W229 - Modulation of Antibody (Ab) Secretion from Human Bone Marrow (BM) Plasma Cells as a Mechanism of Rapid Serum Hla-specific Ab Rebound After Proteasome Inhibitors

Ian Hentenaar; Linda Stempora; Cynthia Breeden; Juliete Silva; Sindhu Chandran; Christian Larsen; Annette Jackson; Stuart Knechtle; F. Eun-Hyung Lee

Abstract Text: In transplantation, proteasome inhibitors (PI) have been used to target plasma cells (PC). However, after an initial decline of Ab titers, with PI removal, serum Ab levels tend to rebound weeks later. Whether old PC die and are replenished by new plasmablasts or newly-minted Ab secreting cells (ASC), or if the existing PC merely modulate Ab secretion without undergoing apoptosis, is unknown. Furthermore, it is unknown if all BM ASC subsets regulate Ab secretion equally. Here, we tested whether human early and late (long-lived) BM ASC subsets modulate Ab secretion with a second-generation PI, carfilzomib (CFZ). Using serum and BM ASC from a pilot trial (ITN089ST) of CFZ in combination with belatacept in five highly-sensitized patients, we measured serum HLA-specific antibodies and BM ASC subsets before and after treatment. We also measured in vitro modulation of IgG secretion by ELISpots and by single cell analysis ± CFZ. In the serum, a reduction of immunodominant serum HLA titers were observed after the first cycle; however, a rapid rebound was observed as early as 10 days. BM ASC were also reduced by, on average, 54% and 23% in early and late subsets, respectively. Interestingly, CFZ initially decreased Ab secretion in all BM ASC, but only late subsets recovered Ab secretion at the single cell level. In conclusions, CFZ selectively depleted early BM ASC and the rapid serum HLA Ab rebound in CFZ treated patients may be due to resilience or modulation of Ab secretion rather than apoptosis of late BM ASC subsets.