Myeloma bone marrow plasma cells: Evidence for their capacity as antigen-presenting cells

Myeloma plasma calls constitute 10% to 90% of the total bone marrow cell count in patients with multiple myeloma (MM). These cells express a variety of cell surface markers, such as HLA-ABC and HLA-DR, and surface antigens that are necessary for professional antigen-presenting cells, including adhesion and costimulatory molecules. In this study, we examined the expression of major histocompatability complex (MHC) and costimulatory molecules on CD38((bright,++)) plasma cells in bone marrow aspirates from eight MM patients. Small percentages of plasma cells expressed weak but detectable levels of HLA-DR, HLA-DQ, CD40, CD80, and CD86, which could be upregulated by interferon-γ (IFN-γ) and tumor necrosis factor-α. CD38⁺⁺ plasma cell and CD38((dlm,+)) cells were sorted from freshly isolated bone marrow mononuclear cells and tested for their capacity to act as antigen- presenting cells. Indeed, both CD38⁺⁺ plasma cells and CD38⁺ cells were able to stimulate allogeneic T cells and present the soluble antigens purified protein derivative and tetanus toxoid to autologous T cells. Recognition of the antigens led to T-cell proliferation and secretion of IFN- γ and was MHC class-I and -II restricted. Antigen processing and presentation by CD38⁺⁺ and CD38⁺ cells were abolished by treatment of the cells with chloroquine. Hence, our study provides for the first time evidence that myeloma plasma cells may act as antigen-presenting cells. Further studies are warranted to examine in detail the molecules required for inducing T-cell stimulation.