Antibody Molecules, Genetic Engineering of
337
This product is cloned and sequenced, and
then used for the construction of chimeric
antibodies or for further genetic modiFca-
tions, as described below.
±or the most part, chimeric antibodies
retain their target speciFcity and show
reduced HAMA responses. An example
of a successful mouse–human chimeric
antibody approved for clinical use is Rit-
uximab (Rituxan/Mabthera), which targets
the CD20 antigen and is now widely used
to treat non-Hodgkin’s lymphoma.
2.3
Humanized Antibodies
Although mouse–human chimeric anti-
bodies are less immunogenic than mouse
antibodies, in some cases they can still
elicit a signiFcant human antichimeric an-
tibody (HACA) response. One approach
to overcome this problem is to further
manipulate the antibody variable region
encoding for the antigen binding site re-
sulting in humanized antibodies (±ig. 4).
Each variable domain consists of a
β
-
barrel with seven antiparallel
β
-strands
connected by loops. Among the loops are
the CDR regions. It is feasible to move
the CDRs and their associated speciFcity
from one scaffolding
β
-barrel to another,
thereby creating ‘‘CDR-grafted’’ or ‘‘hu-
manized’’ antibodies. However, it is rarely
sufFcient to move only the CDRs from a
murine antibody onto a completely human
framework because the resulting antibody
frequently has reduced or no binding ac-
tivity. In these cases, additional mouse
residues near the CDRs are incorporated
until binding is restored.
An example of a successful humanized
monoclonal antibody that has been ap-
proved for clinical use is Trastuzumab
(Herceptin),
which
has
demonstrated
signiFcant antitumor activity in patients
affected with breast cancer overexpressing
the tumor-associated antigen HER2/
neu
.
2.4
Human Monoclonal Antibodies in Mice
Recently, mice have been produced that
make antigen-speciFc antibodies that are
totally human. To accomplish this goal,
transgenic mice carrying portions of the
human IgH and Ig
κ
loci [in germ line
conFguration using megabase-sized YACs
(yeast artiFcial chromosome)] were ob-
tained that included the majority of the
variable region repertoire, the genes for
C
µ
,C
δ
and C
γ
1, C
γ
2, or C
γ
4, as well
as the
cis
elements required for their
function. The IgH and Ig
κ
transgenic
was then bred into a genetic background
deFcient in the production of murine im-
munoglobulin. Therefore, the resulting
mouse model, named
XenoMouse
, has ele-
ments of the human heavy- and light-chain
loci in a murine context in which en-
dogenous mouse heavy and light chains
were disrupted.
The large and complex human variable
region repertoire encoded on the im-
munoglobulin transgenes in XenoMouse
strains support the development of large
peripheral B-cell compartments and the
generation of a diverse primary immune
repertoire similar to that of adult humans.
The human genes are compatible with
mouse enzymes mediating class switch-
ingf
romIgMtoIgGaswe
l
lassoma
t
ic
hypermutation and afFnity maturation.
Importantly, the immune system of the
XenoMouse recognizes human antigens
as foreign, with a concomitant strong
human humoral immune response. The
use of XenoMouse mice in conjunction
with well-established hybridoma proce-
dures (see ±ig. 3) reproducibly results in
human IgG monoclonal antibodies with
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