Droplet Size Distribution in Emulsions.

The droplet size in emulsions is known to affect the rheological properties and plays a crucial role in many applications of emulsions. Despite its importance, the underlying mechanisms governing droplet size in emulsification remain poorly understood. We investigate the average drop size and size distribution upon emulsification with a high-shear mixer for model oil-in-water emulsions stabilized by a surfactant. The size distribution is found to be a log-normal distribution resulting from the repetitive random breakup of drops. High-shear emulsification, the usual way of making emulsions, is therefore found to be very different from turbulent emulsification given by the Kolmogorov-Hinze theory, for which power-law distributions of the drop size are expected. In agreement with this, the mean droplet size does not follow a scaling with the Reynolds number of the emulsification flow but rather a capillary number scaling based on the viscosity of the continuous phase.

Biparatopic nanobodies protect mice from lethal challenge with SARS-CoV-2 variants of concern.

The ongoing COVID-19 pandemic and the emergence of new SARS-CoV-2 variants of concern (VOCs) requires continued development of effective therapeutics. Recently, we identified high-affinity neutralizing nanobodies (Nbs) specific for the receptor-binding domain (RBD) of SARS-CoV-2. Taking advantage of detailed epitope mapping, we generate two biparatopic Nbs (bipNbs) targeting a conserved epitope outside and two different epitopes inside the RBD:ACE2 interface. Both bipNbs bind all currently circulating VOCs with high affinities and are capable to neutralize cellular infection with VOC B.1.351 (Beta) and B.1.617.2 (Delta) in vitro. To assess if the bipNbs NM1267 and NM1268 confer protection against SARS-CoV-2 infection in vivo, human ACE2 transgenic mice are treated intranasally before infection with a lethal dose of SARS-CoV-2 B.1, B.1.351 (Beta) or B.1.617.2 (Delta). Nb-treated mice show significantly reduced disease progression and increased survival rates. Histopathological analyses further reveal a drastically reduced viral load and inflammatory response in lungs. These data suggest that both bipNbs are broadly active against a variety of emerging SARS-CoV-2 VOCs and represent easily applicable drug candidates.

Pulmonary Surfactant Proteins Are Inhibited by Immunoglobulin A Autoantibodies in Severe COVID-19.

Rationale: Coronavirus disease 2019 (COVID-19) can lead to acute respiratory distress syndrome with fatal outcomes. Evidence suggests that dysregulated immune responses, including autoimmunity, are key pathogenic factors. Objectives: To assess whether IgA autoantibodies target lung-specific proteins and contribute to disease severity. Methods: We collected 147 blood, 9 lung tissue, and 36 BAL fluid samples from three tertiary hospitals in Switzerland and one in Germany. Severe COVID-19 was defined by the need to administer oxygen. We investigated the presence of IgA autoantibodies and their effects on pulmonary surfactant in COVID-19 using the following methods: immunofluorescence on tissue samples, immunoprecipitations followed by mass spectrometry on BAL fluid samples, enzyme-linked immunosorbent assays on blood samples, and surface tension measurements with medical surfactant. Measurements and Main Results: IgA autoantibodies targeting pulmonary surfactant proteins B and C were elevated in patients with severe COVID-19 but not in patients with influenza or bacterial pneumonia. Notably, pulmonary surfactant failed to reduce surface tension after incubation with either plasma or purified IgA from patients with severe COVID-19. Conclusions: Our data suggest that patients with severe COVID-19 harbor IgA autoantibodies against pulmonary surfactant proteins B and C and that these autoantibodies block the function of lung surfactant, potentially contributing to alveolar collapse and poor oxygenation.

NeutrobodyPlex-monitoring SARS-CoV-2 neutralizing immune responses using nanobodies.

In light of the COVID-19 pandemic, there is an ongoing need for diagnostic tools to monitor the immune status of large patient cohorts and the effectiveness of vaccination campaigns. Here, we present 11 unique nanobodies (Nbs) specific for the SARS-CoV-2 spike receptor-binding domain (RBD), of which 8 Nbs potently inhibit the interaction of RBD with angiotensin-converting enzyme 2 (ACE2) as the major viral docking site. Following detailed epitope mapping and structural analysis, we select two inhibitory Nbs, one of which binds an epitope inside and one of which binds an epitope outside the RBD:ACE2 interface. Based on these, we generate a biparatopic nanobody (bipNb) with viral neutralization efficacy in the picomolar range. Using bipNb as a surrogate, we establish a competitive multiplex binding assay ("NeutrobodyPlex") for detailed analysis of the presence and performance of neutralizing RBD-binding antibodies in serum of convalescent or vaccinated patients. We demonstrate that NeutrobodyPlex enables high-throughput screening and detailed analysis of neutralizing immune responses in infected or vaccinated individuals, to monitor immune status or to guide vaccine design.

