Wild-type human chorionic gonadotropin (hCG)
has been used as a contraceptive vaccine. However,
extensive sequence homology with LH elicits
production of cross-reactive antibodies. Substitution
of arginine68 of the β-subunit (hCGβ) with glutamic
acid (R68E) profoundly reduces the cross-reactivity
while refocusing the immune response to
the hCG β -specific C-terminal peptide (CTP). To investigate
the molecular basis for this change in
epitope usage, we immunized mice with a plasmid
encoding a truncated hCG β-R68E chain lacking
the CTP. The animals produced LH-cross-reactive
antibodies, suggesting that the refocused immunogenicity
of R68E is a consequence of epitope
masking by a novel disposition of the CTP in the
mutant rather than a structural change in the
cross-reactive epitope region. This explanation
was strongly supported by surface plasmon resonance
analysis using a panel of anti-hCGβ-specific
and anti-hCGβ/LH cross-reactive monoclonal antibodies
(mAbs). Whereas the binding of the LH
cross-reactive mAbs to hCGβ-R68E was eliminated,
mAbs reacting with hCGβ-specific epitopes
bound to hCGβ and hCGβ-R68E with identical affinities.
In a separate series of experiments, we
observed that LH cross-reactive epitopes were silent
after immunization with a plasmid encoding a
membrane form of hCGβ-R68E, as previously observed
with the soluble mutant protein itself. In
contrast, the plasmid encoding the soluble secreted
form of hCGβ-R68E evoked LH cross-reactive
antibodies, albeit of relatively low titer, suggesting
that the handling and processing of the
proteins produced by the two constructs differed
