Is immunesenescence a contributing factor for periodontal diseases?

Abstract

Current concept in periodontal diseases (PDs) states that it is the host's response toward the periodontal pathogens which leads to tissue destruction and attachment loss. Hence the role of immune response in the progression and resolution of PD must be considered vital. Any alteration in the immune system disturbs the homeostasis of the periodontium. Decline in immune system is the hallmark of aging, leading to increased susceptibility of elderly individuals to bacterial infections. The periodontal apparatus which is being constantly exposed to plaque biofilm is more vulnerable to destruction in aged individuals. Ageing related alterations in immune system has been discussed elsewhere as a contributor to various chronic inflammatory diseases like atherosclerosis, preterm, and low birth weight, etc. This paper reviews on the possible role of aging in periodontal destruction through altered immunity. Aging has long been associated with altered systemic inflammation. It has been discussed whether (1) this systemic inflammation is a consequence of increased occurrence of chronic inflammatory diseases upon aging or (2) aging associated systemic inflammation leads to such diseases. The immune responses which are protective at the first stages of life might result detrimental in the elderly. Hence it might be very difficult to individuate genetic profiles that might allow to identify individuals with a major risk for one or more age related diseases. Taking this into consideration, the cause of PDs in elderly is addressed with a systemic approach in order to understand the complex interplay between the aging immunity and PDs.

INTRODUCTION

Improvement in both social living conditions and health care has led to greater life span in the world. Human aging is associated with increased morbidity and mortality from infectious diseases and impaired response to immunization. It could be argued that the increased severity of periodontitis in old age could be simply the cumulative effect of prolonged exposure to microbial challenge. Though aging per se leads to physiological loss of attachment and alveolar bone, severe periodontitis is not a natural consequence of aging. It is a well-known fact that aging causes histophysiological and clinical alterations in oral tissues, it must be clearly differentiated from the pathological conditions arising from the altered tissues as a result of aging. Age dependent alterations affect all the components of periodontium including the innate and the adaptive arms of immunity. Recent evidence suggests that the destruction of periodontium due to periodontal diseases (PDs) is mainly due to the host immune response toward the micro-organisms and their products than the organism as such. Immunosenescence refers to the declined function of the immune system with advanced age resulting in increased susceptibility of elderly individuals to microbial infections. The antigenic load from the bacterial flora that colonize all the mucosal surfaces including the periodontium is responsible for the chronic immune system activation and hyper production of pro-inflammatory cytokines. Chronic elevation of inflammatory markers in the elderly could be due to the inability to control infections in old age. Understanding the potential effects of aging on immunity and inflammation, and its complex relationship with PD is important to design appropriate treatment strategies for the elderly. This paper reviews on the contribution of immunesenescence in PD.

AGING ASSOCIATED STRUCTURAL AND FUNCTIONAL ALTERATIONS IN PERIODONTIUM

Aged individuals are increasingly exposed to infection because of decreased efficiency of physical barriers such as the skin or the epithelial lining and the respiratory, gastrointestinal, and genitourinary tracts.

Anatomical and functional changes in the periodontal tissues have been reported as being associated with the aging process. Thinning of the epithelium and diminished keratinization are the age related changes associated with the gingiva. Furthermore, there are conflicting reports regarding the shape of the rete pegs, such as a flattening of rete pegs, and an increase in height of the epithelial ridges associated with aging. In a morphological three-dimensional study of epithelium-connective tissue interface, connective tissue ridges were observed to be more prevalent in young individuals, whereas connective tissue papillae were predominant in elderly individuals. The change from ridges to papillae involves the formation of epithelial cross ridges with increasing age.[1]

Connective tissue changes include a decrease in the number of cellular components with functional and structural alterations in gingival fibroblasts with aging. The gingival fibroblasts which are being constantly affected by the oral bacteria and their products like lipopolysaccharide, releases inflammatory cytokines like prostaglandin E2, interleukin (IL)-1-β, and plasminogen activator.[2,3] It is these inflammatory cytokines which are responsible for periodontal destruction.[2] With a 5-fold decrease in collagen synthesis[4] and an increased collagen intracellular phagocytosis by old fibroblasts, the balance between the synthesis and degradation of collagen is altered with aging.[5] Furthermore, the extracellular matrix proteoglycans secreted by old fibroblasts contains an increase in rates of heparin sulfate and a reduction in chondroitin sulphate.[6]

