Background/Purpose: Systemic lupus erythematosus (SLE) is an autoimmune disease that results in significant morbidity and mortality. In SLE, endosomal TLR7 and TLR9 are known to mediate the anti-self response. We found that TLR9 deletion in the MRL.Fas^lpr model of SLE exacerbates disease, despite the requirement of TLR9 for the formation of anti-DNA antibodies. This is dependent on TLR7, suggesting that TLR9 is protective while TLR7 drives pathology. Recently, we found that TLR9 mediates protection specifically in B cells; however, it is unknown whether TLR9 regulates TLR7 in the same cell (cis) or in different cells (trans). For example, while TLR9 regulates disease in B cells, TLR7 may exacerbate disease in DCs. Thus, a major goal of our lab is to understand the B cell-specific role of TLR7 to inform the development of targeted therapeutics for SLE.

Methods:

The MRL.Fas^lpr model replicates many features of human SLE including ANA production, dermatitis, and renal disease starting from 9-11 weeks of age. MRL.Fas^lpr mice were generated with a B cell specific deletion of TLR7 (CD19-Cre TLR7^fl/fl) and compared to Cre- littermates. Deletion was assessed via cell sorting and qPCR of genomic DNA. Lupus pathology was analyzed in female and male mice at 19 and 22 weeks of age, respectively, with parameters including proteinuria, renal histology for interstitial and histologic disease, dermatitis, organ weight, and immune cell activation. CD19-Cre TLR7^fl/fl mice were also crossed to TLR9^-/-MRL.Fas^lpr mice and analyzed at 16 and 19 weeks of age for females and males, respectively, to study B-cell intrinsic interactions between TLR7 and TLR9.

Results: Validation via qPCR showed specific but incomplete deletion of TLR7, with a 78.60% deletion efficiency in B cells, which was further reduced in plasmablasts. CD19-Cre TLR7^fl/fl mice had reduced proteinuria but no analogous decrease in kidney pathology or other disease parameters. Given the interaction between TLR9 and TLR7, and that TLR9 deficiency accelerates disease, we then studied the effects of B cell-specific TLR7 deficiency in TLR9^-/- mice. The CD19-Cre TLR7^fl/fl TLR9^-/- mice exhibited reduced spleen weights (p=0.002) and an increased proportion of splenic naïve CD4+ T cells. More importantly, they exhibited a significant reduction in proteinuria (p=0.005), glomerulonephritis (p=0.0001), and interstitial nephritis (p=0.0002).

Conclusion:

The inefficient deletion of TLR7 suggests that activated, “escapee” B cells could preferentially expand and explain the lack of disease regulation in the CD19-Cre TLR7^fl/fl cohort. Conversely, a lack of regulation by TLR9 could allow B-cell specific TLR7 to exacerbate disease to an even greater degree than it could when TLR9 is present. This is suggested by the CD19-Cre TLR7^fl/fl TLR9^-/- cohort which shows a marked reduction in TLR9-driven lupus exacerbation when TLR7 is deleted in B cells. This supports the hypothesis that TLR9 regulation acts in cis, and that B cell specific TLR signaling is a major mediator of SLE. Ongoing studies will continue to address the cell specific roles of TLR7 via a mixed bone marrow chimera and DC specific deletion. Overall, these results indicate that targeting TLRs in B cells could be a viable therapeutic strategy in SLE.

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ACR Meeting Abstracts - https://acrabstracts.org/abstract/b-cell-specific-tlr7-regulates-lupus-in-tlr9-deficient-mice/