2. • ANTIBODY STRUCTURE
• GERMLINE AND AFFINITY MATURATION
• DEFINITION
• MONOREACTIVE VS POLYREACTIVE
• MOLECULAR MECHANISMS OF AB POLYREACTIVITY.
• CORRELATES
• PHYSIOLOGICAL ROLE
• PATHOLOGICAL ROLE
3. DEFINITION
• Antibody molecules are Monoreactive or Polyreactive types
Monoreactive antibodies are exactly fitting to their respective antigens
with high affinity.
Polyreactive Antibody are :
• Able to bind at least two structurally different Ags from a broad Ag
group with low affinity reactions.
A large percentage of natural antibodies are polyreactive.
4. MONOREACTIVE vs POLYREACTIVE
Polyreactive Ab Monoreactive Ab
Antigen Structurally-diverse Ags
(e.g., proteins, carbohydrates,
DNA)
Single fitting(cognate) Ag
Affinity Low High
Sequence Germline Somatically mutated
Ig class Mainly IgM, IgG, IgM, IgA
Half-life Short (IgM: 8 h) Long (IgM: 35 h; IgA: 26 h; IgG: 280 h)
6. During B-cell development, one V segment and one J segment (encode parts of the light-chain variable region)
join together with the C (constant which) region to form the gene for the antibody light chain.
This gene rearrangement occurs in the DNA, prior to gene transcription into mRNA.
GERMLINE MATURATION
8. AFFINITY MATURATION
Somatic hypermutation (SHM): Mutations in the variable, antigen-binding
sequences i.e complementarity-determining regions (CDR)) of the immunoglobulin
genes occurs. The mutations alter the binding specificity and binding affinities of
the resultant antibodies.Mutated plasma cells are produced.
Clonal selection. B cell progeny (which have undergone SHM) with the highest
affinities for antigen will be selected to survive.
B cell progeny that have undergone SHM, but bind antigen with lower affinity will
be deleted .The resultant secreted antibodies produced will have HIGH AFFINITY for
the respective antigen.
Certain antibodies increase their structural flexibility by affinity maturation
9. MONOREACTIVE vs POLYREACTIVE
Polyreactive antibody PAb2E4 binds strongly to b-
galactase (b-gal) and single-stranded DNA (ssDNA)
and moderately to insulin, thyroglobulin (Tg) and
LPS
Monoreactive antibody MAb GAL-40 only
recognizes its related antigen, b-gal.
10. MOLECULAR MECHANISMS OF AB POLYREACTIVITY.
Ab molecule (in gray) with flexible
Ag-binding site can adapt to
different Ags (shapes in red) by
structural changes that occur at
the time of interaction.
11. MOLECULAR MECHANISMS OF AB POLYREACTIVITY.
Conformational isomerism model
Here the antibody exist in equilibrium
with different isomers of a single antibody
before Antigen contact.
12. MOLECULAR MECHANISMS OF AB POLYREACTIVITY.
Some Abs can use different
regions from their Ag binding
sites for binding to various Ags
• Human rheumatoid factor binds the Fc
portion of IgG by using amino acid residues
on the edge of the Ag-binding site
• The actual binding site of this Ab is not
occupied even after binding to Fc fragment
and thus could still establish interactions
with the target Ag.
13. CORRELATES OF POLYREACTIVITY
• There is no sequence that can predict the polyreactive or
monoreactive behaviour of an Antibody.
• CDR (Complimentarity determining region)3 region of the H chain
• Main factor that determines the polyreactivity of an Ab molecule is
the structural dynamics of the Ag-binding site.
• It is the intrinsic ability of this region to assume various
configurations/shapes that defines polyreactivity.
14. INDUCED ANTIBODY POLYREACTIVITY
• Few circulating Igs acquire polyreactive characteristics after
exposure to certain protein-destabilizing agents.
• Such agents are
chaotropic agents(break the hydrogen bonds)
low pH
high salt concentrations
reactive oxygen species, iron ions, and heme
• pro-oxidative molecules - inflammatory conditions
• Also Abs with induced polyreactivity demonstrate powerful anti-
inflammatory activity (septic shock and autoimmune diabetes)
15. PHYSIOLOGICAL ROLE
1) Increases diversity of immune stock.
