B Cell Responses

Acute B Cell and Antibody Responses
The humoral immune response is critical for the clearance of cytopathic viruses and is a major part of the memory response that prevents reinfection. SARS-CoV-2 elicits a robust B cell response, as evidenced by the rapid and near-universal detection of virus-specific IgM, IgG and IgA, and neutralizing IgG antibodies (nAbs) in the days following infection. The kinetics of the antibody response to SARS-CoV-2 are now reasonably well described (Huang et al., 2020a).
Similar to SARS-CoV-1 infection (Hsueh et al., 2004), seroconversion occurs in most COVID-19 patients between 7 and 14 days after the onset of symptoms, and antibody titers persist in the weeks following virus clearance (Figure 4 ) (Haveri et al., 2020, Lou et al., 2020, Okba et al., 2020, Tan et al., 2020b, Wölfel et al., 2020, Wu et al., 2020b, Zhao et al., 2020a). Antibodies binding the SARS-CoV-2 internal N protein and the external S glycoprotein are commonly detected (Amanat et al., 2020, Ju et al., 2020, To et al., 2020). The receptor binding domain (RBD) of the S protein is highly immunogenic, and antibodies binding this domain can be potently neutralizing, blocking virus interactions with the host entry receptor, ACE2 (Ju et al., 2020, Wu et al., 2020b). Anti-RBD nAbs are detected in most tested patients (Ju et al., 2020, To et al., 2020, Wu et al., 2020b). Although crossreactivity to SARS-CoV-1 S and N proteins and to MERS-CoV S protein was detected in plasma from COVID-19 patients, no crossreactivity was found to the RBD from SARS-CoV-1 or MERS-CoV. In addition, plasma from COVID-19 patients did not neutralize SARS-CoV-1 or MERS-CoV (Ju et al., 2020).
Figure 4 Antibody-Mediated Immunity in SARS-CoV-2
Virus-specific IgM and IgG are detectable in serum between 7 and 14 days after the onset of symptoms. Viral RNA is inversely correlated with neutralizing antibody titers. Higher titers have been observed in critically ill patients, but it is unknown whether antibody responses somehow contribute to pulmonary pathology. The SARS-CoV-1 humoral response is relatively short lived, and memory B cells may disappear altogether, suggesting that immunity with SARS-CoV-2 may wane 1–2 years after primary infection.
RBD-specific CD19+IgG+ memory B cells were single-cell sorted from a cohort of eight COVID-19 donors between days 9 and 28 after the onset of symptoms (Ju et al., 2020). From their antibody gene sequences, 209 SARS-CoV-2-specific monoclonal antibodies were produced. The monoclonal antibodies had a diverse repertoire, relatively low or no somatic mutations, and variable binding reactivity, with dissociation constants reaching 10−8 to 10−9, similar to antibodies isolated during acute infections. Two potent neutralizing SARS-CoV-2 RBD-specific monoclonal antibodies were characterized that did not crossreact with the RBD of SARS-CoV-1 or MERS-CoV (Ju et al., 2020). Together, these results demonstrate that antibody mediated neutralization is virus specific and likely driven by binding of epitopes within the RBD.

