85 MG vaccines against a local MG strain and, in order to enhance signs and mimic a typical field situation, we co-infected birds with a virulent strain of QX-like infectious bronchitis virus (IBV). Both vaccines showed similar ability to protect infected chickens from clinical signs, although ts-11 performed slightly better. Despite the lower protection against clinical disease, 6/85-vaccinated birds had significantly (P ≤ 0.05) lower tracheal lesion scores and mucosal thickness at day 28 post-vaccination (7 days post-challenge [dpc] with MG, 2 dpc IBV) and day 31 post-vaccination (10 dpc MG challenge, 5 dpc IBV) compared to ts-11 vaccinated birds, but these difference was not significant at day 33 (12 dpc MG, 7 dpc IBV). Pathogen infection and replication was assessed by qPCR, and the 6/85 vaccine produced a more significant (P ≤ 0.05) reduction in MG replication in the lungs, kidneys and livers but enhanced late replication in bursae and caecal tonsils. In contrast, the ts-11 vaccine had a more pronounced reductive effect on replication in tracheas, air sacs, bursae and heart at days 28 and 31, yet increased replication in lungs. Interestingly, both vaccines provided non-specific protection against IBV challenge. The co-challenge model provided useful data on vaccine efficacy, especially on days 31 and 33, and tracheas, lungs, air sacs, kidneys, liver and caecal tonsils were the best organs to assess.鸡毒支原体（Mycoplasma gallisepticum, MG）是引发家禽慢性呼吸道疾病的主要病原体。本研究评估了减毒活疫苗ts-11与6/85株MG疫苗对本地MG毒株的保护效力；为加重感染症状、模拟典型野外感染场景，本研究采用强毒QX样传染性支气管炎病毒（QX-like infectious bronchitis virus, IBV）与MG共感染试验鸡只。两款疫苗在保护感染鸡只免于出现临床症状方面效果相近，其中ts-11株的保护表现略优。尽管在临床疾病防护效果上稍显逊色，但经6/85株免疫的鸡只，在免疫后第28天（即MG攻毒后7天、IBV攻毒后2天）与免疫后第31天（MG攻毒后10天、IBV攻毒后5天）的气管病变评分与黏膜厚度均显著低于ts-11疫苗免疫组（P ≤ 0.05）；而在免疫后第33天（MG攻毒后12天、IBV攻毒后7天），该差异不再显著。本研究通过定量聚合酶链反应（quantitative real-time PCR, qPCR）检测病原体感染与复制水平，结果显示：6/85株疫苗可更显著地（P ≤ 0.05）降低MG在肺脏、肾脏与肝脏中的复制量，却会增强法氏囊与盲肠扁桃体中的病毒后期复制。与之相反，ts-11株疫苗在免疫后第28天与第31天，对气管、气囊、法氏囊与心脏中的病毒复制抑制效果更为显著，却会促进肺脏内的病毒复制。值得关注的是，两款疫苗均可为鸡只提供针对IBV攻毒的非特异性保护。本次共感染攻毒模型可为疫苗效力评估提供极具价值的数据，其中免疫后第31天与第33天为最佳检测节点，气管、肺脏、气囊、肾脏、肝脏与盲肠扁桃体则是评估疫苗效力的最优样本器官。