The present invention relates to an active polypeptide comprising the amino acid sequence of SEQ ID NO:3, or having at least 50%, preferably 70%, more preferably 90% identity with the amino acid sequence of SEQ ID NO:3, or fragments thereof having at least 21 contiguous amino acids, or peptides having at least 50%, preferably 70%, more preferably 90% identity with the amino acid sequence of the fragments, provided that the polypeptide is not SEQ ID NO:2.