Elicitation of potent serum neutralizing antibody responses in rabbits by immunization with an HIV-1 clade C trimeric Env derived from an Indian elite neutralizer
Authors: Kumar Rajesh , Deshpande Suprit , Sewall Leigh M. , Ozorowski Gabriel , Cottrell Christopher A. , Lee Wen-Hsin , Holden Lauren G. , Richey Sara T. , Chandrawacar Antra Singh , Dhiman Kanika , Ashish  , Kumar Vivek , Ahmed Shubbir , Hingankar Nitin , Kumar Naresh , Murugavel Kailapuri G , Srikrishnan Aylur K , Sok Devin , Ward Andrew B. , Bhattacharya Jayanta 
Published: September 2020

Evaluating the structure-function relationship of viral envelope (Env) evolution and the development of broadly cross-neutralizing antibodies (bnAbs) in natural infection can inform rational immunogen design. In the present study, we examined the magnitude and specificity of autologous neutralizing antibodies induced in rabbits by a novel HIV-1 clade C Env protein (1PGE-THIVC)vis-à-visthose developed in an elite neutralizer from whom theenvsequence was obtained that was used to prepare the soluble Env protein. The thermostable 1PGE-THIVC Env displayed a native like pre-fusion closed conformation in solution as determined by small angle X-ray scattering (SAXS) and negative stain electron microscopy (EM). This closed spike conformation of 1PGE-THIVC Env trimers was correlated with weak or undetectable binding of non-neutralizing monoclonal antibodies (mAbs) compared to neutralizing mAbs. Furthermore, 1PGE-THIVC SOSIP induced potent neutralizing antibodies in rabbits to autologous virus variants. The autologous neutralizing antibody specificity induced in rabbits by 1PGE-THIVC was mapped to the C3/V4 region (T362/P401) of viral Env. This observation agreed with electron microscopy polyclonal epitope mapping (EMPEM) of the Env trimer complexed with IgG Fab prepared from the immunized rabbit sera. While the specificity of antibodies elicited in rabbits associated with neutralizing autologous viruses were distinct to those developed in the elite neutralizer, EMPEM analysis demonstrated significant changes to Env conformations when incubated with polyclonal antibody sera from the elite neutralizer, suggesting these antibodies lead to the destabilization of Env trimers. Our study not only shows distinct mechanisms associated with potent neutralization of sequence matched and unmatched autologous viruses by antibodies induced in rabbits and in the elite neutralizer, but also highlights how neutralizing antibodies developed during the course of natural infection can impact viral Env conformations.

Author Summary

The interplay between circulating virus variants and broadly cross neutralizing polyclonal antibodies developed in a subset of elite neutralizers is widely believed to provide strategies for rational immunogen design. In the present study, we studied the structural, antigenic and immunogenic properties of a thermostable soluble trimeric protein with near native pre-fusion conformation prepared using the primary sequence of an HIV-1 clade Cenvisolated from the broadly cross neutralizing plasma of an elite neutralizer. This novel SOSIP Env trimer demonstrated comparable antigenic, structural and immunogenic properties that favoured several ongoing subunit vaccine design efforts. The novel clade C SOSIP induced polyclonal neutralizing antibody response developed in rabbits not only differed in its epitope specificity compared to that elicited in natural infection in presence of pool of viral quasispecies but also showed how they differ in their ability to influence Env structure and conformation. A better understanding of how vaccine-induced polyclonal neutralizing antibody responses compares to responses that developed in natural infection will improve our knowledge in designing better vaccine design strategies.