ABSTRACT Dengue vaccine development is complicated by antibody-dependent enhancement (ADE)—a phenomenon in which antibodies paradoxically boost viral growth. Signals of immune-mediated enhancement were observed in the CYD-TDV (Dengvaxia) vaccine trials, but have not yet been reported for trials of TAK-003 (Qdenga). To better understand how prior immunity influences dengue infection outcomes, we developed a mechanistic within-host model of acute dengue infection dynamics that incorporates ADE as well as humoral and CD8+ T-cell immunity. Model simulations illustrate a potentially very important difference between enhancement by antibody, enhancement by humoral immunity, and enhanced severity of secondary infection. Model results also suggest that recipients of CYD-TDV may have been more likely to experience immune-mediated enhancement because of its yellow fever virus backbone and corresponding lack of dengue-specific CD8+ T-cell epitopes. The model provides a possible explanation for differences in reported outcomes of dengue vaccine trials and suggests incorporating DENV backbones from all four serotypes as a potentially promising way forward in the design of future dengue vaccines. IMPORTANCE Vaccines are typically designed to elicit humoral (antibody-mediated) immunity, and antibody titers are commonly used as correlates of protection. Because CD8+ T-cell responses are more challenging to measure, they are not typically used as correlates of protection and have received comparatively less attention. For viruses such as dengue, where antibodies may increase viral replication and thus the risk of severe disease through antibody-dependent enhancement, it is necessary to understand how humoral and cellular immunity interact during infection. Our model suggests that low antibody levels can enhance viral replication but that this effect can be mitigated by humoral and CD8+ T-cell immunity. This result offers a hypothesis to explain the observed differences in outcomes between the CYD-TDV and TAK-003 trials and suggests that evaluating CD8+ T-cell responses alongside antibody titers may be important for designing and assessing the safety and efficacy of future dengue vaccines.
communication, we investigate interactions of poxvirus core and lateral body proteins using proximity analysis. Poxviruses are large DNA viruses that replicate exclusively in the cytoplasm (1). The first recognizable viral structures following infection with vaccinia virus (VACV), the prototype member of the family, are bilayer membrane crescents that enclose electron-dense amorphous material containing core protein precursors (2–4). The crescents extend to form spherical immature virions (IVs) that enclose the viral genome. The spherical shape is imposed by the reticulon-like A17 membrane protein (5, 6) and the D13 external scaffold (7–9). D13 is shed following processing of A17, leading to a profound shape change resulting in a mature virion (MV) with a dumbbell-shaped core, two lateral bodies in the concavities, and a surrounding membrane envelope (2, 10, 11). The core wall is composed of three major proteins: A3, A4, and A10. Cryo-EM aided by AlphaFold modeling revealed that trimers of A10 are arranged in a pseudohexagonal lattice that forms the outer palisade layer and that the helical N-terminus of A4 is associated with A10 (12–14). A4 is a 39 kDa protein with predicted intrinsically disordered regions that is required for morphogenesis. F17 is an abundant 101-amino acid protein that is a component of the lateral bodies, is also required for morphogenesis, and has an immunomodulatory role (15–18). Evidence for the association of the C-and N-termini of A4 with the C-terminus of F17 was obtained by chemical cross-linking proximity analysis of purified virions (
ABSTRACT Respiratory syncytial virus (RSV) is a major cause of lower respiratory tract infections in infants, immunocompromised individuals, and older adults. A hallmark of RSV infection is the formation of multinucleated syncytia through fusion between infected and adjacent uninfected cells. This process is mediated by the RSV fusion protein (RSV F) expressed on infected cells, which interacts with receptors on neighboring cells to drive membrane fusion. Syncytium formation may facilitate cell-to-cell spread while reducing virion exposure to extracellular neutralizing antibodies and immune cells. Clinically used monoclonal antibodies (mAbs) target RSV F and are routinely evaluated using conventional virus neutralization assays. However, their ability to inhibit syncytium formation remains poorly characterized because these assays primarily measure virus-to-cell entry rather than cell-to-cell fusion. Here, we present a quantitative flow cytometry-based assay to measure RSV-induced syncytium formation during infection. Using this assay, we evaluated clinically relevant anti-F mAbs and compared their potency in syncytium inhibition and conventional virus neutralization assays in HEp-2 and A549 cells. Palivizumab, nirsevimab, and clesrovimab inhibited syncytium formation in a dose-dependent manner, with only modest differences in potency. In contrast, nirsevimab and clesrovimab exhibited substantially greater virus neutralization potency than palivizumab. These findings indicate that inhibition of RSV-induced cell-to-cell fusion represents a distinct functional property of anti-F antibodies that is not fully captured by conventional neutralization assays. Our assay provides a quantitative approach to assess syncytium inhibition and highlights the value of complementary functional readouts for evaluating antibodies targeting RSV and other syncytium-forming viruses. IMPORTANCE Respiratory syncytial virus (RSV) is a leading cause of respiratory disease in infants, older adults, and immunocompromised individuals. A hallmark of RSV infection is syncytium formation, although its contribution to viral spread and disease pathogenesis remains incompletely understood. Increased fusogenicity has been associated with disease severity, and syncytium formation may promote cell-to-cell spread while limiting virion exposure to extracellular neutralizing antibodies. Although RSV fusion (F)-specific monoclonal antibodies (mAbs) are increasingly used for disease prevention, their ability to inhibit syncytium formation remains unexplored, as conventional neutralization assays primarily measure virus-to-cell entry. Here, we describe a quantitative flow cytometry-based assay to measure RSV-induced syncytium formation and evaluate inhibition by clinically relevant anti-F mAbs. Our results indicate that syncytium inhibition represents a distinct functional property of anti-F antibodies that is not fully captured by conventional neutralization assays, highlighting the value of complementary functional readouts for evaluating antibodies targeting RSV and other syncytium-forming viruses.
