Gram-positive bacterial infections are a major cause of organ failure and mortality in sepsis. Cell wall peptidoglycan (PGN) is shed during bacterial replication, and Bacillus anthracis PGN promotes a sepsis-like pathology in baboons. Herein, we determined the ability of polymeric Bacillus anthracis PGN free from TLR ligands to shape human dendritic cell (DC) responses that are important for the initiation of T cell immunity. Monocyte-derived DCs from healthy donors were incubated with PGN polymers isolated from Bacillus anthracis and Staphylococcus aureus. PGN activated the human DCs, as judged by the increased expression of surface HLA-DR, CD83, the T cell costimulatory molecules CD40 and CD86, and the chemokine receptor CCR7. PGN elicited the DC production of IL-23, IL-6, and IL-1β but not IL-12p70. The PGN-stimulated DCs induced the differentiation of naïve allogeneic CD4+ T cells into T helper (TH) cells producing IL-17 and IL-21. Notably, the DCs from a subset of donors did not produce significant levels of IL-23 and IL-1β upon PGN stimulation, suggesting that common polymorphisms in immune response genes regulate the PGN response. In sum, purified PGN is a highly stimulatory cell wall component that activates human DCs to secrete proinflammatory cytokines and promote the differentiation of TH17 cells that are important for neutrophil recruitment in extracellular bacterial infections.
Keywords: Bacillus anthracis; T helper differentiation; dendritic cell; peptidoglycan.