The fiber and cellular contents decrease and the structure of the periodontal ligament (pdl) becomes irregular with age. Furthermore, the pdl cells exhibit decrease in chemotaxis, motility and proliferation rates with aging,[7] which will have drastic effect on the integrity of the pdl upon mechanical loading during function. With age, the cementum increases in width, by acellular cementum deposition. The alveolar bone formation steadily declines with age. This is due to the effect of hormones on osteoblastic cells, decrease in osteoblastic precursors and to decreased synthesis and secretion of essential bone matrix proteins.[2]

This age related oral changes are based on the same pathological dynamics as those generally recognized in all tissues: From tissue desiccation to diminished reparative ability, from reduced elasticity to altered cell permeability. Though these changes could be a cause for loss of periodontal integrity and subsequent destruction upon function and infection, aging does not spare the immune system too.

THE MICROFLORA

The periodontium is constantly exposed to microorganisms. Hundreds of different microbial species are permanently present and form a complex and dynamic ecosystem, the microbiota. Host genotype, oral hygiene habits and environmental factors influence the composition of this microflora. It is the shift in microflora toward Gram-negative anaerobic organisms which leads to PDs.[8] The constant influx of neutrophils into the gingival crevice through the sulcular epithelium and the secretion of antimicrobial peptides by the gingival epithelium protect the periodontium from these organisms. Certain periodontal pathogens like Pophyromonas gingivalis have the potential to escape these defense systems to invade into the connective tissue, triggering the immune system – innate followed by adaptive response. Pathological modifications of oral flora in the elderly have been reported with certain oral diseases, but alterations in periodontal microflora with ageing have not been clearly stated.

IMMUNESENESCENCE

Immunosenescence is the consequence of the continuous attrition caused by lifelong antigenic load which is responsible for the chronic immune system activation and hyperproduction of cytokines.[9] The adaptive immune system exhibits a reduced response with age, whereas the innate immune system ramps up, resulting in chronic inflammation.[10] In 2000, Franceschi et al. coined the term “inflammaging” in order to refer to a low grade pro-inflammatory status appearing during the aging process.[11]

Changes in innate immune system

The innate immune system is a diverse collection of host defenses that lack specificity. It includes physical and chemical barriers, reflexes, cellular components and local responses including inflammation and production of antibacterial peptides.

Neutrophils, the predominant phagocytic cell type, are normally found in circulating blood and are in gingival crevice following chemotactic signals from bacteria and its products, complement components or chemokines. Their characteristic ability to generate and release reactive oxygen species and granular enzymes on encountering the pathogen or host derived proinflammatory molecules renders them ideal cells for pathogen killing but also important effectors of host tissue damage in periodontitis. Several age related studies have proved that although the total number of circulating neutrophils remains constant with advancing age, their capacity to chemotactically migrate in vitro in response to granulocyte-macrophage colony stimulating factor (GM-CSF) or the N-formyl-met-leu-phe peptide is significantly reduced in old age, even when the neutrophils are obtained from healthy elderly individuals.[12] Similar age dependent functional defects are seen in phagocytosis and the production of reactive oxygen species, although, the latter defect seems to be stimulus specific.

The ability of priming agents to induce anti-apoptotic signals is reduced in neutrophils isolated from elderly subjects, although there is no significant difference in the apoptosis of neutrophils obtained from young and old individuals. Hence the primed neutrophils isolated from elderly individuals die off rapidly. Several limitations associated with the use of phagocytes from the elderly includes various clinical conditions and frequent use of drugs, which are likely to be confounding factors in interpreting intrinsic effects of aging on innate immune cells. Thus more valid results could be obtained if the cells are isolated from “healthy elderly subjects.” Interestingly, it must be noted that the surface expression of several receptors like GM-CSF receptor (GM-CSFR), TLR2, TLR4, CD11b, and CD14 were preserved with age.[12,13]

However, the intra-cellular signaling could be affected as a consequence of age associated alterations in the composition of membrane lipid rafts, which serve as cellular signaling platforms. Similarly aging leads to reduced chemotaxis, signal transduction and cytokine production in macrophages while the expression of cell surface receptors like INFγ, TLR2, TLR4, TLR5, TLR6 and phagocytic receptors like CD14, CD11b, CD18, CD36, CD206, dectin-1, scavenger receptor–AI were maintained. Furthermore, the receptors involved in inflammation amplification C5aR, TREM-1 were increasingly expressed.[14,15]

Thus, the products of primed phagocytic cells which include reactive oxygen species, may not be produced at high enough concentration on a per cell basis in old age, however in the setting of chronic inflammation like in PD, the tissue injury could result from repeated exposure to toxic neutrophil products. Furthermore, the accumulations of oxidized, cross-linked or aggregated proteins (advanced glycation end products (AGEs)) in aged individuals, which cannot be handled by the proteolytic systems, are identified as self-antigens by the macrophages. The response of phagocytic cells to AGEs includes the increased expression of pro-inflammatory cytokines,[16] release of toxic enzymes and generation of reactive oxygen species, thus, triggering the vicious cycle.

