Recent advances in host defense mechanisms/therapies against oral infectious diseases and consequences for systemic disease

The innate and adaptive immune systems are both crucial to oral disease mechanisms and their impact on systemic health status. Greater understanding of these interrelationships will yield opportunities to identify new therapeutic targets to modulate disease processes and/or increase host resistance to infectious or inflammatory insult. The topics addressed reflect the latest advances in our knowledge of the role of innate and adaptive immune systems and inflammatory mechanisms in infectious diseases affecting the oral cavity, including periodontitis and candidiasis. In addition, several potential links with systemic inflammatory conditions, such as cardiovascular disease, are explored. The findings elucidate some of the defense mechanisms utilized by host tissues, including the role of IL-17 in providing immunity to oral candidiasis, the antimicrobial defense of mucosal epithelial cells, and the pro-resolution effects of the natural inflammatory regulators, proresolvins and lipoxins. They also describe the role of immune cells in mediating pathologic bone resorption in periodontal disease. These insights highlight the potential for therapeutic benefit of immunomodulatory interventions that bolster or modulate host defense mechanisms in both oral and systemic disease. Among the promising new therapeutic approaches discussed here are epithelial cell gene therapy, passive immunization against immune cell targets, and the use of proresolvin agents.

A new inflammatory cytokine on the block: re-thinking periodontal disease and the Th1/Th2 paradigm in the context of Th17 cells and IL-17

For almost two decades, the Th1/Th2 paradigm has offered a productive conceptual framework for investigating the pathogenesis of periodontitis. However, as with many other inflammatory diseases, the observed role of T-cell-mediated immunity in periodontitis did not readily fit this model. A new subset of CD4+ T-cells was recently discovered that explains many of the discrepancies in the classic Th1/Th2 model, and has been termed "Th17" based on its secretion of the novel pro-inflammatory cytokine IL-17. The identification of Th17 cells as a novel effector T-cell population compels re-examination of periodontitis in the context of the new subset and its signature cytokines. This review aims to offer a clarifying insight into periodontal pathogenesis under the extended Th1/Th2/Th17 paradigm, and is predicated on the principle that periodontal disease activity is determined by a complex interplay between the immune system and periodontal pathogens. The re-examination of existing periodontal literature and further studies in the light of these new discoveries may help explain how the inflammatory response results in damage to the periodontium while generally failing to control the pathogens. This knowledge is essential for the development of immunomodulatory intervention strategies for fine-tuning the host response to maximize the protective and minimize the destructive aspects of the periodontal host response. Moreover, with the advent of anti-cytokine biologic drugs that target the Th1 and Th17 pathways in autoimmunity, the potential consequences to periodontal disease susceptibility in humans need to be understood.

Signaling domains of the interleukin 2 receptor

Interleukin (IL-)2 and its receptor (IL-2R) constitute one of the most extensively studied cytokine receptor systems. IL-2 is produced primarily by activated T cells and is involved in early T cell activation as well as in maintaining homeostatic immune responses that prevent autoimmunity. This review focuses on molecular signaling pathways triggered by the IL-2/IL-2R complex, with an emphasis on how the IL-2R physically translates its interaction with IL-2 into a coherent biological outcome. The IL-2R is composed of three subunits, IL-2Ralpha, IL-2Rbeta and gammac. Although IL-2Ralpha is an important affinity modulator that is essential for proper responses in vivo, it does not contribute to signaling due a short cytoplasmic tail. In contrast, IL-2Rbeta and gammac together are necessary and sufficient for effective signal transduction, and they serve physically to connect the receptor complex to cytoplasmic signaling intermediates. Despite an absolute requirement for gammac in signaling, the majority of known pathways physically link to the receptor via IL-2Rbeta, generally through phosphorylated cytoplasmic tyrosine residues. This review highlights work performed both in cultured cells and in vivo that defines the functional contributions of specific receptor subdomains-and, by inference, the specific signaling pathways that they activate-to IL-2-dependent biological activities.

Genetic evidence for an additional factor required for erythropoietin-induced signal transduction

Erythropoietin (EPO) and its receptor (EPOR) are required for the development of mature erythrocytes. After binding of ligand, the EPOR activates a variety of signaling pathways that ultimately control cellular proliferation, survival, and specific gene expression. Although erythroid progenitors appear to be the principal EPO-responsive cell type in vivo due to the restricted expression of the EPOR, many growth factor-dependent cell lines expressing the EPOR can respond to EPO by activating many or all of these pathways. In the present study, we have identified a cellular context (the interleukin-2 [IL-2]-dependent HT-2 line) in which the EPO stimulation of the EPOR fails to support cellular proliferation, STAT-5 induction, or MAPK activation, despite efficient phosphorylation of the EPOR and JAK2 and inhibition of apoptosis after withdrawal of IL-2. Interestingly, when we fused HT-2 cells expressing the EPOR with Ba/F3 cells in a complementation assay, the resulting hybridomas proliferated and potently activated STAT-5 and MAPK in response to EPO. These data indicate that an unidentified cellular factor is needed to mediate signaling by the EPOR. Moreover, Ba/F3 cells apparently express this factor(s) and somatic fusions can, therefore, confer EPO-responsiveness to HT-2 cells that lack this factor.

