p97 is a human
tumor-associated Ag present on most
melanoma cells that represents a possible target for immunologic attack. To evaluate the capacity of T cells reactive with this
protein to promote elimination of
melanoma cells expressing p97, a murine model was developed by transfecting a C3H/HeN
melanoma with the p97
cDNA, generating p97-specific CD4+ T cells by in vivo immunization of C3H/HeN mice with a
vaccinia/p97 recombinant virus followed by in vitro cloning with soluble p97
protein, and determining whether these CD4+ T cells could mediate rejection of pulmonary
metastases. Characterization of the T cell clones demonstrated the presence of both I-Ak and I-Ek-restricted clones, although the majority of clones recognized p97 in the context of I-Ek. Analysis of clonal specificity using truncated p97
proteins revealed that at least three
epitopes were immunogenic, and further studies with overlapping 15-amino
acid peptides from a region of the p97 molecule defined by these truncated
proteins identified an
immunodominant epitope responsible for the majority of the I-Ek response. The T cell clones were not capable of directly recognizing the p97-expressing
melanoma cells but responded to the
tumor if syngeneic APC were present to process the
tumor-derived p97 Ag. The therapeutic efficacy of these CD4+ T cell clones was evaluated in an adoptive
therapy model in which mice bearing metastatic pulmonary lesions were treated by i.v. administration of the p97-specific cells. Despite the inability of the CD4+ clones to directly respond to or lyse the
tumor cells, the clones were effective in promoting
tumor eradication. In vitro studies demonstrated that this may have reflected secretion of
lymphokines that activated macrophages to lyse the
tumor. The results suggest that noncytolytic p97-specific CD4+ T cell clones can be effective in
therapy of pulmonary
melanoma metastases. Moreover, if human T cells reactive with the p97
protein could be generated, the expression of this
tumor-associated Ag in
melanoma cells might be adequate for such T cells to mediate a therapeutic antitumor response.