Reduced immune effector cell NKG2D expression and increased levels of soluble NKG2D ligands in multiple myeloma may not be causally linked.

BACKGROUND: There is limited understanding of the dysregulation of the innate immune system in multiple myeloma (MM). We analysed the expression of the activating receptor NKG2D on NK cells and T cells of MM patients and investigated the impact of soluble versus membrane-bound NKG2D ligands on the expression of NKG2D. DESIGN: NKG2D expression on NK cells and CD8+ alphabeta T cells from patients with MM or monoclonal gammopathy of uncertain significance and healthy controls was examined flow-cytometrically. Sera from patients and controls were analysed for soluble NKG2D ligands (sNKG2D ligands). RESULTS: Significantly fewer NK cells and CD8+ alphabeta T cells from patients expressed NKG2D compared to healthy controls (NK cells: median 54% interquartile range (IQR) 32-68 versus 71% IQR 44-82%, P = 0.017, CD8+ alphabeta T cells: median 63% IQR 52-81 versus 77% IQR 71-90%, P = 0.018). The sNKG2D ligand sMICA was increased in patients [median 175 (IQR 87-295) pg/ml] versus controls [median 80 (IQR 32-129) pg/ml, P < 0.001], but levels of sMICA did not correlate with NKG2D expression on effector cells. To elucidate the mechanism of NKG2D down-regulation, we incubated lymphocytes from healthy donors in the presence of sNKG2D ligands or in co-culture with MM cell lines. sNKG2D ligands in clinically relevant concentrations did not down-regulate NKG2D expression, but co-culture of effector cells with myeloma cells with high surface expression of NKG2D ligands reduced NKG2D expression significantly. CONCLUSIONS: These results indicate that MM is associated with a significant reduction in NKG2D expression which may be contact-mediated rather than caused by soluble NKG2D ligands.