However, detailed insight into the nature and longevity of the Bmem cell compartment specific to SARS-CoV-2 is currently still unresolved ( 26 ).

Hartley 1, View ORCID Profile Emily S.J.

Edwards 1, View ORCID Profile Pei M.

Aui 1, View ORCID Profile Nirupama Varese 1, 2, View ORCID Profile Stephanie Stojanovic 3, View ORCID Profile James McMahon 4, 5, View ORCID Profile Anton Y.

Peleg 4, 6, View ORCID Profile Irene Boo 7, View ORCID Profile Heidi E.

However, functional immunity is mediated by long-lived memory T and B (Bmem) cells.

Therefore, we generated fluorescently-labeled tetramers of the spike receptor binding domain (RBD) and nucleocapsid protein (NCP) to determine the longevity and immunophenotype of SARS-CoV-2-specific Bmem cells in COVID-19 patients.

While serum IgG to RBD and NCP was identified in all patients, antibody levels began declining at 20 days post-symptom onset.

RBD- and NCP-specific Bmem cells predominantly expressed IgM or IgG1 and continued to rise until 150 days.

RBD-specific IgG Bmem were predominantly CD27, and numbers significantly correlated with circulating follicular helper T cell numbers.

Thus, the SARS-CoV-2 antibody response contracts in convalescence with persistence of RBD- and NCP-specific Bmem cells.

Flow cytometric detection of SARS-CoV-2-specific Bmem cells enables detection of long-term immune memory following infection or vaccination for COVID-19.

INTRODUCTION Coronavirus disease (COVID)-19 is a global health emergency.

The causative agent, severe acute respiratory syndrome coronavirus-2 (SARS-CoV-2) is highly contagious and has infected tens of millions worldwide and caused over 1.2 million deaths since its discovery in Wuhan, China in December 2019 ( 1, 2 ).

Although mild or asymptomatic in many cases, SARS-CoV-2 infection in the elderly and individuals with chronic health problems can result in severe COVID-19 requiring invasive ventilation or in death ( 3 6 ).

Since early 2020, many insights have been obtained into the pathology of severe COVID-19.

It appears that high viral loads induce strong inflammatory responses that cause systemic disease, especially in the elderly and in individuals requiring immunosuppressive treatment ( 6 ).

Immunomodulation with corticosteroids has improved survival in hospitalized individuals, and anti-SARS-CoV-2 monoclonal antibody treatments have shown early evidence of alleviating symptoms and decreasing SARS-CoV-2 viral loads in mild disease ( 7, 8 ).

The COVID-19 pandemic has led to a huge global effort to identify a safe therapeutic vaccine to induce a protective immune response.

Our current understanding of SARS-CoV-2 immunity is based mainly on previous experiences with SARS-CoV, supplemented with recent studies in patients infected with and recovered from SARS-CoV-2 infection.

Similar to SARS-CoV infection ( 9, 10 ), the main antibody targets in SARS-CoV-2 are the spike and nucleocapsid proteins (NCP) ( 11 14 ).

These antibodies are detectable from approximately 6 days after PCR confirmation of infection, and those directed against spike receptor binding domain (RBD) show neutralizing capacity and hence, can prevent infection ( 15, 16 ).

However, the rapid decline of anti-SARS-CoV-2 serum IgG levels beyond 20 days post-diagnosis and the transient presence of circulating plasmablasts have led to questions about the longevity of immunity ( 17 21 ).

In contrast, antigen-specific memory T cells and memory B (Bmem) cells can be detected in convalescence ( 22 24 ).

As these memory cells are programmed to respond rapidly upon subsequent antigen encounter, it is reasonable to hypothesize that these long-lived memory cells provide durable long-term immunity ( 4, 25 ).

Graphpad Plasma Serial Dilution