Immune memory is one of the major public health challenges raised by the COVID-19 pandemic. Although memory B lymphocytes play a fundamental role in the body's defense against viruses, their precise contribution in the case of SARS-CoV-2 remained uncertain. Several questions were still open: serological studies had yielded contradictory results regarding the persistence of humoral immunity in asymptomatic, moderate, and severe patients. Some work even suggested that severe infection might impair the germinal center reaction, or that the strong extrafollicular activation observed after symptomatic infection might delay a germinal center–dependent response. Understanding the longevity and functionality of the SARS-CoV-2–specific memory B cell response was therefore essential to anticipate herd immunity and guide vaccination strategies.
To address this issue, the authors conducted a longitudinal follow-up of the B lymphocyte response up to six months after infection in patients with moderate and severe disease. The approach relied on a single-cell analysis combining gene expression profiling, B cell receptor repertoire sequencing, and labeling with viral proteins, complemented by viral neutralization assays and the culture of individual B cell clones.
The results show that activated B cell clones specific for the SARS-CoV-2 Spike protein fuel both an early wave of antibody-secreting cells and a durable, synchronous germinal center response. Early in the response, highly mutated memory B lymphocytes are recruited, among which are preexisting clones cross-reactive with seasonal Betacoronaviruses. Over time, neutralizing clones specific for the receptor-binding domain (RBD) accumulate and contribute substantially to a remarkably stable late memory B cell compartment. These cells display evidence of germinal center maturation, illustrated by the progressive accumulation of somatic mutations in the genes of their variable regions.
Taken together, these observations establish that antigen-driven activation persists and continues to refine itself up to six months after SARS-CoV-2 infection. Far from confirming the hypothesis of a germinal center reaction compromised by severe disease, this work highlights a mature and durable humoral response, capable of conferring long-term protection.