gp120 Immunogen Design And Characterization

Abstract

HIV-1 is the causative agent for AIDS and has been a major focus of research for the past two decades. Though there is a combination therapy in place known as the “Highly Active Anti-Retroviral Therapy” (HAART), its usefulness is confounded by the generation of escape mutants, a host of side effects, and its prohibitive cost. The most useful alternative would be the prevention of infection by vaccination. Vaccine research has been focused on the use of recombinant protein sub-units of the virus or combinations thereof to elicit a neutralizing response against the virus. These approaches have mostly resulted in a failure to generate broadly cross reactive neutralizing response against primary strains of the virus. The work reported herein is aimed at designing a rigidified version of gp120/gp120 derivatives and understanding the scope of the various antigenic regions in gp120 in generating a neutralization response. Chapter one discusses some general features of the virus and the immune system. The general nature of AIDS, its spread and its immunological characteristics are also described in this chapter. Chapter two discusses the design and NMR structural analysis of gp120 bridging sheet peptide mimics in methanol and water. The structure of gp120 can be loosely divided into two domains (the outer domain and the inner domain) that are linked together by a discontinuous four stranded antiparallel beta sheet known as the bridging sheet. The bridging sheet is known to overlap with the coreceptor binding site of gp120 and hence is a suitable target for designing virus-entry inhibitors. 17b, a neutralizing antibody isolated from an infected individual, is known to bind to this region of gp120. Our aim in this part of the work was to design a four stranded antiparallel beta sheet, based on the sequence of the bridging sheet, that would contain most of the residues involved in 17b binding. NMR and CD studies confirmed that the peptide was well structured in methanol but the structure was largely lost on addition of aqueous solvent. A small population of the peptide was found to be well-folded in aqueous solution. Chapter three discusses the design and characterization of a gp120-CD4D12 single chain. It is well known that the conformation of gp120 changes upon binding CD4 to expose cryptic epitopes, known as CD4i epitopes. In this work we report the generation of a single chain gp120-CD4 construct that has the cryptic epitopes exposed. The construct bound to 17b, a conformation specific antibody against the bridging sheet of gp120, a cryptic epitope, as well as a non-covalent complex of gp120:CD4D12. There was also very insignificant secondary structural change in gp120 upon complex formation with CD4D12 as observed by CD spectroscopy. Immunological studies with DNA and protein vaccination in guinea-pigs indicated that though 17b like antibodies are generated after immunization, they did not contribute towards the neutralization of primary isolates of the virus. It was also observed that it was the anti-CD4D12 antibodies that were responsible for the neutralization by the sera. These studies indicated towards the inability of the bridging sheet to generate effective neutralization response in case of vaccination with gp120/CD4 complexes. Chapter four discusses the design of a mimic of the gp120/CD4 complex. Since it was seen from our previous work that gp120/CD4 complexes generate a large fraction of antiCD4 antibodies and hence are unsuitable for vaccination purposes, we generated a construct with the minimal binding region of CD4. The small fragment of CD4 spanning from 21st residue to 64th residue was inserted in the V1/V2 loop of gp120. The insertion site was designed based on the region of gp120 closest to this fragment and capable of tolerating insertions. This protein did not bind to 17b as well as gp120/CD4 complex but showed a higher binding compared to full length gp120. Further immunological characterization with this protein revealed that it was not capable of generating neutralizing antibodies against the virus. Chapter five discusses the design and execution of a SPR based solution phase competition experiment to find the solution phase binding constant of CD4 and CD4 analogs to gp120. A major problem during the analysis of binding data obtained by SPR is the accurate determination of Rmax, a parameter needed to obtain an accurate equilibrium dissociation constant. In this chapter we have developed a binary as well as a ternary solution phase SPR based assay to accurately determine a solution phase equilibrium binding constant. The binding constants were determined for gp120 binding to CD4D12 and other CD4 analogs. To confirm the validity of the assay, a control antigen:antibody interaction whose equilibrium dissociation constant has been determined by other methods has been used as a test case. Chapter six discusses the design and characterization of V3 peptides inserted in the loop regions of E. coli Thioredoxin (Trx). Trx has earlier been used to display random peptide libraries between the 33rd and the 34th residue. We have constructed three constructs where the peptide has been inserted between the 33rd and 34th residue, between the 74th and 75th residue and between the 84th and 85th residue. The insertion between 74th and 75th position (74V3Trx) was found to be superior to the other two and would be a suitable alternative for display of a random peptide library. The binding of these constructs to 447-52D, a V3 peptide specific antibody was characterized. These were also characterized immunologically, and 74V3Trx was found to generate weakly neutralizing activity against the MN strain of HIV-1. Competition experiments with 447-52D with these sera indicated that there were antibodies generated that could compete out 447-52D binding to gp120 but not in sufficient concentration to provide broad neutralization. Appendix 1 discusses the rational design of disulfides to stabilize proteins based on the analysis of naturally occurring disulfides. In our attempts to design a rigidified version of gp120 we had designed disulfides in gp120 based on its crystal structure. Many of these were disulfides that would span antiparallel adjacent strands. In order to improve the design principles, we analyzed naturally occurring disulfides that span antiparallel adjacent strands and characterized them in terms of their positional preference in a beta sheet. It was found that these disulfides mostly occur on edge strands and are found exclusively between non-hydrogen bonded registered pairs of adjacent antiparallel strands. Mutagenesis on Thioredoxin was performed to verify our results. It was found that disulfides designed between the non-hydrogen bonded pairs of antiparallel strands could significantly stabilize the protein whereas the ones between hydrogen bonded pairs destabilized the protein.