Listeria Monocytogenes



(Our listeria projects are currently on hold as we focus on the autoimmune diabetes projects.)


We study the early stages of Listeria infection, focusing on the cellular events and the interaction between phagocytes and T cells. Our interest in Listeria began with the initial demonstration of antigen processing and presentation to T cells. These studies were followed by examination of many early events in microbial infection including the role of inflammatory cytokines (IL-1, IL-6, IL-10, IL-12, TNF, and IFN-g), reactive oxygen/nitrogen species, and listeriosis in lymphocyte deficient strains of mice (SCID). This work has led to the three active areas of investigation examining different aspects of the immune response to Listeria. (Studies with Listeria monocytogenes lead the way, Adv Immunol. 2012;113:1-5).

Lymphocyte apoptosis. One of the major investigations centers on an early transitory stage of lymphocyte apoptosis that takes place in infectious foci. A few hours after infection, Listeria is transported into the T cell dependent areas of lymph nodes and spleen. It is there where the infectious foci then takes hold and the microbe grows exponentially. By about 48 hours there is a transitory but extensive lymphocyte apoptosis. The lymphocytes do not harbor the organisms, which are found in phagocytic cells next to them or free in the extracellular spaces. Lymphocyte apoptosis is potentiated by type I interferon, which primes the lymphocyte to undergo cell death. The lymphocyte's death is caused by the release of the pore-forming protein, Listeriolysin-O (LLO). Lymphocyte apoptosis is important because it is detrimental to host handling of the infection. In the absence of cell death, resistance to Listeria increases markedly. Indeed, and paradoxically, mice lacking the receptor for type I interferons are highly resistant to listeriosis and show limited lymphocyte apoptosis. The scenario that is emerging is that following the death of lymphocytes, the macrophages phagocytize the dead cells and release IL-10, which limits the anti-bacterial resistance. The mechanism of apoptosis and the biological consequence are current investigations. 

Role of interferons in early listeriosis.
 In collaboration with the laboratory of Dr. Robert D. Schreiber, we investigated the role of type I and type II interferon receptor signaling during the earliest stages of infection. Mice deficient in type I and type II interferon receptor signaling are both susceptible to viral infection. This is not the case in Listeria. Type II interferon (Interferon-gamma) is required for clearance of Listeria from infected tissues and mice devoid of interferon-gamma activity succumb to very low infectious doses. In contrast, the type I interferons sensitize mice to increased infection due to the presence of an apoptotic lesion. This counterintuitive result is being investigated in the context of very early infection. We are interested in the role that two cytokines play in lymphoid versus myeloid cell types.

 TNF-α, potentially produced by infected splenic marginal zone macrophages, sits at the top of the initiating cytokine cascade and induces the first IFN-γ from NK/NKT cells that, in turn, induces the early IL-12 from CD8α+ DCs. TNF-α was originally identified as a major participant with IL-12 in the induction of IFN-γ by NK cells. The transient IFN-γ–dependent induction of IL-12 from CD8α+ DCs represents a key step in forming an amplification loop that enhances IFN-γ by NK/NKT cells, depresses the latter’s ability to produce IL-4, and establishes an environment within the myeloid compartment that remains receptive to stimulation of enhanced antimicrobial function. Despite the defect in early IL-12 production within several hours p.i., the biggest difference in spleen colony counts was observed in Itgax-cre+Ifngr1f/f mice at day 3 after Listeria infection. This timing supports the conclusion that defects in early IL-12 production require a certain time window before they manifest changes in bacterial burdens. Strikingly the cellular source of IL-12 shifts, in a relatively short period of time, from CD8α+ DCs to Ly6Chi inflammatory monocytes, whose vastly greater numbers induce higher levels of IFN-γ from NK/NKT cells.

 Thus, our studies and those of other investigators indicate at least two distinct functions for CD8α+ DCs in the innate immune response to L. monocytogenes. First, CD8α+ DCs play an obligate role in transporting L. monocytogenes into the PALS and, thus, are required for establishing a productive infection . Second, CD8α+ DCs play a critical role in producing the first IL-12 that initiates an IFN-γ– and IL-12–dependent amplification loop. The selective abrogation of IFN-γ responsiveness in CD8α+ DCs does not impede L. monocytogenes transport, spatial regulation of innate cell clustering, or L. monocytogenes proliferation. This result demonstrates that the two critical functions of CD8α+ DCs are largely independent of one another.

In sum, this study reveals that NK/NKT cell production of IFN-γ and subsequent IL-12 production by CD8α+ DCs are critical initiators of the innate response to L. monocytogenes and, thus, illustrate how genetically homogeneous mice with tissue-selective defects in IFN-γ responsiveness help to refine our understanding of IFN-γ’s physiologic roles in vivo

 Immunogenicity of LLO. LLO is a cholesterol-dependent cytolysin and the key virulence factor of infection with Listeria. LLO deficient strains are avirulent both in vivo and in vitro. LLO deficient strains are also poorly immunogenic even at high infectious doses. LLO binds all cell membranes with high affinity and triggers multiple biological effects in cells. In the context live infection of phagocytic cells it is responsible for the transport of the microbe from vacuoles into the cytosol where it can divide and spread from cell to cell. LLO is pleitropic and causes MAPK signaling, calcium fluxes, and programmed cell death in cells. Despite its negative effects on cells, LLO is also an immunogen that induces both CD4 and CD8 responses. Recently, we have shown that the cytotoxic and immunogenic activity of LLO are independent, suggesting that multiple domains of the protein have biological effects in cells. At present the immunogenicity of LLO and the pathways of presentation are the subject of intensive investigations.