The 75-kD tumour necrosis factor (TNF) receptor is specifically up-regulated in monocytes during Q fever endocarditis

Caspase-8 activity mediates TNFα production and restricts Coxiella burnetii replication during murine macrophage infection

Coxiella burnetii is an obligate intracellular bacteria which causes the global zoonotic disease Q Fever. Treatment options for infection are limited, and development of novel therapeutic strategies requires a greater understanding of how C. burnetii interacts with immune signaling. Cell death responses are known to be manipulated by C. burnetii, but the role of caspase-8, a central regulator of multiple cell death pathways, has not been investigated. In this research, we studied bacterial manipulation of caspase-8 signaling and the significance of caspase-8 to C. burnetii infection, examining bacterial replication, cell death induction, and cytokine signaling. We measured caspase, RIPK, and MLKL activation in C. burnetii-infected TNFα/CHX-treated THP-1 macrophage-like cells and TNFα/ZVAD-treated L929 cells to assess apoptosis and necroptosis signaling. Additionally, we measured C. burnetii replication, cell death, and TNFα induction over 12 days in RIPK1-kinase-dead, RIPK3-kinase-dead, or RIPK3-kinase-dead-caspase-8-/- BMDMs to understand the significance of caspase-8 and RIPK1/3 during infection. We found that caspase-8 is inhibited by C. burnetii, coinciding with inhibition of apoptosis and increased susceptibility to necroptosis. Furthermore, C. burnetii replication was increased in BMDMs lacking caspase-8, but not in those lacking RIPK1/3 kinase activity, corresponding with decreased TNFα production and reduced cell death. As TNFα is associated with the control of C. burnetii, this lack of a TNFα response may allow for the unchecked bacterial growth we saw in caspase-8-/- BMDMs. This research identifies and explores caspase-8 as a key regulator of C. burnetii infection, opening novel therapeutic doors.

Immune response and Coxiella burnetii invasion

Coxiella burnetii, the causative agent of Q fever, has evolved a wealth of mechanisms in order to persist within hosts. Two tissues, namely adipose tissue and placenta, are candidates to house C. burnetii, but the mechanisms governing C. burnetii survival in these tissues are still unknown. In contrast, monocytes and macrophages are well-known targets of C. burnetii. First, C. burnetii has developed a specific strategy of phagocytosis subversion that consists of the inhibition of integrin interplay. Second, C. burnetii persistence is associated with macrophage activation profiles. Indeed, monocytes (in which C. burnetii survives without replication) exhibit a proinflammatory M1-type response, whereas macrophages (in which C. burnetii slowly replicates) are polarized towards an M2-type. Third, interleukin-10 produced by monocytes is a main factor of the chronic development of Q fever, and murine models confirm the key role of interleukin-10 in C. burnetii persistence. Fourth, apoptotic cells may play a key role in chronic Q fever. The uptake of apoptotic cells by circulating monocytes increases C. burnetii replication by redirecting monocytes toward a non-protective M2 profile. In the presence of interferon-γ, apoptotic cell engulfment is inhibited and monocytes polarized toward an M1 program are able to kill C. burnetii; this is the situation observed in patients with uncomplicated acute Q fever. Finally, we cannot exclude that regulatory T cells may play a role in C. burnetii persistence because their number is increased in patients with chronic Q fever.

Immune response genes in the post-Q-fever fatigue syndrome, Q fever endocarditis and uncomplicated acute primary Q fever

BACKGROUND

The influence of immune response gene variations on the development of chronic complications of Q fever is presently unclear.

AIM

To compare the frequencies of allelic polymorphisms in immune response genes in different Q fever patient groups.

DESIGN

Genetic association study.

METHODS

We measured the frequencies of immune response gene variants in: (i) an expanded group of 31 post-Q-fever fatigue patients (QFS); (ii) 22 Q fever endocarditis patients (QFE); and (iii) 22 patients who made an uncomplicated recovery from their initial attack of primary acute Q fever, comparing them with various standard control panels from the general population.

RESULTS

There were significant differences between the three Q fever groups. QFS patients differed from both QFE and uncomplicated patients and controls in the frequency of carriage of HLA-DRB1*11 and of the 2/2 genotype of the interferon-gamma intron1 microsatellite. Carriage of the HLA DRB1*11 allele was associated with reduced interferon-gamma and IL-2 responses from PBMC stimulated with ligand in short-term culture. QFE showed differences in the IL-10 promoter microsatellites R and G and had higher frequencies of the TNF-alpha receptor II 196R polymorphism. Q fever patients who had made an uncomplicated recovery differed from those with QFS or QFE, but were not significantly different in allelic frequencies to the control panels.

DISCUSSION

These immunogenetic differences support the concept of different immune states in chronic Q fever, determined by genetic variations in host immune responses, rather than by solely properties of Coxiella burnetii.

