Background/Purpose:

We used the B6.Sle1.Sle2.Sle3 triple congenic (TC) mouse, which expresses three lupus susceptibility loci derived from NZM2410 onto the C57BL/6 (B6) background, to investigate how marginal zone B (MZB) cells, a B cell subset enriched with autoreactive clones, contribute to autoimmune pathogenesis.   Early B cells entering the spleen are transitional B cells which can differentiate into MZB cells in the presence of Notch2 signaling.  Notch2 signaling is initiated when Notch2 receptor (Notch2R) on B cells engage delta like 1 (DL1) ligand on myeloid or stromal cells, and this leads to the transcription of Notch2 target genes, such as Deltex 1 (Dtx1).  Previous results indicated that contrary to non-autoimmune B6 mice, a large number of MZB cells in TC mice enter the follicle (FO).  This breach of follicular exclusion by TC MZB cells corresponds to a breach of tolerance associated with lupus pathology.  We predict that TC MZB cells contribute to autoimmunity by either becoming anti-DNA IgM secreting plasma cells, and/or entering the FO and contributing to germinal centers, either directly as GC B cells or indirectly by shuttling autoantigen, or by activating autoreactive CD4⁺ T cells which then proceed to activate auto-reactive FOB cells that contribute to the anti-DNA antibody (Ab) production. 

Methods:

Age matched B6 and TC mice from pre- to post-disease onset are used for FACS analysis of MZB and FOB cells, and ELISA detection of autoreactive IgM.  Notch2R and DL1 expression were assessed by FACS on B cell subsets and myeloid cells, respectively, in young B6 and TC spleens. Dtx1 expression was measured by quantitative RT-PCR in FACS-sorted MZB and FOB cells.  In order to determine whether MZB cell depletion from TC mice will correlate with decrease in autoreactive IgM, old B6 and TC mice were subjected to anti-DL1 monoclonal Ab mediated MZB cell depletion.  One week following treatment, the anti-DL1 treated and untreated mice were sacrificed for flow analysis of B cell subsets, immunofluorescence staining of spleen sections, and ELISA detection of autoreactive IgM.

Results:

TC mice displayed a preferential expansion of MZB cells and an increased production of autoreactive IgM with progression of disease.  TC MZB cells expressed a significantly higher level of Notch2R protein and Dtx1 mRNA than B6 MZB cells.  A same trend was observed for TC FOB cells, which express a much lower level of Notch2R than MZB cells.  Although DL1 was expressed at a lower level on TC than B6 myeloid cells; there were significantly more DL1⁺ neutrophils and macrophages in TC than B6 spleens.  As previously reported, anti-DL1 treatment depleted B6 MZB cells. The same treatment, however, only partially depleted TC MZB cells, and did not reduce the level of serum anti-DNA IgM.

Conclusion:

The preferential expansion of MZB cells with disease progression in TC mice corresponds to an enhanced Notch2 signaling provided by increased expression of Notch2R on MZB cells and a large number of DL1⁺ myeloid cells.  Unlike B6 mice, TC mice are resistant to anti-DL1 mediated MZB cell depletion, which is consistent with a stronger Notch2 signaling.  Additional studies are ongoing to determine the role of the Notch2 pathway in the B cells of lupus mice.   

 

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ACR Meeting Abstracts - https://acrabstracts.org/abstract/contribution-of-marginal-zone-b-cells-to-autoimmunity-in-the-b6-sle1-sle2-sle3-lupus-prone-mouse/