In this study, we compared the immunogenicity and
tumor-protective activity of
anti-idiotypic antibodies mimicking a single
tumor-associated
epitope and
tumor-associated
antigen expressing multiple potentially immunogenic
epitopes. We focused our study on the
colorectal-carcinoma(CRC)-associated
antigen GA733 (also known as CO17-1A/KS1-4/KSA/
EpCAM). Monoclonal anti-idiotypic antibody (Ab2) BR3E4 was produced against murine anti-CRC mAb CO17-1A (Ab1) in rats. Full-length native GA733
protein was isolated from human
tumor cells, and the extracellular domain protein (GA733-2E) was isolated from supernatants of recombinant baculovirus-infected insect cells by immunoaffinity chromatography. The immunomodulatory activity of the Ab2 was compared with that of the
antigen, both in rabbits and in mice. Mice, like humans but not rabbits, express a
GA733 antigen homologue on some of their normal tissues. Thus, these in vivo models allow the comparison of the immunogenicity of Ab2 and
antigen in the presence (mice) and absence (rabbits) of normal tissue expression and immunological tolerance of the
GA733 antigen homologue. In rabbits,
aluminum-hydroxide(
alum)-precipitated native
GA733 antigen was superior to
alum-precipitated Ab2 in inducing specific humoral immunity. In mice,
alum-precipitated recombinant GA733-2E
antigen, but not
alum-precipitated Ab2, induced specific humoral immunity. However, when the Ab2 was administered to mice in Freund's complete adjuvant, specific humoral immune responses were elicited. Ab2 in complete
Freund's adjuvant and GA733-2E in
alum were compared for their capacity to induce
antigen-specific cellular immunity in mice. Whereas lymphoproliferative responses were obtained with the recombinant
antigen only, delayed-type
hypersensitivity responses were obtained with both recombinant
antigen and Ab2, although these responses were lower than after
antigen immunization. The recombinant
antigen in
alum did not protect mice against challenge with
antigen-positive syngeneic murine CRC cells. Similar studies with Ab2 BR3E4 mimicking the CO17-1A
epitope were not possible because the
tumor cells do not express this
epitope after transfection with the human GA733-2
cDNA. However, similar studies with Ab2 mimicking the
epitope defined by mAb GA733, which is expressed by the transfected
tumor cells, indicated a lack of
tumor-protective activity of this Ab2. In contrast, the full-length
antigen expressed by recombinant adenovirus inhibited the growth of established
tumors in mice. In conclusion, soluble
antigen is a more potent modulator of humoral and cellular immune responses than Ab2, both administered in adjuvant. However, for induction of protective immunity, the immunogenicity of the
antigen must be further enhanced, e.g., by expression of the
antigen in a viral vector.