Caspase recruitment domains‐containing protein 9 (CARD9) is an adaptor molecule critical for key signalling pathways initiated through C‐type lectin receptors (CLRs). Previous studies demonstrated that Pneumocystis organisms are recognised through a variety of CLRs. However, the role of the downstream CARD9 adaptor signalling protein in host defence against Pneumocystis infection remains to be elucidated. Herein, we analysed the role of CARD9 in host defence against Pneumocystis both in CD4‐depleted CARD9^−/− and immunocompetent hosts. Card9 gene‐disrupted (CARD9^−/−) mice were more susceptible to Pneumocystis, as evidenced by reduced fungal clearance in infected lungs compared to wild‐type (WT) infected mice. Our data suggests that this defect was due to impaired proinflammatory responses. Furthermore, CARD9^−/− macrophages were severely compromised in their ability to differentiate and express M1 and M2 macrophage polarisation markers, to enhanced mRNA expression for Dectin‐1 and Mincle, and most importantly, to kill Pneumocystis in vitro. Remarkably, compared to WT mice, and despite markedly increased organism burdens, CARD9^−/− animals did not exhibit worsened survival during pneumocystis pneumonia (PCP), perhaps related to decreased lung injury due to altered influx of inflammatory cells and decreased levels of proinflammatory cytokines in response to the organism. Finally, although innate phase cytokines were impaired in the CARD9^−/− animals during PCP, T‐helper cell cytokines were normal in immunocompetent CARD9^−/− animals infected with Pneumocystis. Taken together, our data demonstrate that CARD9 has a critical function in innate immune responses against Pneumocystis.

中文翻译：

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CARD9 在肺孢子虫肺炎宿主防御中的关键作用。

含有半胱天冬酶募集结构域的蛋白质 9 (CARD9) 是一种衔接分子，对通过 C 型凝集素受体 (CLR) 启动的关键信号通路至关重要。先前的研究表明，肺孢子菌是通过多种 CLR 识别的。然而，下游 CARD9 适配器信号蛋白在宿主防御肺孢子虫感染中的作用仍有待阐明。在此，我们分析了 CARD9 在CD4 耗尽的 CARD9 ^-/-和免疫活性宿主中对抗肺孢子虫的宿主防御中的作用。Card9基因破坏 ( CARD9 ^-/- ) 小鼠对肺孢子虫更敏感，与野生型 (WT) 感染的小鼠相比，感染肺中的真菌清除率降低就证明了这一点。我们的数据表明，这种缺陷是由于促炎反应受损所致。此外，CARD9 ^-/-巨噬细胞分化和表达 M1 和 M2 巨噬细胞极化标记的能力严重受损，以增强 Dectin-1 和 Mincle 的 mRNA 表达，最重要的是，在体外杀死肺孢子虫。值得注意的是，与 WT 小鼠相比，尽管生物体负担显着增加，CARD9 ^-/-在肺孢子虫肺炎 (PCP) 期间，动物没有表现出生存率下降，这可能与由于炎症细胞流入改变和促炎细胞因子水平降低而导致的肺损伤减少有关。最后，尽管PCP 期间CARD9 ^-/-动物的先天期细胞因子受损，但T 辅助细胞细胞因子在感染肺囊虫的具有免疫活性的 CARD9 ^-/-动物中是正常的。总之，我们的数据表明 CARD9 在针对肺孢子菌的先天免疫反应中具有关键功能。