A chimeric or fusion protein for inducing an immune response to P. gingivalis, the protein comprising a first polypeptide linked to a second polypeptide, wherein: A) the first polypeptide comprises or consists of an amino acid sequence of the active site of an Arg- or Lys-gingipain of P. gingivalis, or a sequence that is at least 80% identical thereto; and B) the second polypeptide comprises or consists of an amino acid sequence of an adhesin domain of an Arg- or Lys-gingipain of P. gingivalis; and wherein the second polypeptide: a) comprises an amino acid sequence corresponding substantially to the full length of the DUF2436 domain of an Arg- or Lys-gingipain, or a sequence at least 80% identical thereto; and/or b) comprises one or more cysteine amino acid substitutions in the adhesin domain, compared to the naturally occurring Arg- or Lys-gingipain sequences in corresponding regions; and optionally comprises one or more amino acid motif substitutions in the adhesin domain for reducing multimerisation.