One option that is being pursued is the
use of ‘‘experimental medicine’’ trials in
humans, which are small Phase-I-like
studies in humans that are not necessarily
positioned on a product development
path but are designed for discovery pur-
poses. The barrier there is that materials
made under Good Manufacturing Prac-
tices (GMP) are required, and the produc-
tion and qualification of such materials are
costly. An alternate strategy, used with
success in this series of papers, is the
use of transgenic mice into which human
antibody genes are ‘‘knocked in’’ in
various combinations. The investigators
in the collaborative network associated
with these papers have picked increas-
ingly challenging tasks using humanized
mouse models.
In the simplest approach, the lowest
bar, human antibody genes for a paired
heavy and light chain are knocked in to
assess whether an immunogen stimu-
lates the final B cell receptor desired.
Next, a recombined RUA germline
mouse can be tested to see whether a
‘‘germline-targeted’’ antigen can initiate
proliferation of a clone for which the re-
combined BCR is in high frequency;
this heavy chain can be paired with the
natural light chain, or the murine light
chain gene segments can be allowed to
recombine and pair, a higher level chal-
lenge. Finally, the antibody V, D, and J
gene segments can be knocked in to
see if germline targeting is effective
when many junctions (encoding diverse
heavy-chain CDR3 regions) are present,
with optimal clones for the antigen likely
to be in low frequency and having to
compete for antigen and space in the
repertoire with many other clones. For
example, Alt and colleagues (Tian et al.,
2016 ) report here the generation and
use of two sophisticated types of mouse
models for study of immunization target-
ing the induction of antibodies similar to
the CD4 binding site targeting mono-
clonal antibody VRC01, which are en-
coded by the particular human heavy-
chain variable gene segment IGHV1-2.
The investigators designed the trans-
genic model such that this gene segment
was expressed in combination with
diverse heavy-chain complementarity
determining region 3 (HCDR3) domains
(‘‘IGHV1-2*02-rearranging mice’’). The
mice were created using some clever en-
gineering strategies by putting the hu-
man gene segment in the position most
proximal to the diversity (D) segments
with which it must combine and inacti-
vating a major V-D-J recombination
regulatory element. Using sequential
immunization with antigens, including a
previously described engineered outerdomain-GT8 protein expressed as part
of a 60-subunit self-assembling nanopar-
ticle, these mouse models revealed that
VRC01 precursor antibodies can be
engaged and further matured into HIV
neutralizing antibody lineages.
The principal leaps forward in these
studies are that germline targeting initia-
tion (previously reported) can be sus-
tained or pushed along even to the point
that B cell clonal derivatives not only
bind diverse HIV Envs, but they begin to
acquire inhibitory (neutralizing) activity. It
is a major accomplishment to bridge the
gap from initiation of proliferation of germ-
line configuration B cell clones to sustain-
ing intermediate members of what are
essentially canonical clonotypes or path-
ways that start to neutralize HIV. Until
now, it was not clear that rational strate-
gies of structure-based design could
push B cell clones along to acquire
neutralizing capacity.
Much remains to be done of course.
These are proof-of-concept experi-
ments, in contrived animal models, with
obvious limitations. Incomplete breadth
of coverage of HIV field strains was
achieved, and the levels of neutralizing
potency that were induced are not alto-
gether sufficient for the final solution. But
the studies do suggest that the overall
strategy of walking clones forward with
structure-based designs focused on
sequential phases of bNAb induction is
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