Innate immunity in sarcoidosis pathobiology

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Purpose of reviewThe immunopathogenesis of sarcoidosis is considered to involve contributions from both adaptive and innate immune responses. Although the identification of adaptive responses to candidate pathogenic antigens will elucidate mechanisms that regulate inflammation in sarcoidosis, innate mechanisms likely represent the ‘missing link’ to the initiation, maintenance, and resolution of noncaseating granulomatous inflammation, the hallmark feature of sarcoidosis. Furthermore, environments that expose patients to candidate pathogenic antigens for sarcoidosis also provide opportunities for engagement with innate ligands.Recent findingsSeveral studies have identified enhanced responsiveness via pattern recognition receptor pathways, potentiating the local induction of cytokines relevant to granulomatous inflammation, such as through stimulation of nucleotide-binding oligomerization domain-like receptor and Toll-like receptor pathways. These pathways contribute to the inherent properties of granulomas including, in some cases, persistent localization of pathogens and antigen. Serum amyloid A has been identified to be abundant in sarcoidosis tissues, and this promiscuous host protein can serve as an innate ligand to regulate experimental granulomatous inflammation. Nascent evidence supports a potential role for alternatively activated macrophages to direct histopathological outcomes in sarcoidosis.SummaryInnate pathways deserve further investigation as potential therapeutic targets for inhibiting granuloma formation in sarcoidosis.

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