Humoral Responses and Serological Assays in SARS-CoV-2 Infections

Abstract

In December 2019, the novel betacoronavirus Severe Acute Respiratory Disease Coronavirus 2 (SARS-CoV-2) was first detected in Wuhan, China. SARS-CoV-2 has since become a pandemic virus resulting in hundreds of thousands of deaths and deep socioeconomic implications worldwide. In recent months, efforts have been directed towards detecting, tracking, and better understanding human humoral responses to SARS-CoV-2 infection. It has become critical to develop robust and reliable serological assays to characterize the abundance, neutralization efficiency, and duration of antibodies in virus-exposed individuals. Here we review the latest knowledge on humoral immune responses to SARS-CoV-2 infection, along with the benefits and limitations of currently available commercial and laboratory-based serological assays. We also highlight important serological considerations, such as antibody expression levels, stability and neutralization dynamics, as well as cross-reactivity and possible immunological back-boosting by seasonal coronaviruses. The ability to accurately detect, measure and characterize the various antibodies specific to SARS-CoV-2 is necessary for vaccine development, manage risk and exposure for healthcare and at-risk workers, and for monitoring reinfections with genetic variants and new strains of the virus. Having a thorough understanding of the benefits and cautions of standardized serological testing at a community level remains critically important in the design and implementation of future vaccination campaigns, epidemiological models of immunity, and public health measures that rely heavily on up-to-date knowledge of transmission dynamics.

Keywords: COVID-19; SARS-CoV-2; coronavirus; humoral immunity; original antigenic sin; serological assays; serology.

Figure 1

Overview of antibody isotype characteristics and an approximate timeline from SARS-CoV-2 infection to possible immunity. Each antibody isotype is represented with their typical form and associated heavy chain. A brief description of their main function as well as a representation of upregulated and downregulated cytokine necessary for each class switching is also included. The approximate timeline of appearance and subsequent decrease of each isotype in relation to the viral RNA is shown. The curves and values are based on recent serological studies discussed in this review. Since limited literature is available on the implication of IgE in the pathogenesis and antibody mediated immunity to SARS-CoV-2, as such the representation of the IgE timeline is purely hypothetical. Figures were generated using (23).

Figure 2

Structure and organization of the spike glycoprotein and phylogenetic tree of all seven human CoVs. (A) A cartoon structure of the Spike protein and its receptor (i.e., ACE2) is shown in relation to its localization on the virion surface. The S1 domain interacts directly with the receptor through its RBD via it’s C-terminal domain (CTD). (B) Graphical representation of the various spike human CoV proteins. The RBD, S1 (blue), and S2 (gray) domain locations and all other relevant sites (cleavage sites), and other topological features are shown with their respective amino acid sequence number. The information for each spike was obtained using Uniprot with the following accession numbers: 229E P15423, NL63 Q6Q1S2, HKU1 Q0ZME7, OC43 P36334, MERS K9N5Q8, SARS-CoV P59594, SARS-CoV-2 P0DTC2. (C) A phylogenetic tree based on the complete genome of all seven human CoVs was made using Clustal Omega multiple alignment tool using the reference genome sequenced from NCBI with the following accession numbers: 229E NC002645.1, NL63 NC005831.2, HKU1 NC006577.2, OC43 NC006213.1, MERS NC019843.3, SARS-CoV NC_004718.3, SARS-CoV-2 NC_045512.2. Figures were generated using (23).

Figure 3

Comparison of various serological assays. (A) The sampling method and subsequent treatment of the blood before performing the serological assay is shown. Either a tube of blood is collected to isolate serum/plasma, or blood from a finger prick is used to fill a dried blood spot card or used directly in a LFA. Here we show the 2 main types of serological assays: on the left, a quantitative ELISA, or on the right, a binary result LFA. (B) The experimental procedure of each test is shown in their most simple form. Many variations are now available and are being used (see Table S1). (C) Comparison of the strengths and weaknesses of each method. POC, point of care; LFA, lateral flow assay. Figures were generated using (23).