The interaction of the MHC class I-related chain molecules A and B (
MICA and MICB) with the corresponding natural killer group 2, member D (
NKG2D) receptor triggers cytotoxic effector activity of natural killer cells and certain T-cell subsets and provides a costimulatory signal for
cytokine production. Thus, the presence of
MICA/B on transformed cells contributes to
tumor immunosurveillance. Consequently, the proteolytic cleavage of
MICA/B is regarded as an important immune escape mechanism of various
cancer cells. To investigate the molecular machinery responsible for the shedding of endogenous
MICA/B, we analyzed different human
tumor entities including mammary, pancreatic and prostate
carcinomas. Flow cytometry and
enzyme-linked
immunosorbent assay (ELISA) revealed that all tested
tumor cells constitutively expressed
MICA and MICB on the cell surface and also released NKG2D
ligands into the supernatant. We demonstrate that the "a
disintegrin and
metalloproteases" (ADAMs) 10 and 17 are largely responsible for the generation of soluble
MICA/B. Pharmacological inhibition of
metalloproteases reduced the level of released
MICA/B and increased cell surface expression. Studies using RNA interference not only revealed a prominent role of ADAM10 and ADAM17 in NKG2D
ligand shedding but also a
tumor cell-specific role of ADAM10 and/or ADAM17 in shedding of
MICA or MICB. Moreover, we report that in the prostate
carcinoma cell line PC-3,
MICA was not shed at all but rather was secreted in exosomes. These data indicate that the release of NKG2D
ligands from individual
tumor entities is by far more complex than suggested in previously reported
MICA/B transfection systems.