Mucosal macrophages combat pathogens through phagocytosis initiated by pathogen recognition receptors and by producing nitric oxide. The chemokine CCL25 is exclusively expressed in the lamina propria, directing the migration of cells with CCR9 receptor.
Mucosal macrophages utilize several methods to recognize and kill pathogens without generating excessive inflammation. Firstly, they rely on manifold pathogen recognition receptors (PRRs) that recognize unique pathogen-associated molecular patterns (PAMPs), initiating phagocytosis without vigorous inflammatory responses. Additionally, these macrophages are adept at producing inducible nitric oxide synthase (iNOS) which generates nitric oxide, a reactive nitrogen species that possesses antimicrobial activity without necessarily invoking a full-scale inflammatory reaction.
Concerning the second question, the chemokine exclusively expressed within the lamina propria that recruits cells expressing the CCR9 receptor is CCL25 (Thymus-expressed chemokine). It plays a crucial role in the selective homing of T cells and plasmablasts to the small intestine, especially within the context of mucosal immunity.
So, mucosal macrophages adopt specific mechanisms to tackle pathogens efficiently while curtailing excessive inflammation, and CCL25 serves as a key chemokine in the recruitment of specific immune cells to the lamina propria, hence contributing to the mucosal immune defense.