First developed and most widely used technology for generation of monclonal antibodies = the use of mouse hybridomas. It is generated from stable fusion of immortalised myeloma cells with B cells from immunnized mice.
- mice first exposed to antigen(Ag) to which AB of interest is specific for –> by series of injections of Ag
- B cells then isolated from spleen
- myeloma cells are aslo selected to ensure that they themselves do not secrete AB and lack hypoxanthine guanin phosphoribosyltransferase (HGPRT) gene , making them sensitive to HAT medium
- myeloma cells + B cells are fused using polyethylene glycol
- fused cells incubated in HAT((hypoxanthine-aminopterin-thymidine) medium ,
- aminopterin blocks nucleotide synthesis. Therefore cells forced to use the purine salvage pathway to synthesise purine nucleotide which require HGPRT , as B cells have this gene , so only B cells that have fused will be able to survive.
- unfused myeloma cells is removed for its potential to outgrow other cells
- B-cell myeloma hybrids produce AB that are immortal
- the incubated medium is diluted into multi well plate to extent that each well contains one cell
- so AB produced in single cell , were generated from single B cells = directed towards same epitope and thus monoclonal AB
- B cells that produces desired AB then cloned in supplemental media containing IL-6
- once hybridoma colony established –> continue to grow in culture medium like RPM1-1640( fetal bovine serum+ antibiotics) and produce AB
The ubiquitous use and broad success of monoclonal antibody in research , is sharp contrast to 3% success rate in drug development.
This high infrequent success rate reflects:
- the high immunogenicity of these foreign proteins in humans as well as,
- weak interactions that mouse antibodies typically have with human complement and FcγRs,which results in inefficient effector functions(fi,g 1)
- mouse AB does not bind human salvage receptor FcRn, resulting in terminal half-life (typically less than 20 hours)
These limitations of mouse AB have largely been overcome by their chimerization or humaization.
Chimerization and humanization
Chimerization involves joining the variable( antigen binding region) domains of mouse monoclonal antibody to the constant domains of human antibody. This process requires the appreciation of structure and function of immunoglobulin domains.
- Starting point= hybridoma cell line generating AB against desired epitope
- reverse trascriptase PCR used to generate copies of DNA–> complementary to mRNA coding for variable region of murine mAB
- Recombinant DNA techniques allow this cDNA for variable region to be combined with DNA coding for human constant region
- recombinant DNA introduced into myeloma cell line using vectors
- chimeric mAB is expressed
- because original murine variable region= bind to same epitope
- because human constant region= less immunogenic and interact with human receptors to elicit therapeutic effects e.g (ADCC, ADCP,CDC)
Rituximab is an example of a chimeric mAb that is used clinically to treat non-Hodgkins lymphoma . Unfortunately, even the murine variable region can still be recognised as foreign—leading in many cases to the same limitations as murine mAbs.
The simplest of many humanization strategies involves transfer of complementarity determining regions (CDR) from mouse AB to human IgG. For generation of high affinity binding , generally requires additional transfer of one or more framework-region residues from parent mouse AB.
Route for completely human Ab
Transgenic mice - human AB generated by target antigen immunization of mice transgenic for human immunoglobulin (Ig) genes and disrupted Ig heavy chain and Igκ light-chain loci.
Subsequent progress in research enabled the inclusion the expression of more V gene segments by transgenic mice, thereby expanding the potential repertoire of recovered AB.
- B cells that express human AB are isolated from immunized mice.
- then the B cells are cloned as for hybridomas, similar to generation of mouse monoclonal AB
- binding affinity of human AB from transgenic mice = often high, reflecting in vivo affinity maturation= integral to secondary immune response.
- initial output of transgenic mice is human IgG, allowing early screening of biological function.
- hybridoma cell lines generated from the transgenic mice= produce AB easily for screening through pre-clinical development.
- recombinant cell lines such as CHO (chinese hamster ovary) or NSO mouse myeloma cell = produce higher AB titres ( than hyridoma) –> chosen for later stage of clinical development if not all.
Phage-display libraries- phage encoding single chain V domain AB fragment (scfv) first described 1990.
- Diverse human Ig heavy chain V (Vh) gene segments and light chain V gene segment were prepared. How? using amplification PCR from peripheral blood lymphocytes of non immunized donors.
- scFv generated randomly combining the variable heavy and light chain gene segments using PCR,
- Combinatorial library(fewer than 107 scFv genes) was then cloned for display on surface of phage,
- then used to identify scFv that bind target antigens.
- More commonly,antibodies need to be optimized to meet the design goals of the project, and this is readily accomplished by selection for high-affinity variants from phage-display
One human antibody isolated by phage-display technology has been approved widely for therapeutic use: adalimumab ( TNF- specific monoclonal AB) used for treatment of rheumatoid and psoriatic arthritis.
A particular strength of phage-display libraries, in contrast to hybridoma technology, is that direct
selection for exquisitely specific binding properties, such as species crossreactivity, is sometimes possible( allowing the biological function of the antibodies to be evaluated in animal models of disease.)