Immunology

Advanced immunoassays are crucial in assessing antibody responses, serving immune surveillance goals, characterising immunological responses to evolving viral variants, and guiding subsequent vaccination initiatives. This protocol outlines an indirect ELISA protocol to detect and quantify virus-specific antibodies in plasma or serum after exposure to viral antigens. The assay enables the measurement of IgG, IgA, and IgM antibodies specific to the virus of interest, providing qualitative and quantitative optical densities and concentration data. Although this protocol refers to SARS-CoV-2, its methodology is versatile and can be modified to assess antibody responses for various viral infections and to evaluate vaccine trial outcomes.

Key features

• This protocol builds upon previously described methodology [1] explicitly tailored for SARS-CoV-2 and broadens its applicability to other viral infections.

• The protocol outlines establishing antibody responses to SARS-CoV-2 infections by determining optical densities and concentrations from blood plasma or serum.

Graphical overview

Summary of the conventional ELISA (A) vs. sensitive ELISA (B) procedures. In both A and B, wells are coated with a capture antigen, such as the spike protein, while in (C) they are coated with human Kappa and Lambda capture antibodies. For the conventional ELISA (A), wells with immobilised capture antigens receive serum/plasma containing the target antibody (A1 and B1). This is followed by an HRP-conjugated detection antibody specific to the captured antibody (A2 and B2) and then a substrate solution that reacts with the HRP, producing a colour proportional to the concentration of the antibody in the serum/plasma (A3 and B3). The reaction is halted, and absorbance is measured. In the sensitive ELISA (B), after the serum/plasma addition (A1 and B1), a Biotin-conjugated primary detection antibody is introduced (A2 and B2). Depending on the target antibody, a secondary streptavidin-HRP conjugated detection antibody is added for IgG or IgM (3a) or a poly-HRP 40 detection antibody for IgA (3b). A substrate is introduced, producing a colour change proportional to the antibody concentration (A4 and B4). The reaction is then stopped, and absorbance is measured. In Panel C, wells are coated with human Kappa and Lambda capture antibodies. Serial dilutions of a known antibody standard are introduced. After undergoing the standard ELISA steps, a detection antibody is added, specifically binding to the Ig standard antibody. Subsequently, a substrate solution causes a colour change proportional to the antibody concentration in the serum/plasma. The reaction is halted, and the absorbance of each well is measured. The resulting optical densities from the coated wells form the standard curve, plotting the absorbance against concentrations.

Surface Plasmon Resonance (SPR) is a label-free optical technique to assess protein–protein interaction kinetics and affinities in a real-time setting. Traditionally, Biacore SPR employs a continuous film of gold to detect any change in the angle of re-emitted light when the refractive index of a ligand conjugated to the flat gold surface is altered by its interaction with a local analyte. In contrast, the Nicoya Lifesciences’ OpenSPR technology uses gold nanoparticles to detect small changes in the absorbance peak wavelength of a conjugated ligand after its engagement by an analyte. Specifically, when broadband white light is shone onto the gold nanoparticles, it produces a strong resonance absorbance peak corresponding to the refractive index of a ligand conjugated to the surface of gold nanoparticles. Upon its interaction with an analyte, however, the absorbance wavelength peak of the conjugated ligand will be changed and timely recorded as sensorgrams of dynamic ligand–analyte interactions. Thus, the improvement in the detection method (from traditional detection of changes in the angle of re-emitted light to the contemporary detection of changes in the wavelength of the absorbance peak) features OpenSPR as a cost-effective and user-friendly technique for in-depth characterization of protein–protein interactions. Here, we describe the detailed method that we used to characterize procathepsin L (pCTS-L) interactions with two putative pattern recognition receptors (TLR4 and RAGE) using the 1st generation of Nicoya Lifesciences’ OpenSPR instrument with a 1-channel detection.

Key features

• Nicoya OpenSPR is a benchtop small-size equipment that provides in-depth label-free binding kinetics and affinity measurement for protein–protein interactions in real-time fashion.

• This technology is relatively intuitive and user-friendly for scientists at any skill level.

• OpenSPR sensors employ nanotechnology to reduce the cost of manufacturing complex optical hardware and Sensor Chips, and similarly reduce the consumption of precious analyte samples.

• The manufacturer provides online training for OpenSPR (Catalog: TRAIN-REMOTE) and TraceDrawer (Catalog: TRAIN-TD) to customer scientists.

