In contrast to the classical HLA class la molecules, the nonclassical HLA-G primary transcript is alternatively spliced to generate several mRNAs that encode four membrane-bound and three soluble isoforms. This study demonstrated that the soluble form of HLA-G can also be generated by metalloprotemase-dependent shedding at post-translational level. These soluble HLA-G1 molecules generated by the cleavage of membrane-bound HLA-G1 associate with beta2-microglobulin and contain bound peptides that are stable at physiological conditions. This report further showed that the soluble HLA-G1 is able to protect HLA class I-negative K562 cells from NK lysis, suggesting that soluble HLA-G could act as an immunoregulator in NK cell recognition and possibly in other immune responses. (C) 2003 Elsevier Inc. All rights reserved.