APRIL regulates immunosuppression via TACI in the MM bone marrow

Overexpression of A Proliferation Inducing Ligand (APRIL) has been linked to multiple myeloma (MM) and is thought to drive both proliferative signals as well as immunosuppressive signals in the bone marrow (BM). Therefore, targeting APRIL with blocking antibodies is a therapeutic strategy in MM. Recently, an APRIL-specific antibody (BION-1301) developed by Aduro Biotech entered an initial phase I/II clinical trial in patients (pts) with relapsed and refractory (RR) MM.

APRIL binds to two receptors, B cell maturation antigen (BCMA) and Transmembrane activator and CAML interactor (TACI), so it has been hard to decipher through which receptor individual signals emanate. However, high BCMA expression occurs on MM cells, and so BCMA is a prime target in MM, with the development of a number of BCMA-directed CAR T-cell already in clinical trials.

In a study published in Leukemia in July 2018, Yu-Tzu Tai from The Jerome Lipper Multiple Myeloma Center and LeBow Institute for Myeloma Therapeutics, Dana-Farber Cancer Institute, Boston, US, and colleagues, studied the impact of APRIL on regulatory T-cells (Tregs) in the context of immunosuppression in the MM BM milieu.

Key Findings:

High levels of TACI were found on Tregs compared to conventional T cells (Tcons)

• T-cell subsets were harvested from freshly isolated peripheral blood (PB) from MM pts (n = 47) and comparisons made between Tregs (CD4⁺CD25⁺Foxp3⁺) and Tcons (CD4⁺CD25⁻):
• Tregs expressed the highest TACI levels and IL-10
• Functionally suppressive CD8⁺ Tregs (CD8⁺CD25⁺Foxp3⁺), found at higher levels in MM pts, also expressed higher levels of TACI than CD8⁺CD25⁻ Tcons
• TACI was significantly higher on Tregs in both PB and BM samples from the same MM pt (n = 9, P < 0.02)
• Fold increase in expression:
  • TACI mean fluorescence intensity (MFI): Tregs > 4–40 vs paired Tcons > 3–15
  • TACI transcripts: Tregs > 4–12 vs Tcons (normal donors) and >17–52 vs Tcons (MM pts)
• Tregs had increased levels of Foxp3 (>7–16 fold) and CTLA4 (>3–9-fold) vs paired Tcons
• TACI levels also significantly correlated with CTLA- 4 (r = 0.9715, p < 0.0001)
• Significant increase in TGFβ (p < 0.0001) and IL-10 (p <0.0003) for Tregs vs paired Tcons

APRIL signals drive viability and block apoptosis of Tregs via TACI-mediated signals

• Soluble APRIL promoted the viability of Tregs more effectively than Tcons
• APRIL blocked apoptosis in Tregs by inhibition of caspase-3/7 and caspase-8
• Blocking monoclonal antibodies (mAbs) against APRIL and TACI (used separately) reversed this effect; though anti-TACI was only effective on Tregs
• APRIL was found to induce expression of cell cycle progression genes CCND1 and CCND2, and anti-apoptotic genes BCL2 and BCL2L1/BCLxL in Tregs but not Tcons
• Continued addition of APRIL sustained this upregulation

APRIL-TACI signals drive immunosuppressive effects through Tregs

• APRIL stimulation led to:
  • Increased expression of key suppressive molecules in Tregs: > 11-fold, 4-fold, and 5-fold higher mRNA expression of Foxp3, IL-10, and TGFβ
  • Enhanced Foxp3 and IL-10 expression after prolonged stimulation
  • Time-dependent induction of PDL1 and TGFβ1
• No effects were observed in paired Tcons
• Tregs blocked proliferation of Tcons in co-culture experiments via APRIL-mediated signals

APRIL enhances production of T-regs driven by MM cells in an IL-10 dependent manner

• APRIL enhanced the production of iTregs generated from co-cultures of MM cells and CD3 T-cells, which mimics the increased Tregs generated during disease progression
• MM cells were found to significantly stimulate proliferation of iTregs
• APRIL upregulated the proliferative iTreg fraction from 7.24 ± 0.27% to 11.28 + 1.1% (P < 0.02)
• At low iTreg to Tcon ratios, APRIL enhanced the iTreg block on Tcon proliferation (p <0.005)
• APRIL further increased the percentage of IL10⁺TGFβ⁺ iTreg cells (p < 0.05)

Tregs contribute to Osteoclast (OC)-induced immune suppression on Tcons

• OCs significantly induced generation of CD4 and CD8 iTreg from T cells (7 days coculture)
• OCs inhibited expansion of Tcons  
• Combined treatments of anti-APRIL with either anti-PD1 or anti-PD-L1 further overcame OC suppression on Tcons

BM-derived Bregs (CD19⁺CD24^highCD38^high) from MM pts express TACI and specifically mediate APRIL-induced IL-10 production

• APRIL upregulated the percentage of Breg in B cell fraction from 14.59 ± 1.36% to 25.2 ± 0.69% (P = 0.0004)
• APRIL enhanced IL-10 production in Bregs from 15.02 ± 0.88% to 29.22 ± 3.33% (P < 0.007)

 Conclusion

TACI is expressed at significantly higher levels on Tregs than on conventional T cells from the same patient, and APRIL stimulates survival of these cells. In turn, these cells drive immunosuppressive signals in the BM of myeloma patients. In all experiments where an effect with APRIL was observed, the use of a blocking anti-APRIL mAb reversed this effect. Thus, this study provides a strong rationale for the use of an anti-APRIL antibody as a therapy in MM, and also suggests that combination with other agents, such as anti-PD1 may be beneficial.