B Cell Memory: Development and Lifespan
The B cell response to a virus serves not only to protect from the initial challenge, but also to offer extended immunity against reinfection. Following resolution of an infection, plasma cells formed during the acute and convalescent phases continue to secrete antibodies, giving rise to serological memory. Memory B cells that are also formed during the primary infection constitute the second arm of B cell memory. Memory B cells can quickly respond to a reinfection by generating new high-affinity plasma cells. Long-term protection is achieved through the induction of long-lived plasma cells and memory B cells.
There is great interest in understanding the lifespan of B cell memory responses to SARS-CoV-2. Protection from reinfection has direct medical and social consequences as the world works to develop vaccination strategies and resume normal activities. In COVID-19 patients, evidence of near-universal seroconversion and the lack of substantial descriptions of reinfection point to a robust antibody response, which, along with the T cell memory response, would offer protection to reinfection. Indeed, a case study of a single patient described induction of CD38HiCD27Hi antibody-secreting cells (ASCs), concomitant with an increase in circulating follicular T helper cells (Tfh) cells (Thevarajan et al., 2020), and a scRNA-seq study of PBMCs from critically ill and recently recovered individuals revealed a plasma cell population (Guo et al., 2020). In addition, IgG memory cells specific to the RBD have been identified in the blood of COVID-19 patients (Ju et al., 2020). Consistent with the development of immunity after COVID-19 infection, a recent study of SARS-CoV-2 infection in rhesus macaques found that two macaques that had resolved the primary infection were resistant to reinfection 28 days later (Bao et al., 2020b).
Due to the timing of this outbreak, it is not yet possible to know the nature and extent of long-term memory responses, but lessons may again be learned from other human CoVs. In the case of the human CoV 229E, specific IgG and nAbs are rapidly induced but wane in some individuals around a year after infection, with some residual protection to reinfection (Callow et al., 1990, Reed, 1984). The lifespan of the humoral response following SARS-CoV-1 infection is also relatively short, with the initial specific IgG and nAb response to SARS-CoV-1 diminishing 2–3 years after infection and nearly undetectable in up to 25% of individuals (Cao et al., 2007, Liu et al., 2006). A long-term study following 34 SARS-CoV-1-infected healthcare workers over a 13-year period also found that virus-specific IgG declined after several years, but the authors observed detectable virus-specific IgG 12 years after infection (Guo et al., 2020). In the case of MERS-CoV, antibodies were detected in six of seven volunteers tested 3 years after infection (Payne et al., 2016).
IgG specific to SARS-CoV-2 trimeric spike protein was detectable in serum up to 60 days after symptom onset, but IgG titers began decreasing by 8 weeks post symptom onset (Adams et al., 2020). Long-term protection from reinfection may also be mediated by reactive memory B cells. A study that analyzed SARS-CoV-1 S protein-specific IgG memory cells at 2, 4, 6, and 8 months post infection found that S-specific IgG memory B cells decreased progressively about 90% from 2 to 8 months after infection (Traggiai et al., 2004). A further retrospective study of 23 individuals found no evidence of circulating SARS-CoV-1-specific IgG+ memory B cells 6 years after infection (Tang et al., 2011). This is in contrast to the memory T cell response, which was robustly detected based on induced IFN-γ production (Tang et al., 2011).
Studies of common CoVs SARS-CoV-1 and MERS-CoV indicate that virus-specific antibody responses wane over time and, in the case of common CoVs, result in only partial protection from reinfection. These data suggest that immunity to SARS-CoV-2 may diminish following a primary infection, and further studies will be required to determine the degree of long-term protection (Figure 4).

Consequences of the B Cell Response: Protection versus Enhancement
Several studies have demonstrated that high virus-specific antibody titers to SARS-CoV-2 are correlated with greater neutralization of virus in vitro and are inversely correlated with viral load in patients (Figure 4) (Okba et al., 2020, Wölfel et al., 2020, Zhao et al., 2020a). Despite these indications of a successful neutralizing response in the majority of individuals, higher titers are also associated with more severe clinical cases (Li et al., 2020b, Okba et al., 2020, Zhao et al., 2020a, Zhou et al., 2020a), suggesting that a robust antibody response alone is insufficient to avoid severe disease (Figure 4).
This was also observed in the previous SARS-CoV-1 epidemic, where neutralizing titers were found to be significantly higher in deceased patients compared to patients who had recovered (Zhang et al., 2006). This has led to concerns that antibody responses to these viruses may contribute to pulmonary pathology via antibody-dependent enhancement (ADE) (Figure 4). This phenomenon is observed when non-neutralizing virus-specific IgG facilitate entry of virus particles into Fc-receptor (FcR) expressing cells, particularly macrophages and monocytes, leading to inflammatory activation of these cells (Taylor et al., 2015). A study in SARS-CoV-1-infected rhesus macaques found that anti-S IgG contributed to severe acute lung injury (ALI) and massive accumulation of monocytes and macrophages in the lung (Liu et al., 2019). Furthermore, serum containing anti-S Ig from SARS-CoV-1 patients enhanced the infection of SARS-CoV-1 in human monocyte-derived macrophages in vitro (Yip et al., 2014). ADE was also reported with a monoclonal antibody isolated from a patient with MERS-CoV (Wan et al., 2020c). Somewhat reassuringly, there was no evidence of ADE mediated by sera from rats vaccinated with SARS-CoV-2 RBD in vitro (Quinlan et al., 2020) nor in macaques immunized with an inactivated SARS-CoV-2 vaccine candidate (Gao et al., 2020c).
As of now, there is no evidence that naturally developed antibodies toward SARS-CoV-2 contribute to the pathological features observed in COVID-19. However, this possibility should be considered when it comes to experimental design and development of therapeutic strategies. Importantly, in all of the descriptions of ADE as it relates to CoV, the FcR was necessary to trigger the antibody-mediated pathology. High-dose intravenous immunoglobulin (IVIg), which may blunt ADE, has been trialed in COVID-19 patients (Cao et al., 2020b, Shao et al., 2020), but further studies are needed to determine the extent to which IVIg is safe or beneficial in SARS-CoV-2 infection. Vaccine trials will need to consider the possibility of antibody-driven pathology upon antigen rechallenge; strategies using F(ab) fragments or engineered Fc monoclonal antibodies may prove particularly beneficial in this setting (Amanat and Krammer, 2020).