Background/Purpose:

Autoreactive B cells are central in the development of many autoimmune diseases, such as systemic lupus erythematosus (SLE), and are normally eliminated or inactivated by B cell tolerance mechanisms. Our lab developed a mouse model, called TDN-B, whereby inhibition of the microphthalmia transcription factor (MITF) and its family members, TFE3, TFEB, and TFEC occurs specifically in B cells. Prior work showed that B cell homeostasis and tolerance were compromised in this model, as evidenced by splenomegaly, elevated autoantibody titers, and altered B cell compartments. Further, this model worsened SLE-like disease when superimposed on an SLE-susceptible genetic background (B6.lpr), driving accelerated mortality, production of pathologic autoantibodies, and hastened renal disease. To define the signaling pathways regulated by Mitf and the MiT family that could explain their involvement in B cell tolerance, the presented work evaluated the TDN-B model, and a genetic model in which Mitf was not expressed in any cell type (Mitf^mi-vga9).

Methods:

B cell and T cell subsets were characterized by flow cytometry of bone marrow and spleen, serum immunoglobulin and autoantibody serum titers were quantified by ELISA, in-vitro isolated B cell cytokine secretion was quantified by Luminex, B cell and T cell organization within splenic follicles was visualized through wide-field and confocal microscopy, and comprehensive investigation of mRNA expression in ex-vivo B cells was defined through RNA sequencing. All experiments compared TDN-B, Mitf^mi-vga9, and wildtype mice.

Results:

Both TDN-B and Mitf^mi-vga9 mice showed increased serum rheumatoid factor, splenomegaly, increased numbers of splenocytes, and disorganization of splenic follicles. Uniquely, Mitf^mi-vga9 mice, with Mitf absent in all cells, showed increased serum levels of IgG anti-dsDNA, increased splenic germinal center (GC) B cells, and increased splenic plasma cells (PCs). While increased splenic GC B cells and PCs were not observed in TDN-B mice, increased numbers of pre-B/immature B cells and PCs were observed in the bone marrow. RNA sequencing of ex-vivo B cells showed that in both models, upregulated mRNA pools were significantly enriched for genes with roles in GC growth and/or regulation. Further, pathways related to regulation of cell cycle, MHCII antigen presentation, and cytokine signaling were all significantly enriched for in mRNA from both Mitf^mi-vga9 and TDN-B B cells. Additional experiments in Mitf^mi-vga9 mice uniquely demonstrated increased numbers of B cells with surface expression of activation markers (CD69, CD25) and antigen presentation molecules (MHCII, CD86), and that B cells in culture had increased secretion of TNF-alpha after LPS stimulation.

Conclusion:

These data demonstrate that the functional impairment of Mitf and the MiT transcription factor family results in a breach of B cell tolerance that is coincident with cellular and molecular changes, which lead to dysregulation of B cell activation, antigen presentation, cytokine secretion, germinal center organization, plasma cell differentiation, and autoantibody production. The underlying mechanisms responsible for these effects are under investigation.

« Back to 2019 ACR/ARP Annual Meeting

ACR Meeting Abstracts - https://acrabstracts.org/abstract/functional-impairment-of-mitf-and-the-mit-transcription-factor-family-dysregulates-b-cell-activation-and-function/