AFFINITY MATURATION The process by which B-cells produce antibodies with

increased affinity for antigen during the course of an immune response.

A secondary response can elicit antibodies with greater affinities than in a primary response.

IN VIVO AFFINITY MATURATION The process is thought to evolve two interrelated

processes,occuring in the germinal centers of the secondary lymphoid organs.

1) Somatic hypermutation(SHM)-Mutations in the variable regions of the immunoglobulin genes.

2) Clonal selection-B-cells that have undergone SHM must compete for limiting growth resources,including the availability of antigen.

•The follicular dentritic cells(FDCs) of the germinal centers present antigen to B-cells and only the B-cell progeny 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 out completed and deleted.

IN VITRO AFFINITY MATURATION This is based on the principle of mutation and

selection.

In vitro affinity maturation has successively been used to optimize antibodies,antibody fragments,antibody mimetics.

Random mutation can be introduced using radiation,chemical mutagens or error prone PCRs.

SOMATIC HYPERMUTATION Somatic hypermutation or SHM is a diversity

generating regulated cellular mechanism displayed by adaptive immune response.

Somatic hypermutation adds diversity in already rearranged gene segments.

The mutation rate is 10-3/base pair/generation than the spontaneous mutation of about 10-8/base pair/generation.

•Hence the name hypermutation.•These mutations occur mostly at “hotspots” in the DNA, known as hypervariable regions.•These regions correspond to the complementarity determining regions, the sites involved in antigen recognition on the antibody

•These mutation result from nucleotide substitution rather than addition or deletion.•They influence the process of affinity maturation of B cells.

MOLECULES INVOLVED IN SHM IgM antibodies expressing V regions that have

undergone SHM, as well as IgG and IgA antibodies with no somatic mutations.

Activation-induced cytidine deaminase: A

B cell-specific protein required for SHM.

Uracil N- glycosylase : Removing uracil generated by AID deamination.

•Mismatch repair proteins: Mismatch repair (MMR) normally maintains genomic integrity.

•MMR eliminates mutations that arise spontaneously in the genome.

•Error-prone DNA polymerases: increase mutation rate

CELLULAR EVENTS