Induction of Protective T Cells against Listeria monocytogenes in Mice by Immunization with a Listeriolysin O-Negative Avirulent Strain of Bacteria and Liposome-Encapsulated Listeriolysin O

Only listeriolysin O (LLO)-producing strains of Listeria monocytogenes generate protective immunity in mice. Based on the findings that endogenous gamma interferon (IFN-γ) production was induced only by such strains and that purified LLO could induce IFN-γ from NK cells, we have postulated that LLO may play a pivotal role in the induction of Th1-type protective T cells, which are highly dependent on IFN-γ. In this study, mice were immunized with L. monocytogenes ATCC 15313, an LLO-nonproducing avirulent strain, along with LLO encapsulated in liposome (LLO-liposome). LLO-liposome was highly potent in the induction of various cytokines, including IFN-γ. Immunization of mice with either LLO-liposome or the viable strain ATCC 15313 alone did not induce protection against challenge infection. In contrast, the combination of LLO-nonproducing bacteria plus LLO-liposome induced a significant level of protective immunity mediated mainly by Th1-type cells capable of producing a large amount of IFN-γ in an antigen-specific manner. The protection afforded by the combination was not dependent on LLO-specific cytotoxic T cells. These results support the idea that the inability of an LLO-nonproducing avirulent strain or killed bacteria to induce the generation of protective T cells is due not to the lack of a central T-cell epitope(s) but to the lack of ability to induce the production of endogenous cytokine during the early stage of immunization; the results also suggest that an appropriate use of LLO at least in an animal model may be effective in the induction of antigen-specific Th1-dependent protective immunity to various kinds of intracellular parasitic bacteria.

仅产李斯特菌溶血素O（listeriolysin O, LLO）的单核细胞增生李斯特菌（Listeria monocytogenes）菌株可在小鼠体内诱导保护性免疫。基于仅此类菌株可诱导内源性γ干扰素（IFN-γ）产生，且纯化LLO可从自然杀伤（NK）细胞中诱导IFN-γ生成的研究结果，我们推测LLO在高度依赖IFN-γ的Th1型保护性T细胞诱导过程中可能发挥关键作用。本研究中，我们使用LLO非产毒的减毒株单核细胞增生李斯特菌ATCC 15313，与包被于脂质体（liposome）的LLO（LLO-脂质体）联合免疫小鼠。LLO-脂质体在诱导包括IFN-γ在内的多种细胞因子方面表现出极强的活性。仅单独使用LLO-脂质体或活菌ATCC 15313免疫小鼠，均无法诱导针对攻击感染的保护性免疫。与之相反，LLO非产毒菌株与LLO-脂质体联合免疫可诱导显著水平的保护性免疫，该免疫主要由以抗原特异性方式大量产生IFN-γ的Th1型细胞介导。该联合免疫所提供的保护性免疫并不依赖LLO特异性细胞毒性T细胞。上述研究结果支持以下观点：LLO非产毒减毒株或灭活细菌无法诱导保护性T细胞生成，并非因其缺乏核心T细胞表位，而是由于其无法在免疫早期阶段诱导内源性细胞因子产生；此外，结果还表明，至少在动物模型中，合理使用LLO可有效诱导针对多种胞内寄生菌的抗原特异性、Th1依赖性保护性免疫。