Abstract
NLRs are recently discovered PRRs detecting substructures of peptidoglycans and triggering innate immunity. NLRs are expressed in several cell types, but the presence in human B lymphocytes is still unknown. This study aimed to investigate expression and function of NLRs in human B lymphocytes. B cells were isolated and analyzed for mRNA and protein expression. The functional responsiveness of NOD1 and NOD2 was investigated upon stimulation with the cognate ligands, with or without stimulation via IgM/IgD/CD40 and/or selected TLR agonists. A differential expression of NLRs was demonstrated in blood-derived and tonsillar B cells, whereas no variations were found among naive, germinal center, or memory B cells. Stimulation with the ligands alone did not induce B cell activation. However, upon concomitant BCR triggering, an increase in proliferation was seen, together with an induction of cell surface markers (CD27, CD69, CD71, CD80, CD86, and CD95) and prolonged survival. Peripheral B cells were activated by NOD1 and NOD2 ligands, whereas tonsil-derived B cells responded solely to NOD1. In contrast, costimulation with CD40L failed to induce activation. Additionally, it was found that NLR ligands could enhance TLR-induced proliferation of B cells. The present study demonstrates expression of functional NLRs in human B cells. We show that NOD1 and NOD2 have the ability to augment the BCR-induced activation independently of physical T cell help. Hence, NLRs represent a new pathway for B cell activation and a potentially important host defense system against bacterial infections.