In a recent study posted to the bioRxiv* preprint server, researchers evaluated memory B and T cell responses to the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) before and after the third dose of the BNT162b2 vaccine.
BNT162b2 is a messenger ribonucleic acid (mRNA) platform-based coronavirus disease 2019 (COVID-19) vaccine. Like mRNA-1273, it also encodes SARS-CoV-2 spike (S) protein and has shown immense potential to combat COVID-19, together bringing down fatality rates from 3% in early 2020 to less than 0.3% currently.
COVID-19 vaccination programs have been immensely successful in controlling severe cases and deaths worldwide. They induce anti-SARS-CoV-2 neutralizing antibodies to prevent infection, but these antibodies wane in about eight months after vaccination. Thus, it is crucial to continue monitoring the persistence of immunological memory following COVID-19 vaccination.
Amid the emergence of SARS-CoV-2 variants of concern (VOCs) that evade immunity (e.g., Omicron), it is also crucial to elucidate the interaction between humoral and T-cell immunity that defend against VOCs. So far, studies have barely documented the relationship between circulating follicular helper T (cTfh) cells and humoral responses in vaccinated subjects. It is worth noting that Tfh cells regulate antibody production and class switching by B cells by facilitating germinal center reactions and promoting their differentiation for antibody secretion.
About the study
In the present study, researchers investigated T and B cell immunological memory in vaccinated subjects using several approaches. They assessed B cell memory using fluorescence-activated cell sorting (FACS) for surface expression levels of anti-receptor binding domain (RBD) immunoglobulin (Ig)G antibodies. Next, the researchers used the ELISpot assay to measure the number of B cells producing anti-RBD antibodies.
Further, they used FACS analysis of surface activation markers to evaluate T cell memory. The team also cultured peripheral blood mononuclear cells (PBMCs) with S protein peptide pools to evaluate levels of interferon-gamma (IFN-γ) in the study participants. Lastly, they comprehensively evaluated the vaccination effects, including cross-reactivity of memory cells to Omicron.
The 43 study participants had no pre-existing systemic diseases, including cancers. They received the second and third (booster) vaccinations at an eight-month interval and the first and second vaccinations at a three-week interval. The 88 COVID-19 convalescent patients provided nose swabs or saliva samples during hospitalization at Keio University Hospital between April and December 2020 and blood samples during outpatient visits between six months and one year.
Similar to their observed kinetics in unvaccinated infected patients, memory B cells increased, and T cells decreased slowly after the second dose of BNT162b2. After boosting, memory B cells increased more rapidly than before the booster, while memory T cells regained levels attained after the second vaccination.
Memory T and B cells reside inside the bone marrow, spleen, lung, and multiple lymph nodes for up to six months after pathogenic infections. The study results could not elucidate why memory B cells continued to increase in the absence of infection after the second vaccination. It is possible that the mRNA vaccine had such high amounts of S protein that B cells continued differentiating into plasma cells three weeks after the second vaccination but could not accumulate. It is also possible that they were being produced in the lymph nodes but had not emerged in the bloodstream. Measuring both the IgM- and IgG-type memory B cells in lymph nodes could help verify such possibilities.
The study results also highlighted that the ELISpot method has significant advantages over the FACS analysis. The FACS method requires a huge number of PBMCs of human origin, while the ELISpot yielded results with just 105 cells and detected cells that secreted anti-RBD antibodies instead of the expression of surface Ig bound to RBD.
Although with low affinity, while 60% to 80% of memory B cells could bind the Omicron RBD, memory T cells showed an adequate response to the Omicron S. Further, the authors observed a positive correlation between memory B cells and CXCR3+ CCR6- cTfh1 and a negative correlation between memory B cells and CXCR3- CCR6+ cTFh17. The cTfh1 levels positively correlate with neutralizing antibodies in COVID-19 convalescent patients and influence the subsequent antibody response in vaccinated subjects. Overall, these findings highlight the significance of secretion of IFN-γ from cTfh1 for the class switch to IgG.
Overall, the study results showed that while antibodies gradually decrease after SARS-CoV-2 vaccination and infection, memory B cells producing anti-RBD antibodies gradually increase in the plasma of vaccinated and infected individuals. They persist up to eight months after the second vaccination and increase even after the booster shot. On the other hand, memory T cells, which respond similarly to Wuhan-Hu1 and Omicron, persist more than eight months after vaccination. Also, the memory B and T cell repertoire showed the potential to quickly upregulate antibody production following Omicron infection and prevent severe illness and death. Thus, the study raises questions regarding the need for fourth and fifth booster vaccination for young healthy individuals who acquire enough memory B and T cells by a single booster shot.
bioRxiv publishes preliminary scientific reports that are not peer-reviewed and, therefore, should not be regarded as conclusive, guide clinical practice/health-related behavior, or treated as established information.