V3 recombinants indicate a central role for CCR5 as a coreceptor in tissue infection by human immunodeficiency virus type 1

Binding of the human immunodeficiency virus type 1 (HIV-1) envelope glycoprotein gp120 to both CD4 and one of several chemokine receptors (coreceptors) permits entry of virus into target cells. Infection of tissues may establish latent viral reservoirs as well as cause direct pathologic effects that manifest as clinical disease such as HIV-associated dementia. We sought to identify the critical coreceptors recognized by HIV-1 tissue-derived strains as well as to correlate these coreceptor preferences with site of infection and dementia diagnosis. To reconstitute coreceptor use, we cloned HIV-1 envelope V3 sequences encoding the primary determinants of coreceptor specificity from 13 brain-derived and 6 colon-derived viruses into an isogenic (NL4-3) viral background. All V3 recombinants utilized the chemokine receptor CCR5 uniformly and efficiently as a coreceptor but not CXCR4, BOB/GPR15, or Bonzo/STRL33. Other receptors such as CCR3, CCR8, and US28 were inefficiently and variably used as coreceptors by various envelopes. CCR5 without CD4 present did not allow for detectable infection by any of the tested recombinants. In contrast to the pathogenic switch in coreceptor specificity frequently observed in comparisons of blood-derived viruses early after HIV-1 seroconversion and after onset of AIDS, the characteristics of these V3 recombinants suggest that CCR5 is a primary coreceptor for brain- and colon-derived viruses regardless of tissue source or diagnosis of dementia. Therefore, tissue infection may not depend significantly on viral envelope quasispeciation to broaden coreceptor range but rather selects for CCR5 use throughout disease progression.

JAK/STAT signaling by cytokine receptors

The JAK/STAT pathway is recognized as one of the major mechanisms by which cytokine receptors transduce intracellular signals. This system is regulated at multiple levels, including JAK activation, nuclear trafficking of STAT factors, and negative feedback loops. Gene deletion studies have implicated selected STAT factors as predominant mediators for a limited number of lymphokines. This signaling pathway influences normal cell survival and growth mechanisms and may contribute to oncogenic transformation.

Janus kinases in interleukin-2-mediated signaling: JAK1 and JAK3 are differentially regulated by tyrosine phosphorylation

BACKGROUND

Cytokines mediate a variety of effector cell functions, including cellular proliferation, differentiation, and modulation of the immune response. Many cytokines activate receptor-associated Janus kinases (JAKs) that promote tyrosine phosphorylation of signal transducers and activators of transcription (STAT) factors. Although JAK activation has been correlated with phosphorylation, the role of this tyrosine phosphorylation in the regulation of JAK1 and JAK3 remains unclear. Furthermore, the relative roles of JAK1 and JAK3 in the activation of STAT5 by interleukin-2 (IL-2) remain poorly understood.

RESULTS

We targeted two conserved tyrosine residues within the activation loop of the JAK1 and JAK3 kinase domains for substitution with phenylalanines. In an overexpression system, the catalytic function of JAK1 strictly required the presence of the first of these tyrosines, Y1033. In contrast, JAK3 retained catalytic activity when either or both of these activation-loop tyrosines were mutated. Analysis of JAK1/3 chimeras demonstrated that JAK activity was also controlled by intramolecular interactions involving the amino-terminal domain of the JAK as well as by the inherent signaling properties of the kinase domain. Finally, we have reconstituted IL-2-dependent STAT5 induction in a cell line that lacks detectable expression of JAK1 and JAK3. Catalytically active versions of both JAK1 and JAK3 must be present for effective induction of STAT5.

CONCLUSIONS

JAK1 and JAK3 are differentially regulated by specific tyrosines within their respective activation loops. Additionally, the amino-terminal domain of JAK3 appears to contain regulatory sequences that modify the function of the kinase domain. Finally, both JAK1 and JAK3 must retain catalytic function for IL-2-induced STAT5 activation.

