Autologous NK cells recognize freshly explanted bone marrow myeloma cells. (A) Freshly explanted autologous NK and myeloma cells were used in cytotoxicity assays. The results obtained in patients 1, 7, 8, 9, 10, and 11 are shown. ♦ indicates recognition by autologous fresh NK cells; •, percentage of lysis obtained using as effectors IL-2 activated autologous NK cells. The E/T ratios used were 25:1 (1), 12:1 (2), 6:1 (3), and 3:1 (4). Myeloma cells from patient 10 were obtained by immunomagnetic selection using BB-4 (anti-CD138) mAb and kept without exogenous IL-6 treatment. Five of 6 patients showed a very high autologous NK recognition of BM-derived myeloma cells, while patient 1 had less efficient autologous myeloma elimination. In patient 1, 10, and 11, IL-2 activation of NK cells had a beneficial effect on autologous myeloma recognition. (B) CD38⁺ cells from bone marrow aspirates were double stained with anti-MHC class I, W6-32 (i-iii) or anti-MICA, BAM195 (iv-vi) antibodies. Open areas are isotype controls; filled areas are specific antibody staining. Patient 1 had higher MHC class I expression than patient 7 and patient 8, while MICA was not detectable. Patient 7- and patient 8-derived myeloma cells were readily stained with anti-MICA antibody. After subtracting the background value the MHC class I surface expression median value for the patients 1, 7, and 8 were, respectively, 30, 8, and 10, while the MICA median values were 3, 10, and 8. For patients 9, 10, and 11 it was not possible to evaluate the complete phenotype because of the limited number of cells obtained.