The chronic inflammation associated with aging could reflect an intrinsic innate or adaptive-related impaired ability to clear up foreign antigens totally. The interdependence of the innate and adaptive systems makes it difficult to identify the origin of this impairment. The decline in the adaptive system's ability to recognize antigens and the changes in cytokine profiles are expected to reduce the recruitment of the cellular components of the innate system in charge of killing invaders.

Changes in adaptive immune system

Adaptive immunity relies on three types of lymphocytes: B cells, cytotoxic T cells, and helper T cells. These lymphocyte populations are altered by age. Aging is associated with a reduced proportion of naive T cells relative to their memory counterparts.[17] This could result from an increased number of memory cells because of lifelong exposure to foreign antigens and/or a reduced production of naive cells. The latter is not supported by recent findings that, despite its involution during aging, the thymus is able to support adequate T lymphopoiesis late in life.[18] The importance of the naive/memory ratio is suggested by several studies. Homeostasis of the immune system requires that expanded populations of T cells undergo apoptosis after an infection has been resolved.

In addition to shifts in population types, immune cells also exhibit reduced proliferative responses, altered cytokine production and responsiveness, diminished antigen recognition and signal transduction defects.[17,19,20] Reduced proliferative capacity among the memory cell pool should manifest itself functionally in a diminished response to previously encountered immunological challenges. Consistent with this prediction, older animals are less efficient in fighting off recurrent infections despite a surplus of memory cells. It is also generally accepted that aging is associated with a shift of cytokine profiles in helper T cells from the Th1 (interferon-γ (IFN-γ) and IL-2) to the Th2 (IL-4, IL-5, IL-6, IL-10, and IL-13) type.[10]

The correct recognition of an infectious antigen is the most crucial task performed by the immune system. The binding of major histocompatibility complex (MHC)/peptide to a T-cell receptor initiates T-cell activation characterized by intracellular events, including the trigger of phosphorylation/dephosphorylation cascades, modification of calcium concentrations, reorganization of the cytoskeleton, transcription of specific genes and cell division.[21] Within the first few minutes of T-cell activation, there is increased activity of a variety of protein and lipid kinases. Abundant studies have described age-dependent alterations in the activity of these kinases as well as defects in both their re-localization and that of their substrates[22,23] because all intracellular events associated with T-cell activation are linked, it is not surprising that calcium mobilization and cytoskeletal reorganization are also impaired in old cells.[17,24]

The antibody-mediated response is also modified with age. The dependency of B cells on helper T cells for activation has complicated efforts to isolate the direct consequences of aging on this response. The quantity of natural serum antibodies specific for foreign antigens declines with age, as does the antigen induced antibody production in response to most vaccines.[19] This phenomenon is correlated with a shift in the variety of antibodies B cells generate. Overall, the total level of immunoglobulins is unchanged because of increased production of antibodies directed against autoantigens. In addition to their smaller number in the elderly, antibodies specific for foreign antigens belong to different isotypes and/or display reduced affinity.[19] It remains unclear if these changes are a direct consequence of an age-related modification of B cells or an indirect consequence related to antigen presenting cells (APCs). The latter is supported by recent studies consistent with reduced antigen trapping and presentation by old follicular dendritic cells.[20]

OXIDATIVE STRESS AND IMMUNE SYSTEM

The proliferation of T and B cells, natural killer cells, and lymphokine-activated killer cells that is required to mount an effective defense against pathogens and tumor cells appears to be inhibited markedly with age and upon exposure to oxidants. Both polymorphonuclear leukocytes and macrophages can inhibit proliferation of various lymphocyte subpopulations through the production of reactive oxygen intermediates and prostaglandin E2 and NO. Conditions like chronic periodontitis where chronic inflammation persists could result in compromised lymphocyte function. The age associated decrease in cell mediated immunity may be due to decreased levels of small molecule antioxidants and antioxidant enzyme activities.