• further evolution of highly specific Abs.
• help in magnifying the Ag detection power of the immune system.
• provide the required dynamics for evolution of highly specific receptors.
2) Immune surveillance
• Immunoregulatory activity.
• Transport and tolerogenic presentation of antigens to T cells.
• Clearance of apoptotic cells and macromolecules.
16. PHYSIOLOGICAL ROLE
• A complete and healthy immune system should consist of
specific(stringent) and non-specific(promiscuous) receptors.
The specific receptors will provide costancy of the immune reactions
and help in building the memory of the system.
The non- specific receptors will give the immune system the
adaptability and evolvability.
17. PATHOLOGICAL ROLE
• 1) Infectious disease
• Provide a first line of defence against pathogens.
• Bacteria induced polyreactive Abs contribute to the direct neutralization of
the pathogen.
• Dampen the inflammation by clearing apoptotic cells and cellular debris.
• HIV(Beneficial) :
Virus neutralisation
Higher tolerance to mutation to target epitopes
• HIV(harmful) :
Initiate cytotoxic reactions against T lymphocytes
May synergize with the virus and contribute to AIDS.
• Dengue(harmful) : the binding of polyreactive Abs to the virion may
promote infection by facilitating entry into the target cells
18. PATHOLOGICAL ROLE
2) B cell malignancy
• Direct role in pathogenesis of CLL, MALToma & splenic marginal zone
lymphoma.
• Provide continuous stimulation and transmission of survival signals to
transformed B cell clone.
• CLL : monoreactive vs polyreactive antibodies
19. PATHOLOGICAL ROLE
3)Autoimmune disease
• High prevalence of naive B cells expressing polyreactive BCRs is observed in
patients with systemic lupus erythematosus and rheumatoid arthritis
4)Allergy
• Polyreactive IgE Abs contribute to the pathogenesis by providing
continuous stimulation of the mast cells upon interaction with endogenous
auto-allergens(atopic dermatitis).
20.
21. References
1) Antibody Polyreactivity in Health and Disease: Statu Variabilis
Tchavdar L. Vassilev, Srinivas V. Kaveri and Sebastien,Jordan D. Dimitrov, Cyril Planchais, Lubka T. Roumenina,
Lacroix-Desmazes. J Immunol 2013; 191:993-999.
2) Properties and function of polyreactive antibodies and polyreactivenantigen-binding B cells
Zhao-Hua Zhou, Athanasios G. Tzioufas, Abner Louis Notkins. Journal of Autoimmunity 29
(2007) 219e228
23. • Affinity maturation is a process of affinity-selected differentiation of activated B cells. Repeated
exposures to the same antigen provokes greater antibody ligating affinity in the antibody secreted
by successive generations of plasma cells.
The mechanisms by which affinity maturation is achieved are somatic hypermutation and clonal
selection.
• Somatic hypermutation (SHM) is a diversity generating, regulated cellular mechanism through
which antibodies are produced against an enormous variety of different potential antigens. The
binding affinities of the variable regions of immunoglobulins are altered by AID-enzyme-
promoted mutations during antigen-stimulated proliferation of B cells. These somatic
hypermutations are transcribed and translated into thousands of slightly different
immunoglobulins coded by the hypermutated V regions. The complementarity determining
regions of these antibodies possess different affinities for the encountered antigen, and clonal
selection will favor cells equipped with highest affinity antibodies because these B cells are
favoured in terms of activation and co-operation with T cells.
Clonal selection is the phenomenon whereby a previously unencountered cognate antigen
(epitope) can stimulate naïve B lymphocytes to proliferate and differentiate into clones of
memory B cells and plasma cells that produce antibodies with the highest affinity for the antigen.
Those B cells that have highest affinity BCR against the encountered antigen will be selected for
proliferation, antibody production, and committment to an antigen-specific memory lineage.
Thus, SHM prepares a spectrum of antibodies with different affinities for the antigen, while clonal
selection ensures that the immune system will react increasingly effectively (highest affinity) to an
encountered antigen and will be ready for rapid response to subsequent encounters with the
antigen.
