Adaptive Immunity Carranza.docx

Adaptive Immunity Adaptive immunity has evolved to provide a focused and intense defense against infections that overwhelm innate immune responses. Adaptive immunity contrasts with innate immunity where adaptive immunity is slower and reliant on complex interactions between APCs and T and B lymphocytes. A key element is the antigen specificity of the responses that facilitates the specific targeting of a diverse range of effector elements, including cytotoxic T cells and antibodies. Another facet is the ability of adaptive immune responses to improve during exposure to antigen and on subsequent infection events. Our current understanding suggests that the cellular and molecular elements of adaptive immunity are more diverse than those of innate immunity, and, although a role for many of these factors in periodontal disease has been identified, our knowledge is far from complete. The importance of adaptive immune responses in periodontal pathogenesis is endorsed by histologic studies of established lesions in periodontal disease.91,132 The population of leukocytes in the periodontium in gingivitis (i.e.,the early stages of responses to the plaque biofilm) and in stable periodontal lesions (i.e., those in which tissue destruction is apparently not progressing) is dominated by T cells, and these cells are clustered mainly around blood vessels. Cell surface marker studies suggest that these cells are activated but not proliferating.59 In addition, there is a predominance of the helper T-cell subset (i.e., CD4expressing T cells) over the cytotoxic T-cell subset (i.e., CD8-expressing T cells). These T cells are considered to be proactively maintaining tissue homeostasis in the face of the microbial challenge of the plaque biofilm. By contrast, in active (progressing) periodontitis, B cells and plasma cells predominate and are associated with pocket formation and the progression of disease. Antigen-Presenting Cells. A central element of the activation and function of T cells and B cells is the presentation of antigen by specialized APCs to T cells and the development of a specific cytokine milieu that influences the development of T cells with a particular effector function. APCs are sentinel cells in mucosal tissues such as the periodontium. These cells detect and take up microorganisms and their antigens, after which they may migrate to lymph nodes and interact with T cells to present antigen. The periodontium is often compared to other mucosal tissues and the skin in terms of its repertoire of immune cells, and it contains a number of “professional” APCs, including B cells, macrophages, and at least two types of dendritic cells (i.e., dermal dendritic cells and Langerhans cells).36 These cells naturally express the majo histocompatibility complex class II molecules necessary for antigen presentation to T-cell receptors, and they may take up specific antigens and transport them to local lymph nodes, thereby facilitating the activation of specific effector T cells and the generation of an antigen-specific immune response to periodontal pathogens. Although these cells have been identified in periodontal tissues, shown to stimulate antigen specific T-cell responses in experimental systems, and found to generally increase in the presence of periodontitis, their relative contribution to antigen presentation in vivo remains to be determined. The expression of major histocompatibility complex class II molecules may be induced in other cells that are present in the periodontium (e.g., fibroblasts, epithelial cells), which then also take up antigen and present antigen locally in the periodontium. It is increasingly recognized that the engagement of PRRs (and in particular TLRs) by MAMPs from pathogenic microorganisms is not only central to signaling innate immunity in the form of cytokine upregulation; it is also a critical element of the activation of APCs and the elaboration of T-cell effector function. Thus, TLR activation increases the expression of

