Activation of B cells requires two signals. Depending on the nature of the antigen, B-cell activation proceeds by two different routes, one dependent on helper T cells (TH cells), the other not. In the case of T dependent antigen the interaction between CD40 of B Cells and CD40 ligand of T cells gives second signal but in T independent antigen, cross-linking of membrane bound immunoglobulin to polymeric carbohydrate gives the needed signal.
The B-cell response to thymus-dependent (TD) antigens requires direct contact with TH cells, not simply exposure to TH-derived cytokines.
Antigens that can activate B cells in the absence of this kind of direct participation by TH cells are known as thymus-independent (TI) antigens. TI antigens are divided into types 1 and 2, and they activate B cells by different mechanisms whereas polymeric proteins e.g., bacterial flagellin acts as Type 2 thymus-independent (TI-2) antigens.
T Independent (TI) Antigen
A typical large polysaccharide is made up of repeating sequences of few simple sugars so it has multiple copies of identical antigenic determinants.
When a specific naïve B cells come in contact with such antigens, these antigenic determinants bind the surface IgM and IgD receptors. This binding leads to clustering of surface immunoglobulins which generates a signal, strong enough to activate the naïve B cells. This activated B cells produces and releases first immunoglobulin i.e. IgM.
Most TI-1 antigens are polyclonal B-cell activators (mitogens); i.e. they are able to activate B cells regardless of their antigenic specificity. At higher concentrations, some TI-1 antigens will stimulate proliferation and antibody secretion by as many as one third of all B cells but in lower concentrations of TI-1 antigens, only those B cells specific for epitopes of the antigen will be activated.
TI-2 antigens activate B cells by extensive crosslinking of membrane bound immunoglobulin (mIg) receptor.
Unlike TI-1 antigens, TI-2 antigens do not act as polyclonal activator, activate only mature B cells and may require cytokines derived from TH cells.
Main features of T Independent Antigen (Ti-Ag)
- Antigens that stimulate B-cells directly, without co-stimulation by helper T-cells
- Usually polysaccharides or lipopolysaccharides (e.g. bacterial capsules)
- Cross link antigen receptors on the surface of B-cells to activate them
- Don’t generate strong immune response (no memory cells, IgM is the only antibody class produced, and the immunity doesn’t last long).
T Dependent Antigen (Td-Ag)
Humoral response to protein and most other antigens requires interaction of B cells with helper T cells. These are thymus-dependent or simply T-dependent (TD) responses. B cell activation by T dependent antigens require contact dependent help delivered by the interaction between CD40 on B cells and CD40L on activated TH cells.
Main steps during B-cell activation by a thymus-dependent antigen:
- Soluble protein antigens which bind to membrane bound immunoglobulin on the surface of B Cell are internalized, processed and are displayed as peptide-MHC-II complexes.
- TH cell recognizes class II MHC-antigen complex on B-cell surface via TCR. It also interacts with costimulatory molecule B7 via CD28. These interactions activates TH cell. Activated TH cells produces various cytokines.
- TH cell begins to express CD40L and interacts with CD40 of the B Cell. The interaction between CD40 and CD40L provides second signal to activate B cell.
- B cells begins to express receptors for various cytokines and binds to cytokines released from TH cells. Which activates B cell and differentiates them to plasma cells.
- The activated B-cell clonally proliferates to produce a population of plasma cells and memory cells, which all recognize the same antigen
This CD40/CD40L interaction is essential for B-cell survival, the formation of germinal centers, the generation of memory-cell populations and somatic hypermutation (for affinity maturation).
Difference between T dependent Antigen and T independent Antigen
|T dependent (TD) Antigen|| |
T independent Antigen
|Soluble proteins|| |
Bacterial cell wall components Lipopolysaccharide (LPS), Capsular polysaccharide, flagella etc.
Antigen is processed and displayed in the surface of Antigen Presenting Cells (B Cells) in association with MHC-II.
Antigen processing is not needed
Immunogenic over wide range of dose
Dose dependent immunogenicity
|No polyclonal activation i.e. Activate B cells monoclonally|| |
Polyclonal activation of B cells occur in high doses of Type-I TI Antigens
|Immunologic memory present|| |
No Immunologic Memory
|Affinity Maturation- Yes||Affinity Maturation- No|
|Isotype Switching occurs (i.e. antibodies of all classes are produced)||No Isotype switching |
( Antibody response is restricted to IgM and IgG3)
|Activate mature B cells only|| |
Activate both mature and immature B cells
Conjugate vaccines: A way of developing IgG response against polysaccharide antigen.
Capsular polysaccharides (e.g. of Neisseria meningitidis, Haemophilus influenzae, Streptococcus pneumoniae) and/or lipopolysaccharides (major cell wall component of Gram negative bacteria e.g. Salmonella typhi, E.coli etc) are the major structural components of bacterial pathogens. But these polysaccharide antigens are mostly poor immunogen; the antibody response to these antigens is mostly restricted to IgM (lack of isotype switching) because of their T-lymphocyte independent (TI) nature. IgM antibodies though excellent in activating complement penetrates poorly into tissues and is not itself opsonizing. Anti-polysaccharide immune response is also characterized by lack of T-lymphocyte memory.
Immunity against these surface components confers protection against the disease caused by these pathogens but the most vulnerable age group (children below 2 years of age and elderly) responds poorly to carbohydrate antigen. If the B Cells is switched to produce IgG, the vaccine would be more effective. To overcome the problem arise due to TI nature of carbohydrate antigen and to produce IgG response against such antigen conjugate vaccine is being used.
Antibody response to polysaccharide antigen like pneunmococcal or meningococcal capsular antigen are restricted to IgM, which Polysaccharide antigens are mostly poor immunogen due to their T-lymphocyte independent (TI) nature. Often, anti-polysaccharide immune response is characterized by lack of T-lymphocyte memory and lack of isotype switching. Children below 2 years of age and elderly (which are mostly vulnerable to disease caused by these pathogens) poorly respond to polysaccharides antigens.
In the conjugate vaccine, carbohydrate is coupled to an immunogenic protein also known as carrier molecule/peptide. Generally formalin inactivated diphtheria toxin is used as carrier molecule.
Here is how the conjugate vaccine works:
- In conjugate vaccine, capsular polysaccharide is coupled with highly immunogenic protein (which is commonly diphtheria toxin).
- B cell specific to the polysaccharide antigen, binds to polysaccharide antigen, and endocytose polysaccharide antigen along with the coupled protein.
- The protein is broken down in to various peptides. Some of the processed peptides are loaded on MHC Class II molecules and move to the surface for the recognition by the T cells (i.e. Processing and Presentation of Peptide antigen presentation by B cell)
- Carrier peptide specific T cells (mainly follicular helper T Cell), recognizes the MHC-II peptide antigen by its T Cell Receptor (TCR). Engagement of CD40 and CD40 ligand also takes place which gives switch signal.
- The activated B cell first secrete anti-carbohydrate IgM antibodies and because of the T cell help than switches to secreting IgG antibodies.
- Thus by coupling the protein with polysaccharide antigen we make the B cell able to do something that it could not have done on its own.
References and further reading:
- Cellular and Molecular Immunology, 9th Edition
- Kuby Immunology, 7th Edition
- Roitt’s Essential Immunology, 13th Edition