Immunization by an insoluble fraction extracted from Brucella melitensis: immunological and chemical characterization of the active substances.

A peptidoglycan-containing fraction called fraction P.I. (phenol insoluble), extracted from Brucella melitensis and previously described by some of us, had immunogenic and protective properties and did not produce any allergic reactions. Since it is well known that bacterial peptidoglycans studied so far have immunoadjuvant properties, the isolation of the active factor(s) of Brucella was undertaken. By successive enzymatic and chemical treatments, a new, much more purified fraction, called "4A" (approximately 5% of fraction P.I.), is obtained, retaining the same properties as P.I. and giving better protection against infection by Brucella. Immunogenicity, immunoadjuvant activity, allergizing capacity, and specific and nonspecific protective effects of fractions P.I. and 4A are compared. Chemically, fraction 4A is constituted by a lipoprotein covalently linked to peptidoglycan and by a few (lipo)proteins that could be solubilized by hot sodium dodecyl sulfate. Intrinsic properties of peptidoglycan could not be studied, but it does not seem to be essential for the activity. In conclusion, fractions P.I. and 4A are not agglutinogenic and, since fraction 4A induces better protection against infection by Brucella, it could advantageously replace fraction P.I. as a vaccine for humans.