Chimeric and Hybrid Monoclonal Antibodies Have Potent Clinical Potential

One approach to engineering an antibody is to clone recombinant DNA containing the promoter, leader, and variableregion sequences from a mouse antibody gene and the constant-region exons from a human antibody gene (Figure 5-20). The antibody encoded by such a recombinant gene is a mouse-human chimera, commonly known as a humanized antibody. Its antigenic specificity, which is determined by the variable region, is derived from the mouse DNA; its isotype, which is determined by the constant region, is derived from the human DNA. Because the constant regions of these chimeric antibodies are encoded by human genes, the anti-

LIGHT-CHAIN GENES

HEAVY-CHAIN GENES

Mouse VL Human CL Mouse VH Human CH

Promoter

Promoter

Promoter

Promoter

Selection gene (ampR)

Plasmid DNA

Light-chain \\ Transfect U Heavy-chain chimeric T into Ab- w chimeric vector / myeloma cells \ vector

Selection gene (ampR)

Light-chain \\ Transfect U Heavy-chain chimeric T into Ab- w chimeric vector / myeloma cells \ vector

Transfected antibody-secreting myeloma cell

Plasmid DNA

Transfected antibody-secreting myeloma cell

Chimeric mouse-human antibody

Chimeric mouse-human antibody

FIGURE 5-20

Production of chimeric mouse-human monoclonal antibodies. Chimeric mouse-human heavy- and light-chain expression vectors are produced. These vectors are transfected into Ab~ myeloma cells. Culture in ampicillin medium selects for transfected myeloma cells that secrete the chimeric antibody. [Adaptedfrom M. Verhoeyen and L. Reichmann, 1988, BioEssays 8:74.]

bodies have fewer mouse antigenic determinants and are far less immunogenic when administered to humans than mouse monoclonal antibodies (Figure 5-21a). The ability of the mouse variable regions remaining in these humanized antibodies to provide the appropriate binding site to allow specific recognition of the target antigen has encouraged further exploration of this approach. It is possible to produce chimeric human-mouse antibodies in which only the sequences of the CDRs are of mouse origin (Figure 5-21b). Another advantage of humanized chimeric antibodies is that they retain the biological effector functions of human antibody and are more likely to trigger human complement activation or Fc receptor binding. One such chimeric human-mouse antibody has been used to treat patients with B-cell varieties of non-Hodgkin's lymphoma (see Clinical Focus).

Chimeric mouse-human antibody

Chimeric Antibody Human Mouse

Mouse

Human

Chimeric mouse-human antibody

Toxin

Chimeric immunotoxin

FIGURE 5-21

Mouse

Human

Mouse monoclonal antibody (anti-tumor)

Anti-tumor antibody

Toxin

Chimeric immunotoxin

Anti-T-cell receptor

Anti-T-cell receptor

Heteroconjugate

FIGURE 5-21

Chimeric and hybrid monoclonal antibodies engineered by recombinant DNA technology. (a) Chimeric mouse-human monoclonal antibody containing the VH and VL domains of a mouse monoclonal antibody (blue) and CL and CH domains of a human monoclonal antibody (gray). (b) A chimeric monoclonal antibody containing only the CDRs of a mouse monoclonal antibody (blue bands) grafted within the framework regions of a human monoclonal antibody is called a "humanized" monoclonal antibody. (c) A chimeric monoclonal antibody in which the terminal Fc domain is replaced by toxin chains (white). (d) A heteroconjugate in which one-half of the mouse antibody molecule is specific for a tumor antigen and the other half is specific for the CD3/T-cell receptor complex.

Chimeric monoclonal antibodies that function as im-munotoxins (see Figure 4-23) can also be prepared. In this case, the terminal constant-region domain in a tumor-specific monoclonal antibody is replaced with toxin chains (Figure 5-21c). Because these immunotoxins lack the terminal Fc domain, they are not able to bind to cells bearing Fc receptors. These immunotoxins can bind only to tumor cells, making them highly specific as therapeutic reagents.

Heteroconjugates, or bispecific antibodies, are hybrids of two different antibody molecules (Figure 5-21d). They can be constructed by chemically crosslinking two different antibodies or by synthesizing them in hybridomas consisting of two different monoclonal-antibody-producing cell lines that have been fused. Both of these methods generate mixtures of monospecific and bispecific antibodies from which the desired bispecific molecule must be purified. Using genetic engineering to construct genes that will encode molecules only with the two desired specificities is a much simpler and more elegant approach. Several bispecific molecules have been designed in which one half of the antibody has specificity for a tumor and the other half has specificity for a surface molecule on an immune effector cell, such as an NK cell, an activated macrophage, or a cytotoxic T lymphocyte (CTL). Such heteroconjugates have been designed to activate the immune effector cell when it is crosslinked to the tumor cell so that it begins to mediate destruction of the tumor cell.

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Responses

  • galdino
    What are chimeric and hybrid monoclonal antibodies?
    8 years ago
  • Aaran
    What is the expression vector in hybrid monoclonal antibody?
    2 years ago
  • ora
    Which heavy light chain antibody to use with chimeric antibodies?
    5 months ago

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