Qin Yali, Shi Heliang, Banerjee Saikat, Agrawal Aditi, Banasik Marisa, Cho Michael W
Department of Biomedical Sciences, College of Veterinary Medicine, Iowa State University, 1600 S 16th Street, Ames, IA, 50011-1250, USA.
Center for Advanced Host Defenses, Immunobiotics and Translational Comparative Medicine, Iowa State University, Ames, IA, 50011, USA.
Retrovirology. 2014 Dec 20;11:125. doi: 10.1186/s12977-014-0125-5.
We recently reported induction of broadly neutralizing antibodies (bnAbs) against multiple HIV-1 (human immunodeficiency virus type 1) isolates in rabbits, albeit weak against tier 2 viruses, using a monomeric gp120 derived from an M group consensus sequence (MCON6). To better understand the nature of the neutralizing activity, detailed characterization of immunological properties of the protein was performed. Immunogenic linear epitopes were identified during the course of immunization, and spatial distribution of these epitopes was determined. Subdomain antibody target analyses were done using the gp120 outer domain (gp120-OD) and eOD-GT6, a protein based on a heterologous sequence. In addition, refined epitope mapping analyses were done by competition assays using several nAbs with known epitopes.
Based on linear epitope mapping analyses, the V3 loop was most immunogenic, followed by C1 and C5 regions. The V1/V2 loop was surprisingly non-immunogenic. Many immunogenic epitopes were clustered together even when they were distantly separated in primary sequence, suggesting the presence of immunogenic hotspots on the protein surface. Although substantial antibody responses were directed against the outer domain, only about 0.1% of the antibodies bound eOD-GT6. Albeit weak, antibodies against peptides that corresponded to a part of the bnAb VRC01 binding site were detected. Although gp120-induced antibodies could not block VRC01 binding to eOD-GT6, they were able to inhibit VRC01 binding to both gp120 and trimeric BG505 SOSIP gp140. The immune sera also efficiently competed with CD4-IgG2, as well as nAbs 447-52D, PGT121 and PGT126, in binding to gp120.
The results suggest that some antibodies that bind at or near known bnAb epitopes could be partly responsible for the breadth of neutralizing activity induced by gp120 in our study. Immunization strategies that enhance induction of these antibodies relative to others (e.g. V3 loop), and increase their affinity, could improve protective efficacy of an HIV-1 vaccine.
我们最近报道，使用源自M组共有序列（MCON6）的单体gp120，在兔体内诱导出了针对多种HIV-1（人类免疫缺陷病毒1型）毒株的广泛中和抗体（bnAbs），尽管对2级病毒的中和作用较弱。为了更好地理解中和活性的本质，我们对该蛋白的免疫学特性进行了详细表征。在免疫过程中鉴定出了免疫原性线性表位，并确定了这些表位的空间分布。使用gp120外结构域（gp120-OD）和基于异源序列的蛋白eOD-GT6进行了亚结构域抗体靶点分析。此外，通过使用几种具有已知表位的单克隆中和抗体（nAbs）进行竞争试验，进行了精细的表位定位分析。
基于线性表位定位分析，V3环最具免疫原性，其次是C1和C5区域。V1/V2环出人意料地缺乏免疫原性。许多免疫原性表位即使在一级序列中相距甚远，也聚集在一起，这表明蛋白表面存在免疫原性热点。尽管有大量抗体反应针对外结构域，但只有约0.1%的抗体与eOD-GT6结合。虽然检测到了针对与bnAb VRC01结合位点一部分相对应的肽段的抗体，但其活性较弱。尽管gp120诱导的抗体不能阻断VRC01与eOD-GT6的结合，但它们能够抑制VRC01与gp120和三聚体BG505 SOSIP gp140的结合。免疫血清在与gp120结合时，也能有效地与CD4-IgG2以及nAbs 447-52D、PGT121和PGT126竞争。
结果表明，在我们的研究中，一些在已知bnAb表位或其附近结合的抗体可能部分导致了gp120诱导的中和活性的广度。相对于其他抗体（如V3环抗体），增强这些抗体诱导并提高其亲和力的免疫策略，可能会提高HIV-1疫苗的保护效果。