Flow cytometry based-FRET: basics, novel developments and future perspectives.

Förster resonance energy transfer (FRET) is a widespread technology used to analyze and quantify protein interactions in multiple settings. While FRET is traditionally measured by microscopy, flow cytometry based-FRET is becoming popular within the last decade and more commonly used. Flow cytometry based-FRET offers the possibility to assess FRET in a short time-frame in a high number of cells thereby allowing stringent and statistically robust quantification of FRET in multiple samples. Furthermore, established, simple and easy to implement gating strategies facilitate the adaptation of flow cytometry based-FRET measurements to most common flow cytometers. We here summarize the basics of flow cytometry based-FRET, highlight recent novel developments in this field and emphasize on exciting future perspectives.

Absence of detectable SARS-CoV-2 replication in ex vivo cultured cornea and cornea-derived epithelial cells.

PURPOSE: To study the possibility of SARS-CoV-2 to infect human corneal cells and tissues under standard corneal culture conditions using explants of COVID-19 donors and primary cornea-derived epithelial cells. METHODS: Cornea isolated from deceased COVID-19 donors was cultured for 4 weeks, and SARS-CoV-2 replication was monitored by qRT-PCR. Furthermore, primary corneal epithelial cells from healthy donors were cultured ex vivo and infected with SARS-CoV-2 and human cytomegalovirus (HCMV) as a control. Infection status was assessed by western blotting and reporter gene expression using green fluorescent protein-expressing viral strains. ACE2 and TMPRSS2 receptor expression levels in cornea and epithelial cells were assessed by qRT-PCR. RESULTS: We did not detect SARS-CoV-2 replication in 10 corneas isolated from deceased COVID-19 patients and cultured for 4 weeks, indicating absence of infection under natural conditions. Furthermore, high-titer SARS-CoV-2 infection of ex vivo cultured cornea-derived epithelial cells did not result in productive virus replication. In contrast, the same cells were highly permissive for HCMV. This phenotype could potentially be explained by low ACE2 and TMPRSS2 transcriptional activity in cornea and cornea-derived epithelial cells. CONCLUSIONS: Our data suggest that cornea and limbal epithelial cells are refractory to productive SARS-CoV-2 infection. This could be due to the absence of robust receptor expression levels necessary for viral entry. This study adds further evidence to support the very low possibility of transmission of SARS-CoV-2 from an infected corneal transplant donor to a recipient in corneal organ cultures.

Multiomics surface receptor profiling of the NCI-60 tumor cell panel uncovers novel theranostics for cancer immunotherapy.

BACKGROUND: Immunotherapy with immune checkpoint inhibitors (ICI) has revolutionized cancer therapy. However, therapeutic targeting of inhibitory T cell receptors such as PD-1 not only initiates a broad immune response against tumors, but also causes severe adverse effects. An ideal future stratified immunotherapy would interfere with cancer-specific cell surface receptors only. METHODS: To identify such candidates, we profiled the surface receptors of the NCI-60 tumor cell panel via flow cytometry. The resulting surface receptor expression data were integrated into proteomic and transcriptomic NCI-60 datasets applying a sophisticated multiomics multiple co-inertia analysis (MCIA). This allowed us to identify surface profiles for skin, brain, colon, kidney, and bone marrow derived cell lines and cancer entity-specific cell surface receptor biomarkers for colon and renal cancer. RESULTS: For colon cancer, identified biomarkers are CD15, CD104, CD324, CD326, CD49f, and for renal cancer, CD24, CD26, CD106 (VCAM1), EGFR, SSEA-3 (B3GALT5), SSEA-4 (TMCC1), TIM1 (HAVCR1), and TRA-1-60R (PODXL). Further data mining revealed that CD106 (VCAM1) in particular is a promising novel immunotherapeutic target for the treatment of renal cancer. CONCLUSION: Altogether, our innovative multiomics analysis of the NCI-60 panel represents a highly valuable resource for uncovering surface receptors that could be further exploited for diagnostic and therapeutic purposes in the context of cancer immunotherapy.

