Many
antigens associated with human
tumors are overexpressed in
tumor cells as compared with normal tissues; these "self"
tumor-associated
antigens are also expressed during fetal development, and it is, thus, not surprising that they are either weakly immunogenic or functionally nonimmunogenic in the
tumor-bearing host. In the studies reported here, we have used different
vaccines and
vaccine strategies in an attempt to develop antitumor immunity in a stringent animal model. The
tumor antigen used was human
carcinoembryonic antigen (CEA). The model used was CEA transgenic mice, in which the human CEA transgene is under the control of the endogenous CEA promoter; CEA is expressed in fetal tissues and normal gastrointestinal tissues, and CEA
protein is found in sera. Previous studies have shown these CEA transgenic mice to be tolerant to the induction of CEA immunity using CEA
protein in adjuvant as an immunogen. CEA-expressing
tumor cells were implanted 14 days before vaccine therapy. The
vaccines used were recombinant vaccinia virus containing the transgenes for CEA and three T-cell
costimulatory molecules [B7-1,
ICAM-1, and
LFA-3, designated recombinant
vaccinia (rV)-CEA/TRICOM], with each transgene under the control of individual poxvirus promoters, and a replication-defective avipox virus (
fowlpox; rF) containing the same four transgenes (designated rF-CEA/TRICOM). The results demonstrate that (a) continued boosting with
vaccine is required to maintain CEA-specific T-cell responses, and boosting with rF-CEA/TRICOM is superior to boosting with rF-CEA; (b) a diversified vaccination protocol consisting of primary vaccination with rV-CEA/TRICOM followed by boosting with rF-CEA/TRICOM is more efficacious than homogeneous vaccination with rF-CEA/TRICOM in the induction of both CEA-specific T-cell responses and antitumor activity; and (c) the use of
cytokines, local
granulocyte macrophage colony-stimulating factor (
GM-CSF) and low-dose systemic
interleukin 2, in combination with
vaccine is essential in inducing antitumor activity, as compared with the use of
cytokines alone, or the use of
vaccines without
cytokine. Both
GM-CSF and
interleukin 2 were shown to contribute to the induction of CEA-specific T-cell responses. These studies thus provide a "proof of concept" that potent
vaccines and
vaccine strategies, in combination with
cytokines, may be essential to obtain the level of T-cell responses directed against a
self-antigen that is necessary to achieve antitumor responses.