PAMP processing in host–pathogen interactions: shaping innate immune recognition

The immunological activity of pathogen-associated molecular patterns (PAMPs) is not determined solely by the molecules a pathogen produces. Pattern-recognition receptors (PRRs) often encounter ligands that have undergone release, degradation, transport, chemical modification or spatial reorganization. Host nucleases, metabolic enzymes, membrane trafficking and organelle programs govern these processes, while pathogen adaptations and effectors reshape both substrates and processing conditions. Consequently, the same microbial molecule can differ in receptor accessibility and inflammatory activity across tissues, host states and stages of infection. In this review, we examine this layer of regulation upstream of receptor engagement, focusing on Toll-like receptors (TLRs) with selected examples from other innate immune sensors. We first discuss how hosts generate, expose, reorganize and inactivate microbial ligands, and then consider how pathogens respond to host cues and reprogram phagosomes, autophagy and membrane transport. These interactions support a context-dependent view of PAMP activity in which the ligand available to a receptor is a product of ongoing host–pathogen interactions. We distinguish established processing mechanisms from the broader, testable hypothesis that reciprocal adaptation generates distinct receptor-accessible ligand repertoires. This review identifies therapeutic opportunities to restore host processing, expose protective ligands and limit persistent stimulation, while emphasizing the need to measure ligand states directly and match interventions to the stage of disease.