Natural and induced immunity against the tumour-associated antigen, Ep-CAM
Abstract: The tumour-associated antigen (TAA), Ep-CAM is overexpressed on various human carcinomas, including colorectal carcinoma (CRC). TAAs or their immunodominant epitopes that are spontaneously recognised by the immune system might constitute a suitable target for immunotherapy. Fifteen % of sera of CRC patients with no previous immunotherapy elicited IgG antibodies against Ep-CAM. No Ep-CAM specific antibodies were detected in healthy controls or patients with Crohn´s disease or colitis ulcerosa. Further analyses revelaed that 50% of the Ep-CAM-reactive sera bound to peptide residues 29-46 of Ep-CAM. The results provide evidence for spontaneous immune recognition of Ep-CAM in CRC patients and identify an immunodominant B cell epitope of human Ep-CAM. Anti-idiotypic antibodies (anti-Id) may serve as surrogate TAAs for vaccination. The optimal design of an anti-Id vaccine, however remains unclear. Moreover, whether vaccination with anti-Id or the original antigen is superior is controversial. SM262 is a human anti-Id raised against mAb 17-1A that recognises Ep-CAM. Vaccination of mice with anti-Id induced antibodies that shared idiotopes with mAb 17-1A and recognised Ep-CAM. Fusion of GM-CSF to anti-Id enhanced the magnitude of the antibody responses, while xenogeneic Fc domain had no significant modulatory effect. Recombinant anti-Id protein vaccine evoked a more potent humoral immunity as compared to DNA delivered by gene gun. Our study provides the fist evidence that immune tolerance in mice expressing the transgene for human Ep-CAM can be circumvented by anti-Id vaccination. The results may have implications for future anti-Id vaccine design. Vaccination of CRC patients with recombinant Ep-CAM protein, in combination with GM-CSF, induced Ep-CAM specific T and NK-like T cells producing cytotoxic cytokines. In addition, a long-lasting Th1 biased humoral and proliferative T cell response was elicited against Ep-CAM. The original antigen, Ep-CAM induced a more potent overall immune response as compared to anti-Id mimicking Ep-CAM. Analysis of TCR BV gene repertoire revealed that BV19+ CD8+ T cells might be involved in the vaccine induced anti-Ep-CAM immune response. The results collectively suggest that immunisation with Ep-CAM protein may serve as a novel approach to CRC immunotherapy. Furthermore, immunogenic MHC class I and II restricted Ep-CAM epitopes were identified that may provide new opportunities for developing effective multiepitope cancer vaccines targeting Ep-CAM. Vaccination with a recombinant canarypox virus (ALVAC) encoding human Ep-CAM in combination with GM-CSF induced a potent Ep-CAM specific, type 1 cellular immune response in CRC patients. However, no anti-Ep-CAM antibody or proliferative T cell responses were elicited. Combining ALVAC-Ep-CAM and recombinant Ep-CAM in a prime-boost vaccination approach may represent an effective strategy to induce a coordinated antigen specific cellular and humoral immune response. In conclusion, the results suggest that Ep-CAM is a promising target structure for immunotherapy. The present studies may form a basis for further enlarged clinical trials targeting Ep-CAM by active specific vaccination.
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