Combinatorial Phage Antibody Libraries
87
diverse libraries covering all functional
germ line V genes.
Also critical to library preparation is the
choice of tissue source from which spe-
ciFc mRNA can be accessed. In principle,
peripheral blood, bone marrow, spleen,
tonsil, or lymph node can be used as
a starting point for library construction.
In humans, bone marrow is known to
be a major reservoir of plasma cells that
serve to maintain circulating antibody
titers and has proven to be an excellent
starting point for library construction. In
contrast to bone marrow, the number of
antibody-producing cells in the peripheral
circulation is normally very low. However,
if a potential donor is exposed to secondary
contact with the antigen, the level of spe-
ciFc plasma cells increases dramatically
within around three days postexposure, re-
turning to a resting level a few days later.
It has been demonstrated that libraries
constructed from peripheral blood lym-
phocytes obtained during this time frame
can yield speciFc antibodies. Recombinant
±abs have also been isolated from libraries
prepared from spleen and tonsil, although
these and lymph tissues are not so read-
ily obtained.
To date, the major application of com-
binatorial libraries derived from immune
donors has been in the area of infectious
diseases, particularly viruses, although hu-
man antibodies have also been selected
against bacteria, protozoans, and self and
tumor antigens. With respect to viruses,
current clinical management of viral in-
fections continues to be inadequate, and
at present there are only a handful of an-
tiviral drugs in clinical use in a restricted
number of disease states. A signiFcant
number of studies, however, suggest that
human monoclonal antibodies may prove
to be a viable alternative. ±irst, it is well
known that speciFc antibodies are capable
of extremely efFcient neutralization of a
broad spectrum of viruses
in vitro
through
a variety of mechanisms. In addition,
numerous studies with human serum an-
tibody preparations or rodent monoclonal
antibodies have clearly shown the utility
of antibodies in preventing an array of
viral diseases, if these agents are adminis-
tered before or shortly after exposure to the
pathogen. In a smaller number of cases,
neutralizing antibodies have been found to
resolve established viral infections, an ac-
tivity once considered to be mediated exclu-
sively by cytotoxic T-lymphocytes. In hu-
mans, the lives of patients suffering from
Argentine hemorrhagic fever, induced by
Junin virus, can be saved by administration
of pooled human sera containing a high
titer of Junin virus neutralizing antibodies
withineightdaysoftheonsetofsymptoms.
In an animal model of RSV, infection-
neutralizing antibodies, administered in-
travenously or via aerosol at the height of
infection, efFciently cleared the virus from
the lungs of cotton rats. Antibodies have
also been shown to clear viral infections
from immunocompromised mice, includ-
ing persistent virus infections of neurons,
which are not recognized by CD8+ cyto-
toxic T-lymphocytes because the cells are
deFcient in molecules of the class I major
histocompatibility complex. The potential
of antibodies has also been demonstrated
in passive transfer studies in animal mod-
els in which sterilizing immunity against
viral challenge has been achieved.
Nevertheless, most commonly, an anti-
body does not provide sterilizing immunity
against natural viral infection. Rather, the
prevailing view has been that an antibody
acts by reducing the viral load to allow the
innate and the adaptive cellular arm of im-
munity to clear the remaining infection.
Indeed, neutralizing antibodies are essen-
tial to the success of the vaccines against
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