Antibodies belonging to the immunoglobulin G (IgG) class are a crucial part of SARS-CoV-2 immunity. But while many studies have explored how SARS-CoV-2 vaccines lead to an overall increase in IgG, few have tracked how the levels of individual subsets within this antibody class evolve after repeated immunization.
Now, a longitudinal study in Science Immunology on this less-examined topic has yielded an unexpected finding: Within the total amount of vaccine-induced, SARS-CoV-2-specific IgG, there appears to be a notable increase in the subtype called IgG4. It begins to accumulate late after the second round of SARS-CoV-2 mRNA vaccination and continues to do so following the third.
More SARS-CoV-2-specific IgG4 is not harmful. It does not create additional susceptibility to other pathogenic infections, and it does not impact the effectiveness of the vaccine. Instead, IgG4's elevated presence is noteworthy, because not much is known about how the subtype's levels transform during infection in the first place.
"This IgG subclass is very rarely described for antibodies after viral infection," said Thomas Winkler, one of the study's corresponding authors and scientist at Friedrich-Alexander-Universität Erlangen-Nürnberg. "Generally, IgG3 and IgG1 are the dominant isotypes for virus-specific antibodies."
Charting how and when SARS-CoV-2 IgG subsets change post-vaccination could lead to more information about the immune system's many strategies against infection. The surprising discovery tied to the SARS-CoV-2 Pfizer/BioNTech mRNA vaccine could one day help scientists enhance future vaccines against SARS-CoV-2 and design other mRNA vaccines that build upon individual IgG responses to infection protect against other pathogenic diseases.
"We revealed an interesting immunological observation, which warrants further research to uncover the underlying mechanisms and potential clinical consequences, but there is no reason to worry about vaccine failure," said Matthias Tenbusch, a corresponding author of the study and virologist at the Institute of Clinical and Molecular Virology at Universitätsklinikum Erlange.
Training the Immune System
One of the ways the immune system learns to recognize intruders is through a process called antibody maturation that involves a type of T cells, called follicular helper T (Tfh) cells, and B cells. The T fh cells help B cells produce antibodies — such as IgG1, IgG2, IgG3, and IgG4 — against select antigens, or molecules associated with foreign substances. In order to proactively initiate immunity, vaccines introduce certain antigens that drive antibody maturation against specific bacteria and viruses.
"By vaccination, we are mimicking a previous infection. If we then meet a pathogen for the first time, our immune system is already in a much better position," said Kilian Schober, an immunologist at Institute of Microbiology - Clinical Microbiology, Immunology and Hygiene, Universitätsklinikum Erlangen and one of the corresponding authors on the study.
SARS-CoV-2 vaccines present the immune system with an antigen derived from the virus's spike protein. Exposure to this antigen drives antibody maturation, leading the body to produce SARS-CoV-2 specific IgGs that can both slow the pathogen's spread and encourage immune cells to attack it. Upon re-exposure to the virus, those vaccine-induced, "spike-specific" antibodies block binding sites on the surface of SARS-CoV-2, neutralizing its ability to attach to and invade other cells. They also draw other immune cells to the virus, so that those cells can attack it.
Because all spike-specific IgGs play such an active role in protecting against SARS-CoV-2, their total abundance in the body post-immunization has sometimes been used as a metric for SARS-CoV-2 vaccines' efficacy. However, until now there has been little investigation into how the various IgG subtypes change in response to those vaccines.
Tracking Antibody Subtypes
To address this knowledge gap, Schober and his colleagues looked at how concentrations of IgG subsets fluctuated over time in serum samples from two groups of health-care workers — one with 29 volunteers and one with 38 volunteers — who received two to three Pfizer/BioNTech monovalent mRNA vaccines.
Throughout the course of their experiment, the group observed a distinct rise in spike-specific IgG4 in roughly half of the serum sampled around five to seven months post-second round of vaccination. After the third vaccine dose, the team noticed that the amount of spike-specific IgG4 grew even more markedly, going from constituting 0.4% to 19.27% of all spike-specific IgG. The antibody subtype — which is sometimes labeled "non-inflammatory" — also became detectable in serum from nearly all the volunteers.
"We describe this subclass as 'non-inflammatory' since the antibody blocks other antibodies to bind to the same spike molecules and their inflammatory activity is reduced thereby," said Winkler. This "non-inflammatory" moniker can seem misleading, but the antibody subtype is like other IgGs in its skillful ability to combat viruses. It is very adept at facilitating the immune system's response against SARS-CoV-2.
Beyond this unforeseen uptick in IgG4, the analysis produced another nuanced finding. The researchers noticed that the percentage of spike-specific IgG3 began decreasing in collected serum around the same time that the percentage of spike-specific IgG4 started increasing. IgG3 contains a region that is especially adept at facilitating two cellular mechanisms by which the immune system kills virus.
Yet, the absolute amount of vaccine-induced, spike-specific IgG3 remained relatively the same during the experiment. The mRNA vaccine was not negatively impacting spike-specific IgG3 abundance, the researchers concluded, and the antibody was still significantly present in the serum samples.
"Our findings do not question the excellent protection that mRNA vaccines provide against severe disease with SARS-CoV-2, which has been shown in a plethora of clinical trials and through real-world evidence. In the majority of individuals we tested, pro-inflammatory IgG3 antibodies were still dominant," said Schober.
Overall, this work from Schober, Winkler, Tenbusch and their team underscores the need for further research into how IgG subtypes shift over the course of repeated mRNA vaccination. Understanding these subtleties will boost the advancements research on how to maximize mRNA vaccines' effectiveness.