The coronavirus disease (COVID-19) is caused by severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2). Infection with SARS-CoV-2 produces B cell responses that persist for at least one year. Thus, immunological memory is essential to prevent re-infection, and B cells play a vital role in this aspect of the defense mechanism provided by the immune system.
B cell memory develops in response to infections or vaccinations. The rapid recall responses elicited by memory B cells help provide long-term protection against pathogens.
A new study published in the journal Nature attempted to investigate memory B cell evolution 5 months after vaccination with mRNA vaccines such as Moderna (mRNA-1273) or Pfizer-BioNTech (BNT162b2) in SARS-CoV-2 naïve individuals. The study explored various aspects of memory B cell antibody response like neutralizing response, affinity, epitope targeting, and neutralizing breadth.
Study population
The study recruited 32 volunteers with no prior history of SARS-CoV-2 infection who had received any of the two mRNA vaccines Moderna or Pfizer-BioNTech.
Blood samples were collected sequentially from 8 subjects who had received Moderna and 24 subjects treated with Pfizer-BioNTech. Samples were collected from study subjects at an average of 2.5 weeks after their first dose, and they were identified as “prime.”
Similarly samples were also collected from subjects who had taken the second vaccine dose and completed their vaccination regime. In this group of individuals, samples were drawn at two time points, one at an average of 1.3 months and the other at around 5 months after the booster doses were administered. The age of the study population ranged between 23-78 years with a median age of 34.5 years, and 53% of the population was male while 47% was female.
Significant increase in plasma neutralizing activity seen in naïve individuals who had received the COVID-19 mRNA vaccines
ELISA (enzyme-linked immunosorbent assay) was performed to assess the plasma Immunoglobulin M (IgM) , Immunoglobulin G (IgG), and Immunoglobulin A (IgA) responses to the SARS-CoV-2 receptor-binding domain (RBD).
This study showed that during the period between the prime and booster vaccine doses, a significant increase in IgG reactivity to RBD was detected, which correlated with earlier evidence.
After the booster dose, the levels of IgM and IgA titers were found to be low when compared to IgG. An inverse correlation was significantly identified between the magnitude of response and age after the prime vaccine dose. However, this correlation was not significantly detected in this small study population at 1.3 or 5 months after the booster dose. Further, during the period between 1.3 and 5 months after the booster dose, there was a significant decrease in the IgG and IgA titers against RBD. In the case of IgG titers, a 4.3 fold decrease was observed and the reduction in their activity showed a significant direct correlation with time after vaccination.
HIV-1 pseudotyped with the SARS-CoV-2 spike were employed for assessing neutralizing activity. In naïve individuals who received the first vaccine dose, a significant inverse correlation was observed between age and neutralizing activity.
In the study population that received the booster dose, there was a similar 12 fold increase in the neutralizing activity that did not differ with gender or age. Further, a significant positive correlation was observed in binding and neutralizing activity between the booster dose and the first vaccine dose.
In naïve individuals who received the booster dose, it was found that at 1.3 and 5 months after the booster, they showed 4.9 and 3.6 fold higher neutralizing titers when compared to infected individuals who were assessed 1.3 and 5 months after COVID-19 symptom onset.
This study's findings suggest a significant increase in plasma neutralizing activity in naïve individuals who had received the COVID-19 vaccine, which correlated with enhanced vaccine efficacy after the booster dose. Furthermore, higher neutralizing titers were observed in individuals who had completed the vaccination regime when compared to COVID-19 infected individuals.
When 28 of the study subjects were assessed 5 months after their booster dose, it was identified that neutralizing activity inversely correlated with time and directly correlated with IgG anti-RBD binding titers. As reported earlier, at 1.3 months post booster dose, the binding to neutralizing serum titers was higher in vaccinated individuals when compared to convalescent patients, but this difference in effect disappeared at later time intervals.
Similar to previous reports, the neutralizing activity against the variants tested were found to be lower when compared to the original Wuhan Hu-1 strain. In addition, the neutralizing activity was also found to decrease against the original strain and the variants as the time from vaccination increased further.
Memory B cells may continue to evolve for up to 5 months post-COVID-19 mRNA vaccinations
The effect of the mRNA vaccines on the memory B cell compartments was assessed by flow cytometry wherein the B cells expressing receptors that facilitate binding to Wuhan Hu-1 (wild type, WT) and the B.1.351 K417N/E484K/N501Y variant RBDs were measured. Memory B cells expressing receptors for Wuhan-Hu RBD were detected after the initial mRNA vaccine dose, and their numbers continued to increase 5 months post vaccination.
Memory B cells expressing receptors for binding to RBD of the variants tested were detected at very low levels compared to the wild type. After the booster dose, an increase in the IgG memory cells was observed, but IgM expressing memory B cells that were initially increased after the prime vaccine dose was nearly absent after the booster dose.
Plasmablasts are antibody-producing stem cells, and RBD-specific plasmablasts were detected after the first vaccine dose and were detected significantly after the booster doses.
