By assessing the development of Y-linked autoimmune acceleration (Yaa) gene-induced systemic lupus erythematosus in C57BL/6 (B6) × (New Zealand Black (NZB) × B6.Yaa)F1 backcross male mice, we mapped three major susceptibility loci derived from the NZB strain. These three quantitative trait loci (QTL) on NZB chromosomes 1, 7, and 13 differentially regulated three different autoimmune traits: anti-nuclear autoantibody production, gp70-anti-gp70 immune complex (gp70 IC) formation, and glomerulonephritis. Contributions to the disease traits were further confirmed by generating and analyzing three different B6.Yaa congenic mice, each carrying one individual NZB QTL. The chromosome 1 locus that overlapped with the previously identified Nba2 (NZB autoimmunity 2) locus regulated all three traits. A newly identified chromosome 7 locus, designated Nba5, selectively promoted anti-gp70 autoantibody production, hence the formation of gp70 IC and glomerulonephritis. B6.Yaa mice bearing the NZB chromosome 13 locus displayed increased serum gp70 production, but not gp70 IC formation and glomerulonephritis. This locus, called Sgp3 (serum gp70 production 3), selectively regulated the production of serum gp70, thereby contributing to the formation of nephritogenic gp70 IC and glomerulonephritis, in combination with Nba2 and Nba5 in NZB mice. Among these three loci, a major role of Nba2 was demonstrated, because B6.Yaa Nba2 congenic male mice developed the most severe disease. Finally, our analysis revealed the presence in B6 mice of an H2-linked QTL, which regulated autoantibody production. This locus had no apparent individual effect, but most likely modulated disease severity through interaction with NZB-derived susceptibility loci.

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Retroviral envelope glycoprotein gp70 is present in the sera of immunologically normal and autoimmune-prone strains of mice. However, only lupus-prone mice spontaneously develop gp70-anti-gp70 immune complexes (gp70IC), and these have been implicated in the development of nephritis. We investigated the genetic factors that affect the production of both free serum gp70 and gp70IC in the lupus-prone BXSB mouse strain by analyzing (BXSB × (C57BL/10 × BXSB)F1)- and (C57BL/10 × (C57BL/10 × BXSB)F1)-backcrossed male mice. Production of gp70 mapped to a single major locus located on chromosome 13 (Bxs6) with a maximum log likelihood of the odds of 36.7 (p = 1.6 × 10−38). The level of gp70IC was highly dependent on Bxs6-related gp70 production, and high titer autoantibody production only occurred when serum gp70 levels were greater than a threshold value of ∼4.0 μg/ml. The subdivision of the (BXSB × (C57BL/10 × BXSB)F1)-backcrossed mice into those homozygous or heterozygous for Bxs6 enabled a remarkable association to be observed between high levels of gp70IC and severe nephritis in the Bxs6 homozygote population. A further mapping study in these two subgroups identified a previously unrecognized interval associated with the production of autoantibodies.

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In a subset of systemic lupus erythematosus (SLE) patients, antiphospholipid syndrome, characterized by occurrence of anti-cardiolipin (CL) antibodies, thrombocytopenia, thrombosis and recurrent intrauterine fetal death occurs. Male (NZW x BXSB)F1 mice, carrying the BXSB Yaa gene, serve as a model for SLE-associated antiphospholipid syndrome. Using microsatellite markers in the NZW x (NZW x BXSB)F1 backcross male progeny, we mapped BXSB alleles contributing to the generation of anti-CL antibodies, platelet-binding antibodies, thrombocytopenia and myocardial infarction. Generation of each disease character was controlled by two major independently segregating dominant alleles, i.e. those on chromosomes (Chr.) 4 and 17 for anti-CL antibodies, Chr. 8 and 17 for both anti-platelet antibodies and thrombocytopenia and, to our surprise, Chr. 7 and 14 for myocardial infarction, and that a combination of the two alleles appeared to produce full expression of each character, as a complementary gene action. The alleles on Chr. 17 linked to the above three characters were all mapped in close proximity to the H-2 complex. Therefore, no single factor such as anti-CL antibodies can explain the pathogenesis of SLE-associated antiphospholipid syndrome. Rather, a combination of susceptibility alleles such as described here, along with additional modifying loci, i.e. BXSB Yaa and some from NZW, characterizes unique SLE features in male (NZW x BXSB) F1 mice. There are potentially important candidate genes which may be linked to the syndrome.