A single, continuous metric to define tiered serum neutralization potency against HIV

Congratulations, we are pleased to inform you that your article, "A Single, Continuous Metric to Define Tiered Serum Neutralization Potency against HIV", has been accepted for publication in eLife.

This well written manuscript from Hraber and colleagues describes a novel method for applying a single score to quantify polyclonal antibody responses directed to HIV Envelope that result in blocking of virus in vivo, called virus neutralization. This approach of classifying HIV-positive serum samples is analogous to what has been done for viral isolates. Neutralization results from the combined activities of multiple single monoclonals in a given individual, each of which possesses potentially different affinities and activities. It has been a challenge for the vaccine field to evaluate the activity of antibodies in a consistent manner. The proposed approach will be extremely useful in providing an objective, quantitative, high throughput approach for evaluating the potential of vaccine regimens geared towards neutralizing antibodies and/or classifying patient plasma or antibody potency and breadth. Advantages of this approach include: factoring in the sensitivity of the variants; cutting down on the time and cost needed to perform these evaluations; and importantly enhance standardized comparisons between studies.

The statistical approach consists of cascading the neutralization sensitivity level of HIV-1 variants to generate a neutralization index score (NI) to report a serum neutralization potency (NP) score from 1 to 4, with 1 being the most sensitive variants and scores > 2 being able to neutralize the more relevant patient derived variants. In summary, a vaccine approach that achieves a higher potency score would be considered more promising and worthy of further investigation, compared to those that elicited lower scores.

The authors provide a link to a tool that they will make available for calculating NP on the Los Alamos HIV Database website. This tool includes pre-calculated NI values for several hundred envelope variants that are widely available and would be commonly used by labs that do this type of work. The acceptability of IC50 or binary data increases the utility of the tool.

The value of this approach cannot be underscored sufficiently, as it has been extraordinarily challenging to compare serum responses from infected humans, monkeys and other animals immunized with various vaccines or infected with SHIV (monkeys) or HIV (humans). At present, the evaluation of neutralizing antibodies requires that a set of virus isolates be tested individually, as the authors note, in a dilution series, in a cumbersome although painstakingly validated assay. Despite the use of common 'panels' of virus it is difficult to interpret hence the need for a simpler method to advance the field which these authors do. The proposed approach also works on smaller virus panels and may provide a higher-throughput and more cost-effective method for screening sera. Importantly, as part of the development of the methodology, they have studied the longitudinal responses in HIV-infected subjects (n=2 shown in the manuscript) to demonstrate how this value might be applied to understanding the changing landscape of the humoral immunity to the escape variants in vivo.

Given its significance to the field it is important to share these findings and methodology with the research community for further validation in different groups and settings.

The paper could be strengthened by: i) The application of a retrospective analyses to understand the immunity generated in humans and monkeys following vaccination, and to further correlate these findings with protection outcomes.

ii) Inclusion of data from a panel of patient serum/plasma samples that had been previously ranked for breadth and potency by other methods to see how this approach compares.

iii) Clarifying how this approach is better than simply using the geometric mean titer (GMT). Figure 5 shows very clearly that the NP and GMT track very closely so more discussion of the added value of using the NP is needed.

iv) Introduction, first paragraph: Suggest replacing "complicated" with "requires standardization".

v) Last line of the introductory paragraph: Since Env panels vary a lot between labs, the inclusion of the something to the effect that "serum breadth and potency therefore depend strongly on the Env panels used that vary markedly between studies” would make that sentence more complete.

vi) Second paragraph of the Introduction: Minimise the repetition in discussing that tier 2 viruses are more difficult to neutralize.

vii) Subsection “Case studies”, last paragraph: Make it clear that this data is using monoclonal antibodies.

viii) Clarify what a concentration series is?

ix) Figure 1: It would be helpful to show the NP scores in blocks a and b as is done for block d. Is the scale between NP 1, 2 and 3 not linear? The spacing between the numbers on the x-axis suggests they are not.

x) Figure 6: The labelling is confusing. The same symbol is used to identify individual antibodies as well as sets of antibodies.