alpha(2)-Macroglobulin modulates the immunoregulatory function of the lipocalin placental protein 14

Human placental protein 14 (PP14; also known as glycodelin and progesterone -associatcd endometrial protein) is an immuno-suppressive protein of the li pocalin structural superfamily. Mechanisms regulating serum PP14's inmunosu ppressive activity remain to be elucidated. In the present study, an intera ction between PP14 and a major serum protein carrier, alpha (2)-macroglobul in (alpha M-2), was documented for the first time. Using native gel electro phoresis, we showed that PP14, as well as its alternative splice variant PP 14.2, binds to both alpha M-2 and methylamine-activated (MA)-alpha M-2. Cro ss-competition studies demonstrated that the variants compete for binding t o alpha M-2. PP14 bound to alpha M-2 and MA-alpha M-2 with K-d values of 16 7+/-70 and 221+/-56 nM (means+/-S.D.) respectively, as determined by surfac e plasmon resonance. Significantly, the addition of alpha M-2 or MA-alpha M -2 to a T-cell proliferation assay strongly potentiated the inhibitory capa city of PP14. On the basis of these findings, alpha M-2 emerges as the firs t serum protein that can physically associate with, and thereby regulate, P P14. Moreover, this represents the first documented interaction between the protein carrier alpha M-2 and a lipocalin protein.