Molecular cloning and immunologic characterization of a novel cDNA coding for progesterone-induced blocking factor.

Previous studies from our laboratory showed that the immunomodulatory effects of progesterone are mediated by a 34-kDa protein, named the progesterone-induced blocking factor (PIBF). Lymphocytes of women with threatened abortion fail to produce this factor. Via inducing a Th2 biased cytokine production and blocking of NK activity, PIBF prevents induced pregnancy loss in mice, suggesting that substitution therapy with PIBF could be useful as an alternative treatment of certain forms of recurrent spontaneous abortions. Our study was aimed at mapping the sequence and structure of PIBF coding cDNA and characterizing the encoded protein product. Screening of a human liver cDNA library revealed a 2765-bp clone with a 2271-bp open reading frame. The PIBF1 cDNA encodes a protein of 757 amino acid residues with an 89-kDa predicted molecular mass, which shows no significant amino acid sequence homology with any known protein. PIBF produced via recombinant technique is recognized by the Ab specific for the secreted lymphocyte PIBF Ab, and possesses the biological activities of the secreted lymphocyte PIBF. The full-length PIBF is associated with the nucleus, whereas secretion of shorter forms, such a 34-kDa protein is induced by activation of the cell. The 48-kDa N-terminal part of PIBF is biologically active, and the part of the molecule, responsible for modulating NK activity is encoded by exons 2-4. These data provide an initial step for exploiting the possible diagnostic and therapeutic potential of this immunomodulatory molecule.