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Potent Selection of Antigen Loss Variants of B16 Melanoma following Inflammatory Killing of Melanocytes In vivo

¹ Molecular Medicine Program and Departments of ² Immunology and ³ Oncology, Mayo Clinic, Rochester, Minnesota; ⁴ Department of Pathology, St George's Hospital Medical School, London, United Kingdom; ⁵ Cancer Research UK Oncology Unit, St James' Hospital, Leeds, United Kingdom; and ⁶ Van Andel Research Institute, Grand Rapids, Michigan

Requests for reprints: Richard G. Vile, Molecular Medicine Program, Mayo Clinic, Guggenheim 1836, 200 1st Street Southwest, Rochester, MN 55902. Phone: 507-284-9941; Fax: 507-266-2122; E-mail: vile.richard{at}mayo.edu.

We have reported that i.d. injection of plasmids encoding hsp70 and a suicide gene transcriptionally targeted to melanocytes generates specific proinflammatory killing of melanocytes. The resulting CD8⁺ T cell response eradicates systemically established B16 tumors. Here, we studied the consequences of that CD8⁺ T cell response on the phenotype of preexisting tumor. In suboptimal protocols, the T cell response selected B16 variants, which grow extremely aggressively, are amelanotic and have lost expression of the tyrosinase and tyrosinase-related protein 2 (TRP-2) antigens. However, expression of other melanoma-associated antigens, such as gp100, was not affected. Antigen loss could be reversed by long-term growth in culture away from immune-selective pressures or within 96 hours by treatment with the demethylating agent 5-azacytidine (5-Aza). When transplanted back into syngeneic animals, variants were very poorly controlled by further vaccination. However, a combination of vaccination with 5-Aza to reactivate antigen expression in tumors in situ generated highly significant improvements in therapy over treatment with vaccine or 5-Aza alone. These data show that inflammatory killing of normal cells activates a potent T cell response targeted against a specific subset of self-antigens but can also lead to the immunoselection of tumor variants. Moreover, our data indicate that emergence of antigen loss variants may often be due to reversible epigenetic mechanisms within the tumor cells. Therefore, combination therapy using vaccination and systemic treatment with 5-Aza or other demethylating agents may have significant therapeutic benefits for antitumor immunotherapy.

Key Words: Melanocytes • melanoma • immunotherapy • heat shock protein • HSVtk • antigen loss variants • 5-Azacytidine

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