Graft-versus-tumor response in patients with multiple myeloma is associated with antibody response to BCMA, a plasma-cell membrane receptor

Abstract

Donor lymphocyte infusions (DLIs) induce effective graft-versus-tumor responses in patients with multiple myeloma who relapse after allogeneic hematopoietic stem-cell transplantation. The graft-versus-myeloma response is presumably mediated primarily by donor T cells, but recent studies have also demonstrated the presence of antibodies specific for a variety of myeloma-associated antigens in patients who achieve complete remission after DLI. One of the B-cell antigens identified in these studies was B-cell maturation antigen (BCMA), a transmembrane receptor of the tumor necrosis factor (TNF) superfamily that is selectively expressed by mature B cells. The present studies were undertaken to characterize the functional significance of antibodies to BCMA in vivo. Using transfected cells expressing BCMA, antibodies in patient serum were found to react with the cell-surface domain of BCMA. Post-DLI patient serum was able to induce complement-mediated lysis and antibody-dependent cellular cytotoxicity (ADCC) of transfected cells and primary myeloma cells expressing BCMA. BCMA antibodies were only found in post-DLI responders and not in other allogeneic transplant patients or healthy donors. These results demonstrate that BCMA is a target of donor B-cell immunity in patients with myeloma who respond to DLI. Antibody responses to cell-surface BCMA may contribute directly to tumor rejection in vivo.

Figure 1.

Quantitative expression of the BCMA gene in primary myeloma cells, plasma cells from patients with MGUS and normal bone marrow, CLL, T-ALL, and pre-B-ALL measured by Affymetrix U95Av2 microarray. Box plots define the median values, 25% to 75% of values around the median and the range of values. ○ indicates outliers.

Figure 2.

Western blot analysis with patient serum. (A) Western blot analysis of immune precipitates from BCMA-293 stable transfectants, BCMA-293 transient transfectants, empty vector-293, and nontransfected 293 cells. (B) Immune precipitates (IP) from BCMA-293 (ST) and vector-293 (VT) cell lysates were blotted with patient serum (1:200 dilution) obtained before BMT, before DLI, and 4 months after DLI.

Figure 3.

Flow cytometric analysis with patient serum. (A) Flow cytometric analysis of BCMA-293 (blue filled histogram), vector-293 (red open histogram), and nontransfected 293 cells (black open histogram) with control anti–BCMA antibody. BCMA-293 (red filled histograms) and vector-293 cells (black open histograms) incubated with pre-BMT (Bef. BMT), pre-DLI (Bef. DLI), and 4 months post-DLI serum (4 mo). PE indicates phycoerythrin. (B) Mean fluorescence intensity (MFI) of serial patient samples tested for reactivity against BCMA-293 (—) and vector-293 control cells (–). ♦ indicates patient 1; ▪, patient 2.

Figure 4.

Complement-mediated cytotoxicity and ADCC using patient serum. (A-B) — indicates BCMA-293 cells; –, vector-293 cells; (A-D) ♦, patient 1; ▪, patient 2. (A) Complement-mediated cytotoxicity of post-DLI patient serum against BCMA-293 or vector-293 target cells. (B) ADCC mediated by post-DLI patient serum against BCMA-293 or vector-293 target cells. (C-D) ADCC mediated by post-DLI patient serum against BCMA-positive multiple myeloma (MM) cells. Serum from 2 healthy donors (• and ▴) was also tested at 1:10 dilution.

Figure 5.

Detection of BCMA antibodies in posttransplantation patient serum compared with serum from healthy donors. BCMA-specific fluorescence was measured by flow cytometry. Box plots describe the median values, 25% to 75% values around the median, and the range of values for each group. • indicates outliers.