Site-Resolved Partitioning of Neutralization and FcγR Activation on Herpesvirus gH Complexes Is Demonstrated Only for HCMV

Question: How do antibody responses against herpesvirus gH/gL envelope complexes such as EBV gH/gL and HSV gH/gL partition neutralizing versus FcγR-activating functions across distinct antigenic sites?

Answer

Taken across the retrieved literature, the answer to this question is largely unbuilt for the two viruses named. Only one herpesvirus gH-associated complex — HCMV gH/UL116 — has been mapped at the resolution the question demands, and there the partition is clean and subunit-determined: gH binding is required for neutralization while UL116 targeting carries potent, broad FcγR activation ([5]). For HSV, the evidence is one level coarser: gH/gL-directed immunity protects disproportionately to its neutralization titer, working through fusion blockade that conventional neutralization assays do not score ([3]). Across these papers, the defensible position is that neutralization and FcγR activation are separable, site-determined properties rather than a single potency gradient — but the partitioning is documented on different axes in different herpesviruses, and no retrieved paper resolves a single HSV or EBV gH/gL antigenic site against both readouts simultaneously. Any assumption that EBV gH/gL or HSV gH/gL follows the HCMV gH/UL116 pattern is extrapolation, not evidence.

Neutralization and FcγR activation map to different subunits within a single gH-associated complex

The clearest demonstration of functional partitioning comes from human monoclonal antibodies isolated from HCMV-seropositive donors against the gH/UL116 complex, where epitope mapping assigned the two functions to different subunits: gH binding was required for virus neutralization, whereas UL116 targeting was associated with potent and broad Fcγ receptor activation ([5]). Cryo-EM structures of UL116-specific antibodies bound to gH/UL116 defined how that non-neutralizing, Fc-competent footprint is physically achieved ([5]). This is a stronger claim than mere dissociation: it locates the two functions on distinct, structurally resolvable surfaces of the same complex, and it implies that the neutralization potency of a polyclonal gH/UL116 response does not predict its FcγR-activating capacity. The same study showed that gH/UL116 immunization in mice elicited serum IgG whose binding titers correlated with both neutralization and mouse FcγR activation ([5]), which means the partition is visible at the monoclonal level but collapses into correlated bulk titers at the serum level — a caution for any serology-based inference of function.

Neutralizing activity within a gH/gL complex is epitope-restricted and non-additive, not graded by affinity

High-resolution cryo-EM of seventeen human monoclonal antibodies against HCMV gH and gL resolved a landscape of vulnerable epitopes with defined binding modes and spatial orientations, and these structural features tracked distinct neutralization phenotypes ([1]). Critically, single antibodies only partially neutralized HCMV, while synergistic combinations achieved substantially more complete inhibition ([1]). That combination-dependent ceiling argues that neutralization is distributed across non-equivalent sites whose individual occupancy is insufficient, rather than concentrated in one dominant site that scales with affinity. The functional correlate of a given gH/gL site is therefore partly a property of the site's position in the fusion-activation cascade: neutralizing antibodies potently block gH/gL/gO–PDGFR-regulated cell-cell fusion, and mutations in gL that leave complex formation and receptor binding intact still abolish fusion activation ([10]). Site assignment is also constrained by which subunit is presented in which complex — for the murine gammaherpesvirus MHV68, adsorption with either the full gH ectodomain or domain I alone produced comparable reductions in serum neutralization capacity, identifying the Eph-binding domain I as the major target of gH/gL-directed neutralizing antibodies ([8]).

For HSV, gH/gL-based protection is real but invisible to neutralization assays

Immunization with HSV-2 nanoparticles displaying gD together with gH/gL elicited high neutralizing antibody titers in mice and non-human primates, and gH/gL contributed substantially to protection despite lower neutralization titers than gD ([3]). The mechanism assigned was fusion-blocking rather than neutralization, and fusion-blocking activity was an important correlate of protection, leading the authors to conclude that standard neutralization assays incompletely capture fusion-blocking activity ([3]). This is the clearest retrieved statement that for an HSV gH/gL response, the conventional neutralizing/non-neutralizing dichotomy misclassifies functionally important antibodies. It is also a direct challenge to importing the HCMV gH/UL116 framework wholesale: the HCMV partition places non-neutralizing, FcγR-activating antibodies on a separate accessory subunit, while the HSV gH/gL result places protective activity outside neutralization but attributes it to fusion blockade, not to Fc effector recruitment ([5]; [3]).

Fc effector dependence is a separate axis from neutralization potency and from antigen conformation

For HSV gB-specific monoclonal antibodies, viral neutralization contributed to protection but Fc effector functions were critical for protection against neonatal HSV mortality, with the balance depending on dose ([2]). Antigen conformation can move Fc function while leaving neutralization untouched: prefusion-like and postfusion HCMV gB elicited comparable fibroblast neutralization titers against a heterologous strain, yet postfusion gB elicited significantly higher plasma IgG binding to cell-associated gB and higher antibody-dependent cellular phagocytosis ([9]). Because IgG binding to cell-associated antigen, not virion binding, is what drives ADCP, this result shows that an immunogen's conformational state can selectively tune the Fc arm without registering in a neutralization assay ([9]). Whether the same lever operates inside gH/gL complexes has not been tested in the retrieved set.

