$$\rightleftharpoonup{xx}$$
$$\longleftharp{xx}$$,
$$\longrightharp{xx}$$,
B lymphocytes are the principal player in humoral immunity and the only cell type in mammals that are capable of producing antibodies, also termed as immunoglobulins (Ig)1,2. Antibodies secreted by B cells provide a formidable defense against invading pathogens via diverse mechanisms including neutralization, opsonization and complement activation, leading to protective immunity3. Secretion of antibodies by B cells is only achieved after full activation of specific B cells, which normally requires two distinct signals3. Signal 1 is relayed by direct binding of the antigen (Ag) to the B cell receptor (BCR) expressed on the surface of specific naïve B cells3. Depending on the source of Signal 2, B cell activation can be divided into thymus-dependent (TD) or thymus-independent (TI)3,4. In a TD antigen response, Signal 2 is provided by activated cognate CD4 T helper (TH) cells, which express CD154, the ligand for the co-stimulatory receptor CD40 expressed on B cells1,2,3. In a TI antigen response, Signal 2 comes from either engagement of Toll-like receptors (TLRs in the case of type 1 TI Ag) or extensive cross-linking of the BCRs (in the case of type 2 TI Ag) on the B cells3,4. Type 1 TI (TI-1) antigens are microbial ligands of TLRs, including bacterial lipopolysaccharides (LPS), viral RNAs, and microbial CpG DNA4,5. Type 2 TI (TI-2) antigens have highly repetitive structure, and are able to deliver prolonged and persistent signaling to the B cell by multiple cross-linking of the BCRs4,6. Typical examples of TI-2 antigens include pneumococcal polysaccharides and hapten-conjugated polysaccharide6,7. Both TD and TI antigens can elicit robust antigen-specific IgM responses and can also induce the production of isotype-switched antibodies (IgG, IgA and IgE) with the help provided by antigen presenting cells (APCs) such as dendritic cells (DCs)1,2,3. Furthermore, both TD and TI antigens are able to induce memory responses with the help of APCs, but TD antigens are more efficient at inducing memory B cell generation3,8.
In this protocol, TD and TI Ig responses are elicited in mice by intraperitoneal (i.p.) immunization with hapten-conjugated model antigens 2,4,6-trinitrophenyl-keyhole limpet hemocyanin (TNP-KLH) and TNP-polysaccharide (neutral, highly branched and high-mass), respectively9,10,11. TD antigens are usually used with an adjuvant to enhance the production of antibodies12. Here in our protocol, TNP-KLH is injected with alum, a commonly used adjuvant in immunization studies12. Other examples of adjuvants that can be used include complete or incomplete Freund's adjuvant (CFA or IFA), monophosphoryl-lipid A/trehalose dicorynomycolate ("Ribi" adjuvant), and CpG oligodeoxynucleotides, etc.13,14. After immunization, mouse sera are harvested at different time points and TNP-specific antibodies in sera are quantified using Ig isotype-specific enzyme-linked immunosorbent assay (ELISA)9,10,11.
ELISA is a plate-based assay that is widely used as a diagnostic tool in medicine and also as an analytical tool in biomedical research15,16. It is used to detect and quantify analytes including antibodies, hormones, cytokines, chemokines, and various antigens, etc. ELISA can be performed in several different formats, including direct, indirect, sandwich and competitive ELISA15,16. In general, it involves the immobilization of the antigen to a solid surface, usually a 96-well microtiter plate, which is incubated with a primary antibody. After incubation, the unbound antibody is washed away. In a direct ELISA, the primary antibody is directly conjugated to an enzyme (typically horseradish peroxidase or alkaline phosphatase), which can cleave a chromogenic substrate to yield a visible color change detected by a signal-detection instrument such as a spectrophotometer15,16. In contrast, if an enzyme-linked secondary antibody is used to bind the primary antibody, then this is considered as an indirect ELISA15,16. Direct ELISA is faster whereas indirect ELISA is more sensitive15,16. In a sandwich ELISA, the plates are coated with a "capture" antibody used to immobilize the antigen of interest in the samples, and then the captured antigen can be detected by another "detection" antibody in a direct or indirect manner15,16. Sandwich ELISA offers high specificity since the antigen is detected by two different antibodies of the antigen. In a competitive ELISA, the competition is established between the sample antigen and the plate-bound antigen for binding to the primary antibody, and then the antigen concentration in sample is quantified by measuring the reduction in signal from the substrate15,16. Competitive ELISA can be performed using the above mentioned direct or indirect format and is useful for the detection of small antigens with only one epitope15,16.
Alternative techniques for the measurement of antibodies include radio-immunoassay (RIA), electrochemiluminescence (ECL) assay and surface plasmon resonance (SPR) assay17. RIA was the first immunoassay developed that measures the presence of an antigen (or antibody) with high specificity and sensitivity using radiolabeled reagents18,19. However, due to the concerns of radioactive toxicity, disposal costs, shelf-life and special licenses to work with radioactive materials, ELISA is a better and more convenient technique for common uses20,21. ECL is a highly sensitive assay in which chemiluminescent reactions are initiated using electricity to generate highly reactive species from stable precursors on the surface of an electrode, and can be used to measure the amount of analytes (such as antigens or antibodies)22. However, ECL requires a special instrument and thus is not as broadly used as ELISA23. SPR is a direct assay that can be used to measure the binding of ligands (e.g., antibodies) to immobilized molecules (e.g., antigens) on a sensor chip surface24. SPR detects the interactions in real time very specifically and does not require the use of labelled reagents as in ELISA. However, SPR also requires a special equipment and has lower sensitivity than ELISA17. Given the limitations of the alternative methods, ELISA is the most suitable and convenient technique for our purpose in this protocol. Here, we describe the use of sandwich ELISA for the analysis of total Ig isotype levels and the procedures of indirect ELISA for the analysis of antigen-specific Ig isotypes.