Multiple Mechanisms Contribute to the Robust Rapid Gamma Interferon Response by CD8⁺T Cells duringListeria monocytogenesInfection

Abstract

A subset of CD8؉ T cells can rapidly secrete gamma interferon (IFN-␥) in an antigen-independent and interleukin-12 (IL-12)-and IL-18-dependent manner within 16 h of infection with the intracellular bacterial pathogen Listeria monocytogenes. This rapid IFN-␥ response is robust enough to be detected directly ex vivo and is not observed following infection with intracellular bacterial pathogens that remain sequestered within host cell vacuoles. We demonstrate here that three distinct pathways can lead to rapid secretion of IFN-␥ by CD8 ؉ T cells during L. monocytogenes infection: (i) a direct cytokine-inducing activity encoded by the cholesteroldependent cytolysin (CDC) listeriolysin O (LLO) acts within the infected cell, (ii) the pore-forming activity of LLO promotes cytosolic localization of bacterial products that trigger cytosol-specific signaling pathways, and (iii) the sustained presence of high concentrations of bacterial products can exogenously trigger cytokine production. Although it has been suggested that CDC protein toxins may act as Toll-like receptor 4 (TLR4) agonists to trigger proinflammatory cytokine secretion, we show in this report that TLR4 signaling is not required to induce a maximal rapid IFN-␥ response by CD8 ؉ T cells. The results presented here indicate that multiple mechanisms contribute to the induction of rapid IFN-␥ secretion by CD8 ؉ T cells during Listeria infection and that care must be taken when interpreting the results of in vitro assays, since the contribution of each pathway can vary depending on how the assay is performed.Gamma interferon (IFN-␥) is a multifunctional cytokine that contributes to both innate and adaptive immune responses and is critical for the clearance of intracellular bacterial pathogens such as Listeria monocytogenes. IFN-␥ activates macrophages, which leads to enhanced production of antimicrobial oxygen and nitrogen intermediates and limits bacterial spread during infection (33, 41). IFN-␥ also increases major histocompatibility complex class I expression on macrophages, which promotes recognition of infected cells by CD8 ϩ cytotoxic T lymphocytes (11). Mice that lack IFN-␥ or its receptor are significantly more susceptible to infection with L. monocytogenes (8, 17).It has long been known that T cells are an important source of IFN-␥ during the adaptive phase of the immune response against L. monocytogenes; however, more recently it was demonstrated that a subset of CD8 ϩ T cells can also secrete IFN-␥ during the early, innate phase of the immune response. Berg et al. showed that up to 6% of splenic CD8 ϩ T cells were actively secreting IFN-␥ 16 h after infection of mice, a robust response that could be detected directly ex vivo without any further stimulation of the T cells (2, 3). All of the IFN-␥ ϩ CD8 ϩ T cells had a "memory" phenotype (CD44 hi ); however, the rapid IFN-␥ response was not antigen specific and instead depended only on the presence of the cytokines interleukin-12 (IL-12) and IL-18 (2).In addition to CD8 ϩ T cells, many other cell types ...

Listeria monocytogenes-Infected Human Peripheral Blood Mononuclear Cells Produce IL-1β, Depending on Listeriolysin O and NLRP3

“…Whether this "more direct NLRP3 activation" depends on a direct interaction of LLO and NLRP3, or more likely on still unidentified cytosolic mediator molecules upstream of NLRP3, is unknown. The hypothesis that LLO activates host cell responses through different pathways is supported by recently published results suggesting multiple mechanisms of LLO-stimulated, IL-18-dependent IFN-g production (56).…”

Dendritic Cells Inhibit the Progression ofListeria monocytogenesIntracellular Infection by Retaining Bacteria in Major Histocompatibility Complex Class II-Rich Phagosomes and by Limiting Cytosolic Growth

“…DC derived from murine bone marrow with GM-CSF are known to be phenotypically heterogeneous (23) and most closely related functionally to inflammatory, monocyte-derived DC (48) Although our data implicate defective LLO function in DC phagosomes, they must be considered in light of the fact that the precise mechanism by which LLO forms pores in phagosomal membranes is not clear and is known to be influenced by multiple bacterial and host-derived factors (37). Further, LLO has the ability to transiently delay phagosome/lysosome fusion in macrophages (39), as well as to transmit signals to the host cell (7,29). Therefore, the factors that influence LLO activity in DC are likely to be complex and will require further study.…”

Dysfunctional expansion of hematopoietic stem cells and block of myeloid differentiation in lethal sepsis

“…27,28 Previous studies conducted on the burn P aeruginosa model revealed the existence of myelosuppression in addition to neutrophil consumption and apoptosis during sepsis. 13,29 Interestingly, a recent study from Zhu et al showed that Listeria monocytogences, a Gram-positive bacteria with a TLR4 agonist component, 30,31 can induce myelosuppression and neutropenia correlating with lethality. 32 Although these studies clearly indicated the presence of a myelosuppressive activity during lethal sepsis, they were limited to the analysis of progenitor colonies and terminal differentiation of BM myeloid cells, 33,34 and the mechanisms mediating this process were not elucidated.…”