Macrophages and dendritic cells play essential roles in initiating inflammation by releasing cytokines and chemokines in response to pathogen-associated molecular patterns (PAMPs) detected by innate immune receptors. Surface receptors, such as surface Toll-like receptors (TLRs) and C-type lectin receptors, detect extracellular PAMPs.

In addition, microbes internalized by phagocytes are enzymatically degraded, releasing small molecules that are screened for potential danger by a panel of intracellular innate immune receptors, such as intracellular TLRs and Nod-like receptors. Researchers have found that degradation of Staphylococcus aureus in phagosomes is a key factor in determining the types and amounts of inflammatory cytokines produced following phagocytosis. In particular, authors noted that production of interleukin (IL)-1β and IL-18 required the degradation of S. aureus cell wall peptidoglycan (PGN) and that this response is suppressed when the organism modifies its PGN to become resistant to degradation.

IL-1β and IL-18 play essential roles in controlling bacterial infections, in part, by recruiting neutrophils to sites of infection and polarizing T cell responses. Unlike many other cytokines, IL-1β and IL-18 are transcribed as pro-cytokines in the cytosol. Signaling to multiprotein complexes known as inflammasomes activates caspase-1 to process and secrete the cytokines. While there are several varieties of inflammasomes, the one responsible for responding to PGN is defined by the presence of NOD-like receptor family, pyrin domain-containing 3 (NLRP3).

The mechanism by which NLRP3 is activated by PGN is not known. It is generally thought that all of the immunomodulatory activity of the S. aureus PGN comes from the degradative release of muramyl dipeptide (MDP), which is detected by cytosolic NOD2 receptor. However, authors observed that NLRP3 inflammasome activation in response to S. aureus PGN was not affected by the loss of NOD2. Thus, the fragment of PGN that must be generated through degradation to activate the inflammasome and how it is sensed have not been established.

Diverse particulate stimuli that activate the NLRP3 inflammasome have been identified, including crystals such as silica, alum, asbestos, uric acid, and cholesterol. Like PGN particles, phagocytosis of these crystals is a necessary step in the process leading to inflammasome activation. For crystalline particles, which are non-degradable and non-microbial, it has been suggested that disruption of the phagosomal compartment leads to NLRP3 inflammasome activation. However, authors have previously observed that phagosomes containing PGN remain intact, and this, together with the observation that lysosomal degradation is necessary, suggests the existence of an alternative mechanism for specifically sensing PGN degradation products.

In this study, researchers identified N-acetylglucosamine (NAG), a sugar subunit of the backbone of PGN, as an activator of the NLRP3 inflammasome. Anthrax bacteria specifically de-acetylate NAG in PGN, and they show that this PGN becomes a poor activator of IL-1β secretion in vitro and in vivo.

Mechanistically, authors observed that purified NAG and NAG released upon degradation of PGN in phagosomes are detected via inhibition of the glycolytic enzyme hexokinase, resulting in its dissociation from the mitochondrial outer membrane.

Using a peptide that competes with hexokinase for binding to mitochondria and induces its dissociation from the outer membrane, authors observed that hexokinase dissociation alone is sufficient to induce NLRP3 inflammasome activation. These conclusions are further supported by the observation that specific metabolic perturbations that affect hexokinase function also induce inflammasome activation.

Together, the data suggest a model in which hexokinase effectively acts as a pattern recognition receptor, alerting the cell to degradation of bacterial PGN in phagosomes and activating an inflammatory response via disruption of the glycolytic pathway and mitochondrial function.

http://www.cell.com/cell/abstract/S0092-8674(16)30723-1