Diverse Heterologous Primary Infections Radically Alter Immunodominance Hierarchies and Clinical Outcomes Following H7N9 Influenza Challenge in Mice

The recent emergence of a novel H7N9 influenza A virus (IAV) causing severe human infections in China raises concerns about a possible pandemic. The lack of pre-existing neutralizing antibodies in the broader population highlights the potential protective role of IAV-specific CD8+ cytotoxic T lymphocyte (CTL) memory specific for epitopes conserved between H7N9 and previously encountered IAVs. In the present study, the heterosubtypic immunity generated by prior H9N2 or H1N1 infections significantly, but variably, reduced morbidity and mortality, pulmonary virus load and time to clearance in mice challenged with the H7N9 virus. In all cases, the recall of established CTL memory was characterized by earlier, greater airway infiltration of effectors targeting the conserved or cross-reactive H7N9 IAV peptides; though, depending on the priming IAV, each case was accompanied by distinct CTL epitope immunodominance hierarchies for the prominent KbPB1703, DbPA224, and DbNP366 epitopes. While the presence of conserved, variable, or cross-reactive epitopes between the priming H9N2 and H1N1 and the challenge H7N9 IAVs clearly influenced any change in the immunodominance hierarchy, the changing patterns were not tied solely to epitope conservation. Furthermore, the total size of the IAV-specific memory CTL pool after priming was a better predictor of favorable outcomes than the extent of epitope conservation or secondary CTL expansion. Modifying the size of the memory CTL pool significantly altered its subsequent protective efficacy on disease severity or virus clearance, confirming the important role of heterologous priming. These findings establish that both the protective efficacy of heterosubtypic immunity and CTL immunodominance hierarchies are reflective of the immunological history of the host, a finding that has implications for understanding human CTL responses and the rational design of CTL-mediated vaccines.近期，一种可导致人类重症感染的新型甲型流感病毒（influenza A virus, IAV）H7N9亚型在中国出现，引发了人们对其可能引发大流行的担忧。普通人群中普遍缺乏预先存在的中和抗体，这凸显了针对H7N9与既往感染过的甲型流感病毒之间保守表位的IAV特异性CD8+细胞毒性T淋巴细胞（CTL）记忆细胞的潜在保护作用。本研究中，既往感染H9N2或H1N1亚型病毒所诱导的异型交叉免疫，可显著但存在差异地降低感染H7N9亚型病毒的小鼠的发病率与死亡率、肺部病毒载量以及病毒清除所需时间。在所有实验分组中，已建立的CTL记忆的召回均表现为：靶向保守或交叉反应性H7N9 IAV肽段的效应细胞更早、更大量地浸润气道；不过，根据致敏所用IAV亚型的不同，各组针对主要的KbPB1703、DbPA224及DbNP366表位的CTL免疫显性层级均存在差异。尽管致敏所用的H9N2、H1N1亚型与攻毒用H7N9 IAV之间存在的保守、可变或交叉反应性表位，确实会影响免疫显性层级的变化，但这种变化模式并非仅由表位保守性所决定。此外，与表位保守程度或二次CTL扩增相比，致敏后IAV特异性记忆CTL池的总规模更能预测实验的良好结局。调控记忆CTL池的规模可显著改变其后续对疾病严重程度或病毒清除的保护效力，这进一步证实了异源致敏的关键作用。本研究结果表明，异型交叉免疫的保护效力与CTL免疫显性层级均能反映宿主的免疫史，这一发现对于理解人类CTL应答以及合理设计基于CTL的疫苗具有重要意义。