Inactivation of aerosolized SARS-CoV-2 by 254 nm UV-C irradiation.

Surface residing SARS-CoV-2 is efficiently inactivated by UV-C irradiation. This raises the question whether UV-C-based technologies are also suitable to decontaminate SARS-CoV-2- containing aerosols and which doses are needed to achieve inactivation. Here, we designed a test bench to generate aerosolized SARS-CoV-2 and exposed the aerosols to a defined UV-C dose. Our results demonstrate that the exposure of aerosolized SARS-CoV-2 with a low average dose in the order of 0.42-0.51 mJ/cm2 UV-C at 254 nm resulted in more than 99.9% reduction in viral titers. Altogether, UV-C-based decontamination of aerosols seems highly effective to achieve a significant reduction in SARS-CoV-2 infectivity.

Discovery of Ircinianin Lactones B and C-Two New Cyclic Sesterterpenes from the Marine Sponge Ircinia wistarii.

Two new ircinianin-type sesterterpenoids, ircinianin lactone B and ircinianin lactone C (7 and 8), together with five known entities from the ircinianin compound family (1, 3-6) were isolated from the marine sponge Ircinia wistarii. Ircinianin lactones B and C (7 and 8) represent new ircinianin terpenoids with a modified oxidation pattern. Despite their labile nature, the structures could be established using a combination of spectroscopic data, including HRESIMS and 1D/2D NMR techniques, as well as computational chemistry and quantum-mechanical calculations. In a broad screening approach for biological activity, the class-defining compound ircinianin (1) showed moderate antiprotozoal activity against Plasmodium falciparum (IC50 25.4 µM) and Leishmania donovani (IC50 16.6 µM).

Investigations on the E/Z-isomerism of neonicotinoids.

We investigate the minimum-energy path for the rotation of formal C=N double bonds in molecules with guanidine-like substructures as present in the chemical class of neonicotinoids. The transitions between the E- and Z-isomers of several neonicotinoids using scans of the torsional potential energy hypersurfaces are quantified at the DFT-level of theory. The validity of using this ansatz is checked by single-point CCSD(T) calculations for model systems like nitroguanidine. A combined approach of theory and experiment permits to unambiguously identify the relevant isomers present at ambient conditions. As an example, MP2-GIAO predictions of the NMR spectra of E- and Z-Clothianidin are experimentally confirmed by low-temperature NMR-experiments identifying for the first time the hitherto unknown Z-Isomer of Clothianidin.

Activated integrins identify functional antigen-specific CD8+ T cells within minutes after antigen stimulation.

Immediate ß2-integrin activation upon T cell receptor stimulation is critical for effective interaction between T cells and their targets and may therefore be used for the rapid identification and isolation of functional T cells. We present a simple and sensitive flow cytometry-based assay to assess antigen-specific T cells using fluorescent intercellular adhesion molecule (ICAM)-1 multimers that specifically bind to activated ß2-integrins. The method is compatible with surface and intracellular staining; it is applicable for monitoring of a broad range of virus-, tumor-, and vaccine-specific CD8+ T cells, and for isolating viable antigen-reacting cells. ICAM-1 binding correlates with peptide-MHC multimer binding but, notably, it identifies the fraction of antigen-specific CD8+ T cells with immediate and high functional capability (i.e., expressing high levels of cytotoxic markers and cytokines). Compared with the currently available methods, staining of activated ß2-integrins presents the unique advantage of requiring activation times of only several minutes, therefore delivering functional information nearly reflecting the in vivo situation. Hence, the ICAM-1 assay is most suitable for rapid and precise monitoring of functional antigen-specific T cell responses, including for patient samples in a variety of clinical settings, as well as for the isolation of functional T cells for adoptive cell-transfer immunotherapies.

Rapid, dose-dependent and efficient inactivation of surface dried SARS-CoV-2 by 254 nm UV-C irradiation.

