Introduction
The COVID-19 pandemic has led to devastating health outcomes with the
death toll exceeding one million cases by the end of 2020. Furthermore,
the global lockdowns impeded social and commercial interactions, with
psychological and economic impacts on the public1.
Vaccines are currently in the final stages of approval or have already
been authorized under emergency use authorization2.
However, there is still limited data on long-term immunity against
SARS-CoV-23. Studying long term immunity is essential
to aid decision making for reopening and easing social distancing,
assess the need for booster doses of vaccines, and estimate the
potential of reinfections4,5. Focusing mainly on
antibody-based assays as readouts for long-term immunity may be strongly
impeded due to lack of consistency in circulating antibody levels
regardless of disease severity. Indeed, there are conflicting results
about the longevity of antibody response in SARS-CoV-2 infected
individuals6–10, suggesting that long-lived plasma
cells may not have been efficiently established in a proportion of
COVID-19 patients. Another study found that about 20 % of German
SARS-CoV-2 PCR-positive patients with mild to moderate symptoms did not
have detectable spike-specific antibodies when tested 3 weeks or longer
after infection11. In line with these findings, Long
et al. reported that 40 % of non-symptomatic patients, as well as
12.9% with symptoms, became seronegative with no detectable antibodies
in the blood eight weeks after discharge from
hospital12. Therefore, it is of uttermost importance
to develop and test other diagnostic assays to study the long-term
immunity against SARS-CoV-2.
For diagnostic assays, the right target, the test accuracy, and
minimizing false-negative and false-positive outcomes are the major
challenges13. One potential approach that can help to
improve the detection of long-term immunity is the analysis of
SARS-CoV-2-specific memory T and B cell formation. We and others have
reported on the detection of a strong SARS-CoV-2 specific memory T cell
response in immune-competent and immunosuppressed
patients14–18. Similar studies of B cell-related
immunity have so far focused mainly on characterizing antibody
specificities19–24. Other studies identified and
isolated SARS-COV-2 B cells to generate neutralizing antibodies or to
characterize the B cell receptor usage20–22.
BMEMORY cells can remain for decades or potentially
lifelong, located within lymph nodes, spleen, bone marrow, and the lung,
or circulate in the blood25–28. Upon reinfection,
they become re-activated by encountering the same antigen and
immediately start to proliferate and differentiate into plasma cells
secreting neutralizing antibodies. Thus, quantifying
BMEMORY cell levels by a reliable assay may be used as
an indicator of long-term immunity in convalescent patients. Although
the formation of BMEMORY cells was described, controlled
studies comparing SARS-COV-2 specific memory B cell formation in
unexposed individuals and COVID 19 patients are limited. To our
knowledge, only studies with Australian and US-American cohorts assessed
the long-term immunity through characterization of the
BMEMORY cells for more than 6
months29,30. Since the reports on duration of
S1-specific antibody detection upon SARS-CoV-2 resolution are
conflicting, several studies on antibody-producing
BMEMORY cells performed within different ethnic origin
populations are required. In this work, we focus on detecting SARS-CoV2
spike (S)-protein specific memory B cells (BMEMORY) in a
well characterized cohort of central European COVID-19 patients as
compared to unexposed controls. Furthermore, we analysed the
sustainability for several months after infection as a specific
long-term marker of adaptive immunity.