1. Periodontology 2000, Vol. 35, 2004, 9±13 Copyright # Blackwell Munksgaard 2004
Printed in Denmark. All rights reserved PERIODONTOLOGY 2000
Shouts and whispers: an
introduction to immunoregulation
in periodontal disease
Gregory J. Seymour & John J. Taylor
There is little doubt that patient susceptibility to and control of both the innate and adaptive immune
chronic periodontal disease plays a major role in responses. As pointed out by Marshall (15) The
determining the ultimate disease outcome. Suscept- sulcular and junctional epithelia have been thought
ibility to periodontal disease involves the interplay to represent the apparent weak link in the body's
between bacteria, the host and environmental factors ability to seal out the outside environment i.e. the
(Fig. 1). It is well established that bacteria in the plaque bio®lm. In reality however, it acts more as a
dental plaque bio®lm are the cause of the in¯amma- gatekeeper, selectively allowing the passage of anti-
tion. However, since the 1970s it has been clear that gens and cells as well as producing a range of defen-
not all plaques result in progressive disease. In 1996 sive molecules. While the physical barrier function of
the consensus report of the World Workshop on Clin- the epithelium cannot be understated, it is now
ical Periodontics (1) concluded that three bacterial recognised that epithelia throughout the body pro-
species, Porphyromonas gingivalis, Actinobacillus duce a diverse range of antimicrobial peptides. To
actinomycetemcomitans and Tannerella forsythia, date, at least four families of different antimicrobial
should be considered as the major periodontal peptides (a-defensins, b-defensins, Cathelicidins,
pathogens. Subsequently, Socransky et al. (24) des- Saposins) have been found in humans. An overview
cribed a number of bacterial complexes with the of these molecules and their possible role in period-
major `red complex', consisting of P. gingivalis, Tre- ontal disease is presented by Marshall (15) in this
ponema denticola and T. forsythia, being associated issue. This is not an extensive review of defensins but
with advanced forms of chronic periodontitis. rather an overview to highlight the important contri-
Although there is general support for this concept, bution of these molecules in disease pathogenesis and
it is also well recognized that the presence of the as possible therapeutic modalities. For a more com-
pathogenic bacteria per se is insuf®cient to cause prehensive review, readers are referred to Dale (7).
disease. It has recently been shown that there is a Once the epithelial barrier with its antimicrobial
high degree of volatility in terms of acquisition and peptides is breached the adaptive immune response
loss of the putative periodontal pathogens over a comes into play. Cytokines are central to this
5-year period. Relatively few subjects had these response, such that the production of `appropriate'
organisms on multiple occasions (4). Whether or cytokines results in development of protective
not the bacteria were actually lost or just declined immunity and the production of `inappropriate'
to levels below the detection limit of the assay cytokines leads to tissue destruction and disease pro-
remains to be determined. Nevertheless, this study gression (10). Just how the immune system chooses
clearly showed the limitations in cross-sectional and regulates the right cytokines is unclear, although
microbiological studies and periodontal disease genetic factors are most likely involved. Recently,
and highlights the fact that many individuals may Cullinan et al. (3) in a 5-year longitudinal study
harbor the organisms without manifesting progres- showed that a speci®c interleukin (IL)-1 genotype
sive periodontal destruction (4). was a contributing but nonessential factor in the
Host factors are clearly of fundamental importance progression of periodontal disease. Equally they
(21). In this context, the way in which the host showed smokers with P. gingivalis had signi®cantly
responds to the bacteria is determined by the nature more probing depths greater than 3.5 mm compared
9

2. Seymour & Taylor
Environmental factors Plaque ties. It is therefore erroneous to compare the various
Smoking cytokines quantitatively and suggest that any differ-
Stress ences are of greater or lesser biological signi®cance.
Within the context of this analogy it can be seen that
immunoregulation or control of the immune res-
Specific pathogens ponse in periodontal disease is a series of `shouts'
and `whispers' as cells talk to one another while they
combat the bacterial attack.
Non-susceptible Susceptible Cytokines also have a tremendous built-in redun-
dancy, many cytokines having overlapping functions.
