Male NZW/BXSB.Yaa (W/B) mice express two copies of TLR7 and develop pathogenic autoantibodies, whereas females with only one copy of TLR7 have attenuated disease. Our goal was to analyze the regulation of the autoantibody response in male and female W/B mice bearing the autoreactive site-directed H chain transgene 3H9. Serum anti-dsDNA Abs appeared in males at 12 wk, and most had high-titer IgG anti-dsDNA and anti-cardiolipin Abs and developed >300 mg/dl proteinuria by 8 mo. Females had only low-titer IgG anti-cardiolipin Abs, and none developed proteinuria by 1 y. Males had a smaller marginal zone than females with a repertoire that was distinct from the follicular repertoire, indicating that the loss of marginal zone B cells was not due to diversion to the follicular compartment. Vk5-43 and Vk5-48, which were rare in the naive repertoire, were markedly overrepresented in the germinal center repertoire of both males and females, but the VJ junctions differed between males and females with higher-affinity autoreactive B cells being selected into the germinal centers of males. Administration of IFN-α to females induced anti-cardiolipin and anti-DNA autoantibodies and proteinuria and was associated with a male pattern of junctional diversity in Vk5-43 and Vk5-48. Our studies are consistent with the hypothesis that presence of the Yaa locus, which includes an extra copy of Tlr7, or administration of exogenous IFN-α relaxes the stringency for selection in the germinal centers resulting in increased autoreactivity of the Ag-driven B cell repertoire.