Nasal-gut microbiome axis in health and disease

The nasal and gut microbiomes are increasingly recognised as key regulators of mucosal and systemic immunity. While each has been studied extensively in isolation, emerging evidence suggests they are connected through a bidirectional network of immune signalling, microbial metabolites, and barrier integrity, forming what may be termed “the nasal–gut microbiome axis”. This review synthesises current knowledge on the composition and function of these microbiomes, highlighting shared features, environmental influences, and patterns of dysbiosis observed in conditions such as asthma, allergic rhinitis, and chronic rhinosinusitis. We examine potential mechanisms of cross-talk, including cytokine and chemokine exchange, short-chain fatty acid mediated epigenetic regulation, and dendritic cell–driven immune priming across mucosal sites. Clinical implications are explored, with particular attention to dual-site microbiome modulation strategies, concurrent nasal–gut microbial profiling for diagnostics, and microbiome-informed precision therapies. Despite promising early evidence, knowledge gaps persist, particularly the scarcity of longitudinal, multi-omic studies and mechanistic human data. Framing the nasal and gut microbiomes as components of an integrated mucosal network, this review aims to advance understanding of their interplay and encourage research that could transform prevention and treatment strategies for immune-mediated respiratory disease. • Proposes a novel "nasal–gut microbiome axis" influencing immune responses across mucosal sites. • Discusses environmental factors like diet, pollution, and antibiotics in shaping microbiome diversity. • Identifies immune crosstalk mechanisms, such as short-chain fatty acids and cytokine signalling. • Evaluates dual-site microbiome modulation as a potential therapeutic strategy for allergic diseases. • Highlights knowledge gaps and calls for longitudinal studies to better understand microbial interactions.