Inflammatory response of TLR4 deficient spleen macrophages (CRL 2471) to Brucella abortus S19 and an isogenic ΔmglA deletion mutant

Brucella infection and Toll-like receptors

Brucella consists of gram-negative bacteria that have the ability to invade and replicate in professional and non-professional phagocytes, and its prolonged persistence in the host leads to brucellosis, a serious zoonosis. Toll-like receptors (TLRs) are the best-known sensors of microorganisms implicated in the regulation of innate and adaptive immunity. In particular, TLRs are transmembrane proteins with a typical structure of an extracellular leucine-rich repeat (LRR) region and an intracellular Toll/interleukin-1 receptor (TIR) domain. In this review, we discuss Brucella infection and the aspects of host immune responses induced by pathogens. Furthermore, we summarize the roles of TLRs in Brucella infection, with substantial emphasis on the molecular insights into its mechanisms of action.

The peroxisome: an up-and-coming organelle in immunometabolism

Peroxisomes are essential metabolic organelles, well known for their roles in the metabolism of complex lipids and reactive ionic species. In the past 10 years, peroxisomes have also been cast as central regulators of immunity. Lipid metabolites of peroxisomes, such as polyunsaturated fatty acids (PUFAs), are precursors for important immune mediators, including leukotrienes (LTs) and resolvins. Peroxisomal redox metabolism modulates cellular immune signaling such as nuclear factor kappa-light-chain-enhancer of activated B cells (NF-κB) activation. Additionally, peroxisomal β-oxidation and ether lipid synthesis control the development and aspects of the activation of both innate and adaptive immune cells. Finally, peroxisome number and metabolic activity have been linked to inflammatory diseases. These discoveries have opened avenues of investigation aimed at targeting peroxisomes for therapeutic intervention in immune disorders, inflammation, and cancer.

Survival of Brucella abortus S19 and other Brucella spp. in the presence of oxidative stress and within macrophages

The evolutionary “success” of the genus Brucella depends on the ability to persist both in the environment as well as inside of even activated macrophages of the animal host. For that, the Brucellae produce catalase and superoxide dismutase to defend against oxidative stress. Since the deletion of the mglA gene in the B. abortus S19 vaccine strain resulted not only in an increased tolerance to H₂O₂ but also in the induction of cytokines in macrophages, we here investigated the effect of oxidative stress (Fe²⁺ and H₂O₂) on the survival of B. abortus S19 and the isogenic B. abortus S 19 ∆mglA 3.14 deletion mutant in comparison with B. neotomae 5K33, Brucella strain 83/13, and B. microti CCM4915. These Brucellae belong to different phylogenetic clades and show characteristic differences in the mgl-operon. From the various Brucellae tested, B. abortus S19 showed the highest susceptibility to oxidative stress and the lowest ability to survive inside of murine macrophages. B. abortus S19 ∆mglA 3.14 as well as B. neotomae, which also belongs to the classical core clade of Brucella and lacks the regulators of the mgl-operon, presented the highest degree of tolerance to H₂O₂ but not in the survival in macrophages. The latter was most pronounced in case of an infection with B. 83/13 and B. microti CCM4915. The various Brucellae investigated here demonstrate significant differences in tolerance against oxidative stress and different survival in murine macrophages, which, however, do not correlate directly.

Peroxisomes in Immune Response and Inflammation

The immune response is essential to protect organisms from infection and an altered self. An organism’s overall metabolic status is now recognized as an important and long-overlooked mediator of immunity and has spurred new explorations of immune-related metabolic abnormalities. Peroxisomes are essential metabolic organelles with a central role in the synthesis and turnover of complex lipids and reactive species. Peroxisomes have recently been identified as pivotal regulators of immune functions and inflammation in the development and during infection, defining a new branch of immunometabolism. This review summarizes the current evidence that has helped to identify peroxisomes as central regulators of immunity and highlights the peroxisomal proteins and metabolites that have acquired relevance in human pathologies for their link to the development of inflammation, neuropathies, aging and cancer. This review then describes how peroxisomes govern immune signaling strategies such as phagocytosis and cytokine production and their relevance in fighting bacterial and viral infections. The mechanisms by which peroxisomes either control the activation of the immune response or trigger cellular metabolic changes that activate and resolve immune responses are also described.