Abstract 3832: A proliferation-inducing ligand (APRIL) directly impacts immune regulatory subsets to regulate suppressive multiple myeloma bone marrow microenvironment
Authors: Tai Yu-Tzu , Lin Liang , Xing Liji , Xing Liji , Wen Kenneth , Munshi Nikhil , Richardson Paul , Anderson Kenneth 
Published: August 2018

Abstract
A proliferation inducing ligand (APRIL), abundant in serum samples of multiple myeloma (MM) patients, promotes MM cell progression in vivo predominantly via MM-specific receptor B cell maturation antigen (BCMA) (Blood. 2016; 127:3225). Here, we study whether APRIL directly regulate non-MM subpopulations to influence immunosuppressive bone marrow (BM) milieu and further define molecular mechanisms regulating these processes. First, transmembrane activator and calcium modulator (TACI) protein, the other APRIL receptor expressed at lower levels than BCMA on MM cell membrane, is consistently expressed at higher levels on CD4+CD25highFoxP3high T regulatory (Treg) vs autologous CD4+CD25- conventional T cells (Tcon) from MM patients (n=10, p&lt;0.02). In contrast, BCMA is confirmed undetectable in T cells. TACI levels are even higher in the IL10+Foxp3+ subset within Foxp3+ Tregs of paired BM and PB samples from MM patients (n=17). TACI mRNA is also increased by &gt;10-fold in purified Tregs vs autologous Tcon (n=12, p&lt;0.01), which is strongly associated with upregulated immune inhibitory genes including Foxp3, CTLA4, IL-10, and CD38. APRIL preferentially induces growth and survival genes in Treg than paired Tcon, confirming elevated TACI levels in Treg vs autologous Tcon (n=4). Importantly, APRIL stimulates proliferation of Treg more potently than Tcon (n=3). In the presence of Treg, APRIL further suppresses proliferation of autologous Tcon (n=3) stimulated by anti-CD3/CD28 beads in a dose-dependent manner. MM-induced Treg (iTreg) can be generated in ex vivo co-cultures of MM cells with PB mononuclear cells or Tcon, mimicking increased Tregs linked to disease progression. Importantly, APRIL further augments generation of iTreg. Osteoclasts, a major physiological source of APRIL in the BM, significantly enhance iTreg generation that blocks Tcon proliferation. APRIL, without MM cells, cannot convert Tcon into iTreg, confirming a lack of direct impact due to the absence of TACI in paired Tcon. APRIL preferentially induces IL-10 and PD-L1 in TACIhigh Treg vs paired TACIlow/-Tcon. Finally, APRIL upregulates CD19+CD24highCD38high B regulatory cells (Breg) which interact with MM cells in the BM to confer an immunosuppressive microenvironment. Importantly, APRIL-induced Breg has higher IL-10, which is blocked by a blocking APRIL monoclonal antibody. Taken together, we are the first to characterize significantly high TACI expression that is functional in immune checkpoint Treg and Breg subsets which barely express BCMA. Importantly, we identify additional biological functions of APRIL in regulating immunosuppression via specific TACI- but not BCMA-mediated signaling cascades in the MM BM milieu, further supporting targeting this mechanism-based immunotherapeutic pathway to simultaneously target MM cells and revert immunosuppression.
Citation Format: Yu-Tzu Tai, Liang Lin, Liji Xing, Liji Xing, Kenneth Wen, Nikhil Munshi, Paul Richardson, Kenneth Anderson. A proliferation-inducing ligand (APRIL) directly impacts immune regulatory subsets to regulate suppressive multiple myeloma bone marrow microenvironment [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2018; 2018 Apr 14-18; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2018;78(13 Suppl):Abstract nr 3832.