Invasion of the central nervous system by intracellular bacteria

Infection of the central nervous system (CNS) is a severe and frequently fatal event during the course of many diseases caused by microbes with predominantly intracellular life cycles. Examples of these include the facultative intracellular bacteria Listeria monocytogenes, Mycobacterium tuberculosis, and Brucella and Salmonella spp. and obligate intracellular microbes of the Rickettsiaceae family and Tropheryma whipplei. Unfortunately, the mechanisms used by intracellular bacterial pathogens to enter the CNS are less well known than those used by bacterial pathogens with an extracellular life cycle. The goal of this review is to elaborate on the means by which intracellular bacterial pathogens establish infection within the CNS. This review encompasses the clinical and pathological findings that pertain to the CNS infection in humans and includes experimental data from animal models that illuminate how these microbes enter the CNS. Recent experimental data showing that L. monocytogenes can invade the CNS by more than one mechanism make it a useful model for discussing the various routes for neuroinvasion used by intracellular bacterial pathogens.

Q fever pneumonia

PURPOSE OF REVIEW

In this era of emerging infectious diseases and bioterrorism it is important to be up to date with the diagnosis and management of Q fever pneumonia.

RECENT FINDINGS

A considerable amount of new information has emerged regarding the pathogenesis of Coxiella burnetii infection. The complete genome of this microorganism has now been sequenced and there are several unique features. The spectrum of manifestations of infection due to C. burnetii continues to expand. Some of the more recently described findings are acalculous cholecystitis, rhabdomyolysis, long-term persistence of Coxiella, post Q fever fatigue syndrome, and hemolytic uremic syndrome. Pneumonia as a manifestation of acute Q fever shows tremendous geographic variation, being common in one area of a country such as Spain but not in another area.

SUMMARY

Pneumonia continues to be an important manifestation of infection with C. burnetii. It responds to treatment with doxycycline, quinolones or macrolides.

Interleukin-4 induces Coxiella burnetii replication in human monocytes but not in macrophages

Coxiella burnetii, an obligate intracellular bacterium, is the agent of Q fever. The chronic disease is characterized by impaired cell-mediated immune response and microbicidal activity of monocytes. We hypothesized that interleukin(IL)-4, a Th2 cytokine, interferes with the fate of C. burnetii inside monocytes. C. burnetii survived without multiplication in resting monocytes, but replicated in IL-4-treated monocytes. The effect of IL-4 is specific for monocytes since IL-4 did not stimulate C. burnetii replication in monocyte-derived macrophages. The effects of IL-4 on bacterial replication and on tumor necrosis factor (TNF) production in monocytes were apparently not related. Although IL-4 inhibited C. burnetii-stimulated release of TNF, the addition of recombinant TNF to IL-4-treated monocytes did not prevent the IL-4 effect. These results suggest that IL-4 enables monocytes to support C. burnetii replication and a Th2 polarization of immune response that may interfere with immune control of Q fever.

Variation in immune response genes and chronic Q fever. Concepts: preliminary test with post-Q fever fatigue syndrome

Acute primary Q fever is followed by various chronic sequelae. These include subacute Q fever endocarditis, granulomatous reactions in various organs or a prolonged debilitating post-infection fatigue syndrome (QFS). The causative organism, Coxiella burnetii, persists after an initial infection. The differing chronic outcomes may reflect variations within cytokine and accessory immune control genes which affect regulation of the level of persistence. As a preliminary test of the concept we have genotyped QFS patients and controls for gene variants spanning 15 genes and also examined HLA-B and DR frequencies. QFS patients exhibited a significantly increased frequency of HLA-DR-11 compared with controls and also significant differences in allelic variant frequencies within the NRAMP, and IFNgamma genes. These results indicate a possible genetic role in the expression of overt chronic Q fever. Further studies will be undertaken to increase sample sizes, to survey other forms of chronic Q fever and to examine Q fever patients who have recovered without sequelae.

Interleukin-10 stimulates Coxiella burnetii replication in human monocytes through tumor necrosis factor down-modulation: role in microbicidal defect of Q fever

Coxiella burnetii, an obligate intracellular bacterium, is the agent of Q fever. The chronic form of the disease is associated with the overproduction of interleukin-10 and deficient C. burnetii killing by monocytes. We hypothesized that the replication of C. burnetii inside monocytes requires a macrophage-deactivating cytokine such as interleukin-10. In the absence of interleukin-10, C. burnetii survived but did not replicate in monocytes. C. burnetii replication (measured 15 days) was induced in interleukin-10-treated monocytes. This effect of interleukin-10 is specific since transforming growth factor beta1 had no effect on bacterial replication. C. burnetii replication involves the down-modulation of tumor necrosis factor (TNF) release. First, interleukin-10 suppressed C. burnetii-stimulated production of TNF. Second, the addition of recombinant TNF to interleukin-10-treated monocytes inhibited bacterial replication. Third, the incubation of infected monocytes with neutralizing anti-TNF antibodies favored C. burnetii replication. On the other hand, deficient C. burnetii killing by monocytes from patients with chronic Q fever involves interleukin-10. Indeed, C. burnetii replication was observed in monocytes from patients with Q fever endocarditis, but not in those from patients with acute Q fever. Bacterial replication was inhibited by neutralizing anti-interleukin-10 antibodies. As monocytes from patients with endocarditis overproduced interleukin-10, the defective bacterial killing is likely related to endogenous interleukin-10. These results suggest that interleukin-10 enables monocytes to support C. burnetii replication and to favor the development of chronic Q fever.