Gram-Positive and Gram-Negative Probiotic Extracellular Vesicles: Mechanisms of Immune Modulation and Therapeutic Opportunities in Atopic Dermatitis.

Atopic dermatitis (AD) is a prevalent chronic inflammatory skin disease characterized by epidermal barrier dysfunction, type 2 immune skewing, and microbiome dysbiosis. Although live probiotics can modulate the gut-skin axis, their clinical use is limited by variable efficacy and safety concerns. Probiotic extracellular vesicles (PEVs) have emerged as cell-free nanotherapeutics that retain microbial bioactivity while circumventing the risks associated with live bacteria. However, the comparative immunological roles of vesicles from Gram-positive and Gram-negative probiotics remain incompletely understood. This review systematically compares Gram-negative outer membrane vesicles (OMVs) and Gram-positive membrane vesicles (MVs) in the context of AD. OMVs, enriched with lipopolysaccharide (LPS) and other bioactive cargo, mainly signal through toll-like receptor 4 (TLR4) to regulate innate immunity, strengthen antimicrobial defense, and modulate inflammation. In contrast, Gram-positive MVs, containing lipoteichoic acid (LTA), peptidoglycan (PG), proteins, and small RNAs, preferentially activate TLR2-linked pathways that support immune tolerance, regulatory T cell (Treg) responses, and barrier repair. Both PEV classes demonstrate significant potential in preclinical models by attenuating inflammation, improving epithelial integrity, and restoring microbiome balance. Despite this potential, clinical translation requires standardized isolation protocols, scalable manufacturing, rigorous safety testing, and regulatory alignment. Advances in microbial engineering and EV design may enable targeted, microbiota-directed therapies for AD.