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with CRPV. Rabbits immunized with intact CRPV L1-capsids develop
high-titer antibodies detectable by an ELISA using intact L1-capsids as
antigen and are resistant to CRPV challenge. However, CRPV chal-
lenge to rabbits immunized with denatured CRPV L1/L2-capsids
results in development of extensive papillomas. Transfer of serum or
immunoglobulin G from rabbits immunized with CRPV L1-capsids
protects naive rabbits against the challenge, strongly suggesting that
the protection is mediated by neutralizing antibodies. Similarly, immu-
nization of calves with L1- or L1/L2-capsids of BPV4 is extremely
effective for protecting animals against BPV4 challenge. 44 The data
obtained from these animal experiments clearly indicate that the
properties of L1-capsids are suitable for prophylactic vaccine antigen.
Neutralization with Anti-L2 Antibodies in vitro
Roden et al . 45 showed that sheep antisera obtained by immunization
with bacterially produced L2s of HPV6, 16, and 18 neutralize the
pseudovirions of the homologous HPV type and cross-neutralize those
of the heterologous HPV types, indicating that a cross-neutralization
epitope(s) is present in L2.
We indicated the presence of a cross-neutralization surface epitope
in HPV16 L2 by characterizing anti-HPV16 L2 MAbs. 21,33 Seventeen
anti-HPV16 L2 MAbs were produced by immunizing Balb/c mice
with HPV16 L1/L2-capsids. All of the MAbs bind HPV16 L1/L2-
capsids in ELISA, indicating that they recognize epitopes displayed
on the surface of L1/L2-capsids. ELISA using synthetic peptides as
antigens showed that seven MAbs recognized linear epitope(s) within
HPV16 L2 (473 amino acids) aa69-81, and two MAbs (#5 and #13)
recognized those within aa108-120. 21,33 Amino acid sequences cor-
responding to the HPV16 aa108-120 are highly conserved among dif-
ferent HPVs (Fig. 3), and indeed MAb#5 and #13 bind to HPV6 L2. 33
MAb#5 and #13 inhibit infection with HPV16 and 6 pseudovirions of
COS-1 cells, suggesting that the binding of antibody to the region of
aa108-120 neutralizes the infectivity of HPV. 33
We then showed that the peptide having the amino acid sequence
of HPV16 L2 aa108-120 induces a cross-neutralizing antibody against
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