This seemed to be confirmed by in vivo data, which showed that both dimer types were functional. Yet, B2M-HLA-G5 was more efficient than B2M-HLA-G1s-Fc, and alpha1-Fc was more efficient than alpha1_peptide in vivo. It is possible that Fc-dimers and natural dimers might not be structurally identical: whereas dimers formed via C42-C42 bonds are likely to closely resemble “natural�?HLA-G dimers, dimers formed via Fc might not. As far as HLA-G structure is concerned, B2M-HLA-G1s-Fc dimers might actually be two HLA-G monomers next to each other rather than “real�?HLA-G dimers, although additional dimerization through the C42 residues of HLA-G molecules cannot be ruled out and might happen through the C42 of B2M-HLA-G1sFc HLA-G portions located within the same homodimer or not. The same hypothesis can be made for alpha1 constructs: alpha1_peptides may only dimerize through C42-C42 disulfide bridging, whereas alpha1-Fc proteins may multimerize further. Our data seem to indicate that for B2M-HLA-G structures, natural multimers are more efficient than Fc-multimers, and that multimers are more efficient than dimers. Wortmannin molecular weight Whether this will hold true when soluble HLA-G multimers and not bead-bound multimers are used is currently under investigation. It was reported that all isoforms of HLA-G have immunosuppressive functions, including HLA-G3 which extracellular part is constituted of the alpha-1 domain only and which was shown to block the functions of NK cells and CTLs. The other goal of this study was to determine if tolerance induction in vivo could be induced by the alpha-1 domain of HLA-G only. For this purpose, alpha1-Fc molecules and a synthetic peptide of HLA-G alpha-1 domain were produced. Alpha1-Fc molecules multimerized, whereas alpha1_peptide molecules dimerized. Interestingly, in vivo data showed that the alpha-1 domain of HLA-G prolonged the survival of allo-transplanted skin in mice. This was especially true of alpha1-Fc molecules. Once again, this was unexpected because HLA-G-induced tolerance in mice is mediated through HLA-G binding to PIR-B. This receptor shares sequence similarity with the human ILT family of molecules, and particularly with ILT4 which is known to bind HLA-G alpha-3 domain. One explanation for this could be that when it is not part of the HLA-G1:B2M:peptide complex, the HLA-G alpha-1 domain adopts a conformation that allows it to bind to PIR-B, in which case it might also bind ILT molecules. One other explanation could be that HLA-G alpha-1 domain cross-reacts with inhibitory molecules other than PIR-B, such as murine KIRs for instance. In order to discriminate between these two hypotheses, we tested B2M-HLA-G5 and alpha1-Fc in skin transplantation experiments in which the recipient was an ILT4-transgenic mouse. In these experiments, B2M-HLA-G5 retained its tolerogenic capability.