The mechanisms whereby the hemi-allogeneic human fetus and placenta avoid rejection by the maternal immune system remain unknown. It has long been proposed that HLA-G, a non-classical class I molecule expressed specifically on fetal-derived placental cells, protects them from attack by maternal natural killer (NK) cells which are abundant in the pregnant uterus. Mechanisms for NK cell down-regulation by class I molecules are well-described; these leukocytes bear so-called inhibitory receptors (IRs) that recognized class I molecules on target cells. IR engagement by a ligand on a target cell protects the target from NK cell-mediated lysis. Attempts to identify IRs specific for HLA-G have yielded controversial results. One explanation for the discrepancies is the difference between the HLA-G glycoform(s) produced by the placenta, and the glycoforms used in receptor-binding studies. Whereas the placental glycoform(s) contains a large N-linked glycan, the recombinant and cell line-derived molecules used to assess binding lacked this important modification. In this regard, it is particularly noteworthy that one of the two classes of IRs are lectins. This proposal aims to 1) completely characterize the glycan on placental-derived HLAG, and 2) determine its effect on receptor binding. Results from this work may help identify novel HLAG receptors and elucidate the mechanisms involved in maternal tolerance.