costimulatory molecules on APCs, which are critical to the interaction of these cells with T cells. In addition, TLR activation enhances antigen uptake and processing. Different APCs process and present antigens via different pathways and mechanisms, and this variation is one of the actors— along with the presence of specific combinations of cytokines—that influences the phenotype of T-cell effector function produced during specific immune responses.59 T Cells. There are a number of different subsets of thymic lymphocytes (i.e., T cells) that develop in the bone marrow and thymus and migrate to the peripheral tissues to participate in adaptive immune responses. The expression of the cell surface molecules (CD4 or CD8) or particular Tcell antigen receptors (αβ or γδ) broadly defines functional T-cell subsets that emerge from the thymus. The role of T-cells in periodontal disease has been established through immunohistologic studies of diseased tissues.159 CD4+ helper T cells are the predominant phenotype in the stable periodontal lesion, and it is thought that alterations in the balance of effector T-cell subsets within the CD4+ population may lead to progression toward a destructive, B-cell–dominated lesion.59 CD4+ T-cell subsets are defined on the basis of their phenotypic characteristics and effector functions. The nature of the APCs, which present antigen to cognate T-cell receptors on T cells, and the presence of specific combinations of cytokines and chemokines influence the nature of the CD4+ T-cell effector subset, which develops from naive T-cells (Figure 5-5). CD4+ T-cell subsets are defined by the expression of specific transcription factors, and their functional characteristics are associated with their cytokine secretion profile. The best-defined functional subsets of CD4+ T cells are the Th1 and Th2 cells, and a dynamic interaction between Th1 and Th2 cells may provide, in part, an explanation for fluctuations in disease activity and the progression of periodontal disease (Box 5-3). Th1 cells secrete IFN-γ, which activates cell-mediated immunity (i.e., macrophages, NK cells, and CD8+ cytotoxic T cells) against pathogenic microorganisms. The activation of macrophages promotes phagocytosis and killing of microbial pathogens, whereas NK cells and CD8+ T cells are cytotoxic T cells that kill infected host cells. Conversely, Th2 cells regulate humoral (antibodymediated) immunity and mast cell activity through the secretion of the cytokines IL-4, IL-5, and IL-13. Thus, the predominance of Th2 cells leads to a B-cell response. The B-cell response may be protective, for example, as a result of the production of specific antibodies that would serve to clear tissue infections through interaction with the complement system and by enhancing neutrophil phagocytosis. However, B cells are also a source of pro-inflammatory cytokines that contribute to tissue destruction. Treg cells have an immunosuppressive action that is mediated by the secretion of TGF-β and that is important to the prevention of autoimmune disease. These cells are increased in periodontitis lesions and may therefore have a role in disease pathogenesis.120 A number of lines of evidence suggest that the pathogenesis of periodontal disease may involve some elements of autoimmunity.59 For example, there is immunologic cross-reactivity between HSP60 expressed on human cells and the GroEL molecule of P. gingivalis, and specific serum antibodies and antigen-specific T cells to these molecules have been detected in periodontal disease. Similarly, autoantibodies and specific T cells against other host (i.e., self) molecules, such as type I collagen, have been identified in periodontal disease. Th17 cells are another subset of T cells, and they have a proinflammatory action that is important in immune responses against extracellular infections mediated by the cytokine IL-17. Infections with a diverse range of pathogens have been shown to activate strong Th17 cell

responses, and Th17 cells are thought to provide a substantial inflammatory response to clear microorganisms that Th1/Th2 cells have failed to eradicate. IL-17 has a number of activities in common with pro-inflammatory cytokines, such as IL-1β and TNF-α, and it has a synergistic activity with these cytokines, particularly TNF-α. IL-17 induces proinflammatory cytokine expression (including IL-1β and TNF-α) in macrophages, stimulates chemokine expression, and thereby activates neutrophil infiltration. There is increasing evidence for a role of IL-17 and Th17 cells in periodontal disease.54 IL-17 has been detected in periodontal tissues at sites of advanced disease. IL-17 induces IL-6 and IL-8 secretion by gingival fibroblasts and also upregulates MMP1 and MMP-3 in these cells. IL-17 also induces IL-1β and TNF-α secretion from macrophages and gingival epithelial cells. In a mouse model of periodontitis induced by P. gingivalis, IL-17 receptor deficiency (IL-17RA knockouts) resulted in increased susceptibility to alveolar bone loss, thereby suggesting a protective role of IL-17 in bone homeostasis, possibly via an effect on neutrophil function. A number of other effector CD4+ T-cell subsets have been defined on the basis of their cytokine secretion profile: these include Th9 cells and Th22 cells. In addition, T-cell subsets have been defined on the basis of their specific anatomic location. For example, Th22 cells home to the skin in which they likely stimulate antimicrobial peptide production and the differentiation of keratinocytes. In addition, T follicular helper cells are located in germinal centers in lymph nodes in which they provide B-cell help and stimulate Ig class switching. The homing of particular Tcell subsets to specific anatomic locations is defined by the expression of specific chemokine receptors that confer responsiveness to specific chemotactic signals that are produced in those locations. However, thus far, the majority of the work on these novel T-cell subsets has been carried out in mouse models and in vitro systems; their relevance to human biology in vivo and in disease remains to be fully elucidated. Cytokines produced by differentiated T-cell subsets feedback to stimulate differentiation and to sustain the activity of the cells from which they are derived (i.e., in a positive feedback loop). Simultaneously, they inhibit the development of other competing subsets. For example, IL4 from Th2 cells inhibits the development of Th1 cells, and IFN-γ from Th1 cells inhibits Th2 cells. It is increasingly appreciated that individual CD4+ T-cell clones—after they have encountered antigen and differentiated in response under the influence of a specific cytokine environment (i.e., a milieu)—may not be terminally differentiated cells. Rather, there appears to be functional flexibility between T-cell subsets and in particular within the memory T-cell population (Figure 5-5). For example, Th17 and Treg cells can interconvert, depending on the local concentrations of IL-6, IL-23, and TGF-β.175 It is thought that the plethora of functional subsets of T cells, their anatomic location, and their ability to switch phenotype is a reflection of the requirement for effective responses against diverse pathogens. Other immune cells also have subsets that are defined by the expression of cell surface markers and diverse functional and anatomic locations (e.g., myeloid immune cells, NK cells), but these are less well defined than CD4+ T-cell subsets. The complexities of the interactions between cellular and molecular aspects of innate and adaptive immune functioning are presented in Figure 5-5. It is clear that multiple pro- and antiinflammatory pathways, positive and negative feedback loops, and agonists and antagonists all play a role in determining the nature of the immune–inflammatory response to bacterial plaque and the degree of tissue damage that is experienced. Furthermore, the nature of the inflammatory

