Evaluation of SARS‐CoV‐2 antibody titers and potency for convalescent plasma donation: a brief commentary

摘要

Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) is the causative agent of the ongoing COVID-19 pandemic. It is responsible for more than 1 million deaths worldwide already [1]. Because preventive and anti-viral treatment options are still limited, COVID-19 convalescent plasma (CPP) has been suggested as a potential therapy [2-4]. ‘Convalescent’ implies that anti-SARS-CoV-2 antibodies are present in plasma collected from individuals recovered from COVID-19. However, the dose and nature of antibodies required to effectively interfere with a SARS-CoV-2 infection is unclear. Most ongoing observational studies and prospective clinical trials currently focus on neutralizing antibodies (nAbs) that interfere with viral binding to host cells, but non-neutralizing antibodies might mediate a therapeutic effect as well. These and other unknowns highlight the importance of testing CCP efficacy in randomized trials. This commentary consequently does not claim to provide evidence on how to select potent CCP, but does want to provide an opinion-based discussion on how to investigate CCP potency. The antibody level in CCP varies greatly between donors. Therefore, it is required to measure antibody titer and/or to assess the neutralization potency of CCP. The current gold standard for the latter is in vitro viral neutralization like in the plaque reduction neutralization test (PRNT) or microneutralization (MN) assay. Both measure the ability of nAbs to prevent infection in vitro calculated either as a reduction in the formation of plaques or as the inhibition of viral infectivity in a cell monolayer, respectively [5, 6]. These assays utilize live SARS-CoV-2 virus and, hence, require a biosafety level 3 (BSL-3) facility. In addition, it is time-consuming (5–7 days). Furthermore, the output data cannot be compared among laboratories because different assay readouts (e.g. virus concentration or % inhibition) and protocols are currently being used. In addition, an international standard is not yet available. Blood establishments may choose to partner with a virology laboratory that can perform viral neutralization on donor samples. Alternatively, other assays are available using pseudoviruses (i.e. a recombinant virus expressing a SARS-CoV-2 protein) that require lower biosafety levels [7]. Anti-SARS-CoV-2 antibody titers can also be measured using immunoassays such as enzyme-linked (ELISA) and chemiluminescent immunoassays (CLIA) which are based on biochemical detection of antibody binding to viral proteins. Recently, the FDA suggested that all putative CCP donations should be tested in the Ortho VITROS SARS-CoV-2 IgG CLIA-based test and donations with a signal to cut-off of 12 or higher to be qualified as a high titer plasma [8]. In contrast, European blood establishments are using a variety of commercial immunoassays (Table 1), making it more difficult to compare data across the region. Sensitivities and specificities of the commercial assays presented in Table 1 can differ from those provided by the respective manufacturers. Thresholds, sensitivities and specificities may change depending on sample size, the timing post-symptom onset and the seroprevalence in the population [9, 10]. Immunoassays allow the detection of total or isotype-specific antibody binding the spike (S), receptor binding domain of spike (RBD) or nucleocapsid (N) proteins. In our opinion, immunoassays for IgG targeting RBD are most likely to be relevant because (i) most potent neutralizing antibodies are directed towards RBD, (ii) IgG is efficiently transported across the epithelial lung barrier [11] and (iii) IgG has a longer half-life. Finally, immunoassays are compatible with BSL-1 facilities, do not require sophisticated technology and may be emulated on robots to increase throughput. As viral neutralization assays are not high-throughput and thus may become rate limiting for CCP release to patients, immunoassays may be used to select CCP donations. However,