Competition between influenza A virus subtypes through heterosubtypic immunity modulates re-infection and antibody dynamics in the mallard duck

Our overall hypothesis is that host population immunity directed at multiple antigens will influence the prevalence, diversity and evolution of influenza A virus (IAV) in avian populations where the vast subtype diversity is maintained. To investigate how initial infection influences the outcome of later infections with homologous or heterologous IAV subtypes and how viruses interact through host immune responses, we carried out experimental infections in mallard ducks (Anas platyrhynchos). Mallards were pre-challenged with an H3N8 low-pathogenic IAV and were divided into six groups. At five weeks post H3N8 inoculation, each group was challenged with a different IAV subtype (H4N5, H10N7, H6N2, H12N5) or the same H3N8. Two additional pre-challenged groups were inoculated with the homologous H3N8 virus at weeks 11 and 15 after pre-challenge to evaluate the duration of protection. The results showed that mallards were still resistant to re-infection after 15 weeks. There was a significant reduction in shedding for all pre-challenged groups compared to controls and the outcome of the heterologous challenges varied according to hemagglutinin (HA) phylogenetic relatedness between the viruses used. There was a boost in the H3 antibody titer after re-infection with H4N5, which is consistent with original antigenic sin or antigenic seniority and suggest a putative strategy of virus evasion. These results imply competition between related subtypes that could regulate IAV subtype population dynamics in nature. Collectively, we provide new insights into within-host IAV complex interactions as drivers of IAV antigenic diversity that could allow the circulation of multiple subtypes in wild ducks.本研究的核心假说为：针对多种抗原的宿主群体免疫，会对维持大量亚型多样性的禽种群中甲型流感病毒（influenza A virus, IAV）的流行率、多样性及进化过程产生影响。为探究初次感染如何影响同源或异源甲型流感病毒亚型后续感染的结局，以及病毒如何通过宿主免疫应答产生相互作用，本研究以绿头鸭（Anas platyrhynchos）为对象开展了感染实验。研究人员先以低致病性甲型流感病毒H3N8对绿头鸭进行预攻击，并将其分为6组。在H3N8接种后第5周，对每组分别施加不同的甲型流感病毒亚型（H4N5、H10N7、H6N2、H12N5）攻击，或施加同源的H3N8病毒攻击。另外设置2个已完成预攻击的组别，分别在预攻击后第11周和第15周接种同源H3N8病毒，以评估免疫保护的持续时长。实验结果显示，绿头鸭在预攻击后15周仍可抵御再次感染。与对照组相比，所有预攻击组的病毒排毒量均显著降低；而异源攻击的结局与所用病毒的血凝素（hemagglutinin, HA）系统发育相关性相关，呈现出差异。研究人员在以H4N5进行再次感染后，观测到H3抗体滴度升高，这一结果与原始抗原原罪或抗原优先级效应相符，提示病毒可能存在某种免疫逃逸策略。上述结果表明，相关亚型之间存在竞争关系，该竞争或可调控自然环境中甲型流感病毒亚型的种群动态。综上，本研究揭示了宿主体内甲型流感病毒的复杂相互作用，该相互作用作为甲型流感病毒抗原多样性的驱动因素，可促成多种亚型在野生鸭群中循环传播。