Episomal Expression of Truncated Listeriolysin O in LmddA-LLO–E7 Vaccine Enhances Antitumor Efficacy by Preferentially Inducing Expansions of CD4+FoxP3− and CD8+ T Cells

Abstract

Studies have shown that Listeria monocytogenes (Lm)-based vaccine expressing a fusion protein comprising truncated listeriolysin O (LLO) and human papilloma virus (HPV) E7 protein (Lm-LLO-E7) induced a decrease in regulatory T cells (Treg) and complete regression of established, transplanted HPV-TC-1 tumors in mice. However, how the Lm-based vaccine causes a decrease in Tregs remains unclear. Using a highly attenuated Lm dal dat ΔactA strain (LmddA)-based vaccine, we report here that the vector LmddA was sufficient to induce a decrease in the proportion of Tregs by preferentially expanding CD4+FoxP3− T cells and CD8+ T cells, by a mechanism dependent on and directly mediated by LLO. Episomal expression of a nonhemolytic truncated LLO in Lm (LmddA-LLO) significantly augmented the expansion, thus further decreasing Treg frequency. While adoptive transfer of Tregs compromised the antitumor efficacy of the LmddA-LLO-E7 vaccine, a combination of LmddA-LLO and an Lm-based vaccine expressing E7 protein (Lm-E7) induced complete regression against established TC-1 tumors. An engineered LLO-minus Lm expressing perfringolysin O (PFO) that enables the recombinant bacteria to exit from the phagolysosome without LLO confirmed that the adjuvant effect was dependent on LLO. These results suggest that LLO may serve as a promising adjuvant by preferentially inducing the expansions of CD4+FoxP3− T cells and CD8+ T cells, thus reducing the ratio of Tregs to CD4+FoxP3− T cells and to CD8+ T cells favoring immune responses to eradicate tumor.

Current state in the development of candidate therapeutic HPV vaccines

“…Listeria based E7 vaccines have been shown to cause regression of implanted, E6/E7 expressing solid tumors as well as to limit the growth of spontaneous E6/E7 expressing tumor in HPV-16 E6/E7 transgenic mice [23]. Furthermore, another study has demonstrated that listeriolysin induces a decrease in regulatory T cell population by preferentially expanding CD4+ FoxP3− T cells and CD8+ T cells in tumor bearing mice, thus augmenting the therapeutic and antitumor effects of vaccine against E6/E7 expressing TC-1 tumors [29]. …”

The current state of therapeutic and T cell-based vaccines against human papillomaviruses

“…Listeria -based E7 vaccines have been shown to reduce implanted, E6/E7-expressed solid tumors as well as limit the growth of spontaneous E6/E7-expressing tumors in HPV-16 E6/E7 transgenic mice (Sewell et al, 2008). Additionally, LLO was shown to induce a decrease in regulator T cells, resulting in the augmentation of the therapeutic and antitumor effects of the vaccine against E6/E7-expressing TC-1 tumors (Chen et al, 2014). Other attenuated bacterial vectors can be generated by vector transformation with plasmids containing selected genes of interest.…”

Immunotherapy for head and neck cancer: latest developments and clinical potential

“…This vaccine has been shown to decrease regulatory T cells and cause complete regression of established, transplanted HPV-TC-1 tumors in mice [Wallecha et al 2012;Chen et al 2014]. A phase II trial of ADX11 [ClinicalTrials.gov identifier: NCT02002182] is accruing newly diagnosed patients with stage II-IV HPV16-positive squamous cell carcinoma of the oropharynx who are scheduled to receive ablative transoral robotic surgery.…”