Distinct tyrosine residues within the interleukin-2 receptor beta chain drive signal transduction specificity, redundancy, and diversity

To explore the basis for interleukin (IL)-2 receptor (IL-2R) signaling specificity, the roles of tyrosine-based sequences located within the cytoplasmic tails of the beta and gammac chains were examined in the murine helper T cell line HT-2. Activation of the Janus kinase/signal transducer and activator of transcription (JAK/STAT) pathway, cellular proliferation, and the induction of various genes were monitored. All four of the cytoplasmic tyrosine residues as well as the distal portion of the gammac proved dispensable for the entire spectrum of IL-2R signaling responses studied. Conversely, select tyrosine residues within the beta chain were essential and differentially required for various signaling events. Specifically, activation of c-fos gene expression was found to occur exclusively through the most membrane proximal tyrosine, Tyr-338, whereas proliferation and the activation of STAT-5 were induced either through Tyr-338 or through the two C-terminal tyrosine residues, Tyr-392 and Tyr-510. These tyrosine residues mediated the induction of two different STAT-5 isoforms, which were found to form heterodimers upon receptor activation. In contrast to the tyrosine dependence of c-fos and STAT-5 induction, bcl-2 gene induction proceeded independently of all IL-2Rbeta tyrosine residues. Thus, the tyrosine-based modules present within the IL-2Rbeta cytoplasmic tail play a critical role in IL-2R signaling, mediating specificity, redundancy, and multifunctionality.

Expression of the immunoglobulin J chain in a murine B lymphoma is driven by autocrine production of interleukin 2

Expression of the immunoglobulin J chain is initiated by lymphokine signals delivered to activated B cells during a primary immune response. In the mature murine B cell line, CH12.LX, IL-5 and LPS but not IL-2 were found to greatly enhance basal levels of J chain gene expression. Analysis of the IL-2 receptor (IL-2R) showed two defects: an unusually low expression of the IL-2R alpha chain and little or no IL-2R beta chain. Treatment with IL-5 strongly amplified IL-2R alpha chain expression in CH12.LX cells, yet failed to confer IL-2 responsiveness. However, when the IL-2R beta chain was introduced by stable transfection, the cells expressed 400-500 high affinity IL-2R and responded to IL-2 with increased J chain expression. Surprisingly, in the absence of exogenous lymphokine stimulation, the basal levels of J chain and IL-2R alpha in all IL-2R beta transfectants became significantly elevated over time. Analysis showed that CH12.LX cells constitutively synthesized IL-2 and, given a functional IL-2R, responded to the lymphokine in an autocrine fashion to upregulate both J chain and IL-2R alpha. Thus, CH12.LX cells provide a model cell line in which the role of the IL-2R beta chain in differentiative events such as J chain upregulation can be examined.

The molecular role of the common gamma c subunit in signal transduction reveals functional asymmetry within multimeric cytokine receptor complexes

The specific signal transduction function of the gamma c subunit in the interleukin (IL) 2, IL-4, IL-7, IL-9, and IL-15 receptor complexes remains undefined. The present structure-function analyses demonstrated that the entire cytoplasmic tail of gamma c could be functionally replaced in the IL-2 receptor (IL-2R) signaling complex by a severely truncated erythropoietin receptor cytoplasmic domain lacking tyrosine residues. Heterodimerization of IL-2R beta with either gamma c or the truncated erythropoietin receptor chain led to an array of specific signals normally derived from the native IL-2R despite the substitution of Janus kinase JAK2 for JAK3 in the receptor complex. These findings thus suggest a model in which the gamma c subunit serves as a common and generic "trigger" chain by providing a nonspecific Janus kinase for signaling program initiation, while signal specificity is determined by the unique "driver" subunit in each of the gamma c- containing receptor complexes. Furthermore, these results may have important functional implications for the asymmetric design of many cytokine receptor complexes and the evolutionary design of receptor subfamilies that share common trigger or driver subunits.

Signaling through the interleukin 2 receptor beta chain activates a STAT-5-like DNA-binding activity

To explore the possible involvement of STAT factors ("signal transducers and activators of transcription") in the interleukin 2 receptor (IL-2R) signaling cascade, murine HT-2 cells expressing chimeric receptors composed of the extracellular domain of the erythropoietin receptor fused to the cytoplasmic domains of the IL-2R beta or -gamma c chains were prepared. Erythropoietin or IL-2 activation of these cells resulted in rapid nuclear expression of a DNA-binding activity that reacted with select STAT response elements. Based on reactivity with specific anti-STAT antibodies, this DNA-binding activity was identified as a murine homologue of STAT-5. Induction of nuclear expression of this STAT-5-like factor was blocked by the addition of herbimycin A, a tyrosine kinase inhibitor, but not by rapamycin, an immunophilin-binding antagonist of IL-2-induced proliferation. The IL-2R beta chain appeared critical for IL-2-induced activation of STAT-5, since a mutant beta chain lacking all cytoplasmic tyrosine residues was incapable of inducing this DNA binding. In contrast, a gamma c mutant lacking all of its cytoplasmic tyrosine residues proved fully competent for the induction of STAT-5. Physical binding of STAT-5 to functionally important tyrosine residues within IL-2R beta was supported by the finding that phosphorylated, but not nonphosphorylated, peptides corresponding to sequences spanning Y392 and Y510 of the IL-2R beta tail specifically inhibited STAT-5 DNA binding.