BACKGROUND: The COVID-19 pandemic urges for cheap, reliable, and rapid technologies for disinfection and decontamination. One frequently proposed method is ultraviolet (UV)-C irradiation. UV-C doses necessary to achieve inactivation of high-titre SARS-CoV-2 are poorly defined. AIM: We investigated whether short exposure of SARS-CoV-2 to UV-C irradiation sufficiently reduces viral infectivity and doses necessary to achieve an at least 6-log reduction in viral titres. METHODS: Using a box and two handheld systems designed to decontaminate objects and surfaces, we evaluated the efficacy of 254 nm UV-C treatment to inactivate surface dried high-titre SARS-CoV-2. RESULTS: Drying for 2 hours did not have a major impact on the infectivity of SARS-CoV-2, indicating that exhaled virus in droplets or aerosols stays infectious on surfaces for at least a certain amount of time. Short exposure of high titre surface dried virus (3-5*10^6 IU/ml) with UV-C light (16 mJ/cm2) resulted in a total inactivation of SARS-CoV-2. Dose-dependency experiments revealed that 3.5 mJ/cm2 were still effective to achieve a > 6-log reduction in viral titres, whereas 1.75 mJ/cm2 lowered infectivity only by one order of magnitude. CONCLUSIONS: SARS-CoV-2 is rapidly inactivated by relatively low doses of UV-C irradiation and the relationship between UV-C dose and log-viral titre reduction of surface residing SARS-CoV-2 is nonlinear. Our findings emphasize that it is necessary to assure sufficient and complete exposure of all relevant areas by integrated UV-C doses of at least 3.5 mJ/cm2 at 254 nm. Altogether, UV-C treatment is an effective non-chemical option to decontaminate surfaces from high-titre infectious SARS-CoV-2.

Lectin from Triticum vulgaris (WGA) Inhibits Infection with SARS-CoV-2 and Its Variants of Concern Alpha and Beta.

Even in the face of global vaccination campaigns, there is still an urgent need for effective antivirals against SARS-CoV-2 and its rapidly spreading variants. Several natural compounds show potential as antiviral substances and have the advantages of broad availabilities and large therapeutic windows. Here, we report that lectin from Triticum vulgaris (Wheat Germ Agglutinin) displays antiviral activity against SARS-CoV-2 and its major Variants of Concern (VoC), Alpha and Beta. In Vero B4 cells, WGA potently inhibits SARS-CoV-2 infection with an IC50 of <10 ng/mL. WGA is effective upon preincubation with the virus or when added during infection. Pull-down assays demonstrate direct binding of WGA to SARS-CoV-2, further strengthening the hypothesis that inhibition of viral entry by neutralizing free virions might be the mode of action behind its antiviral effect. Furthermore, WGA exhibits antiviral activity against human coronavirus OC43, but not against other non-coronaviruses causing respiratory tract infections. Finally, WGA inhibits infection of the lung cell line Calu-3 with wild type and VoC viruses with comparable IC50 values. Altogether, our data indicate that topical administration of WGA might be effective for prophylaxis or treatment of SARS-CoV-2 infections.

Iota-Carrageenan Inhibits Replication of SARS-CoV-2 and the Respective Variants of Concern Alpha, Beta, Gamma and Delta.

The COVID-19 pandemic continues to spread around the world and remains a major public health threat. Vaccine inefficiency, vaccination breakthroughs and lack of supply, especially in developing countries, as well as the fact that a non-negligible part of the population either refuse vaccination or cannot be vaccinated due to age, pre-existing illness or non-response to existing vaccines intensify this issue. This might also contribute to the emergence of new variants, being more efficiently transmitted, more virulent and more capable of escaping naturally acquired and vaccine-induced immunity. Hence, the need of effective and viable prevention options to reduce viral transmission is of outmost importance. In this study, we investigated the antiviral effect of iota-, lambda- and kappa-carrageenan, sulfated polysaccharides extracted from red seaweed, on SARS-CoV-2 Wuhan type and the spreading variants of concern (VOCs) Alpha, Beta, Gamma and Delta. Carrageenans as part of broadly used nasal and mouth sprays as well as lozenges have the potential of first line defense to inhibit the infection and transmission of SARS-CoV-2. Here, we demonstrate by using a SARS-CoV-2 spike pseudotyped lentivirus particles (SSPL) system and patient-isolated SARS-CoV-2 VOCs to infect transgenic A549ACE2/TMPRSS2 and Calu-3 human lung cells that all three carrageenan types exert antiviral activity. Iota-carrageenan exhibits antiviral activity with comparable IC50 values against the SARS-CoV-2 Wuhan type and the VOCs. Altogether, these results indicate that iota-carrageenan might be effective for prophylaxis and treatment of SARS-CoV-2 infections independent of the present and potentially future variants.

