The present invention provides antibodies directed against ICOS or a derivative thereof which neutralize ICOS engagement on Treg by inhibiting the fixation between ICOS and ICOS-L and abrogate proliferation of Treg induced by plasmacytoid dendritic cells. The present invention further provides antibodies directed against ICOS or a derivative thereof which induce IL-10 and IFNγ production, induce CD4+ T cells proliferation, reduce Tconv proliferation, and increase the immunosuppressive function of Treg.