Since the first cases of SARS-CoV-2 were identified in Wuhan, China, late last year, researchers have established that a viral surface structure called the spike protein is highly immunogenic. The receptor-binding domain (RBD) of this viral protein has been shown to elicit neutralizing antibodies as early as 3 days following the onset of symptoms. Still, the question if serological memory persists over time remains unanswered.
The information regarding the dynamics of antibodies and their association with RBD-specific B cells in recovered COVID-19 patients is lacking. This information would help researchers access the breadth of the humoral memory and the fact that it contributes to long-lasting serological memory. Researchers investigated the dynamics of the serological and cellular response in recovered COVID-19 patients using blood serum collected from 54 symptomatic patients with active disease, 57 recovered patients (6 of whom provided follow-up samples), and 26 healthy controls. Serum tests were conducted to determine levels of RBD-specific immunoglobulins (Igs) and their ability to neutralize SARS-CoV-2.
The team found that levels of RBD-specific antibodies quickly increased among symptomatic patients. Among recovered patients, the team found that the RBD-specific antibody titers declined over six months following symptom onset, pointing to decay in serological memory over time. These RBD-specific memory B cells were stable during the six months. Because the virus-specific antibodies declined over time, the team hypothesized that the differentiation of B cells into long-lived antibody-secreting plasma cells (LLPCs) following SARS-CoV-2 infection is impaired. The researchers studied the association between the frequency of RBD-specific subsets of plasmablasts and the frequency of RBD-specific memory B cells, which revealed that the frequency of both subsets was strongly correlated.
They also investigated the association between RBD-specific plasmablasts and antibody titers to test whether the main contributor to serum antibody levels was short-lived plasmablasts. The team reports that the frequency of SARS-CoV-2-specific B cell plasmablasts was indeed associated with levels of virus-specific antibodies. The persistence of RBD-specific memory B-cells could provide a robust recall immune response in cases of re-infection.