Multimeric polypeptides and pharmaceutical uses thereof; multimers comprising alpha3 and alpha1 peptides of an HLA-G antigen and methods of producing such multimers, pharmaceutical compositions comprising the same, as well as their uses for treating various diseases including organ/tissue rejection. Said multimers comprise at least two monomers, each of said monomers being selected in the group consisting of a peptide P2 of formula P1-X3 or X2-X3, wherein P1 is of formula X1-X2, wherein X1 represents a peptidic linker including a cysteine amino acid and X2 represents an alpha1 domain (or alpha1 peptide) of HLA-G and X3 represents an alpha3 domain of HLA-G.