Immunotherapy for Infectious Diseases

Production of Igs and MAbs Targeting Infectious Diseases 83
fragments can then be chemically linked to a variety of substances including plant and
bacterial toxins, enzymes, radionuclides, or cytotoxic drugs. Such modifications may
be useful tools for the imaging of or to attack cancer cells if antibodies recognizing
specific surface marker proteins are available.
Recombinant DNA techniques allow expression of hybrid molecules in different
recombinant cell systems. Especially in human therapy, human/mouse hybrid antibodies
are essential tools in overcoming the problems of interspecies reactions. Mouse
MAbs that have undergone Fc replacement are described as chimeric antibodies.
Humanization of antibodies is achieved by transferring the antigen-specific binding
regions and single-framework amino acids of a mouse antibody into a human antibody
backbone.
Chimeric Antibodies
Chimeric antibodies are generated biochemically or genetically by combining variable
regions of mouse antibodies with human constant regions. Such recombined antibody
cDNAs have been successfully expressed in different host cell lines. These expression
systems use transcription of heavy- and light-chain cDNAs under the control of strong
viral or cellular promoters and RNA processing elements in standard eukaryotic expression
vectors. Such a hybrid chimeric antibody molecule contains less than 10% of mousederived
sequences coupled to approx. 90% sequences of human origin, usually retaining
100% of the functional binding properties of the parental progenitor.
Humanized Antibodies (CDR-Grafted Antibodies)
To improve therapeutic benefit, humanization of antibodies has become essential. It
was observed that not only rodent antibodies induced an immune response in patients,
but also chimeric antibodies switched within the Fc region also induced the generation
of HAMAs.
Humanized antibodies are chimeric molecules with only the six hypervariable loops
of the original non-human antibody transfered into human framework regions (Fig. 5).
Essentially, powerful techniques have been developed for the generation of antibodies
that are nearly capable of substituting for the mammaliam immune system. The simplest
way of performing humanization is to graft the complementary determining
regions (CDRs) on human framework (FR) regions. The relative importance of at least
single residues of the FR regions is determined by conformational adjustment of the
CDRs after interaction with antigen. The first humanized monoclonal antibody of clinical
relevance was CAMPATH-1 (100). Humanization experiments with the murine
anti-human CD3 mAb OKT3 made it evident that distinct amino acids in the FR
regions contribute to the affinity of antibodies. A CDR-grafted version of OKT3 incorporating
only the CDRs from OKT3 was found to be functionally inactive (101).
Phage Antibody Libraries
The intact humoral immune system of the mammal represents the most potent
library of antibodies. In vivo, after antigen contact the most suitable antibodies are
selected, affinity-matured, and amplified in plasma cells. Screening and isolation of
high-affinity human MAbs can be imitated by phage techniques and has been thoroughly
reviewed (102–104). Essentially, human VH and VL chains can be amplified
from B-cell populations and cloned into the genome of phages. Fv or F(ab) fragments