Herpesviral Fc receptors make apparent site-dependence of Fc function partly a property of the virus, not the antibody

HSV gE/gI forms a viral Fc receptor for most human IgG subclasses and allotypes, and a gD-specific human mAb engineered to lack gE binding exhibited enhanced neutralization and in vivo protection relative to its native IgG1 form, with the improvement dependent on the presence of the viral Fc receptor ([4]). Human IgG3 allotypes naturally lacking viral Fc receptor binding showed similarly enhanced antiviral activity in vivo ([4]). In HCMV, the conserved viral FcγRs gp34 and gp68 compete with host receptors for IgG1 Fc, and Fc variants retaining CD16A and FcRn binding while losing gp34/gp68 binding avoided internalization by infected cells and mediated enhanced CD16A activation and reduced viral spread ([6]). Consequently, a measured difference in FcγR activation between two antigenic sites can arise from differences in how the bound IgG-Fc is presented to viral versus host Fc receptors at that site's distance and geometry from the virion membrane, rather than from an intrinsic property of the epitope ([4]; [6]).

Conflicts and gaps

The retrieved corpus contains no antibody study of EBV gH/gL, EBV gH/gL/gp42, or EBV gp42 site-resolved function; EBV entries in this window concern latency, transformation, serology, and gB prefusion stabilization ([7]), not gH/gL antibody specificity. The question as posed about EBV cannot be answered from these papers. The HSV and HCMV literatures also disagree on where the non-neutralizing activity of a gH/gL response lives: FcγR activation on an accessory subunit in HCMV ([5]) versus fusion blockade with no Fc arm implicated in HSV ([3]). A further unresolved conflict is whether FcγR-activating antibodies must be non-neutralizing at all — the HCMV gB conformational comparison uncoupled the two ([9]), whereas the HSV gB panel found both contributing with dose-dependent weighting ([2]). Finally, no retrieved paper measures FcγR activation against virus-infected cells and against free virions for the same gH/gL-specific antibodies, so the target-membrane context of the partition is untested.

What would change the answer

Antigen-specific single B cell sorting from EBV- and HSV-seropositive donors against native gH/gL, gH/gL/gp42, and gH/gL/gO trimers, paired with LIBRA-seq, would generate the site-resolved monoclonal panels that currently exist only for HCMV ([5]). Running each antibody in parallel through neutralization, fusion-blocking ([3]; [10]), and FcγR reporter assays against both infected cells and virions would place neutralization and Fc activation on one coordinate system per epitope. Repeating those assays on viral Fc receptor-null HSV (gE/gI) and HCMV (gp34/gp68) backgrounds, or with Fc variants that cannot engage viral Fc receptors ([6]), would separate genuine epitope effects from evasion artifacts. EM-based polyclonal epitope mapping of polyclonal IgG bound to gH/gL complexes would test whether the subunit-level partition seen for gH/UL116 survives in a polyclonal response, and immunization with gH/gL nanoparticles formulated with Fc-domain variants would establish whether the Fc arm is causal for gH/gL-directed protection in vivo.

References

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[6] Qerqez, A. N., Hoffmann, K., Lee, A. G., Pareek, S., Hager, K. M., Mishra, A. K., Delidakis, G., Bentley, K., Kerr-Jones, L., Cabrera, M., Nguyen, T., Goettler, R. L., Chowdhury, A., Kolb, P., Hengel, H., Georgiou, G., McLellan, J. S., Stanton, R., Nguyen, A. W., Maynard, J. A.. "Selective decoupling of IgG1 binding to viral Fc receptorsrestores antibody-mediated NK cell activation against HCMV-infected cells". bioRxiv. 2025-04-18T00:00:00.000Z. URL: https://doi.org/10.1101/2025.04.12.647817

[7] McCallum, M., Veesler, D.. "Computational design of prefusion-stabilized Herpesvirus gB trimers". bioRxiv. 2024-10-24T00:00:00.000Z. URL: https://doi.org/10.1101/2024.10.23.619923

[8] Grosskopf, A. K., Tobiasson, V., Krug, L. T.. "Eph Receptor Tyrosine Kinases Are Functional Entry Receptors for Murine Gammaherpesvirus 68". bioRxiv. 2025-06-05T00:00:00.000Z. URL: https://doi.org/10.1101/2025.06.05.657996

[9] Karthigeyan, K. P., Connors, M., Binuya, C. R., Gross, M., Fuller, A. S., Crooks, C. M., Wang, H.-Y., Sponholtz, M. R., Byrne, P. O., Herbek, S., Andy, C., Gerber, L. M., Campbell, J. D., Williams, C. A., Miller, I., Yu, D., Bottomley, M. J., McLellan, J. S., Permar, S. R.. "A human cytomegalovirus prefusion-like glycoprotein B subunit vaccine elicits similar humoral immunity to that of postfusion gB in mice". bioRxiv. 2024-11-18T00:00:00.000Z. URL: https://doi.org/10.1101/2024.11.18.624140

[10] Schultz, E. P., Ponsness, L., Lanchy, J.-M., Zehner, M., Klein, F., Ryckman, B. J.. "Human cytomegalovirus gH/gL/gO binding to PDGFRα provides a regulatory signal activating the fusion protein gB that can be blocked by neutralizing antibodies.". bioRxiv. 2025-01-08T00:00:00.000Z. URL: https://doi.org/10.1101/2025.01.08.631902