INFLAMMAGING

The presence of pro-inflammatory phenotype in aged mammals is evident by[25] increased expression of genes linked to inflammation and immune responses in the tissues of old humans and rodents, higher levels of cytokines in serum, e.g., IL-6 and TNF-α, and activation of NF-κB signaling which is the master regulator of inflammatory responses.

The reasons that have been proposed for increased inflammation in older adults include[26] (i) latent infections, such as cytomegalovirus, which are prevalent in up to 80% of older adults, may act as chronic stimuli of the inflammatory system, (ii) there is also growing research into the role oxidative stress in creating pro-inflammatory states, (iii) aging is associated with increased pro-inflammatory prostaglandins such as cycloxygenase and lipooxygenase, (iv) during aging, anti-oxidant systems decline, leading to an imbalance in redox status and activation of redox-sensitive transcription factor NF-kB, and (v) activation of NF-kB leads to the expression of pro-inflammatory mediators, including TNF and IL-6, as well as up-regulation of adhesion molecules.

A pro-inflammatory milieu can serve as a risk factor for infection in at least two ways. Persistent activation of B and T lymphocytes by inflammatory cytokines is thought to extinguish the replicative potential of immune cells.[27] Persistent inflammation has been shown to increase risk of bacterial invasion in rodent models. A critical step during the process of bacterial invasion is the adherence of bacteria to host cell surfaces. Recent studies show that two receptors common on many cell types, polymeric immunoglobulin receptor (pIgR) and platelet activating factor receptor (PAFr), are vulnerable to bacterial attachment.[28] Mice lacking PAFr as well as those treated with PAFr antagonists are resistant to invasive pneumococcus.[29] Both pIgR and PAFr are up-regulated by transcription factor NF-kB. Cells in acute and chronic states of inflammation have been found to express increased pIgR and PAFr and are bound by S. pneumoniae more often than resting cells.[30]

Increased systemic cytokine levels activate the hypothalamus-pituitary-adrenal axis which augments the secretion of cortisol, which is a potent anti-inflammatory agent although it induces bone resorption and protein catabolism. Hence the aging process is simultaneously accompanied by both the features accelerating inflammaging and counteracting, anti-inflammaging characteristics. It is the balance between these opposing forces which determines the outcome of aging–aging with healthy periodontium or not.

Cells of the periodontium are constantly exposed to infections and tissue injuries triggering an inflammatory reaction. This kind of cellular stress can induce the adaptive immune response, a kind of inflammation termed as “para-inflammation.”

PERIODONTAL DISEASE AS A MODEL OF INFLAMMAGING

However, PD is, as a model of chronic inflammatory disease, able to influence the general status of health and the quality of ageing. PDs are a heterogeneous group of diseases that affect the supporting structures of the teeth. Its etiology is complex, clinical manifestations are various and several classifications have been proposed. In 1999, the American Academy of Periodontology successfully classified PD in relation to its etiology[31] in an International Workshop for the Classification of Periodontal Diseases and Conditions.

The chronic stimulation of inflammation sustained by the Gram-negative anaerobic bacteria of dental plaque has been correlated with various systemic diseases, such as pre-term and low birth-weight, atherosclerosis and cardiovascular diseases, worsening control over diabetes and the slow healing of wounds, aspiration pneumonia and osteoporosis.[32]

Different models of the etiopathogenetic mechanisms of oral bacteria exist:[9]

1. Common susceptibility involves a genetically determined phenotype. In the presence of periodontal pathogens, a susceptible patient develops PD. This same person would also be susceptible to atherosclerosis, diabetes or pulmonary infections but, in this model, PD does not cause a systemic condition;

2. At present, a more complex model is accredited, in which direct and indirect dynamics as well as innate, endogenous and exogenous factors are involved. Local adaptive immunity reacts to produce cytokines, which are capable of altering vessel permeability, thereby enabling monocytes to penetrate the inflamed tissue.[33] The chronic stimulation of inflammation by bacterial plaque involves several cell populations and several networks of cytokines, facilitating detachment and the formation of bone defects, since the cell populations amplify the inflammatory reaction and activate the effector mechanism, which is responsible for tissue destruction. At the same time, bacteria reach the blood circulation directly or by means of their lipopolysaccharides;

3. This model offers the basic rationale for the documented link between local effects and various related systemic diseases. Specifically, a bi-directional relationship has been reported to exist for many diseases, the latter which could be reciprocally influenced.[32] Of these, diabetes is one of the most reported diseases. Periodontitis enhances insulin resistance and complicates glycemic control, defining periodontitis as the sixth complication of diabetes. The second bidirectional condition is the presence of osteoporosis (primary and secondary one), which is correlated with the loss of alveolar bone and teeth.[34] PD induces an uncoupling of normal bone homeostasis, an increase in osteoclastic activity and decrease in bone mineral density.

