Molecular IgG3 structure paves the way for new applications of antibodies

A combination of scattering and analytical techniques has provided the first atomic-level structural model for the IgG3 antibody

In humans, Immunoglobulin (IgG) is the most common type of antibody found in blood circulation. IgG molecules are created by plasma B cells, and there are four subclasses. Of the four, IgG3 is the least understood. It has a uniquely long hinge region separating its Fab antigen-binding and Fc receptor-binding regions. The presence of this elongated hinge makes it challenging to perform structural studies, for example, with X-ray crystallography. Due to this lack of structural information, IgG3 is the only subclass not currently exploited for therapeutic uses. In work recently published in the Journal of Biological Chemistry, researchers from University College London and the University of Birmingham have used a combination of imaging and analytical methods to provide the first experimentally determined molecular structural model for a full-length IgG3 antibody. This new information should enable the use of IgG3 to develop new therapies and antibody tests. 

Getting a good look at IgG3

A high-resolution structure for part of the IgG3 molecule, the globular IgG3-Fc fragment, is available. And previous studies of the whole molecule using Small Angle X-ray Scattering (SAXS) and analytical ultracentrifugation (AUC) showed that IgG3 is elongated compared to IgG1, IgG2 and IgG4. SAXS also showed that IgG3 has a more extended central hinge than IgG1 and IgG2 that links its three globular regions together.  

Read more on the Diamond website

Image: The IgG3 structural model is formed from two globular Fab regions, a long hinge in the centre, and one Fc region, as shown from the scattering modelling fits. The structure is reminiscent of a giraffe with an extended and semi-rigid neck.

Credit:
Dr Valentina Spiteri, UCL.

Nanobodies against SARS-CoV-2

Göttingen researchers have developed nanobodies – a type of antibodies – that efficiently block the coronavirus SARS-CoV-2 and its new variants. Those nanobodies, which originate from alpacas inoculated with part of the SARS-CoV-2 virus spike protein – the receptor-binding domain that the virus deploys for invading host cells – could serve as a potent drug against COVID-19. The researchers used the X10SA crystallography beamline at the Swiss Light Source to characterize the interaction between the nanobodies and the coronavirus spikes at the molecular level.


Unlike antibodies, nanobodies can be produced on an industrial scale and at a low cost and therefore meet the global demand for COVID-19 therapeutics. The new nanobodies, which can bind and neutralize the virus up to 1000 times better than previously developed antibodies, are currently in preparation for clinical trials.

Read more on the PSI website

Image: The figure shows how two of the newly developed nanobodies (blue and magenta) bind to the receptor-binding domain (green) of the coronavirus spike protein (grey), thus preventing infection with SARS-CoV-2 and its variants.

Credit: Thomas Güttler / Max Planck Institute for Biophysical Chemistry

How X-rays could make reliable, rapid COVID-19 tests a reality

Vaccines are turning the tide in the pandemic, but the risk of infection is still present in some situations. If you want to visit a friend, get on a plane, or go see a movie, there is no highly accurate, instant test that can tell you right then and there whether or not you have a SARS-CoV-2 infection. But new research from Lawrence Berkeley National Laboratory (Berkeley Lab) could help get reliable instant tests on the market.

A study led by Michal Hammel and Curtis D. Hodge suggests that a highly sensitive lateral flow assay – the same type of device used in home pregnancy tests – could be developed using pairs of rigid antibodies that bind to the SARS-CoV-2 nucleocapsid protein. Such a test would only require a small drop of mucus or saliva, could give results in 15 minutes, and could detect a COVID-19 infection one day before the onset of symptoms. Their work was published in the journal mABs.

The current gold standard tests for COVID-19 use a form of polymerase chain reaction (PCR) to identify the presence of SARS-CoV-2 nucleic acid (RNA) rather than a viral protein. They are quite accurate, with false negative rates ranging less then 5%  (depending primarily on the sampling site, sample type, and stage of infection). However, PCR tests must be sent away for analysis at an accredited lab.

Read more on the Berkeley Lab website

Image: Molecular models constructed from the X-ray data show different antibodies bound to the SARS-CoV-2 nucleocapsid protein (pink). The scientists determined that the linear arrangement (right) has higher detection sensitivity than the sandwich arrangement (left).

Credit: Berkeley Lab

New targets for antibodies in the fight against SARS-CoV-2

An international team of researchers examined the antibodies from a large cohort of COVID-19 patients. Due to the way antibodies are made, each person that is infected has the potential to produce many antibodies that target the virus in a slightly different way. Furthermore, different people produce a different set of antibodies, so that if we were to analyse the antibodies from many different patients, we would potentially be able to find many different ways to neutralise the virus.

The research article in the journal Cell is one of the most comprehensive studies of its kind so far. It is available online now and will be published in print on 15 April. These new results now show that there are many different opportunities to attack the virus using different antibodies over a much larger area than initially thought/mapped.

Professor Sir Dave Stuart, Life Sciences Director at Diamond and Joint head of Structural Biology at the University of Oxford, said:

SARS CoV-2 is the virus that causes COVID-19. Once infected with this virus, the human immune system begins to fight the virus by producing antibodies. The main target for these antibodies is the spike protein that protrudes from the virus’ spherical surface. The spike is the portion of the virus that interacts with receptors on human cells. This means that if it becomes obstructed by antibodies, then it is less likely that the virus can interact with human cells and cause infection.

By using Diamond Light Source, applying X-ray crystallography and cryo-EM, we were able to visualise and understand antibodies interact with and neutralize the virus. The study narrowed down the 377 antibodies that recognize the spike to focus mainly on 80 of them that bound to the receptor binding domain of the virus, which is where the virus spike docks with human cells.

Read more on the Diamond website

Image: Figure from the publication showing how the receptor binding domain resembles a human torso.

Credit: The authors (Cell DOI: 10.1016/j.cell.2021.02.032)