Research Highlights

Scientists determine way to increase number of spikes on HIV virion as first step toward development of an effective vaccine

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Computational models of virions were generated using cellPACK. From left to right, the five models illustrate a random distribution of an increasing number of spikes: 10 (average number of spikes for HIV-1 virions), 49 (average number of spikes for V1 hVLPs), 127 (average number of spikes for V4 hVLPs), 214 (highest number of spikes estimated from an EM image of a single V4 hVLP), and 330 (theoretical maximum number of spikes determined to fit on the same size of particle).

HIV-1 is rare among viruses because of the low number of spikes (potential docking stations) on the envelope that covers the HIV virion. This number is typically in the range of 7-14. It contrasts with the much higher number of spikes on other viruses like influenza that features 400-500. Some viruses feature as many as 1,200.

The consequences and underlying basis for the low number of HIV spikes are not fully understood, but this feature has been associated with HIV’s ability to avoid detection and effective response by the immune system.

Scientists hypothesize that increasing the number of spikes could make it possible to develop an anti-HIV vaccine. But the methods tried so far typically require protein engineering that alters the structure of the spikes, potentially compromising the ability of protective antibodies to bind.

In this context, The Scripps Research Institute (TSRI) team hypothesized that impediments to increase the density of the spikes in previous experiments may be intrinsic to the cells that produce HIV. So they screened for cells able to produce an increased number of spikes on the cell surface.

More specifically, they applied a strategy called fluorescence-activated cell sorting (FACS) to sort for cells that stably express HIV spikes. The cells were screened on the basis of high binding to broadly neutralizing antibodies (bnAbs, that neutralize multiple HIV-1 viral strains) and low binding by nonneutralizing antibodies (that are not able to neutralize HIV) to select for a cell phenotype with high levels of bnAb binding and low levels of non-nAb binding. (bnAbs represent an important area of research with respect to development of vaccines against HIV and other rapidly mutating viruses like influenza.)

Four iterations of this sorting mechanism were run. The resulting cells and their progeny displayed increasingly greater numbers of antigenically correct spikes, on average a 10-fold increase in well-defined, functional spikes per HIV particle. Remarkably, using electron microscopy of virus particles and modeling by the NBCR team, the researchers found an average of >120 spikes per viron. This discovery was confirmed by biochemical methods.

More significantly, these spikes were shown to be superior in producing the desired response: activating B cells, highly specialized “defender cells” that, as part of the adaptive immune system, secrete antibodies against the HIV “invader.”

Study first author Armando Stano, a research associate at TSRI, says, “Our work enables new opportunities for vaccine design that were previously impractical using the membrane-bound envelope.”

NBCR member Arthur Olson said, “In this work, our CellPACK software demonstrated its utility in interpreting and analyzing the distribution of membrane-bound molecular species from low-resolution electron micrographs and points the way to a new approach to mining more data from these types of images.” Olson is a professor of Integrative Structure and Computational Biology at TSRI.

The so-called high-Env virus-like particles (hVLPs) produced by the sorting mechanism showed greater infectivity than standard HIV particles but a similar sensitivity to neutralization. Hence, they represent an adaptable platform to facilitate further study of HIV spikes, the use of these spikes as immunogens (substances capable of inducing an immune response),and development of new vaccines against HIV.

Funding Source:

National Institutes of Health (NIAID) R01s AI098602 and AI114401 (M.B.Z.), P01 AI104722 (R.T.W.), and P41GM103426-23 (A.J.O.) and the James B. Pendleton Charitable Trust (M.B.Z.).

Relevant Publication:

Stano A, Leaman DP, Kim AS, Zhang L, Autin L, Ingale J, Gift SK, Truong J, Wyatt RT, Olson AJ, Zwick MB. 2017. Dense array of spikes on HIV-1 virion particles. J Virol 91:e00415-17. https://doi.org/10.1128/JVI.00415-17.