Recently new links have been hypothesized between PD and renal disease, obesity, dysmetabolic syndrome and pancreatic cancer. However, possibly the most interesting link suggested to date is that with Alzheimer's disease (AD),[9] a chronic age-related disease depending on systemic and local inflammation.[34] The invasion of the brain by oral bacteria was posited as recently as 2002:[35] of the periodontal bacteria, various species such as Actinobacillus Actinomycetemcomitans, P. gingivalis, Treponema denticola, and Fusobacterium nucleatum have been found to be capable of invading the brain, modifying the cytokine milieu and possibly contributing to existing pathological mechanisms. In particular, Treponema species, including T. denticola, have been detected in 14/16 AD and 4/18 non-AD brains. Moreover, AD specimens have displayed a greater number of Treponema species than the controls.[35]

Two mechanisms may be involved in the PD-induced onset/progression of AD:

(1) inflammatory; and (2) bacterial mechanisms.

The first mechanism implies that PD-derived inflammatory molecules increase brain inflammation. In cases of severe PD, these pro-inflammatory molecules may induce a systemic inflammation and may, therefore, access the brain via systemic circulation. Pro-inflammatory molecules, derived locally from periodontal tissue, may stimulate trigeminal nerve fibers, leading to an increase in the number of brain cytokines.[36] These cytokines may act on the already primed glial cells, resulting in an amplified reaction and possible progression of AD.

The second mechanism by which PD could contribute to brain inflammation is direct, through bacteria and/or bacterial products. Several bacteria, including oral bacteria, have been hypothesized as being implicated in the pathogenesis of AD.[37] The mechanism by which periodontal bacteria access the brain is unknown. However, the mechanism described for other bacteria accessing the brain via systemic circulation is possible. A further route by which bacteria may reach the brain is via the peripheral nerves. Riviere’ studies have demonstrated that spirochete species were detected in the trigeminal ganglia, thereby suggesting the ability of oral spirochetes to invade the central nervous system via the peripheral nerves.[35] However, the simple presence of periodontal bacteria in the systemic circulation or in the territory of peripheral nerve fibers does not imply access to the brain. Additional co-factors may be required, such as age, the presence of inflammatory cytokines or other infections.[38] Although it is most probable that infections are not causative in these types of diseases, their possible role as aggravating co-factors in patients with susceptible genetic backgrounds should be seriously considered.

Consequently, every chronic peripheral infection including PDs may contribute to the global infectious/inflammatory burden thus leading to/aggrevate inflammaging and participate in the aetiopathogenesis of relevant diseases. A possible bidirectional relationship could exist between PD and immunesenescence/inflammaging.

CONCLUSION

Though it is postulated that aging leads to immunesenescence, it could be vice versa, as the altered functions of phagocytic cells increases the oxidative stress of an individual leading to further aging. It must also be noted that immunesenescence and inflammaging goes hand in hand triggering each other. The factor which has to be seriously considered is that, are all elderly individuals demonstrating inflammaging with immunesenescence? As discussed earlier, it must be noted that it is the genetic phenotype of an individual which determines the existence and severity of immunesenescence in elderly individuals. Hence individuals with such genetic background are prone to develop chronic inflammatory diseases like atherosclerosis, diabetes mellitus, etc. and are likely candidates to develop severe PDs with aging. Although moderate loss of alveolar bone and periodontal attachment is common in elderly, severe periodontitis is not a natural consequence of aging. As stated earlier, the reduced inflammatory and anti-microbial responses on a per cell basis by neutrophils and macrophages could lead to chronic persistence of the pathogens and hence unresolved destructive inflammation which is slowly progressive in nature. This review also discusses about the possible role of inflammation in recurrent bacterial invasion. Hence in a periodontal apparatus, where there is constant exposure to microflora, both commensal and pathogenic organisms, a pro inflammatory milieu is constantly established which would be more extensive in individuals with immunesenescence leading to severe periodontal destruction. Could this be a sole causative factor for recurrent PDs? Further intensive human studies are needed to arrive at a conclusion.