[Hypereosinophilic syndromes: pathogenic and therapeutic up-to-date]

The hypereosinophilic syndromes (HES), defined by an unexplained and sustained hypereosinophilia, can be associated with heterogeneous hematological conditions. Several molecular mechanisms underlying the eosinophilia, which remained indeterminate for a long time, have been recently identified. These recent advances allowed a better classification of the various forms of HES and the development of targeted therapies. The role of tyrosine kinases, especially PDGFRA, and the efficacy of tyrosine kinases inhibitors dramatically improved the diagnosis and the treatment of myeloproliferative variant of HES. On the other side, eosinophilia can be driven by IL-5 secreting abnormal and often clonal T cell subsets (lymphocytic variant of HES). The crucial role of this cytokine in eosinophil development, activation and survival leads to the assessment of anti-IL-5 monoclonal antibodies which have recently shown to provide a significant corticosteroid sparing effect in FIP1L1-PDGFRA negative HES patients. Despite these major advances, half of HES remains unexplained (idiopathic HES). Some FIPL1-PDGFRA negative patients respond to imatinib, suggesting the role of other tyrosine kinases (or other partners than FIP1L1 in a fusion gene implicating PDGFRA). Development of new biomarkers is needed to help physicians in the diagnosis, classification of HES and in the choice of a targeted therapy.