Induction of Protective Host Immunity to Carcinoembryonic Antigen (CEA), a Self-Antigen in CEA Transgenic Mice, by Immunizing with a Recombinant Vaccinia-CEA Virus

Author: Kass E, et al.
Source:Cancer Research, 59, 1999, pg 676

Background

• CEA is expressed in normal epithelial tissue (along the GI tract and at times in the trachea and respiratory tree) as well as in a high percentage of colon, pancreatic, breast, and lung cancers
• CEA as a tumor antigen has been studied extensively, though most studies show that it has little immunogenicity, likely due to T-cell tolerance (T-cells not reacting to it as foreign since it is present during embryonic development)
• More recently, the use of viral vectors to express CEA has been shown to induce weak antibody and cellular responses to recombinant CEA
• This study reports on the use of a recombinant virus that expresses CEA at high quantities as a vaccine in CEA transgenic mice (meaning they have the gene to produce CEA themselves)
• The authors investigated the response of these transgenic mice to CEA administered as a whole protein adjuvant and to determine if the presentation of CEA as a recombinant vaccinia virus (rV-CEA) could elicit an immune response

Materials and Methods

• The mice used were determined to be CEA-positive transgenic through PCR analysis
• CEA levels were measured in CEA transgenic mice, CEA-negative littermates, and wild-type mice
• The rV-CEA was produced by the homologous recombination of a plasmid containing human cDNA for CEA. A control strain of the viral vaccine (V-Wyeth) was used
• Serum samples were collected from unimmunized and immunized CEA transgenic mice as well as from their CEA-negative littermates and analyzed for the presence of antibodies by ELISA
• T-cell proliferation assays, by the incorporation of labeled thymidine into the cells, were done to determine the T-cell response
• CEA transgenic and CEA-negative mice were injected with CEA tumor cells following the immunization with the vaccine to determine if tumor protection could be achieved

Results

• There was 100% concordance with the presence of the CEA transgene and detectable serum CEA levels in 25 mice
• Prior to immunization, analysis of serum samples of 10 different CEA transgenic mice revealed that there were no antibodies to CEA
• Following the administration of whole protein CEA, there continued to be no antibodies detected, though there was a brisk humoral response measured when these mice were exposed to a different, control antigen
• Three of four CEA-transgenic mice given rV-CEA developed measurable anti-CEA antibody titers. No antibody response was noted to the control vaccine (V-Wyeth)
• The CEA transgenic mice developed an immune response approximately 40 fold lower than the CEA negative mice (IgG titers of 250 compared to 10,000)
• There was no measurable T cell response (by counting the incorporation of labeled thymidine) in unimmunized CEA transgenic mice when exposed to CEA
• The T-cell response was significantly higher in CEA transgenic mice when exposed to rV-CEA as compared to the response to V-Wyeth
• The T-cell response was even higher for the CEA negative mice and the wild type mice than the response was for the CEA transgenic mice
• There was no appreciable response to the administration of whole protein CEA
• Following immunization with rV-CEA, mice were challenged with CEA-expressing tumors. CEA transgenic mice immunized with control vaccine showed progressive tumors. Fifty percent of the CEA transgenic mice who were vaccinated with rV-CEA remained tumor free. Eighty percent of the CEA-negative littermates who were immunized with rV-CEA remained tumor free

Authors' Conclusions

• CEA transgenic mice should serve as excellent models to observe the effect of CEA vaccines, as they should mimic human responses
• Measurable humoral and cellular immune responses were noted in response to the rV-CEA vaccine and immunization with the rV-CEA vaccine could be protective against tumor challenge
• These were all outweighed by the responses of CEA negative mice, though the responses are still encouraging

Scientific Implications

These results are encouraging. While there are obvious differences that exist between murine models and humans, the CEA transgenic mice likely do correspond better to humans than do other mice. The authors therefore have shown that even in mice who should have tolerance to CEA (as was demonstrated), an immune response to CEA vaccination can be achieved. In some cases, tumor protection was conferred by this vaccination. Pre-immunization testing was done and controls were done in every aspect of the study, contributing to the clarity of their results. This was a well run and well-reported study that shows promising implications for the development of a CEA targeted tumor vaccine. Obviously, there are many steps from showing responses in mice to the demonstration of clinical efficacy in humans. Some of these will turn out to be the requirement of co-stimulatory molecules, patient selection, and a population that is already immunosuppressed from the usual many regimens of chemotherapy to which they were exposed. Regardless, these results are inspiring for future research.