IgM antibodies curb Zika and dengue enhancement in laboratory studies

A recombinant IgM monoclonal antibody reduced antibody-dependent enhancement (ADE) of Zika virus infection across a panel of IgG antibodies in laboratory assays, including at concentrations below those required for viral neutralization. A second pentameric IgM antibody produced a similar, though less uniform, effect against Zika virus and all four dengue virus serotypes. The findings indicate that reducing IgG-driven enhancement may be a function of IgM antibodies that is partly distinct from neutralization, but the experiments did not test protection in animals or people.

ADE occurs when antibodies increase rather than reduce viral infection by promoting uptake of virus particles into Fc-gamma receptor-bearing cells. The phenomenon is well documented in dengue and has been implicated in severe disease during secondary infections. It has also been observed in laboratory studies of Zika virus, whose E-glycoprotein antibodies can enhance infection of monocytes, macrophages and other cells. The clinical importance of enhancement during Zika infection remains uncertain, although evidence from pregnant non-human primates, mice and ex vivo human placentas has led researchers to consider whether it could contribute to placental infection and vertical transmission.

For the current study, investigators used DH1017.IgM, a native pentameric IgM antibody isolated 71 days after symptom onset from a pregnant woman who had Zika infection and subsequently delivered a healthy baby. The antibody recognizes a discontinuous quaternary epitope on the Zika virus envelope E-protein dimer, spanning domain II and the interface between domains I and II. Its potent neutralization depends on its multimeric structure. Researchers proposed two possible binding arrangements: an umbrella-like configuration in which the pentamer contacts several E dimers on one virion, and a planar configuration in which it cross-links multiple virions. Earlier work estimated that only three such IgM molecules could bind simultaneously to one virion in the umbrella-like arrangement, leaving part of the recognized epitope space available to other antibodies.

The researchers first examined whether IgG monoclonal antibodies and DH1017.IgM would compete for binding to intact Zika virus particles. A panel of 14 characterized IgG antibodies targeted several regions of the envelope protein, including domain I, domain II, domain III, the fusion loop, the domain I/III linker and the E-dimer epitope 1. Two antibodies were excluded from reciprocal competition assays because they could not compete with their own labeled versions. The competition was incomplete in both directions. Some IgG antibodies displaced DH1017.IgM, while DH1017.IgM efficiently outcompeted several others, but even at saturating concentrations it generally did not eliminate IgG binding. Six of the 12 antibodies tested reduced DH1017.IgM binding by more than 75%, whereas only three blocked more than 75% of IgG binding with DH1017.IgM. This partial overlap is consistent with the large footprint of multivalent IgM and suggests that it can alter how many IgG molecules bind a virion without occupying every potentially accessible epitope.

These binding effects did not come at the expense of neutralization. When DH1017.IgM was combined with neutralizing or non-neutralizing Zika IgG antibodies in assays using GFP-expressing Zika reporter virus particles, its expected neutralizing activity remained intact. Combinations of DH1017.IgM with neutralizing IgG antibodies also tracked the increasing neutralization produced by the IgG alone. A control IgM antibody that did not recognize Zika did not alter the IgG neutralization profiles. The results therefore indicate that the cross-competition between IgM and IgG did not produce an obvious detrimental effect on either antibody class’s capacity to reduce viral infectivity.

The more pronounced effect appeared in assays of ADE, in which antibodies were tested with Fc-gamma receptor-expressing K562 cells. At a fixed concentration corresponding to its reporter-virus neutralization midpoint, DH1017.IgM lowered peak IgG-mediated enhancement by an average of 2.7-fold, with reductions ranging from 1.8- to 3.8-fold. It did not simply shift the concentration at which peak enhancement occurred. Reversing the assay design and holding each IgG antibody at its peak enhancement concentration showed that DH1017.IgM reduced enhancement in a dose-dependent manner. Concentrations of 100 nanograms per milliliter or lower completely abrogated enhancement across the tested antibodies, regardless of which envelope epitope they recognized. The concentration required to reduce enhancement by 50% had a mean of 2.1 nanograms per milliliter and ranged from 0.4 to 9.1 nanograms per milliliter.

