Impaired Antigen Presentation and Effectiveness of Combined Active/Passive Immunotherapy for Epithelial Tumors

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Abstract
Background: Although immunization with tumor antigens can eliminate many transplantable tumors in animal models, immune effector mechanisms associated with successful immunotherapy of epithelial cancers remain undefined. Methods: Skin from transgenic mice expressing the cervical cancer–associated tumor antigen human papillomavirus type 16 (HPV16) E6 or E7 proteins from a keratin 14 promoter was grafted onto syngeneic, non-transgenic mice. Skin graft rejection was measured after active immunization with HPV16 E7 and adoptive transfer of antigen-specific T cells. Cytokine secretion of lymphocytes from mice receiving skin grafts and immunotherapy was detected by enzyme-linked immunosorbent assay, and HPV16 E7–specific memory CD8 + T cells were detected by flow cytometry and ELISPOT. Results: Skin grafts containing HPV16 E6–or E7–expressing keratinocytes were not rejected spontaneously or following immunization with E7 protein and adjuvant. Adoptive transfer of E7-specific T-cell receptor transgenic CD8 + T cells combined with immunization resulted in induction of antigen-specific interferon gamma–secreting CD8 + T cells and rejection of HPV16 E7–expressing grafts. Specific memory CD8 + T cells were generated by immunotherapy. However, a further HPV16 E7 graft was rejected from animals with memory T cells only after a second E7 immunization. Conclusions: Antigen-specific CD8 + T cells can destroy epithelium expressing HPV16 E7 tumor antigen, but presentation of E7 antigen from skin is insufficient to reactivate memory CD8 + T cells induced by immunotherapy. Thus, effective cancer immunotherapy in humans may need to invoke sufficient effector as well as memory T cells.

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