Image provided by Ms. Cassandra Simonich.
The elusive search for a HIV-1 vaccine continues in part due to gaps in our understanding of how HIV-1 neutralizing antibodies are generated by our immune system. Several studies have reported the isolation of broadly neutralizing antibodies (bnAbs) from HIV-infected adults yet these bnAbs have several characteristics that would be difficult to mimic in a vaccine. These include high levels of somatic hypermutation and insertions and deletions, processes that normally occur to increase the diversity of antibodies we generate. Because of this long maturation process, adult bnAbs have been isolated from two to fifteen years post-infection. In contrast, a previous study from the Fred Hutch Laboratory of Dr. Julie Overbaugh (Public Health Sciences and Human Biology Divisions), showed that a subset of infants enrolled in the Nairobi Breastfeeding Clinical Trial produced plasma antibody responses that could neutralize several types of HIV-1 viruses, with these responses developing as early as one to two years post-infection. However, the number and types of infant antibodies that were responsible for plasma breadth and potency were unknown. In a follow-up study led by graduate student Cassandra Simonich and published in Cell, the Overbaugh Lab isolated and characterized ten neutralizing Abs from one infant that contributed to the previously observed plasma neutralization activity. "Our lab had previously discovered that infants can also develop broadly neutralizing plasma responses and that they do so relatively early compared to adults, but we didn’t know anything about the antibodies contributing to plasma activity. As far as we knew, no one had isolated HIV neutralizing antibodies from infants. Because these responses developed over a shorter period of time, we thought studying infant antibodies might provide clues for vaccine strategies to elicit neutralizing antibodies more quickly", said Ms. Simonich.
The study began by confirming that one infant (BF520) from the Nairobi trial (conducted in the 1990s before antiviral therapy was routinely used to prevent mother-to-child transmission) was HIV-negative at eight days of age but was HIV-positive at 3.8 months. Using a robotic system to isolate and culture memory B cells, the authors isolated ten (BF520.1-BF520.10) antibodies with HIV neutralizing activity, with one in particular (BF520.1) contributing to the bulk of plasma activity. Next, the researchers compared BF520.1 with adult bnAbs and found that BF520.1 exhibited neutralization breadth that was similar to second-generation adult bnAbs but with slightly lower potency. A combination of epitope mapping and electron microscopy experiments revealed that BF520.1 targeted the N332 glycan on the HIV-1 Envelope (Env) trimer, similar to the PGT class of adult bnAbs. Further insight into infant bnAbs was gained using flow cytometry and biolayer interferometry to determine that BF520 antibodies bound to autologous Env variants (same variant as the one present in infant BF520) yet failed to neutralize them. In contrast, BF520.1 both bound and neutralized heterologous Env variants.
Finally, the scientists examined the levels of somatic hypermutation of infant-derived nAbs at the nucleotide (nt) level, with the infant nAbs presenting a much lower level of somatic hypermutation (2.0%-6.6% nt) than their adult counterparts (3.8%-32.6% nt). In summary, by successfully isolating bnAbs from an infant, this remarkable study demonstrated the HIV-1 neutralization breadth can occur without either extensive somatic hypermutation or long-term affinity maturation and may provide a new route to HIV-1-specific nAbs. Said Ms Simonich, "We are excited to further investigate antibody responses to HIV-1 in infants because we still have a lot to learn. We know that adult broadly neutralizing antibody responses are shaped by the dynamic interaction between the evolving virus and developing antibodies. We have samples from prior to and throughout infection, so we are looking at changes in the antibodies and the virus over time to better understand how these infant antibodies developed. We are also isolating and characterizing antibodies from another infant who developed an interesting plasma response. The envelope protein of the transmitted virus from this infant has been extensively studied by the field, so understanding the antibodies that developed in response to that virus will be very interesting. We hope that understanding the development of broadly neutralizing antibodies in infants will help guide vaccine design".
Simonich CA, Williams KL, Verkerke HP, Williams JA, Nduati R, Lee KK, Overbaugh J. 2016. HIV-1 neutralizing antibodies with limited hypermutation from an infant. Cell 166(1): 77-87.
Funding for this work was provided by the National Institutes of Health.