Saturday, May 17, 2025
5/17/2025
Proinflammatory B Cells Defined by TIM-4 in the Alloimmune Response - B cells play important Ab-independent roles either promoting or regulating immune responses through the opposing activity of regulatory B cells (Bregs) and proinflammatory effector B cells (Beff). B cell depletion with anti-CD20 can rapidly improve autoimmune diseases such as RA and MS, without depleting auto-Ab. Yet, peri- transplant B cell depletion can markedly increase renal allograft rejection and chronic vasculopathy in heart transplants. These contradictory results are likely due to the presence of both Bregs and Beff, and not knowing which predominates at a given time, in a given clinical setting, or in a given patient. A similar dichotomy is present in mice, where B cell depletion/deficiency can either inhibit or promote autoimmunity and allograft rejection. We contend that targeting B cells in autoimmune and transplant patients would have greater efficacy if Beff and Bregs could be selectively targeted. Unfortunately, until the advent of Tim-1, there was no unifying marker for Bregs, hampering understanding of their biology. Even less was known about Beff cells. In mice, B cells expressing pro-inflammatory cytokines such as IL-6 and IFN play a key role promoting autoimmune responses in EAE and proteoglycan-induced arthritis. Moreover, in response to various infections, B cells exhibit rapid innate-like protective responses through expression of TNF, IFN, IL-2, and IL-17. However, it is unknown whether, or how, any of these responses relate to one another, because no phenotype for Beff was established and individual cytokines were examined in isolation. Thus, major aspects of Beff biology, including what regulates their induction and cytokine expression, and relationship to Bregs, if any, were completely unknown. We discovered that Tim-4 identifies Beff that express IFN and accelerate allograft and tumor rejection. We have now used RNAseq and quantitative PCR to demonstrate that Tim-4+ Beff express a pro-inflammatory module that includes IL-17a, IL-17f, IL-22, GM-CSF, IL-6, and IL-1 - all driven by IL-23 signaling. RORt-driven IL-17 not only reinforces its own expression, but is essential to enforce the proinflammatory module, and prevent dysregulated expression of IL-10 and potent Breg function. While IL-23 inhibits IL-10 expression by Tim-1+ Bregs, it does not induce inflammatory cytokines. Thus, Tim-4 is a unifying marker for Beffs and we are now in a prime position to further identify signaling pathways and transcription factors (TF) that control their induction and cytokine expression, and at the same time counter-regulate regulatory molecule expression. In Aim 1 we will further define the Tim-4+ Beff transcriptome and pathways that regulate the Beff inflammatory module and counter-regulate Breg activity. In Aim 2, we will identify TFs that regulate the Tim-4+ Beff inflammatory module. In Aim 3 we will define the in vivo mechanisms of Tim-4+ Beff activity in transplantation by defining the roles of antigen specificity, plasma cells, and targets of B cell IL-17 in rejection, and the role of IL- 23-blockade in subverting Beff activity to prolong allograft survival. This work will greatly advance our understanding of Beff biology and provide insight into manipulating Beff/Breg balance to promote tolerance.
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