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

Abstract

A peptidoglycan containing fraction called fraction P.I. (phenol insoluble), extracted from Brucella melitensis and previously described by the authors, had immunogenic and protective properties and did no 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 '4 A' (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 effects of fractions P.I. and 4 A are compared. Chemically, fraction 4 A 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 4 A are not agglutinogenic and, since fraction 4 A induces better protection against infection by Brucella, it could advantageously replace fraction P.I. as a vaccine for humans.