TLR7 and TLR9-directed plasma cell formation: Dissecting the molecular basis for their differential dependence on IFN-induced signals

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Autoimmune disease like Rheumatoid Arthritis (RA) or Systemic Lupus Erythematosus (SLE) cause significant suffering and represent a huge financial burden. Since many individuals are refractory to treatment with the available drugs, there is a large unmet need to develop new therapeutics for these patients. Although we know that B cells, antibody (Ab) secreting cells (ASCs), inflammatory cytokines and TLR ligands all play important roles in driving humoral immune responses to pathogens, vaccines and self-antigens, we still lack a fundamental understanding of how the signals provided by cytokines and TLR ligands are integrated by responding B cells to promote the development and expansion of ASCs, which in the case of autoimmunity may produce pathogenic autoAbs. We characterized an unusual subset of B cells (DN2 cells), which are found in healthy donors (HD) and expanded in some SLE and RA patients. We showed that DN2 cells, which correlate with disease severity in SLE, can rapidly differentiate into ASCs, suggesting that these cells are “poised” pre-ASCs. Our data suggest that early signals provided by IFNg control DN2 development in SLE patients and HD. Moreover, ex vivo experiments using SLE patient DN2 cells reveal that differentiation of these IFNg-“primed” pre-ASCs requires additional signals provided by TLR7 ligands and IL-21 and we observed that signals provided by IFNg and IFNa control TLR7-dependent but not TLR9-dependent differentiation of human B cells. We showed that IFNg but not IFNa induces expression of the transcription factor IRF1 in human B cells and that IRF1 promotes TLR7-driven human ASC formation. Therefore, we identified at least 2, and likely 3, independent B cell differentiation pathways that are differentially reliant on IFNs, IFN-induced transcription factors and TLR ligands. To date, no studies have focused on how IFNg regulates B cell differentiation and why B cell differentiation in response to TLR7 and TLR9 are differentially dependent on IFNg. In this proposal, we will test our central hypothesis that IFNg selectively induces IRF1-dependent reprogramming of B cells, thereby licensing these cells to differentiate in response to (auto)antigens that engage the TLR7 signaling network. The immediate objectives of this proposal are to (i) examine the overlapping and distinct roles that IFNg and IFNa play in promoting TLR7-dependent human ASC development; (ii) determine how IRF1 supports B cell differentiation and (iii) evaluate why TLR7- mediated B cell differentiation is reliant on IFN-derived signals. Our long-term goal is to use what we learn about the fundamental mechanisms controlling TLR and cytokine-induced B cell differentiation to identify interventions that can regulate the formation, maintenance or function of ASCs in health and disease. This research is significant because we will, for the first time, define the mechanistic basis for IFNg-dependent TLR7-driven human B cell differentiation. We believe that our studies are important as they will advance our fundamental understanding of the mechanisms controlling human B cell differentiation and may in the future allow selective targeting of TLR7-driven autoAb responses without affecting B cell responses to other types of antigens. !