Lysozyme modulates the immune response.

At the site of infection, extracellular lysozyme (red sector), which is secreted locally by the epithelium, can kill bacteria, leading to the release of PAMPs, including but not limited to monomeric PG. This can initiate an epithelial-driven response that leads to phagocyte recruitment (not depicted here). Resident or recruited macrophages also secrete lysozyme extracellularly and can internalize bacteria, delivering lysozyme to the bacterium-containing phagosome. In macrophages, bacterial degradation by phagosomal lysozyme releases PAMPs that stimulate a robust proinflammatory cytokine response and activate the inflammasome. Neutrophil activities may be similarly enhanced by lysozyme-mediated degradation of phagosomal bacteria, akin to macrophages. Deposition of complement (blue circles) on particles, including bacteria and/or insoluble polymeric PG, enhances bacterial phagocytosis and also produces complement-derived anaphylatoxins (yellow stars) that are chemotactic for phagocytes. Because phagocytes poorly respond to extracellular, monomeric PG and monomeric PG cannot activate complement, the degradation of bacterial PG by extracellular lysozyme serves to restrict phagocyte activation and recruitment. Thus, lysozyme activity can function to enhance or dampen the immune response. Abbreviations: PAMP, pathogen-associated molecular pattern; PG, peptidoglycan.