Pre-existing and de novo humoral immunity to SARS-CoV-2 in humans

Abstract

Several related human coronaviruses (HCoVs) are endemic in the human population, causing mild respiratory infections 1 . Severe Acute Respiratory Syndrome Coronavirus 2 (SARS-CoV-2), the etiologic agent of Coronavirus disease 2019 , is a recent zoonotic infection that has quickly reached pandemic spread 2,3 . Zoonotic introduction of novel coronaviruses is thought to occur in the absence of pre-existing immunity in the target human population. Using diverse assays for detection of antibodies reactive with the SARS-CoV-2 Spike (S) glycoprotein, we demonstrate the presence of pre-existing immunity in uninfected and unexposed humans to the new coronavirus. SARS-CoV-2 S-reactive antibodies, exclusively of the IgG class, were readily detectable by a sensitive flow cytometry-based method in SARS-CoV-2-uninfected individuals with recent HCoV infection and targeted the S2 subunit. In contrast, SARS-CoV-2 infection induced higher titres of SARS-CoV-2 Sreactive IgG antibodies, as well as concomitant IgM and IgA antibodies throughout the observation period of 6 weeks since symptoms onset. HCoV patient sera also variably reacted with SARS-CoV-2 S and nucleocapsid (N), but not with the S1 subunit or the receptor binding domain (RBD) of S on standard enzyme immunoassays. Notably, HCoV patient sera exhibited specific neutralising activity against SARS-CoV-2 S pseudotypes, according to levels of SARS-CoV-2 S-binding IgG and with efficiencies comparable to those of COVID-19 patient sera. Distinguishing pre-existing and de novo antibody responses to SARS-CoV-2 will be critical for serology, seroprevalence and vaccine studies, as well as for our understanding of susceptibility to and natural course of SARS-CoV-2 infection. ResultsImmune cross-reactivity among seasonally spreading human coronaviruses (HCoVs) has long been hypothesised to provide cross-protection, albeit transient, against infection with distinct HCoV types 1,4,5 . To determine the degree of cross-reactivity between HCoVs and the recently introduced zoonotic coronavirus SARS-CoV-2, we developed a sensitive flow cytometry-based assay for detection of SARS-CoV-2-binding antibodies. Sera from COVID-19 patients at University College London Hospitals (UCLH) (Table S1), contained high levels of IgG, IgM and IgA antibodies recognising the wild-type Spike (S) glycoprotein of SARS-CoV-2 expressed on the surface of HEK293T cells, whereas control sera did not (Fig. 1a). Notably, sera from a proportion patients with confirmed HCoV infection collected before or during the early spread of SARS-CoV-2 in the UK (Table S1), also contained SARS-CoV-2 S-specific antibodies (Fig. 1a). However, the latter sera contained only lower levels of S-specific IgG and no IgM or IgA antibodies, which clearly distinguished them from COVID-19 patient sera (Fig. 1a). The SARS-CoV-2 S protein is proteolytically processed into the S1 and S2 subunits that mediate target cell attachment and entry, respectively 6,7 . S2 exhibits a higher degree of homology among coronaviruses than S1 (Extended data Fig. 1...

Deep sequencing of B cell receptor repertoires from COVID-19 patients reveals strong convergent immune signatures

“…In the COVID-19 patients, the largest clonal expansions were highly mutated, equivalent to the level observed in healthy control cohort. Such a secondary response to SARS-CoV-2 has been previously observed 26, and may be due to recall of B cells activated in response to previously circulating human coronaviruses, as recently highlighted 27,28 .…”

Recombinant SARS-CoV-2 spike proteins for sero-surveillance and epitope mapping

“…The globular S1 domain of several coronaviruses is the preferred antigen for sero-surveillance [6,22,25,26] and purified SARS-CoV-2 S1 was used similarly here. Full length S protein has been used elsewhere [11] but the S2 domain which it includes can lead to cross reaction as a result of previous coronavirus infections [27]. Although we used purified S1, we have shown previously that glycosylated antigen from infected insect cells captured to the plate by a mannose specific lectin (GNA) can also be an effective antigen, avoiding the need for protein purification in resource limited situations [28].…”

“…Oxfordshire has a demonstrated positivity rate of 269 per 100,000 (Office for National Statistics) which, using a 50-fold factor for non-tested but exposed individuals suggested from other studies [30], would suggest an infection rate of ~13%, very close to our observed positivity. Several studies of seroconversion have been reported, in hospitalised individuals [22] and in the population generally [27] but the general relationships among disease severity, antibody titre, neutralisation, epitope profile and longevity of response remain to be determined. The set of S resources described here may contribute to studies in these areas.…”

S protein-reactive IgG and memory B cell production after human SARS-CoV-2 infection includes broad reactivity to the S2 subunit

“…IgG MBCs are more broadly reactive than Abs generated against the same antigen, they persist after circulating Ab levels wane, and they are readily activated to generate strong Ab responses or seed germinal centers for additional rounds of affinity maturation (14). Concurrent early production of virus-specific IgM and IgG in the response to SARS-CoV-2 infection suggests a response mediated by IgG MBCs as well as naïve B cells (9,(15)(16)(17). This picture is supported by identification of B cell subsets with high and low immunoglobulin V gene mutation frequencies during the response to SARS-CoV-2 infection (18).…”

“…In two large cohorts of unexposed subjects, approximately 10% had IgG that bound S2, but not S1 or the RBD. Approximately 4% of subjects had IgG against the SARS-CoV-2 nucleocapsid (N) protein, which is highly conserved among coronaviruses (9,10). Although N is an internal viral protein and not a target of neutralizing antibodies (Abs), coronavirus infections typically elicit strong anti-N Ab production (11).…”