Since the discovery of antibodies found to neutralize diverse strains of HIV (known as broadly neutralizing antibodies, bNAbs) vaccine researchers have been trying to trigger an immune response activating these antibodies in naïve individuals. However, in infected individuals bNAbs only develop after many years of chronic infection and only in 10-20% of infected people. This suggests that bNABS only arise after many rounds of somatic hypermutation, a process of affinity maturation through rounds of mutation. This process is very difficult to replicate and may partly explain why current vaccines do not lead to bNAb generation. This apparent lack of sufficient stimulation is studied in a paper by doctors Christina Yacoob and Marie Pancera in the Stamatatos lab (Vaccine and Infectious Disease Division) published in Cell Reports. In the study the researchers focused on VRCO1-class antibodies. This bNAb targets a conserved region on the HIV envelope (Env) allowing it to recognize various HIV strains. Twenty-nine antibodies make up the VRCO1-class and all originate from the same heavy chain variable region (VH), VH1-2*02 allele which is paired with one of four light chains. Another characteristic of VRCO1-class antibodies is their short CDRH3 regions, the hypervariable region that interacts with the HIV epitope. This region is part of the varaiable region and is also under selective pressure to change during affinity maturation. Mature forms of VRCO1 have been found in infected patients and are what researchers would like to trigger using vaccination, however mature antibodies only arise after stimulation and maturation. Therefore, vaccines need to stimulate bNAbs precursor, known as germline antibodies, which after stimulation mature into bNAbs. Unfortunately, the field does not know what these precursors are. The best that can be done is to infer a “germline” like sequence from the mature antibody using the known heavy and light chains, however the CDRH3 germline region, which is important for binding, cannot be inferred because it changes as the B cell developments and thus the mature sequence is used. When comparing binding of the inferred germline antibody and mature antibody to Env the inferred antibodies do not bind naturally occurring Envs. This has caused labs to create modified Envs to try and target these germline bNAbs. Dr. Yacoob stated, “this raises the question of accuracy of our HIV Env immunogens to elicit the correct precursor antibodies. Should we redesign our immunogens to improve binding to these “real” naïve antibodies? Should we redesign them so they can bind precursors with ‘real’ naïve CDRH3 region?”
In order to answer these questions, the lab deep sequenced nine uninfected individuals looking at B cell VH allele usage, specifically at VH1-2 usage for potentially VRCO1-class precursors. They found that seven of the nine genomes studied contained VH1-2 and of that most were dominated by VH1-2*02. They also checked the sequences for known residues that are thought to be important for Env binding. To this end the group expressed 67 antibodies from the sequences. In order to create antibodies heavy chains subunits were mixed with one of the four light chains found in VRCO1-class mature antibodies. The 67 antibodies created were tested against natural Env as well as Env modified to bind predicted “germline” like antibodies. Of the antibodies tested 19 bound to the modified Envs and none to the mature Envs. This suggests that modified Envs may be needed in a vaccine to elicit germline antibodies. By looking at binding, sequence information, and structure the group determined that the CDRH3 domain of the germline antibody is important for Env binding. They also showed that not all individuals have the allele used in VRCO1-class antibodies and thus may need a vaccine that targets the elicitation of a different bNAb. Overall the study helps better understand the elements needed for inducing VRCO1-class bNAbs, contributing to the design of more optimal immunogens.
Image provided by Dr. Pancera.
This work was funded through grants from the National Institutes of Health.
Yacoob C,Pancera M,Vigdorovich V,Oliver BG,Glenn JA,Feng J,Sather DN,McGuire AT,Stamatatos L. (2016). Differences in Allelic Frequency and CDRH3 Region Limit the Engagement of HIV Env Immunogens by Putative VRC01 Neutralizing Antibody Precursors. Cell Rep, 17(6), 1560-1570.