A First in Class APRIL Fully Blocking Antibody Targets Novel Immune Regulation of APRIL in Multiple Myeloma: Further Therapeutic Implication

A proliferation inducing ligand (APRIL), a critical paracine factor for human multiple myeloma (MM), is mainly secreted by myeloid cells, including macrophages and osteoclasts in the bone marrow (BM) microenvironment. Recently, an antagonistic APRIL monoclonal antibody (mAb) 01A significantly reduced myeloma burden in SCID-hu mice (Blood. 2016;127:3225-3236). This data suggests that the humanized 01A named BION-1301, which also potently (IC50 <1 nM) blocks APRIL binding to its receptors B cell maturation antigen (BCMA) and transmembrane activator and calcium modulator (TACI) is likely to exert anti-MM activity in vivo. Here, we further characterize the novel inhibition by 01A and a hIgG4 chimeric version C4 on multiple immune cells affected by APRIL in the immunosuppressive MM BM milieu. First, using gene-modified RPMI8226 MM cells, 01A potently and selectively blocks growth and viability of APRIL-overexpressing but not BCMA-overexpressing RPMI8226 transfectants. Significantly, 01A inhibits MM cell growth and survival induced by osteoclasts, as a physiologically relevant endogenous source of APRIL. Next, we demonstrate that pretreatment of APRIL decreases MM cell lysis induced by Daratumumab-mediated antibody-dependent cellular cytotoxicity (ADCC). In addition, APRIL prevents anti-BCMA (J6M0)-induced ADCC against BCMA-expressing MM cell lines in a dose-dependent manner. Conversely, 01A or C4 (a surrogate of BION-1301) reverts APRIL-reduced MM cell lysis induced by J6M0 in the presence of PBMC or NK effector cells. These data indicate that targeting CD38 and BCMA may not be sufficient in MM. To further define the role of APRIL mediating anti-MM immune responses, we show that T cells do not express BCMA while natural T regulatory (Treg) cells significantly express higher TACI when compared with corresponding conventional T cells (Tcon) from the same individual (Abstract #102467). In parallel, detailed gene expression profiling in multiple solid tumors show that the tumor-resident Treg markers are significantly correlated with TACI, along with a strong association of APRIL expression levels with T cell inflamed signature. In addition to natural Treg, MM cell-induced Tregs (iTregs) also showed significantly increased TACI levels when compared with paired conventional T cells (Tcon) from the same individual in ex vivo cocultures. Importantly, APRIL further enhances MM cell-induced iTregs with IL-10-producing and CD15s⁺[JD2] FOXP3^high fractions; conversely, 01A blocks APRIL-induced upregulation of iTreg. Osteoclasts also upregulated iTreg generation induced by MM cells. On the contrary, 01A or C4 partially blocks such induction. Importantly, APRIL directly promotes proliferation and viability of Treg via the induction of CCND1/2, BCL2, and BCL2L1/BCL-xL, which is blocked by 01A or C4. Finally, APRIL enhances the suppressive function of Treg on the proliferation of autologous conventional T cells, which could also be overcome by 01A or C4. Collectively, our data therefore demonstrate that APRIL, via TACI but not BCMA, critically modulates regulatory T cell function; and, importantly, 01A or C4 targets these immune regulatory subsets that have been linked to MM progression via APRIL-dependent manner. Moreover, since patients with active diseases have severe bone lesions induced by hyperactive osteoclasts, inhibiting the APRIL-TACI interaction may also overcome osteoclast-inhibited T cell killing on MM cells. Anti-APRIL Ab (BION-1301) will be entering clinical trial in late 2017 to target the immunosuppressive BM microenvironment and improve patient outcome in MM.