Virginia-Maryland Regional College of Veterinary Medicine, Virginia Tech, Blacksburg, VA 24061, United States.
Vet Microbiol. 2011 Nov 21;153(1-2):163-72. doi: 10.1016/j.vetmic.2011.02.054. Epub 2011 Mar 5.
Histophilus somni is an etiologic agent of bovine respiratory and systemic diseases. Most pathogenic strains of H. somni that have been tested (36 of 42) are able to utilize N-acetyl-5-neuraminic acid (Neu5Ac) to sialylate their lipooligosaccharide (LOS). Homologs of all the genes required for transport, metabolism, and regulation of Neu5Ac in Haemophilus influenzae were identified in the sequenced genomes of H. somni. Three open reading frames (ORFs) in H. somni strain 2336 were identified that contained homology to genes required for LOS sialylation in related bacteria. ORF-1 (hssT-I), ORF-2 (hssT-II), and ORF-3 (neuA(Hs)) were predicted to encode for putative proteins with 37% amino acid homology to an α-(2-3)-sialyltransferase in H. influenzae, 43% amino acid homology to an Haemophilus ducreyi sialyltransferase, and 72% amino acid homology to an H. influenzae CMP-Neu5Ac synthetase, respectively. The specific enzyme activity of each ORF was determined using synthetic acceptor substrates. The HssT-I sialyltransferase primarily sialylated N-acetyllactosamine (LacNAc, Gal-β-[1-4]-GlcNAc-R), which is expressed on strain 2336, whereas HssT-II preferentially sialylated lacto-N-biose (LNB, Gal-β-[1-3]-GlcNAc-R), which is expressed on a phase variant of strain 2336: strain 738. Phase variation of the terminal galactose linkage in strain 738 from β-(1-3)-(LNB) to β-(1-4)-(LacNAc) was confirmed using monoclonal antibody reactivity and nuclear magnetic resonance spectroscopy. Sialylated LOS induced significantly less chemokine response from macrophages derived from Toll-like receptor (TLR)-4 knockout mice than from de-sialylated LOS. Furthermore, sialylated LOS induced significantly less NF-κB activity from mouse-derived bone marrow macrophages than de-sialylated LOS. Therefore, sialylation inhibited LOS signaling through TLR-4. In conclusion, H. somni utilizes linkage-specific sialyltransferases to sialylate its LOS to avoid innate host defense mechanisms despite simultaneous epitope phase variation.
唾液嗜血杆菌是牛呼吸道和全身疾病的病原体。已测试的大多数致病性唾液嗜血杆菌菌株（42 株中的 36 株）能够利用 N-乙酰-5-神经氨酸（Neu5Ac）来唾液酸化其脂寡糖（LOS）。在已测序的唾液嗜血杆菌基因组中鉴定出与流感嗜血杆菌中 Neu5Ac 转运、代谢和调节所需的所有基因同源的同源物。在 2336 株唾液嗜血杆菌中鉴定出三个开放阅读框（ORF），它们包含与相关细菌 LOS 唾液酸化所需的基因同源。ORF-1（hssT-I）、ORF-2（hssT-II）和 ORF-3（neuA（Hs））分别预测编码具有与流感嗜血杆菌α-(2-3)-唾液酸转移酶 37%氨基酸同源性、与杜克雷嗜血杆菌唾液酸转移酶 43%氨基酸同源性和与流感嗜血杆菌 CMP-Neu5Ac 合成酶 72%氨基酸同源性的假定蛋白。使用合成受体底物测定每个 ORF 的特异性酶活性。HssT-I 唾液酸转移酶主要唾液酸化 N-乙酰乳糖胺（LacNAc，Gal-β-[1-4]-GlcNAc-R），这在 2336 株上表达，而 HssT-II 优先唾液酸化乳糖-N-双糖（LNB，Gal-β-[1-3]-GlcNAc-R），这在 2336 株的一个相变体菌株 738 上表达。通过单克隆抗体反应和核磁共振波谱证实 738 株的末端半乳糖连接从β-(1-3)-(LNB)到β-(1-4)-(LacNAc)的相变体。与去唾液酸化的 LOS 相比，唾液酸化的 LOS 诱导来自 Toll 样受体（TLR）-4 敲除小鼠的巨噬细胞产生的趋化因子反应显著减少。此外，与去唾液酸化的 LOS 相比，唾液酸化的 LOS 诱导来自鼠源性骨髓巨噬细胞的 NF-κB 活性显著降低。因此，唾液酸化通过 TLR-4 抑制 LOS 信号传导。总之，尽管同时发生表位相变体，唾液嗜血杆菌仍利用键特异性唾液酸转移酶唾液酸化其 LOS 以逃避先天宿主防御机制。