Comparisons with neutralization potency showed that the enhancement-reducing action was not simply a consequence of virus neutralization. For most antibodies tested, DH1017.IgM reduced enhancement at sub-neutralizing concentrations, defined in the study by a ratio of the 50% enhancement-reduction concentration to the 50% reporter-virus neutralization concentration of no more than 0.75. Ratios ranged from 0.18 for the E-dimer epitope 1 antibodies C8 and C10 to 0.68 for the domain II antibody MZ20. The fusion-loop antibody 2A10G6 narrowly missed the threshold at 0.78 and showed substantial variation between replicates. A second recombinant IgM, C8.IgM, also reduced IgG-driven Zika enhancement in a dose-dependent fashion and did so at sub-neutralizing concentrations for six of 11 tested antibodies. Its profile differed from that of DH1017.IgM, showing that the effect depends on the particular IgM–IgG pairing rather than being a uniform property of all IgM antibodies.

Because C8 also recognizes dengue virus, the investigators extended its testing to dengue reporter virus particles representing serotypes 1 through 4. C8.IgM reduced enhancement mediated by six tested IgG antibodies for every dengue serotype, although the potency varied with both the IgG antibody and the serotype. It reduced enhancement at sub-neutralizing concentrations in 16 of 24 antibody–serotype combinations. This pattern was consistent for three antibodies—MZ20, C10 and the IgG version of C8—across all four serotypes. The antibody was generally less effective against IgG antibodies targeting the fusion loop. For example, the enhancement-reduction to neutralization ratio for 4G2 ranged from 1.2 for dengue 1 to 5.11 for dengue 2, indicating that concentrations sufficient to neutralize the virus were required to substantially reduce enhancement in those combinations.

DH1017.IgM was also tested against unfractionated sera from a controlled human Zika infection study. Participants received either the SJRP/2016-184 or Nicaragua/2016 Zika strain, with nine and 10 infected participants respectively, while seven additional individuals received placebo. At 90 days after infection, sera from all 19 infected participants produced measurable enhancement of a heterologous Zika strain in vitro, whereas placebo sera did not. Adding DH1017.IgM reduced serum-mediated enhancement in a dose-dependent manner. The mean concentration required for 50% reduction was 6.2 nanograms per milliliter for the SJRP cohort and 6.0 nanograms per milliliter for the Nicaragua cohort, with no statistically significant difference between groups. DH1017.IgM reduced enhancement at sub-neutralizing concentrations in 16 of 19 sera, or 84.2% of the tested samples.

The authors interpret the findings as evidence that IgM antibodies can mitigate IgG-mediated ADE through mechanisms that are probably multifactorial. Altered antibody-binding stoichiometry may contribute, but the strongest reductions also occurred with IgG antibodies whose epitopes were not efficiently displaced by DH1017.IgM, implying that additional mechanisms remain to be defined. The results support further investigation of multivalent IgM biologics for Zika and dengue, including possible effects on placental infection and vertical transmission. They do not establish clinical benefit. All experiments were conducted in vitro or ex vivo, without animal or human efficacy testing, and the activity of DH1017.IgM will be difficult to separate from its exceptionally potent neutralization in vivo. The work therefore identifies a measurable antibody function and a basis for further study, rather than demonstrating that an IgM treatment can prevent severe dengue, Zika disease or fetal harm.

Subject of Research: In vitro reduction of IgG-mediated antibody-dependent enhancement of Zika and dengue virus infection by IgM monoclonal antibodies

Article Title: IgM monoclonal antibodies reduce IgG-mediated antibody-dependent enhancement of Zika and dengue virus infectivity

Article References: Lingemann, M., McGee, T. J., Sidoti Migliore, G., DeLuca, J. M., Whitehead, S. S., & Bonsignori, M. (2026). IgM monoclonal antibodies reduce IgG-mediated antibody-dependent enhancement of Zika and dengue virus infectivity. iScience, 29(11), Article 117897. https://doi.org/10.1016/j.isci.2026.117897

Image Credits: AI Generated

DOI: 10.1016/j.isci.2026.117897

Keywords: Zika virus, dengue virus, IgM antibodies, IgG antibodies, antibody-dependent enhancement, viral neutralization, flaviviruses, immune enhancement, antiviral biologics, monoclonal, antibodies, reduce