Periodontitis in humans is caused by a group of predominantly gram-neg
ative, anaerobic bacteria among which Porphyromonas gingivalis and Bac
teroides for sythus are prominent. A similar group is present and pres
umably plays a similar role in experimental periodontitis in the prima
te Macaca fascicular is. Nevertheless, immunization using a vaccine co
ntaining only killed P, gingivalis suppresses the progress of experime
ntal periodontitis in M. fascicular is. We investigated the hypothesis
that gram-negative periodontopathic bacteria may share antigens, and
immunization with one species may induce antibodies reactive with othe
r gram-negative species. Using enzyme-linked immunosorbent assay (ELIS
A), Western and dot immunoblots with nonabsorbed and absorbed and immu
ne and preimmune sera we show that monkeys immunized with P. gingivali
s produce antibodies reactive not only with antigens of P. gingivalis
but also with those of B. forsythus. Similarly, rabbits immunized with
P. gingivalis or with B. forsythus produce antibodies that react with
antigens of both bacteria. Cross-reactive antibodies bind to epitopes
in lipid A and possibly in core carbohydrate of lipopolysaccharide. U
sing complexes of lipopolysaccharide with polymyxin B, bovine serum al
bumin and apolipoprotein A1 specificity of binding was documented. Usi
ng sera from monkeys immunized with P. gingivalis, cross-reactivity wi
th Actinobacillus actinomycetemcomitans could not be demonstrated by E
LISA, although binding to lipopolysaccharide but not to lipid A was de
monstrated by Western and dot immunoblots. Antibodies to shared lipopo
lysaccharide epitopes of periodontopathic bacteria may account, at lea
st in part, for the immune protection observed in immunized monkeys, a
nd shared epitopes may have potential as a vaccine for periodontitis i
n humans.