[Biology and pathology of coronaviruses]. / Biologie und Pathologie von Coronaviren.

The worldwide novel coronavirus SARS-CoV­2 pandemic is ongoing. SARS-CoV­2 belongs to the coronavirus family, the first representatives of which have been known since the 1960s. Coronaviruses are present in animals and humans and show similarities as well as differences in their biology and pathology regarding each genus. Besides mild flu-like and gastroenterological symptoms, SARS-CoV­2 can lead to dysfunctions of the lungs and other organs including the heart as already observed during SARS and MERS infections.

Tetherin Inhibits Nipah Virus but Not Ebola Virus Replication in Fruit Bat Cells.

Ebola virus (EBOV) and Nipah virus (NiV) infection of humans can cause fatal disease and constitutes a public health threat. In contrast, EBOV and NiV infection of fruit bats, the putative (EBOV) or proven (NiV) natural reservoir, is not associated with disease, and it is currently unknown how these animals control the virus. The human interferon (IFN)-stimulated antiviral effector protein tetherin (CD317, BST-2) blocks release of EBOV- and NiV-like particles from cells and is counteracted by the EBOV glycoprotein (GP). In contrast, it is unknown whether fruit bat tetherin restricts virus infection and is susceptible to GP-driven antagonism. Here, we report the sequence of fruit bat tetherin and show that its expression is IFN stimulated and associated with strong antiviral activity. Moreover, we demonstrate that EBOV-GP antagonizes tetherin orthologues of diverse species but fails to efficiently counteract fruit bat tetherin in virus-like particle (VLP) release assays. However, unexpectedly, tetherin was dispensable for robust IFN-mediated inhibition of EBOV spread in fruit bat cells. Thus, the VLP-based model systems mimicking tetherin-mediated inhibition of EBOV release and its counteraction by GP seem not to adequately reflect all aspects of EBOV release from IFN-stimulated fruit bat cells, potentially due to differences in tetherin expression levels that could not be resolved by the present study. In contrast, tetherin expression was essential for IFN-dependent inhibition of NiV infection, demonstrating that IFN-induced fruit bat tetherin exerts antiviral activity and may critically contribute to control of NiV and potentially other highly virulent viruses in infected animals.IMPORTANCE Ebola virus and Nipah virus (EBOV and NiV) can cause fatal disease in humans. In contrast, infected fruit bats do not develop symptoms but can transmit the virus to humans. Why fruit bats but not humans control infection is largely unknown. Tetherin is an antiviral host cell protein and is counteracted by the EBOV glycoprotein in human cells. Here, employing model systems, we show that tetherin of fruit bats displays higher antiviral activity than human tetherin and is largely resistant against counteraction by the Ebola virus glycoprotein. Moreover, we demonstrate that induction of tetherin expression is critical for interferon-mediated inhibition of NiV but, for at present unknown reasons, not EBOV spread in fruit bat cells. Collectively, our findings identify tetherin as an antiviral effector of innate immune responses in fruit bats, which might allow these animals to control infection with NiV and potentially other viruses that cause severe disease in humans.

ESCRT machinery components are required for Orthobunyavirus particle production in Golgi compartments.

Peribunyaviridae is a large family of RNA viruses with several members that cause mild to severe diseases in humans and livestock. Despite their importance in public heath very little is known about the host cell factors hijacked by these viruses to support assembly and cell egress. Here we show that assembly of Oropouche virus, a member of the genus Orthobunyavirus that causes a frequent arboviral infection in South America countries, involves budding of virus particles toward the lumen of Golgi cisternae. As viral replication progresses, these Golgi subcompartments become enlarged and physically separated from Golgi stacks, forming Oropouche viral factory (Vfs) units. At the ultrastructural level, these virally modified Golgi cisternae acquire an MVB appearance, and while they lack typical early and late endosome markers, they become enriched in endosomal complex required for transport (ESCRT) proteins that are involved in MVB biogenesis. Further microscopy and viral replication analysis showed that functional ESCRT machinery is required for efficient Vf morphogenesis and production of infectious OROV particles. Taken together, our results indicate that OROV attracts ESCRT machinery components to Golgi cisternae to mediate membrane remodeling events required for viral assembly and budding at these compartments. This represents an unprecedented mechanism of how viruses hijack host cell components for coordinated morphogenesis.