ABSTRACT Influenza virus, severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), and respiratory syncytial virus (RSV) are leading causes of acute respiratory infections (ARIs). New Zealand’s coronavirus disease 2019 (COVID-19) public health measures disrupted the circulation of these viruses in 2020–2021, creating a rare opportunity to explore the impact of potential immunity gaps on age-specific infection risk. We analyzed data from 843 participants aged between 0 and 4 years enrolled in the Southern Hemisphere Influenza and Vaccine Effectiveness Research and Surveillance 3 (SHIVERS-III) birth cohort, a longitudinal community-based study in New Zealand, during 2020–2024. During the respiratory season, weekly surveillance captured ARI symptoms and severity. Nasal swabs were collected from symptomatic participants and tested for viral RNA by RT-PCR. Incidence rates (IRs) and incidence rate ratios (IRRs) were estimated for the surveillance period and adjusted for age, sex, ethnic group, socioeconomic status, and influenza vaccination status. No influenza or SARS-CoV-2 infections were detected in 2020 and 2021. From 2021 to 2024, RSV had the highest IR during the surveillance period (63.2/100 person-years [PY]; 95% CI, 56.6–69.8), followed by SARS-CoV-2 (39.3/100 PY; 95% CI, 34.5–44) and influenza (35.7/100 PY; 95% CI, 29–42.3). Age influenced infection risk: infants <6 months had the lowest incidence of influenza and RSV, potentially reflecting maternally derived immunity and limited exposure. Risk increased after 6 months of age, peaking at age 1 year for RSV and at age 2 years for influenza. Influenza vaccination lowered symptomatic influenza risk (IRR 0.66; 95% CI, 0.44–1.00). In the New Zealand community setting in the post-COVID-19 era, individual respiratory virus illness patterns differ in children aged 0–4. Influenza vaccine shows protection, and age influences virus-specific risk. IMPORTANCE Influenza virus, severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), and respiratory syncytial virus (RSV) are major causes of acute respiratory illness in early childhood, yet current understanding is largely reliant on hospital-based surveillance that underestimates the broader community burden. This study addresses this limitation by providing comprehensive, community-based estimates of incidence, severity, and symptom profiles across these viruses within a single longitudinal birth cohort. By leveraging the unique epidemiological context created by pandemic-related disruptions, it offers important insights into how changes in exposure influence subsequent infection risk and virus circulation. The direct comparison of three major respiratory viruses within the same population highlights distinct age-related risk patterns and disease characteristics. In addition, our research provides real-world evidence of the reduced risk of influenza-associated acute respiratory illness among vaccinated children. These findings strengthen the evidence base for age-targeted immunization strategies and improve understanding of early-life respiratory virus epidemiology.
ABSTRACT Human herpesvirus 6A/B (HHV-6A/B) belongs to the Betaherpesvirinae subfamily and mainly replicates in activated T cells. Although HHV-6B establishes lifelong latency after primary infection and infects nearly all adult humans, its immune evasion strategies in vivo remain incompletely understood. CD6 is a key costimulatory receptor that shapes T-cell activation and communication. Here, we identify CD6 as a previously unrecognized target of HHV-6A/B. Infection triggered internalization of CD6 from the plasma membrane into the cytoplasm and led to its proteasome-dependent degradation. CD6 transcription and maturation through the secretory pathway were unaffected, indicating that downregulation occurs through post-transcriptional mechanisms involving altered endolysosomal trafficking. We further demonstrate that the virus-specific U21–U24 gene cluster is required for CD6 downregulation, as deletion of this cluster abolished CD6 degradation in both T-cell lines and primary cord blood mononuclear cells. These findings reveal a viral strategy that disrupts T-cell surface architecture and suggest a role for CD6 modulation in supporting HHV-6A/B persistence. IMPORTANCE Human herpesvirus 6B (HHV-6B) infects nearly all individuals early in life and persists lifelong, yet the mechanisms by which it modulates host immunity remain poorly defined. CD6 is a critical costimulatory receptor that shapes T-cell activation and communication within the immune system. Here, we identify CD6 as a previously unrecognized target of HHV-6A/B and demonstrate that infection triggers its internalization and proteasome-dependent degradation. We further show that this process is driven by the virus-specific U21–U24 gene cluster, revealing a dedicated viral strategy for altering host protein trafficking. Because degradation of immune surface molecules is a common mechanism of immune evasion among persistent viruses, our findings provide new insight into how HHV-6A/B remodels T-cell function to support long-term persistence. Understanding this pathway may inform future approaches aimed at preventing or mitigating diseases associated with HHV-6 reactivation.
