Binding Antibodies, nonneutralizing antibodies (n-NAbs), and Neutralizing Antibodies (NAbs)

Last Monday, Moderna announced the positive results from its phase I clinical trial for its mRNA vaccine against Covid-19. The study was listed under clinicaltrials.gov as "Phase I, Open-Label, Dose-Ranging Study of the Safety and Immunogenicity of 2019-nCoV Vaccine (mRNA-1273) in Healthy Adults". This phase I study contains 9 cohorts with 3 different vaccine doses (25 mcg, 100 mcg, and 250 mcg) in three different age groups (18-55, 56-70, and 71 or above years of age). The announced results were from three dose groups in 18-55 years of age. The phase I study was the first step to test the safety and the immunogenicity (i.e. vaccine's ability to trigger an immune response in the body) of the vaccine, but the released preliminary results caused a strong reaction from the wall street.

The immunogenicity data indicated that the binding antibodies were detected in all 45 subjects in three dose groups in 18-55 years of age and the neutralizing antibodies were detected in all 8 subjects in the first four subjects in 25 mcg and 100 mcg groups. It is worth discussing what the difference is between the binding antibodies and the neutralizing antibodies.

Moderna Announces Positive Interim Phase 1 Data for its mRNA Vaccine (mRNA-1273) Against Novel Coronavirus

"Immunogenicity data are currently available for the 25 µg and 100 µg dose level (ages 18-55) after two doses (day 43) and at the 250 µg level (ages 18-55) after one dose (day 29). Dose dependent increases in immunogenicity were seen across the three dose levels, and between prime and boost within the 25 µg and 100 µg dose levels. All participants ages 18-55 (n=15 per cohort) across all three dose levels seroconverted by day 15 after a single dose. At day 43, two weeks following the second dose, at the 25 µg dose level (n=15), levels of binding antibodies were at the levels seen in convalescent sera (blood samples from people who have recovered from COVID-19) tested in the same assay. At day 43, at the 100 µg dose level (n=10), levels of binding antibodies significantly exceeded the levels seen in convalescent sera. Samples are not yet available for remaining participants.

At this time, neutralizing antibody data are available only for the first four participants in each of the 25 µg and 100 µg dose level cohorts. Consistent with the binding antibody data, mRNA-1273 vaccination elicited neutralizing antibodies in all eight of these participants, as measured by plaque reduction neutralization (PRNT) assays against live SARS-CoV-2. The levels of neutralizing antibodies at day 43 were at or above levels generally seen in convalescent sera."

Moderna reports positive data on early-stage coronavirus vaccine trial, shares surge

"Moderna’s closely watched early-stage human trial for a coronavirus vaccine produced Covid-19 antibodies in all 45 participants, the biotech company announced Monday, sending the company’s shares surging nearly 20%."

"The vaccine also produced neutralizing antibodies against Covid-19 in at least eight participants, the company said. Experts have said neutralizing antibodies appear to be important in acquiring protection."

Vaccines are examples of antigens in an immunogenic form, which are intentionally administered to a recipient (usually healthy volunteers) to induce the memory function of adaptive immune system toward the antigens of the pathogen invading that recipient. In Covid-19 situation, vaccines are developed against the SARS-Cov-2 (the pathogen (the virus) that caused Covid-19. In phase I studies, besides the safety, we want to see if administering the vaccines can generate the antibodies (first binding antibodies, then neutralizing antibodies). Phase I study can fail if there is a safety issue or if no or not enough antibodies are generated in recipients. 

Binding Antibodies: 
Not all antibodies that bind a pathogenic particle are neutralizing. Binding antibodies, or non-neutralizing antibodies (n-NAb), bind specifically to the pathogen (SARS-Cov-2 virus), but do not interfere with their infectivity. that might be because they do not bind to the right region. Non-neutralizing antibodies can be important to flag the particle for immune cells, signaling that it has been targeted, after which the particle is processed and consequently destroyed by recruited immune cells.

Binding antibodies are produced at high levels throughout the life of an infected individual but are characterized by their inability to prevent viral infection. Even though they do not have impact on virus' infectivity, binding antibodies are useful as diagnostic indicators of whether an individual is infected or not.

Neutralizing Antibodies (NAb):
Neutralizing antibodies on the other hand can neutralize the biological effects of the antigen without a need for immune cells. In some cases, non-neutralizing antibodies or insufficient amounts of neutralizing antibodies binding to virus particles can be utilized by some virus species to facilitate uptake into their host cells.

Binding antibodies and neutralizing antibodies are similarly discussed in therapeutic protein products or biological products including monoclonal antibody (mAb) drug. In vaccine studies, immunogenicity is a good thing and is what we want to see. In biological products (protein therapy or mAb), the immunogenicity is a thing we try to avoid - developing the anti-drug antibody will have impact on the efficacy and safety of a biological product or mAb.

Binding antibodies and neutralizing antibodies are measured with different assays (i.e., binding antibody assay and neutralization assay). According to FDA guidance "Immunogenicity Testing of Therapeutic Protein Products — Developing and Validating Assays for Anti-Drug Antibody (ADA) Detection"

"Screening assays, also known as binding antibody assays, are used to detect antibodies that bind to the therapeutic protein product. The specificity of ADA for the therapeutic protein product is usually established by competition with a therapeutic protein in a confirmatory assay. ADAs are characterized further using titration and neutralization assays. Titration assays characterize the magnitude of the ADA response. It is important to characterize this magnitude with titration assays because the impact of ADA on pharmacokinetics, pharmacodynamics, safety, and efficacy may correlate with ADA titer and persistence rather than incidence (Cohen and Rivera 2010). Neutralizing antibodies (NAbs) refer to those ADA with the ability to interfere with interactions between the therapeutic protein product and its target. Neutralization assays assess ADA for neutralizing activity. It is important to characterize neutralizing activity of ADA because the impact of ADA on pharmacokinetics, pharmacodynamics, safety, and efficacy may correlate with NAb activity rather than ADA incidence"