Platelets play an important role in hemostasis by forming clots and stopping bleeding. In immune thrombotic conditions, platelets and leukocytes are aberrantly activated by pathogenic antibodies resulting in platelet aggregates and NETosis, leading to thrombosis and thrombocytopenia. A simple assay that assesses platelet function and antibody activity is light transmission aggregometry. This assay can be used to determine antibody activity in patients with disorders such as heparin-induced thrombocytopenia (HIT) and vaccine-induced thrombotic thrombocytopenia (VITT). Briefly, for detection of pathogenic antibody, platelet-rich plasma (PRP) is treated with a specific agent (e.g., patient sera or purified patient antibodies) with constant stirring. Upon activation, platelets undergo a shape change and adhere to each other forming aggregates. This causes a reduction in opacity allowing more light to pass through PRP. Light transmission through the cuvette is proportional to the degree of platelet aggregation and is measured by the photocell over time. The advantage of this protocol is that it is a simple, reliable assay that can be applied to assess antibody activity in thrombotic conditions. Light transmission aggregometry does not require the use of radioactive reagents and is technically less demanding compared with ¹⁴C-serotonin release assay, another common assay for detecting antibody activity.

Key features

• This protocol can be used to assess platelet function and to detect platelet activating antibodies in diseases such as HIT and VITT.

• Does not require radioactive reagents, requires an aggregometer; based on the light transmission aggregometry protocol, adapted for detection of VITT and other platelet-activating antibodies.

• Two positive controls are required for reliable detection of antibodies in diseases such as HIT/VITT, namely a weak HIT/VITT antibody and a physiological agonist.

• For detection of HIT/VITT antibodies, it is essential to use donors known to have platelets reactive to these antibodies to avoid false negative results.

Development of the hybridoma technology by Köhler and Milstein (1975) has revolutionized the immunological field by enabling routine use of monoclonal antibodies (mAbs) in research and development efforts, resulting in their successful application in the clinic today. While recombinant good manufacturing practices production technologies are required to produce clinical grade mAbs, academic laboratories and biotechnology companies still rely on the original hybridoma lines to stably and effortlessly produce high antibody yields at a modest price. In our own work, we were confronted with a major issue when using hybridoma-derived mAbs: there was no control over the antibody format that was produced, a flexibility that recombinant production does allow. We set out to remove this hurdle by genetically engineering antibodies directly in the immunoglobulin (Ig) locus of hybridoma cells. We used clustered regularly interspaced short palindromic repeats (CRISPR)/CRISPR-associated protein 9 (Cas9) and homology-directed repair (HDR) to modify antibody’s format [mAb or antigen-binding fragment (Fab’)] and isotype. This protocol describes a straightforward approach, with little hands-on time, leading to stable cell lines secreting high levels of engineered antibodies. Parental hybridoma cells are maintained in culture, transfected with a guide RNA (gRNA) targeting the site of interest in the Ig locus and an HDR template to knock in the desired insert and an antibiotic resistance gene. By applying antibiotic pressure, resistant clones are expanded and characterized at the genetic and protein level for their ability to produce modified mAbs instead of the parental protein. Finally, the modified antibody is characterized in functional assays. To demonstrate the versatility of our strategy, we illustrate this protocol with examples where we have (i) exchanged the constant heavy region of the antibody, creating chimeric mAb of a novel isotype, (ii) truncated the antibody to create an antigenic peptide-fused Fab’ fragment to produce a dendritic cell–targeted vaccine, and (iii) modified both the constant heavy (CH)1 domain of the heavy chain (HC) and the constant kappa (Cκ) light chain (LC) to introduce site-selective modification tags for further derivatization of the purified protein. Only standard laboratory equipment is required, which facilitates its application across various labs. We hope that this protocol will further disseminate our technology and help other researchers.

Graphical abstract

Immunoglobulins are proteins produced by the immune system, which bind specifically to the antigen that induced their formation and target it for destruction. Highly purified human immunoglobulins are commonly used in research laboratories for several applications, such as in vitro to obtain hybridomas and in vivo animal immunisation. Several affinity purification methods are used to purify immunoglobulins from human serum, such as protein A/G Sepharose beads, polyethylene glycol, and caprylic acid ammonium sulphate precipitation. Here, we provide a detailed protocol for purification of high-quality IgG from human serum, using affinity chromatography with protein G. The protocol is divided into four main steps (column preparation, serum running, wash, and elution) for IgG purification, and two extra steps (protein dialysis and sucrose concentration) that should be performed when buffer exchange and protein concentration are required. Several IgG affinity purification methods using protein A or G are available in the literature, but protein A has a higher affinity for rabbit, pig, dog, and cat IgG, while protein G has a higher affinity for mouse and human IgG. This affinity-based purification protocol uses protein G for a highly specific purification of human IgG for animal immunization, and it is particularly useful to purify large amounts of human IgG.

Graphical abstract

IgG purification protocol. The IgG purification protocol consists of four main steps (column preparation, serum running, wash, and elution) and two extra steps (protein dialysis and concentration). a. Diluted serum is added to the protein G beads and IgG binds to the Fc receptors on protein G beads. b. Beads are washed in Hartman’s solution to fully remove the complex protein mixture (multicolour shapes, as depicted in the graphical abstract). c. IgG (orange triangles, as depicted in the graphical abstract) are removed from protein G with glycine and collected in Tris buffer. d. The IgG is transferred into a semi-permeable membrane (‘snake skin’) and allowed to dialyse overnight for buffer exchange with a physiological solution (Hartmann’s).