IL-1 and IL-6 are two such cytokines, as are IL-4 and
Genetic factors IL-13. Hence, continuing the `shouts' and `whispers'
Cytokine gene polymorphisms analogy, cells hear the same message from a variety
of sources so that the absence of one cytokine but the
Fig. 1. Susceptibility to periodontal disease.
presence of another may result in the same biological
outcome. Equally, many cytokines are antagonistic
with smokers without P. gingivalis. At the same time, and again the overall biological effect is the result of
IL-1 genotype-positive smokers had 70% more pock- the balance between all cytokines rather than their
ets >3.5 mm than IL-1 genotype-negative smokers individual levels. It should be obvious therefore that
(3). This study clearly shows the interplay between cytokines function as a network and individual ele-
bacteria, host and environmental factors. ments of the network cannot be studied in isolation
As stated above, cytokines are central to the patho- (17). For example, Martuscelli et al. (16) have shown
genesis of an ever-increasing number of diseases, that recombinant human IL-11 (an inhibitor of IL-1
including periodontal disease. Cytokines are intercel- production) effectively inhibits periodontal attach-
lular messengers and as such represent a key ment loss in a dog model. Therefore the study of
mechanism by which cells involved in immune IL-1 without reference to IL-11 (as well as other inhi-
responses communicate. They are usually produced bitors such as IL-10 and IL-1ra) may well be mean-
transiently, often in picomolar concentrations, and ingless or at best dif®cult to interpret. The wealth of
some, such as IL-4, may have a very restricted range new information emerging from the human genome
of activity. Indeed, the majority of immune responses project and related initiatives has revealed many
occur locally and often between two cells conjugated novel cytokines with as yet unknown functions
together (Fig. 2). In this context, the analogy can be (19). The study of cytokines and their role in immuno-
drawn that when two cells are talking together they regulation and disease pathogenesis will therefore
`whisper'. However, when cells talk to one another at remain a critical element of periodontal research
a distance they may `shout' by producing large for some years to come.
amounts of cytokine. Such cytokines such as IL-1 Since it was ®rst presented at the ICPR meeting in
and IL-6 are therefore produced by a large number Osaka in 1992 (22) the T helper (Th)1/Th2 hypothesis
of cells and are produced in relatively large quanti- for the immunoregulation of periodontal disease has
attracted a lot of attention (for review see Seymour &
Gemmell (23)). A number of studies have shown a
decrease in Th1 responses in periodontitis, while
others have shown increased Th2 responses. Some
studies have claimed a dominance of Th1 response
over Th2 responses in periodontitis, whereas others
have shown a predominance of Th0 cells in period-
ontitis (reviewed in 23). These studies are dif®cult to
compare since they used a variety of cells and tissues,
a variety of techniques and a variety of stimulants.
In addition, the inability to determine disease acti-
vity clinically makes interpretation dif®cult, if not
Fig. 2. The majority of immune responses occur locally impossible. Nevertheless, it is agreed that control
rather than systemically and often between two cells con- of the Th1/Th2 balance is central to the immuno-
jugated to one another. regulation of periodontal disease.
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3. Immunoregulation in periodontal disease
There is increasing evidence that the Th1/Th2 bal- probably associated with disease and hence the role
ance is controlled by a range of factors, all of which of coinfection has become increasingly recognized.
are possible within the periodontal tissues. An over- Recent data have suggested that combinations of
view of these factors is presented by Gemmell & organisms may result in shifts in the Th1/Th2 and
Seymour (10) in this issue. These include the following: antibody pro®les (2, 10). In order to understand how
combinations of organisms may in¯uence the
immune response it is ®rst necessary to have a
The nature of the innate immune knowledge of the speci®c antigens of the pathogenic
response bacteria. A comprehensive review of these antigens is
presented by O'Brien-Simpson et al. (20). These
A strong innate immune response will result in large authors point out that each of the periodontal patho-
amounts of IL-12 and IL-18 production by mono- gens produce an array of antigens capable of indu-
cytes and neutrophils that, in turn, will promote a cing both Th1 and Th2 cytokine pro®les and they
Th1 response. Interleukin-18 was discovered in 1989 suggest that multispecies vaccines directed against
as INF-g inducing factor. IL-18 is structurally homo- key bacterial epitopes associated with the acquisition
logous to IL-1b and, together with their receptors are of essential nutrients may restrict proliferation of the
members of the IL-1R/Toll-like receptor (TLR) super- pathogens within the bio®lm and thereby in¯uence
family. This causes similar signalling and signal disease progression.