The memory compartment is known to evolve for up to one year after natural infection and results in the enrichment of cells that produce antibodies effective against pathogens. The study examined the characteristics of the memory compartment after mRNA vaccination.
2,327 paired antibody sequences from 11 individuals obtained at the time points that had already been described in the study were examined. IGHV3-30 and IGHV3-53 were prevalent after the prime and booster doses and remained prevalent 5 months after vaccination. Expanded clones of memory B cells expressing the IGHV and IGHL genes were observed in all the individuals studied.
The study found that the amount of memory cells varied between initial vaccine dose, booster dose, and between individuals and also decreased with time. Detection of unique clones in vaccinated individuals suggests that memory cells evolve for up to 5 months post-vaccination.
It was also observed that memory cells isolated after the booster dose had significantly high levels of somatic mutations when compared to plasmablasts and B cells that were obtained after the initial vaccine dose. The study shows that after mRNA vaccinations the memory B cells may continue to mutate or evolve for 5 months post the booster dose.
The booster dose of the COVID-19 mRNA vaccines enhances the neutralizing response of the antibodies expressed in the memory compartment
ELISA was performed to assess the neutralizing activity of antibodies that were isolated from memory B cells that bind to RBD. Four hundred fifty-eight antibodies were tested, and 94% of the antibodies exhibited binding to Wuhan Hu-1 RBD, confirming the efficiency of the method that was used to isolate the RBD-specific memory B cells. The geometric mean ELISA half-maximal concentration (EC50) of the antibodies obtained at various time points was estimated, and there was no significant difference in binding of these antibodies at the time points tested. The EC50 of the antibodies from samples obtained at prime, 1.3- and 5-months after the second dose was 3.5, 2.9 and 2.7 ng/ml, respectively.
The neutralizing activity of 430 RBD-binding antibodies was evaluated using HIV-1 pseudotyped with the SARS-CoV-2 spike. It was found that the geometric mean half-maximal inhibitory concentration (IC50) of the antibodies tested was enhanced from 376ng/mL after the first vaccine dose to 153ng/mL after the second vaccine dose, and this increase in activity correlated with a decrease in the non-neutralizing antibodies and increase in neutralizing antibodies.
Memory B cells recruited after the second dose of the mRNA vaccine are responsible for the enhanced neutralizing activity observed between the first and second dose. The booster dose of the mRNA vaccines results in an improved neutralizing response of the antibodies expressed in the memory compartment.
The neutralizing activity of the monoclonal activities obtained was not significantly enhanced between 1.3 and 5 months post-vaccination. Interestingly, in convalescent individuals, the antibodies from memory cells showed enhanced activity between the period of 1.33 and 6.2 months after symptom onset, with further improvement observed after one year of symptom onset. This was attributed to the increasing neutralizing activity of persistent clones.
Memory antibodies from convalescent individuals show more improved affinity and neutralization breadth post-infection than in case of COVID-19 mRNA vaccinations
Bio-layer interferometry (BLI) analysis was performed employing Wuhan Hu-1 RBD to assess the affinity maturation, which is the process wherein the immune system produces antibodies of increased affinity during an immune response. Antibodies were obtained after the prime dose, 1.3 months and 5 months after the booster amounting to a total of 147 antibodies.
On comparing the affinity of the antibodies, it was found that there was a 3 fold difference in affinities of the antibodies obtained at prime and 1.3 months post booster dose and a 7.5 fold difference in affinities between the 1.3 months and 5 month post booster antibodies.
Affinities of pairs of antibodies from persisting clones obtained at 1.3 and 5 months post booster showed a 4.5 fold increase in affinity, while in the case of convalescent individuals, antibodies from persisting clones obtained at 1.3 and 6.2 months after symptom onset showed an 11.2 fold increase in affinity.
The study also explored if there is a change in the epitopes targeted by the monoclonal antibodies. It found that there was no significant change in the epitopes targeted by the 52 antibodies studied, which were obtained at 1.3 and 5 months post-vaccination and exhibited similar neutralizing activity.
In convalescent individuals, it had been found that the neutralizing breadth of antibodies from memory cells increases along with their potencies over time. The neutralizing potential of antibodies obtained at prime and 1.3 months post-vaccination were tested against a panel of pseudotypes encoding RBD mutations. It was found that there was only a small change in breadth and an increase in resistance to K417N and A475V substitutions. It was found that there was a little increase in neutralizing breath during the post-vaccination period of mRNA vaccines when compared to a similar period post-COVID-19 infection in convalescent individuals.
Conclusion
The findings from the study suggest that providing booster doses of mRNA vaccines to vaccinated individuals will result in increased plasma neutralizing activity. However, it may not produce antibodies that exhibit neutralizing breadth against variants of concern, as observed in the case of convalescent individuals.
Convalescent individuals, when boosted with mRNA vaccines, develop strong protection against the original Wuhan-Hu-1 strain and the variants of concern.
Further investigation needs to focus on whether an additional boost with Wuhan-Hu-1 based or variant based vaccine or re-infection will result in memory B cells that may produce antibodies showing increased neutralizing breadth.
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