Phage display is a proven and widely used technology for selecting specific antibodies against desired targets. However, an immense amount of effort is required to identify and screen the desired positive clones from large and diverse combinatorial libraries. On the other hand, the selection of positive binding clones from synthetic and semi-synthetic libraries has an inherent bias toward clones with randomly produced amber stop codons, making it more difficult to identify desirable binding antibodies. To overcome the screening of desired clones with amber codons, we present a step-by-step approach for effective phage library screening to isolate useful antibodies. The procedure calls for creating a simple new vector system for soluble production of phage ELISA positive binding clones with one or more amber stop codons in their single-chain antibody fragment (scFv) gene sequences, which is otherwise difficult in standard screening.

Graphical abstract:

Neutralizing antibodies (NAbs) are of particular importance because they can prevent binding of the receptor binding domain (RBD) of the spike protein (S protein) to the angiotensin-converting enzyme 2 (ACE2) receptor present at the surface of human cells, preventing virus entry into the host cells. The gold standard method for detection of NAbs is the plaque reduction neutralization test (PRNT). Based on the measurement of cell lysis due to viral infection, this test is able to detect antibodies that prevent cell infection (Muruato et al., 2020; Lau et al., 2021). This technique requires the use of live pathogens, i.e., SARS-CoV-2 in this case, and must be done in a biosafety level 3 (BL3) laboratory. In addition, it requires expensive installations, skillful and meticulous staff, and a high workload, which prevents its wide implementation even in research laboratories. A SARS-CoV-2 pseudovirus will express the S protein responsible for cell entrance, but will not express the pathogenic genetic material of the virus, making them less dangerous for laboratory staff and the environment.

Graphic abstract:

The SARS-CoV-2 pandemic and vaccination campaign has illustrated the need for high throughput serological assays to quantitatively measure antibody levels. Here, we present a protocol for a high-throughput colorimetric ELISA assay to detect IgG antibodies against the SARS-CoV-2 spike protein. The assay robustly distinguishes positive from negative samples, while controlling for potential non-specific binding from serum samples. To further eliminate background contributions, we demonstrate a computational pipeline for fitting ELISA titration curves, that produces an extremely sensitive antibody signal metric for quantitative comparisons across samples and time.

For enveloped viruses, such as SARS-CoV-2, transmission relies on the binding of viral glycoproteins to cellular receptors. Conventionally, this process is recapitulated in the lab by infection of cells with isolated live virus. However, such studies can be restricted due to the availability of high quantities of replication-competent virus, biosafety precautions and associated trained staff. Here, we present a protocol based on pseudotyping to produce recombinant replication-defective lentiviruses bearing the SARS-CoV or SARS-CoV-2 attachment Spike glycoprotein, allowing the investigation of viral entry in a lower-containment facility. Pseudoparticles are produced by cells transiently transfected with plasmids encoding retroviral RNA packaging signals and Gag-Pol proteins, for the reconstitution of lentiviral particles, and a plasmid coding for the viral attachment protein of interest. This approach allows the investigation of different aspects of viral entry, such as the identification of receptor tropism, the prediction of virus host range, and zoonotic transmission potential, as well as the characterisation of antibodies (sera or monoclonal antibodies) and pharmacological inhibitors that can block entry.

Graphic abstract:

SARS-CoV and SARS-CoV-2 pseudoparticle generation and applications.

The relapsing malaria species, Plasmodium vivax, is the most widely distributed and difficult-to-treat cause of human malaria. The merozoites of P. vivax preferentially invade ephemeral human CD71⁺ reticulocytes (nascent reticulocytes), thereby limiting the development of a robust continuous culture in vitro. Fortunately, P. vivax’s sister species, P. cynomolgi Berok, can be cultured continuously, providing the ability to screen novel therapeutics drug and vaccine candidates in a reliable and high-throughput manner. Based on well-established growth inhibition activity (GIA) assays against P. falciparum and P. knowlesi, this protocol adopts the current flow cytometry assay methodology and investigates P. vivax inhibitory antibodies using the P. cynomolgi Berok invasion model based on the thiol-reactivity and DNA abundance of viable parasites in macaque erythrocytes. Established GIA assays screen antibodies at either a single concentration or high/low dose concentrations to provide quick insights for prioritizing potential antibodies capable of specifically interrupting parasite ligand and host receptor binding with minimal concentrations. Hence, this protocol expands on the existing GIA assay by using serially diluted antibodies and generating a dose-response curve to better quantify the inhibitory efficacy amongst selected vaccine candidates.