Immunoglobulin-A has an irreplaceable role in the mucosal defence against infectious microbes^,,,,,. In human and mouse, IgA-producing plasma cells comprise ∼20% of total plasma cells of peripheral lymphoid tissues, whereas more than 80% of plasma cells produce IgA in mucosa-associated lymphoid tissues (MALT)^,,,,,. One of the most biologically important and long-standing questions in immunology is why this ‘biased’ IgA synthesis takes place in the MALT but not other lymphoid organs. Here we show that IgA class-switch recombination (CSR) is impaired in inducible-nitric-oxide-synthase-deficient (iNOS^-/-; gene also called Nos2) mice. iNOS regulates the T-cell-dependent IgA CSR through expression of transforming growth factor-β receptor, and the T-cell-independent IgA CSR through production of a proliferation-inducing ligand (APRIL, also called Tnfsf13) and a B-cell-activating factor of the tumour necrosis factor (TNF) family (BAFF, also called Tnfsf13b). Notably, iNOS is preferentially expressed in MALT dendritic cells in response to the recognition of commensal bacteria by toll-like receptor. Furthermore, adoptive transfer of iNOS⁺ dendritic cells rescues IgA production in iNOS^-/- mice. Further analysis revealed that the MALT dendritic cells are a TNF-α/iNOS-producing dendritic-cell subset, originally identified in mice infected with Listeria monocytogenes^,. The presence of a naturally occurring TNF-α/iNOS-producing dendritic-cell subset may explain the predominance of IgA production in the MALT, critical for gut homeostasis.