Depletion of human regulatory T cells (Treg) and antigen-specific immune responses to cancer vaccines

Abstract

3010 Background: CD4+CD25+FoxP3+ regulatory T cells (Treg) limit antigen-specific immune responses and are a cause of suppressed anticancer immunity. Conversely, depletion of Treg leads to immune enhancement. The immunotoxin denileukin diftitox which selectively targets lymphocytes expressing CD25 may deplete FoxP3+ Treg. Methods: We evaluated the proliferative potential of PBMC to various antigens in vitro following exposure to denileukin diftitox. We then performed a pilot study in which patients with advanced CEA expressing malignancies, being immunized with autologous dendritic cells modified with a fowlpox vector encoding CEA (rF-CEA(6D)-TRICOM), received denileukin diftitox 18 mcg/kg, once, 4 days before the immunizations began, or 9 mcg/kg prior to each of the 4 immunizations. ELISPOT, cytokine flow cytometry, and ELISA were used to measure the T cell and antibody response. Results: In vitro, escalating doses of denileukin diftitox depleted FoxP3+ Treg, decreased Treg function in vitro, and enhanced antigen-specific T cell responses. In the pilot study (n=15), denileukin diftitox was associated with a 74 ± 6% decrease in Treg in those receiving multiple doses, but not in those receiving a single dose. An earlier peak in the vaccine-induced CEA-specific T cell responses, and significant levels (>0.5%) of circulating CD8+ and CD4+ CEA-specific T cells were also seen in the multiple dose group. Conversely, a single dose of denileukin diftitox enhanced anti-CEA, but not antifowlpox vector, antibody responses. Multiple doses abolished the anti-CEA antibody response. Conclusions: These results indicate the potential for combining Treg depletion with anticancer vaccines to enhance tumor antigen specific immune responses. No significant financial relationships to disclose.