Abstract Genetic loci on NZB c1 lead to antinuclear antibody (Ab) production and glomerulonephritis (GN). Using subcongenic lines, we found at least 5 genetic loci within a 35-106cM interval modulate disease. Notably, mice with a 70-100cM interval (c1(70-100cM)) develop high titre anti-dsDNA Ab, increased numbers of large germinal centers and severe GN a characteristic feature of lupus-prone mice with abnormalities of T follicular helper (TFH) and Th17 cells. In this study we determined whether variations in disease severity in c1 subcongenic mice are associated with differences in these subsets. The proportion of splenic TFH cells was significantly increased in c1(70-100) mice. Splenocytes from all c1 congenic mouse strains demonstrated increased production of IL-21 and IL-17 following anti-CD3/CD28 crosslinking which was greatest in c1(70-100) mice. The majority of IL-21 producing cells were TFH, many of which also secreted IFN-γ, but not IL-17. Immunization of 8-wk-old pre-autoimmune mice with ovalbumin (OVA) led to increased generation of TFH and Th17 cells in c1 (70-100) mice. This appeared to result in part from altered T cell function, as adoptively transferred OVA-specific TCR transgenic c1(70-100) T cells and naïve c1(70-100) T cells cultured in-vitro, demonstrated enhanced differentiation to TFH and IL-17-producing cells. Thus, genetic loci on NZB c1 promote differentiation of T cells to TFH and Th17 cells and this capacity correlates with the severity of disease.