*Purpose: Classically, B cells differentiate into Ab-secreting plasma cells (PCs; B220^lowCD138^hiBlimp1⁺Ig⁺) providing humoral immunity. B cells can also inhibit or promote immune responses through the activity of IL10⁺ Bregs and IFNγ⁺ B effectors (Beff). Recently, PCs were shown to be the major source of B-cell lineage IL10, and PC deficiency(PC^ko) worsened EAE (Matsumoto, Immunity: 2014). This suggest that PCs underlie most “Breg” activity and in prior Breg studies, classical B cells likely differentiate into PCs that actually mediate Breg function. To test this hypothesis, we examined the ability of anti-TIM-1 (which acts through induction of IL-10⁺ Bregs) to prolong islet allograft survival (GS) in recipients lacking PCs.

*Methods: To examine the role of PCs, BALB/c islets were transplanted into chemically (Streptozotocin, STZ) induced diabetic PC^ko B6 mice (Blimp1^fl/flCD19-Cre⁺, congenital deletion of Blimp1, a transcription factor essential for PC differentiation; leading to >95% reduction in PCs), or into Cre-neg Control (Ctl) mice. IFNγ and IL10 were examined in splenic PCs, conventional CD19⁺B and CD4⁺ T cells from alloimmunized recipients (detected by intracellular flow cytometry).

*Results: Surprisingly, GS was prolonged by 42% in untreated PC^ko vs. control recipients (MST 27d vs. 19d; p<0.05). Moreover,anti-TIM-1 (RMT1-10; 250 ug ip d1-,0,5) treatment was more effective in PC^ko than Ctl mice (66% vs. 26% GS>100d; p<0.05). Thus, while anti-TIM-1 requires Bregs, it does not require PCs and, in fact, works better in their absence. Moreover, CD4⁺T cells from PC^ko mice exhibited a ~2.5× increase in IL10 and ~2×decrease in IFNγ expression vs. Ctl mice. This prompted us to re-examine the EAE model where regulatory PCs was first identified. Surprisingly, in our hands, the severity of EAE (induced by MOG 35-55 peptide +CFA) was dramatically reduced in PC^ko vs. Ctl mice (p<0.05). To investigate this discrepancy, we further examined cytokines expression by splenic PCs from alloimmunized B6 mice by Flow. As expected, PCs were enriched for IL10 (~38%⁺). However, we made the discovery that PCs are also highly enriched for IFNγ (~25%⁺). (By comparison, ony 4% of whole B cells express IL-10 and 2% express IFNγ). Furthermore, ~50% PCs express TIM-4, which we previously identified as an inclusive marker for IFNγ-expressing Beff cells that promote transplant rejection (Ding, JI, 2017). (By comparison, only ~8% conventional B cells are TIM-4⁺).

*Conclusions: Our results indicate that PCs are not necessary for Breg activity in either autoimmune or transplant models. Moreover, they suggest that PCs expressing pro-inflammatory cytokines promote allograft rejection and autoimmunity and can offset the loss of regulatory IL10⁺ PCs. PC-deficiency might result in discordant outcomes depending on the relative balance of anti-inflammatory vs pro-inflammatory PCs. This hypothesis is being directly tested in novel mice we developed that exhibit PC-specific KO of either IL10 or inflammatory cytokines.

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