Inducible T cell costimulator (ICOS) signalling is essential for follicular helper T (Tfh) cell activation and has emerged as a therapeutic target in B cell-dependent autoimmune diseases such as systemic lupus erythematosus (SLE). However, its dual role in supporting both Foxp3+ regulatory T (Treg) cells and Tfh cell responses complicates therapeutic interventions. Here, we demonstrate that disruption of ICOS signalling in Sanroque mice impairs Treg cell maintenance, unleashing pathogenic T cell responses. In ICOS ligand-deficient Sanroque mice, defective ICOS signalling disrupts canonical Tfh cell-dependent germinal centre (GC) responses but fails to prevent autoantibody production and reverse SLE-like symptoms. Notably, Treg cell reduction in ICOS ligand-deficient Sanroque mice is associated with augmentation of self-reactive neuropilin-1 (Nrp-1)hi CD4+ conventional T (Tconv) cells. Mechanistically, Nrp-1hi CD4+ Tconv cells effectively produce IFN-γ and outcompete Treg cells for dendritic cell engagement. Nrp-1hi CD4+ Tconv cells acquire PD-1hi CXCR5- peripheral helper T (Tph) cell phenotype, and drive non-GC B cell responses by promoting the induction of age-associated B cells through contact-dependent mechanisms in the absence of Treg cells. Our findings highlight that ICOS signalling orchestrates GC and non-GC B cell responses in SLE via impaired Treg cell maintenance, and also underscore potential risks in therapeutic strategies targeting ICOS signalling for SLE, as it could inadvertently promote T cell-mediated inflammation and non-GC B cell responses through the induction of age-associated B cells.