response varies between individuals; this could explain why certain people appear to be more susceptible to periodontitis than others. Antibodies. Specific antibodies are produced in response to an increasing bacterial challenge in periodontal disease and are the endpoint of B-cell activation. Circulating antibodies may be more important than locally produced antibodies. Even so, these generally appear in a high titer but have low biologic activity, so there is some doubt as to their effectiveness. Commensurate with the appearance of antibodies against plaque bacterial antigens is the appearance of differentiated plasma cells that characterize the established lesion in periodontal disease. High levels of antibodies appear in GCF (in addition to those in the circulation), and these are produced locally by plasma cells in periodontal tissues.9 Antibodies to periodontal pathogens are primarily IgG, with few IgM or IgA types produced. Many species of oral bacteria elicit a polyclonal B-cell response(with the consequent production of specific antibodies against those bacteria). However, these responses augment responses against non-oral bacteria and may lead to the production of autoantibodies (e.g., antibodies against collagen and connective tissue proteins), which may contribute to tissue destruction in periodontal disease.9,59 The incidence and levels of specific serum and GCF IgG antibodies are raised with chronic periodontitis, which suggests that local and peripheral generation of antibodies may be important in the immune response to periodontal pathogens. Antibodies (i.e., IgA) to periodontal pathogens are also found in saliva. Variations in the levels of specific antibodies to different species in different clinical presentations suggest differences in pathogenesis. For example, antibodies to A. actinomycetemcomitans of the IgG2 subclass predominate in aggressive periodontitis.150 Other P. gingivalis molecules (i.e., fimbriae and hemagglutinin) also act as antigens. Specific antibodies are also generated by serotype-specific carbohydrate antigens (e.g., capsular polysaccharide of P. gingivalis, carbohydrate of A. actinomycetemcomitans LPS). The subclass distribution of antibodies is influenced by cytokines that are derived from monocytes.150 For example, IgG2 production is regulated by IL-1α, IL-1β, and PGE2 from monocytes as well as by platelet-activating factor from neutrophils. PGE2 and platelet-activating factor indirectly induce Th1 responses and therefore IFN-γ, which stimulates IgG2 production. Individuals with aggressive periodontitis have monocytes that are hyperresponsive to LPS and that produce elevated quantities of PGE2.9 A. actinomycetemcomitans is commonly associated with aggressive periodontitis, which induces IL-12 production that regulates NK cells and Th1 cells. These cells are a source of IFN-γ, which in turn regulates IgG2. A number of studies have reported an effect of treatment on levels of specific antibodies to periodontal pathogens. For example, plaque removal reduces the titers of antibodies to P. gingivalis and A. actinomycetemcomitans in serum, GCF, and saliva.9 Some studies have observed a transient increase in antibody titers after treatment, which may be due to the release of antigens into the tissue and circulation. The significance of antibodies in periodontitis is not clear. It is not known if these antibodies have a protective function or whether they participate in disease pathogenesis. Although there is some evidence for a correlation between clinical parameters of disease and titers of specific antibodies to periodontal pathogens, other studies report an inverse correlation of antibody levels and avidity with periodontal destruction. In addition, specific antibodies to periodontal pathogens are found in healthy individuals as well as in those with periodontal disease.

Most research into the analysis of specific antibodies has focused on antibodies to P. gingivalis, and antigens derived from this organism have been investigated as potential vaccines for periodontal disease.125,137 For example, a significant reduction in disease progression in nonhuman primate and rodent models was observed after immunization with heat-killed P. gingivalis or antigens from P. gingivalis. In addition, immunization with P. gingivalis proteases (i.e., gingipains) prevents colonization with P.gingivalis and reduces bone loss.125