Functional Studies on Lymphocytes in Adult Human Bone Marrow

In seven preparations from adult human bone marrow (BM), surface IgM-positive (SmIgM+ve) cells were detected with anti-IgM coated ox red blood cells (anti-IgM-OxRBC). On the average, SmIgM+ve cells in the lymphocyte rich fractions from BM bound fewer anti-IgM-OxRBC than peripheral blood (PB) SmIgM+ve cells. After isolation on a Ficoll/metrizoate gradient, BM sIgM+ve cells were cultured with mitomycin C-treated PB T cells and pokeweed mitogen (PWM). These cells produced 0.44 ± 0.52 μg IgM/ml and only 0.10 ± 0.11 IgG/ml. By contrast, PB sIgM+ve cells under similar conditions produced 11.5 ± 10.4 μg IgM/ml and 6.5 ± 7.0 μg IgG/ml. In addition, BM sIgM+ve cells showed very poor PWM-induced thymidine incorporation in cultures with PB Tm (mean stimulation index 2.5 ± 2.0) compared to sIgM+ve cells from PB (mean 29.9 ± 25.3). On the other hand, the stimulator capacity of BM sIgM+ve cells in the mixed lymphocyte reaction was similar to that of PB sIgM+ve cells (mean stimulation ratio, BM: 11.1 ± 7.2; PB: 18.9 ± 14.6). The results suggest that B lymphocytes in adult BM develop IgM surface markers and the ability to stimulate in the MLR without becoming fully responsive to PWM for Ig production or proliferation. In addition, these “immature” B cells appear to have less IgM on their surface than PB B cells.