transduction mechanisms. IL-18 is recognized as a
cytokine that is able to enhance the maturation of
naive T cells to Th1 cells as a cofactor with IL-12, and The nature of the antigen-presenting
hence the production of INF-g. The ability of The cell
importance of IL-18 as co-inducer of INF-g induction
in vitro by the successful reduction of INF-g produc- There is ample evidence that the nature of the anti-
tion in stimulated mouse spleen cells by neutralizing gen-presenting cell can determine the nature of the
antibodies to murine IL-18. The major role of IL-12 in Th1/Th2 pro®le. Cutler et al. (6) have suggested that
this mechanism seems to be enhancing IL-18 recep- the default lesion in periodontal disease is Th1, in
tor expression. Interestingly, Th1 cells express IL-18 which the major antigen-presenting cell is the den-
receptor whereas Th2 do not. Clearly IL-18 could be dritic cell. At the same time, Gemmell et al. (12) have
expected to have a fundamental role in the control of shown that in periodontitis lesions CD19‡, CD83‡ B
the Th1/Th2 response in periodontal disease. How- cells are probably the dominant antigen-presenting
ever, the role of IL-18 in periodontal disease and its cell, supporting the concept of shifts in the Th1/Th2
interplay with IL-12 and indeed IL-15 has not yet pro®le between gingivitis and periodontitis. In their
been elucidated. Nevertheless the biological activity review in this issue, Cutler & Jotwani (5) look at the
of IL-18 is reviewed in detail by Delaleu and Bickel (8). events leading up to antigen presentation by dendri-
Equally, there is some evidence to suggest that P. tic cells and suggest that as antigen presentation is
gingivalis LPS is recognized by Toll-like receptor the limiting step in the generation of an immune
(TLR)-2 and TLR-6, which promote a Th2 response. response, blocking speci®c aspects of this could
Nonpathogenic lipopolysaccharide (e.g. Escherichia represent a valid strategy for the control of period-
coli) is recognized by TLR-4 and CD14, which pro- ontitis.
mote a Th1 response. This is discussed in detail by
Dixon et al. (9).
T-cell receptor (TCR) affinity
The nature of the antigen(s) There is emerging evidence that high T-cell receptor
af®nity with short signaling time favors a Th1
As discussed above, it is generally accepted that response while low T-cell receptor af®nity with a
while different people may respond differently, spe- prolonged signaling time favors a Th2 response. As
ci®c bacteria are nevertheless the cause of period- yet, however, the T-cell receptor af®nity of cells
ontal disease. Irrespective of the host response, from involved in periodontal disease has not been deter-
a clinical perspective, if a patient has no plaque, they mined. In their review in this issue, Yamazaki &
have no disease. In recent years it has also become Nakajima (26) raise the possibility of autoimmunity
apparent that complexes of organisms are most contributing to the periodontal lesion. The expres-
11

4. Seymour & Taylor
sion of heat shock proteins in the periodontal tissues 2. Chooi JL, Borrello MA, Smith ES, Zauderer M. Polarization
of Porphyromonas gingivalis-specific helper T-cell subsets
is presented, together with the possibility that the
by prior immunization with Fusobacterium nucleatum. Oral
immune response to their bacterial homolog (GroEL Microbiol Immunol 2000: 15: 181±187.
antigens) cross-reacts and hence contributes to the 3. Cullinan MP, Westerman B, Hamlet SM, Palmer JE, Faddy
in¯ammatory lesion. They further present evidence MJ, Lang NP, Seymour GJ. A longitudinal study of interleu-
that regulatory T cells, which control autoimmunity, kin-1 gene polymorphisms and periodontal disease in a
are in fact lacking in the periodontal tissues. general adult population. J Clin Periodontol 2001: 28:
1137±1144.
4. Cullinan MP, Hamlet SM, Westerman B, Palmer JE, Faddy
MJ, Seymour GJ. Acquisition and loss of Porphyromonas
Genetics gingivalis, Actinobacillus actinomycetemcomitans and Pre-
votella intermedia over a 5-year period: the effect of a tri-
Using a mouse model, Gemmell et al. (13) have closan/copolymer dentifrice. J Clin Periodontol 2003: 30:
532±541.
shown that susceptibility to P. gingivalis infection
5. Cutler CW, Jotwani R. Antigen-presentation and the role of
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9. Dixon DR, Bainbridge BW, Darveau RP. Modulation of the
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11. Gemmell E, Marshall RI, Seymour GJ. Cytokines and
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