Nucleotide-binding oligomerization domain-containing-2 (NOD2) acts as a bacterial sensor in dendritic cells (DCs), but it is not clear how bacterial recognition links with antigen presentation after NOD2 stimulation. NOD2 variants are associated with Crohn's disease, where breakdown in self-recognition of commensal bacteria leads to gastrointestinal inflammation. Here we show NOD2 triggering by muramyldipeptide induces autophagy in DCs. This effect requires receptor-interacting serine-threonine kinase-2 (RIPK-2), autophagy-related protein-5 (ATG5), ATG7 and ATG16L1 but not NLR family, pyrin domain containing-3 (NALP3).We show that NOD2-mediated autophagy is required for both bacterial handling and generation of major histocompatibility complex (MHC) class II antigen-specific CD4(+) T cell responses in DCs. DCs from individuals with Crohn's disease expressing Crohn's disease-associated NOD2 or ATG16L1 risk variants are defective in autophagy induction, bacterial trafficking and antigen presentation. Our findings link two Crohn's disease-associated susceptibility genes in a single functional pathway and reveal defects in this pathway in Crohn's disease DCs that could lead to bacterial persistence via impaired lysosomal destruction and immune mediated clearance.

Summary Respirable fractions of mouldy hay and grain dusts were incubated with fresh human serum, Immunoelectrophoresis against rabbit anti C3 activator serum showed a conversion of C3 proactivator to C3 activator indicating that these respirable dust fractions are capable of operating the C3 by‐pass mechanism (alternative pathway). It is suggested that this, or other mechanisms, may contribute towards a disease essentially the same as classical immune complex farmer's lung where exposure is sufficiently heavy.

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