There is limited knowledge of influenza-specific immune responses and their kinetics in critically ill patients. We investigated humoral and cellular immune responses after critical influenza A/H1N1 infection and hypothesized that dysfunctionality or absence of immune responses could contribute to more severe illness. We followed 12 patients hospitalized with severe influenza infection; the majority admitted to intensive care unit (ICU). Blood samples were collected at days 10 and 19 and at 5 months. Antibody responses to surface glycoproteins haemagglutinin (HA) and neuraminidase (NA) of A/H1N1pdm09 were quantified by haemagglutination inhibition (HAI), microneutralization (MN), Enzyme-linked immunosorbent assay (ELISA) and Enzyme-linked lectin assay (ELLA). Influenza-specific antibody levels and avidity were measured separately for head and stalk domains of H1. Cytokine secreting CD4⁺ and CD8⁺ T cell responses to conserved influenza epitopes (M1, NP and PB1) were analysed by FluoroSpot. Overall, the patients retained a high level of functional HA- and NA-specific antibodies over the study period. During the acute phase (up to 3 weeks from symptom onset), antibodies specific to H1 stalk increased earlier and were present in higher amount compared with H1 head-specific antibodies. The NA-specific antibodies and the non-neutralizing HA-specific antibody response for H1 head and H1 full-length showed a significant decline from acute to convalescent phase. Despite high total IgG concentrations, avidity to H1 head and H1 full-length protein remained low at all time points. Similarly, CD8⁺ T cell responses were continuously measured at low levels. In conclusion, our study found that critically ill patients were characterized by low HA-specific antibody avidity and CD8⁺ T cell response.

中文翻译：

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危重甲型流感/H1N1 感染患者的体液和细胞免疫反应

对重症患者流感特异性免疫反应及其动力学的了解有限。我们研究了严重 A/H1N1 流感感染后的体液和细胞免疫反应，并假设免疫反应功能失调或缺乏可能导致更严重的疾病。我们跟踪了 12 名因严重流感感染住院的患者；大多数住进重症监护室（ICU）。在第 10 天和第 19 天以及第 5 个月收集血样。通过血凝抑制 (HAI)、微量中和 (MN)、酶联免疫吸附试验 (ELISA) 和酶联凝集素试验 (ELLA) 量化对 A/H1N1pdm09 的表面糖蛋白血凝素 (HA) 和神经氨酸酶 (NA) 的抗体反应。流感特异性抗体水平和亲和力分别测量 H1 的头部和茎结构域。细胞因子分泌 CD4⁺和 CD8 ⁺ T 细胞对保守流感表位（M1、NP 和 PB1）的反应通过 FluoroSpot 进行分析。总体而言，患者在研究期间保留了高水平的功能性 HA 和 NA 特异性抗体。在急性期（从症状出现起最多 3 周），与 H1 头部特异性抗体相比，H1 茎部特异性抗体增加更早，并且数量更多。H1 头部和 H1 全长的 NA 特异性抗体和非中和 HA 特异性抗体反应显示从急性期到恢复期显着下降。尽管总 IgG 浓度很高，但对 H1 头部和 H1 全长蛋白的亲和力在所有时间点仍然很低。同样，CD8 ⁺在低水平下连续测量 T 细胞反应。总之，我们的研究发现危重患者的特点是低 HA 特异性抗体亲和力和 CD8 ⁺ T 细胞反应。