<i>In silico</i> screening and epitope mapping of leptospiral outer membrane protein—Lsa46

Leptospirosis is one of the neglected diseases caused by the spirochete, <i>Leptospira interrogans</i>. Leptospiral surface adhesion (Lsa) proteins are surface exposed outer membrane proteins present in the pathogen. It acts as laminin and plasminogen binding proteins which enable them to infect host cells. The major target for the development of vaccine in the current era focuses on surface exposed outer membrane proteins, as they can induce strong and fast immune response in hosts. Therefore, the present study mapped the potential epitopes of the Leptospiral outer membrane proteins, mainly the surface adhesion proteins. Protein sequence analysis of Lsa proteins was done by <i>in silico</i> methods. The primary protein sequence analysis revealed Lsa46 as a suitable target which can be a potent Leptospiral vaccine candidate. Its structure was modelled by threading based method in I-TASSER server and validated by Ramachandran plot. The predicted epitope’s interactions with human IgG, IgM(Fab) and T-cell receptor TCR(αβ) were performed by molecular docking studies using Biovia Discovery studio 2018. One of the predicted B-cell epitopes and the IgG showed desirable binding interactions, while four of the predicted B-cell epitopes and T-cell epitopes showed desirable binding interactions with IgM and TCR respectively. The molecular dynamic simulation studies carried out with the molecular docked complexes gave minimized energies indicating stable interactions. The structural analysis of the entire simulated complex showed a stable nature except for one of the Epitope-IgM complex. Further the binding free energy calculation of eight receptor-ligand complex predicted them energetically stable. The results of the study help in elucidating the structural and functional characterization of Lsa46 for epitope-based vaccine design. Communicated by Ramaswamy H. Sarma.钩端螺旋体病（Leptospirosis）是由螺旋体问号钩端螺旋体（Leptospira interrogans）引发的被忽视传染病之一。钩端螺旋体表面黏附（Leptospiral surface adhesion, Lsa）蛋白是该病原体表面暴露的外膜蛋白，可作为层粘连蛋白与纤溶酶原结合蛋白，介导病原体侵染宿主细胞。当前疫苗研发的核心靶点多集中于菌体表面暴露的外膜蛋白，因其可在宿主体内诱发强烈且快速的免疫应答。因此，本研究针对钩端螺旋体外膜蛋白（主要为表面黏附蛋白）的潜在表位开展了筛选与鉴定工作。本研究通过计算机模拟（in silico）方法完成了Lsa蛋白的序列分析，初步序列分析结果显示，Lsa46是极具潜力的钩端螺旋体疫苗候选靶点。研究人员借助I-TASSER服务器，采用穿线法完成了该蛋白的结构建模，并通过拉马钱德兰图（Ramachandran plot）对模型进行了验证。本研究利用Biovia Discovery Studio 2018软件开展分子对接实验，以预测表位与人IgG、人IgM的Fab段以及T细胞受体（TCR, αβ）的相互作用情况。结果显示，1个预测得到的B细胞表位可与人IgG形成理想的结合相互作用；另有4个预测B细胞表位与T细胞表位，可分别与IgM及TCR产生理想的结合相互作用。对分子对接得到的复合物开展分子动力学模拟实验，结果显示复合物能量已实现最小化，表明其相互作用具有稳定性。对全部模拟复合物的结构分析表明，除1个表位-IgM复合物外，其余复合物均呈现稳定的结构特性。此外，对8个受体-配体复合物的结合自由能计算结果证实，这些复合物在能量层面均处于稳定状态。本研究结果有助于阐明Lsa46的结构与功能特性，为基于表位的疫苗研发提供理论支撑。本文由Ramaswamy H. Sarma转交刊发。