Chronic inflammation underlies many diseases, including periodontal disease and diabetes mellitus (1
). These diseases are generally categorized by pathological changes, but many factors mediating morbidity likely stem from the systemic inflammation shared among these diseases (3
). In general, pathological inflammation results from inappropriate immune responses to seemingly innocuous stimuli, such as commensal microbes or host-derived ligands. Some chronic inflammatory diseases, such as periodontal disease, are thought to initiate as a failure in homeostasis with commensal bacteria. The resulting local inflammation is mirrored by increased systemic inflammation, which likely explains the link between periodontal disease and cardiovascular disease (5
). Similarly, the systemic inflammatory environment in diabetes patients probably stems from elevated levels of endogenous ligands, such as free fatty acids or advanced glycation end products, that can stimulate innate immune cells to produce proinflammatory cytokines (6
). Elevated circulating endotoxin levels in diabetes patients (9
), perhaps originating from compromised mucosal surfaces, may further exacerbate the systemic inflammatory response implicated in the most serious complications of this disease.
TLRs are involved in mounting pathogenic inflammatory responses to commensal organisms and host-derived ligands in chronic inflammatory diseases, for instance periodontal disease and diabetes (10
). The likely role of TLRs in human systemic inflammatory disease is supported by multiple reports. First, both TLR2 and TLR4 are receptors for products of major periodontal pathogens, including Porphyromonas gingivalis
). Second, TLR4 polymorphisms that alter TLR4 function associate with occurrence of periodontal disease and diabetes, at least in some cohorts (14
). Third, TLR2 and TLR4 are strongly implicated in diabetes by demonstrations that either inhibiting or genetically deleting each receptor in mice protects against a key characteristic of type 2 diabetes, insulin resistance (6
). These observations suggest that altering expression, and thus function, of TLRs can promote inflammation in chronic diseases. Interestingly, multiple studies, including analyses of TLR4+
B cells from periodontal disease patients, show that cellular responses to TLR4 ligands can be a mixture of prototypic pro- and anti-inflammatory responses (18
The concept that cross-talk among TLR family members defines the innate immune response has emerged from studies on immune system sentinel cells, especially dendritic cells and macrophages. TLR2 and TLR4 activate these cells through a MyD88-dependent pathway. TLR4 also activates the TRIF/TRAM pathway in response to selected ligands. Pathway cross talk explains why in at least some cases, TLR4 engagement has the same biological outcome as coengagement of TLR2 and TLR4 (20
). However, TLR2 and TLR4 ligands can also synergize to activate production of proinflammatory cytokines under some conditions (22
). Engagement of other TLR family members, such as TLR9, can alternatively alter cellular responses to either TLR2 or TLR4 ligands in myeloid cells (23
). Appropriate TLR cross-talk therefore plays an important role in mounting an effective immune response to the complex combinations of TLR ligands presented by pathogens, commensal bacteria, and endogenous ligands.
Studies aimed at understanding the role of innate immune cells and TLR function in systemic inflammatory disease, with the exception of studies on TLR9, have largely focused on myeloid cells. However, B cells also function as a critical arm of the innate immune system, in part due to their ability to respond to TLR ligands and secrete cytokines (25
). The role of B cell TLR engagement and subsequent cytokine production in chronic inflammatory diseases, including periodontal disease and diabetes, is poorly characterized. Our studies suggest that activated human B cells can circulate throughout the body (26
); therefore, B cells may play an ongoing role in systemic manifestations of inflammatory diseases. Periodontal disease patients vs healthy donors have an elevated percentage of TLR2- and TLR4-positive B cells. New data show that these B cells constitutively and inducibly secrete elevated levels of cytokines, the latter in response to TLR ligands. These results also uncovered a high degree of specificity in B cell cytokine production in response to combinations of TLR ligands. Finally, B cells from periodontal disease and diabetes patients responded differently to combinations of TLR ligands. Based on this analysis of biologically important outcomes of TLR pathway cross-talk in human inflammatory disease patients, we conclude that clinical treatments and vaccines aimed at regulating immune responses through TLRs must test the complex response of B cells to combinations of TLR ligands.