K. Mark Coggeshall
B. anthracis Peptidoglycan as a Pro-inflammatory Agent in Anthrax Pathogenesis
Following inhalation of spores, B. anthracis dramatically expand to achieve 108-109 organisms per milliliter of blood. The level of bacterial expansion is exceptional in human blood infections. Patients have a poor prognosis and ~40% of cases are fatal, even with aggressive antibiotic therapy. The course of the disease is not understood but has been attributed to the B. anthracis toxins. Our group is testing the idea that patients die of the blood infection (sepsis) rather than the toxin. Gram-positive organisms contain a rigid peptidoglycan (PGN) cell wall that can engage several human pattern recognition receptors. Agents that bind these receptors stimulate the production of proinflammatory cytokines, which act as intercellular mediators to support an immune response but also contribute to the pathology of sepsis. Proinflammatory cytokines are present in sera of animal models infected with B. anthracis. The role of endogenous PGN in the cytokine production and anthrax pathology is unclear.
We have isolated the previously uncharacterized B. anthracis PGN from vegetative bacteria and showed that it stimulates the production of proinflammatory cytokines from cells in human blood. We identified some key elements of the structure of B. anthracis PGN but many of the structural issues concerning PGN remain unsolved. The cytokine response is exclusive to monocytes and neutrophils, which are large, circulating phagocytic white blood cells. Despite the central role of these cells in the response, no PGN receptor has yet been identified.
We are characterizing the structure and biological function in humans of B. anthracis peptidoglycan. The information will be useful in identifying the unknown phagocytic PGN receptor and the biological response to B. anthracis PGN. This issue is important because different PGN structures bind to distinct PGN receptors in innate immune cells. One of our goals is to identify the specific receptor for B. anthracis peptidoglycan. Small molecule inhibitors of these pathways might allow clinicians to reduce the hyper-inflammation associated with B. anthracis sepsis, and could identify pathways that promote monocyte cell death.
Iyer JK, Coggeshall KM. Cutting edge: primary innate immune cells respond efficiently to polymeric peptidoglycan, but not to peptidoglycan monomers. J Immunol 186:3841-3845, 2011. [Abstract]
Iyer JK, Khurana T, Langer M, West CM, Ballard JD, Metcalf JP, Merkel TJ, Coggeshall KM. Inflammatory cytokine response to Bacillus anthracis peptidoglycan requires phagocytosis and lysosomal trafficking. Infect Immun 78:2418-2428, 2010. [Abstract]
Langer M, Malykhin A, Maeda K, Chakrabarty K, Williamson KS, Feasley CL, West CM, Metcalf JP, Coggeshall KM. Bacillus anthracis peptidoglycan stimulates an inflammatory response in monocytes through the p38 mitogen-activated protein kinase pathway. PLoS One 3:e3706, 2008. [Abstract]