Biedroń Rafał, Peruń Angelika, Józefowski Szczepan
Department of Immunology, Jagiellonian University Medical College, Kraków, Poland.
PLoS One. 2016 Apr 13;11(4):e0153558. doi: 10.1371/journal.pone.0153558. eCollection 2016.
Lipopolysaccharide (LPS) is the major pathogen-associated molecular pattern of Gram-negative bacterial infections, and includes smooth (S-LPS) and rough (R-LPS) chemotypes. Upon activation by LPS through CD14, TLR4/MD-2 heterodimers sequentially induce two waves of intracellular signaling for macrophage activation: the MyD88-dependent pathway from the plasma membrane and, following internalization, the TRIF-dependent pathway from endosomes. We sought to better define the role of scavenger receptors CD36 and CD204/SR-A as accessory LPS receptors that can contribute to pro-inflammatory and microbicidal activation of macrophages. We have found that CD36 differently regulates activation of mouse macrophages by S-LPS versus R-LPS. The ability of CD36 to substitute for CD14 in loading R-LPS, but not S-LPS onto TLR4/MD-2 allows CD14-independent macrophage responses to R-LPS. Conversely, S-LPS, but not R-LPS effectively stimulates CD14 binding to CD36, which favors S-LPS transfer from CD14 onto TLR4/MD-2 under conditions of low CD14 occupancy with S-LPS in serum-free medium. In contrast, in the presence of serum, CD36 reduces S-LPS binding to TLR4/MD-2 and the subsequent MyD88-dependent signaling, by mediating internalization of S-LPS/CD14 complexes. Additionally, CD36 positively regulates activation of TRIF-dependent signaling by both S-LPS and R-LPS, by promoting TLR4/MD-2 endocytosis. In contrast, we have found that SR-A does not function as a S-LPS receptor. Thus, by co-operating with CD14 in both R- and S-LPS loading onto TLR4/MD-2, CD36 can enhance the sensitivity of tissue-resident macrophages in detecting infections by Gram-negative bacteria. However, in later phases, following influx of serum to the infection site, the CD36-mediated negative regulation of MyD88-dependent branch of S-LPS-induced TLR4 signaling might constitute a mechanism to prevent an excessive inflammatory response, while preserving the adjuvant effect of S-LPS for adaptive immunity.
脂多糖（LPS）是革兰氏阴性菌感染的主要病原体相关分子模式，包括光滑型（S-LPS）和粗糙型（R-LPS）化学型。LPS通过CD14激活后，TLR4/MD-2异二聚体依次诱导两波细胞内信号传导以激活巨噬细胞：来自质膜的MyD88依赖性途径，以及内化后来自内体的TRIF依赖性途径。我们试图更好地确定清道夫受体CD36和CD204/SR-A作为辅助LPS受体的作用，它们可有助于巨噬细胞的促炎和杀菌激活。我们发现，CD36对S-LPS和R-LPS激活小鼠巨噬细胞的调节方式不同。CD36在将R-LPS而非S-LPS加载到TLR4/MD-2上替代CD14的能力，使得巨噬细胞能够独立于CD14对R-LPS作出反应。相反，S-LPS而非R-LPS能有效刺激CD14与CD36结合，这有利于在无血清培养基中S-LPS低占有率的情况下，S-LPS从CD14转移到TLR4/MD-2上。相比之下，在有血清存在的情况下，CD36通过介导S-LPS/CD14复合物的内化，减少S-LPS与TLR4/MD-2的结合以及随后的MyD88依赖性信号传导。此外，CD36通过促进TLR4/MD-2内吞作用，对S-LPS和R-LPS的TRIF依赖性信号传导激活起正向调节作用。相比之下，我们发现SR-A不作为S-LPS受体发挥作用。因此，通过在将R-LPS和S-LPS加载到TLR4/MD-2上与CD14协同作用，CD36可增强组织驻留巨噬细胞检测革兰氏阴性菌感染的敏感性。然而，在后期，随着血清流入感染部位，CD36介导的对S-LPS诱导的TLR4信号传导MyD88依赖性分支的负调节，可能构成一种机制，既能防止过度的炎症反应，又能保留S-LPS对适应性免疫的佐剂作用。
