Listeriolysin O Expressed in a Bacterial Vaccine Suppresses CD4+CD25high Regulatory T Cell Function In Vivo

CD4+CD25high regulatory T cells (Treg) protect the host from autoimmune diseases but are also obstacles against cancer therapies. An ideal cancer vaccine would stimulate specific cytotoxic responses and reduce/suppress Treg function. In this study, we showed that Escherichia coli expressing listeriolysin O and OVA (E. coli LLO/OVA) demonstrated remarkable levels of protection against OVA-expressing tumor cells. By contrast, E. coli expressing OVA only (E. coli OVA) showed poor protection. High-avidity OVA-specific CTL were induced in E. coli LLO/OVA-vaccinated mice, and CD8+ depletion—but not NK cell depletion, abolished the antitumor activity of the E. coli LLO/OVA vaccine. Phenotypic analysis of T cells following vaccination with either vaccine revealed preferential generation of CD44highCD62Llow CD8+ effector memory T cells over CD44highCD62Lhigh central memory T cells. Unexpectedly, CD4+ depletion turned E. coli OVA into a vaccine as effective as E. coli LLO/OVA suggesting that a subset of CD4+ cells suppressed the CD8+ T cell-mediated antitumor response. Further depletion experiments demonstrated that these suppressive cells consisted of CD4+CD25high regulatory T cells. We therefore assessed these vaccines for Treg function and found that although CD4+CD25high expansion and Foxp3 expression within this population was similar in all groups of mice, Treg cells from E. coli LLO/OVA-vaccinated animals were unable to suppress conventional T cells proliferation. These findings provide the first evidence that LLO expression affects Treg cell function and may have important implications for enhancing antitumor vaccination strategies in humans.