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

Abstract

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.

Development of a live and highly attenuated Listeria monocytogenes-based vaccine for the treatment of Her2/neu-overexpressing cancers in human

“…Interestingly, Treg cells from mice immunized with a recombinant E. coli strain expressing LLO, which provided them with the ability to escape from the phagosomes, were unable to suppress conventional T-cell proliferation. 20 Similarly, Salmonella typhimurium-induced Th1 cells were resistant to suppression by Treg cells, and this effect could be partially blocked by disruption of interleukin-6 or glucocorticoid-induced tumor necrosis factor receptor signals. 21 These results indicate that vaccines based on bacterial vectors might be advantageous in treating diseases associated with immunosupression such as cancer.…”

Listeria monocytogenes-Derived Listeriolysin O Has Pathogen-Associated Molecular Pattern-Like Properties Independent of Its Hemolytic Ability

“…However, LLO and other CDCs are potent signaling molecules that trigger a variety of cellular responses. Stimulation of cells with LLO results in a multifaceted response involving production of cytokines (5), influx of calcium signaling (6), epigenetic modifications (7), alteration of immunosuppression (8,9), and induction of apoptosis in T lymphocytes and dendritic cells (10). It has been suggested that the mechanism of signaling by LLO and other bacterial cytolysins is through their ability to act as pathogen-associated molecular patterns (PAMPs) and to interact with pathogen recognition receptors (PRRs) such as Toll-like receptor 4 (TLR4) (11).…”

Listeriolysin O as a strong immunogenic molecule for the development of new anti-tumor vaccines

“…A previous discovery indicated that an LLO-based engineered E. coli vaccine could promote the generation of CD44 high CD62L low CD8 + effector memory T cells and inhibit the functions of Treg cells that expanded normally but was unable to suppress the proliferation of conventional T cells. 123 Through the use of a tumor-bearing animal model, the researchers showed that E. coli LLO/OVA vaccination could generate high-avidity CTLs and functionally defective Treg cells, which led to the rejection of highly aggressive B16/OVA melanoma, compared with the results obtained with E. coli OVA. 123 These studies suggest that LLO is able to boost the effectiveness of the vaccine through the inhibition of Treg cells, although the exact mechanism is not yet known.…”

“…123 Through the use of a tumor-bearing animal model, the researchers showed that E. coli LLO/OVA vaccination could generate high-avidity CTLs and functionally defective Treg cells, which led to the rejection of highly aggressive B16/OVA melanoma, compared with the results obtained with E. coli OVA. 123 These studies suggest that LLO is able to boost the effectiveness of the vaccine through the inhibition of Treg cells, although the exact mechanism is not yet known. Notably, all of the above mentioned studies prepared the LLO-based E. coli vaccines using two separate plasmids for the expression of OVA/tumor antigen and LLO.…”