Systemic autoinflammatory diseases (SAIDs) are rare disorders characterized by recurrent febrile attacks and systemic sterile inflammation. Mutations in genes that cause dysregulation of the innate immune system underlie the etiology of several SAIDs. The study ofmfinogenic SAIDsled to the discovery of inflammasomes, which are intracellular multiprotein signaling complexes that regulate pro-inflammatory ILID and IL18 cytokine secretion. Despite the identification of the genetic defects in several SAIDs, the pathogenicity of the identified mutations is difficult to assess as the majority are missense variations. The report of patients with a clinical presentation of monogenic SAIDs, but in the absence of germline mutations in disease-causing genes, and the poor understanding of the cellular and molecular bases of complex multifactorial polygenic disorders like adult onset still's disease (AOSD) make diagnosis and effective treatment challenging. The work presented in this thesis aimed at (i) Studying the functional consequences of an identified NLRP3 variant in two apparently unrelated ARPU-AID families; (ii) identifying the molecular bases of chronic inflammatory urticaria, and (iii) Studying the molecular and cellular bases of AOSD. We identified a heterozygous missense NLRP3 variant (c.1322C>T, p.A441V) in a multigenerational French family and in a sporadic case presented with two different NLRP3- AID phenotypes. Although microsatellite analyses flanking NLRP3 showed that the two unrelated families share the same disease haplotype, whole-genome SNP genotyping revealed that the mutation occurred independently in the two families, in keeping with a recurrent mutational event. In vitro functional assays showed that the mutated NLRP3 protein -as compared to the wild-type- is associated with significantly higher ASC speck formation and ILl b secretion, two major readouts of inflammasome activation. Gene expression profile in patients' monocytes showed that is highly correlated with the disease activity explaining therefore the heterogeneous clinical phenotype associated with this mutation. In order to identify the molecular bases of histamine-resistant chronic urticarial skin rash associated with systemic inflammation in two sporadic cases, next generation sequencing (NGS) was performed targeting the main genes implicated in SAIDs. A somatic mosaic NLRP3 variant was identified in both patients. Despite the late-disease onset, the mutations were widely distributed in different patients' cells. In vifro functional studies of the identified variants clearly demonstrated a gain-of-function effect on NLRP3-inflammasome activation. Accordingly, the patients showed a clinical response to anti-IU therapy highlighting the importance of molecular diagnostic tools for targeted treatment. Plasma cytokine profile in AOSD patients revealed significantly elevated plasma IL18 levels as compared to healthy controls. Whether or not IL18 is a disease biomarker remains to eb established. Primary results from cytokine expression analysis in AOSD patients' monocytes and macrophages suggested that monocytes may play an important role in disease pathogenesis. In conclusion, this work showed the importance of functional studies and patients' cells in order to validate the pathogenicity of sequence variants and the use of NGS as a diagnostic tool for the molecular bases of SAIDs. The results obtained on AOSD pave the way for further studies which could open up to targeted therapies.