Shell deformity as a marker for retrospective detection of a pathogenic unicellular alga, Coccomyxa sp., in mytilid mussels: A first case study and research agenda.

An L-shaped shell deformity (LSSD) on the posterior shell edge is known exclusively in wild mytilid mussels infected with photosynthetic Coccomyxa-like algae. LSSD forms due to the appearance of extra shell material; it only occurs if the mussel is heavily infected with the alga. Traditionally, observation of high amount of the green spots (algal colonies) on a large area of host soft tissues (most of the mantle and in adductor muscle) has been used to indicate a high infection rate. We examined 300 Mytilus spp. (100 small, 20-30 mm; 200 large, 40-60 mm) with a high degree of LSSD (parameter "d" > 5 mm) from the Lower St. Lawrence Estuary (Québec, Canada). Green spots were absent in two large mussels, and were only present along the mantle posterior edge in 14 large mussels; other individuals had high infection levels. Our observations suggest that some individuals could be in a state of remission, or, even more optimistically - mussels may be able to resist the pathogen. LSSD is the stable through-time marker for detection of mytilid mussels that are or were infected with Coccomyxa algae, and, thus, may provide information for the study of mussel immunity and control of alga distribution/migration in coastal waters worldwide.

First report of signs of infection by Coccomyxa-like algae in wild blue mussels, Mytilus spp., in the Gulf of Maine (USA, Maine).

In August 2019, visual inspection of intertidal zones of the Gulf of Maine (ME, USA) revealed young and adult wild blue mussels, Mytilus spp., in Alley Bay (Jonesport area) with the distinctive L-shaped shell deformity (LSSD) and green spots (GS) in the mantle and adductor muscle. LSSD is a characteristic sign of current or previous mussel infection by photosynthetic unicellular alga from the group Coccomyxa, while GS are algal colonies. Based on these findings, this study represents the first report of infection signs by pathogenic Coccomyxa-like algae in mussels from the coastal waters of the Northeastern United States, providing a base for future large scale monitoring of the alga in the region.

Domains of the Hepatitis B Virus Small Surface Protein S Mediating Oligomerization.

During hepatitis B virus (HBV) infections, subviral particles (SVP) consisting only of viral envelope proteins and lipids are secreted. Heterologous expression of the small envelope protein S in mammalian cells is sufficient for SVP generation. S is synthesized as a transmembrane protein with N-terminal (TM1), central (TM2), and hydrophobic C-terminal (HCR) transmembrane domains. The loops between TM1 and TM2 (the cytosolic loop [CL]) and between TM2 and the HCR (the luminal loop [LL]) are located in the cytosol and the endoplasmic reticulum (ER) lumen, respectively. To define the domains of S mediating oligomerization during SVP morphogenesis, S mutants were characterized by expression in transiently transfected cells. Mutation of 12 out of 15 amino acids of TM1 to alanines, as well as the deletion of HCR, still allowed SVP formation, demonstrating that these two domains are not essential for contacts between S proteins. Furthermore, the oligomerization of S was measured with a fluorescence-activated cell sorter (FACS)-based Förster resonance energy transfer (FRET) assay. This approach demonstrated that the CL, TM2, and the LL independently contributed to S oligomerization, while TM1 and the HCR played minor roles. Apparently, intermolecular homo-oligomerization of the CL, TM2, and the LL drives S protein aggregation. Detailed analyses revealed that the point mutation C65S in the CL, the mutation of 13 out of 19 amino acids of TM2 to alanine residues, and the simultaneous replacement of all 8 cysteine residues in the LL by serine residues blocked the abilities of these domains to support S protein interactions. Altogether, specific domains and residues in the HBV S protein that are required for oligomerization and SVP generation were defined.IMPORTANCE The small hepatitis B virus envelope protein S has the intrinsic ability to direct the morphogenesis of spherical 20-nm subviral lipoprotein particles. Such particles expressed in yeast or mammalian cells represent the antigenic component of current hepatitis B vaccines. Our knowledge about the steps leading from the initial, monomeric, transmembrane translation product of S to SVP is very limited, as is our information on the structure of the complex main epitope of SVP that induces the formation of protective antibodies after vaccination. This study contributes to our understanding of the oligomerization process of S chains during SVP formation and shows that the cytoplasmic loop, one membrane-embedded domain, and the luminal loop of S independently drive S-S oligomerization.