Placental cytotrophoblast HLA-G5 is synthesized as a β2m-free heavy chain homodimer (42.13)

Abstract Placental villous cytotrophoblast cells (vCTB) contain an unusual member of the HLA class I gene family, HLA-G5. Recently we showed that HEK293-derived recombinant HLA-G5 protein is predominantly composed of H:H homodimers. Here, we examined the HLA-G5 produced in primary vCTB. Determining the structure is critical because recombinant ectodomains of the inhibitory leukocyte Ig-like LILRB1 (ILT2) and LILRB2 (ILT4) receptors bind HLA-G dimers with higher affinity than monomers. Analysis of vCTB cell extracts by immunoblotting demonstrated that vCTB cell HLA-G5 H chain proteins are primarily disulfide bonded dimers. The H chains were not associated with β2m L chains; although β2m mRNA was identified by RT-PCR, immunoblots failed to detect β2m protein even when additional β2m mRNA was introduced. Furthermore, sequencing failed to reveal any abnormalities in the translational start codon of either endogenous β2m mRNA or introduced β2m mRNA, and EGF-induced syncytialization did not promote β2m in vCTB cells. The inability of vCTB cells to translate β2m messages in vitro was reflected in failure to detect β2m in vCTB cells in situ. Thus, vCTB cells may synthesize a disulfide-bonded, β2m-free homodimeric form of HLA-G5 due to an inability to synthesize β2m protein. This molecule could comprise a particularly effective tolerogenic molecule at the maternal-fetal interface.