Impact of pneumococcal NanA-mediated host desialylation in Siglec-Toll-like receptor crosstalk

• Main Authors: Chun-Chi Chang, 張鈞棋
• Other Authors: 張永祺
• Format: Others
• Language: zh-TW
• Published: 2017
• Online Access: http://ndltd.ncl.edu.tw/handle/sjyx57

碩士 === 國立臺灣大學 === 微生物學研究所 === 105 === Streptococcus pneumoniae (SPN) is the single most deadly bacterial infection globally, capable of causing a wide spectrum of disease, including sinusitis, otitis media, pneumonia, bacteremia and meningitis. The mortality caused by pneumococcal disease is sometimes linked to imbalanced hyper-inflammatory responses of the host despite of successful pathogen clearance. All SPN strains identified so far produce a surface-associated sialidase (NanA) and we recently demonstrated that leukocyte inflammatory responses are aggravated by pneumococcal NanA through removing the cis-lignads of Siglec expressed on the same myeloid cells. Cell surface in the immune system are richly equipped with a complex mixture of various lectins and glycans. Siglec binding sites are typically ‘masked’ by cis interactions with other sialoglycan ligands expressed on the same cell. Given the ubiquitous and abundant expression of host Sias and the prominence of cognate ITIM-bearing receptors on innate immune cells, Sia is proposed to act as “self-associated molecular patterns” to control the activation threshold of immune cells. Toll-like receptor (TLR) family, a member of pattern recognition receptors (PRRs), are generally thought to be a first-line defense to recognize pathogens. Upon infection, immune cells sense the environment through their various PRRs, processing and integrating this external information through the intracellular signaling cascade and induce appropriate immune responses to combat infectious microbial agents. Thus, this study we aimed to further investigate how pneumococcal NanA-mediated cell desialylation affects the crosstalk between Siglec and PRR signaling pathway. We hypothesized NanA-mediated host cell desialylation may disrupt the interaction between TLR and Siglec, dysregulate the inflammatory signals downstream of TLR and cause excessive pro-inflammatory substance upon pneumococcal infection. Our data suggested that WT SPN was able to induce stronger TNF-α production that partially rely on lipid rafts structure on cell membrane and activation of signaling molecules involved in MAPK, PI3k/Akt and NF-кB pathways compared to ∆nanA SPN. This pneumococcal NanA-mediated cell activation may result from affecting TLR-2-Siglec-5 interaction and Sigelc-5-associated signaling molecules. In addition to dissect the role of inhibitory Siglec in NanA-mediated pneumococcal inflammation, we also found that expression of an activating Siglec, Siglec-14, on human primary macrophages and THP-1 cells also affects host immune responses upon cell surface desialylation by pneumococcal NanA.