Humoral immune deficiency in multiple myeloma patients due to compromised B-cell function

Patients with multiple myeloma are generally immunodeficient, with pronounced depression in primary antibody responses. We have attempted to delineate the reasons for the humoral immunodeficiency by analyzing the specificity repertoire of the surface immunoglobulin (Ig)-positive B cells in patients with multiple myeloma or monoclonal gammopathy of undetermined significance (MGUS), in comparison with normal donors. B lymphocytes from 26 patients with multiple myeloma, 12 patients with MGUS, and 8 normal donors were transformed with Epstein-Barr virus (EBV) and cultured at limiting dilution for clonal analysis. The Ig secreted by each clone was analyzed for class and anti-tetanus toxoid (TT) specificity to determine the frequencies of IgM, IgG, anti-TT IgM, and anti-TT IgG antibody-secreting clones. Our objective was to establish whether the inability to mount humoral responses to common environmental pathogens was due to a lack of specific B cells or to inhibition of B-cell function. Our results indicate that the quantitative B-cell deficiency in patients was due to a nonrandom loss of selected sets of B cells. Although most patients had a reduced aggregate number of B cells, the number of TT-specific B cells was normal. There was, on average, a threefold increase in the proportion of the B-cell specificity repertoire devoted to recognition of TT. Forty-four percent of the patients with MGUS were also affected. In addition, the TT-specific B cells in multiple myeloma patients were severely compromised in their ability to secrete antibody or to differentiate to antibody-secreting cells in vivo. This arrest in differentiation appears to be extrinsic to the B cells, as they were fully able to secrete anti-TT antibody after transformation and culture in vitro. We postulate the existence of an autoimmune inhibitory network mediating the arrest in B-cell differentiation and the humoral immune deficiency.