This report was reviewed for medical and scientific accuracy by Amy S. Paller, MD, Walter J. Hamlin Professor and Chair, Department of Dermatology; Professor, Department of Pediatrics, The Feinberg School of Medicine, Northwestern University; Chicago, Illinois

Expert Commentary

Brian J. Nickoloff, MD, PhD, Professor and Associate Chairman of Pathology; Director, Oncology Institute; Deputy Director, Cardinal Bernardin Cancer Center; Director, Skin Disease Research Program; Loyola University of Chicago Medical Center; Maywood, Illinois

Historically, dermatology has led the investigative effort to identify and understand the molecular and cellular mechanisms of the human inflammatory response. Over the past several decades, significant progress has been made toward identifying and characterizing the mechanisms responsible for induction and resolution of inflammation. Regulation of inflammation involves positive and negative pathways. For example, in the presence of pathogens, the host-immune response and the acute inflammatory response serve a "helpful" purpose. Pathogens are removed as tissue repair is stimulated; tissue function is restored. When the regulation of the acute inflammatory response is altered, chronic inflammation and tissue dysfunction are "hurtful"; inflammation can lead to disease pathology. Evidence clearly demonstrates that skin diseases, such as psoriasis and atopic dermatitis, are characterized by chronic inflammation that contributes to altered barrier function, scale formation, and other detrimental consequences that require therapeutic intervention by dermatologists.

The history of investigation into the inflammatory response is an interesting one. In 1927, Sir Thomas Lewis elucidated the "triple response of skin" to a cutaneous injury - flush (redline), flare (red zone), and wheal (edema).¹ Lewis could eliminate the flare, but not the other components of the inflammatory response, by eliminating the autonomic nerve supply and preventing the "axon reflex." This seminal work led to the eventual discovery of histamine, an important mediator in the inflammatory response. Myriad investigators followed Sir Lewis's lead and conducted complementary work that revealed further insights into the molecular and cellular mechanisms and mediators of inflammation. In the 1940s, Landsteiner and Chase established that a delayed hypersensitivity response was mediated by the cellular not the humoral arm of the immune system. ² More recently, advancements have been made in genetics, the immunological synapse, the untangling of the cytokine web and signaling pathways, xenotransplantation models, and the growing use of selectively targeted therapies for treating chronic and excessive inflammation disease states, such as psoriasis. ³

The complexities underlying the regulation of inflammation continue to evolve. At a recent CME-certified satellite symposium held during the 67th Annual Meeting of the Society for Investigative Dermatology in May 2006, leading researchers presented new insights on the mechanisms involved with initiation and resolution of inflammation. The presentations demonstrated that both initiation and resolution of inflammation represent highly regulated and active processes. Topics of discussion included initiation and resolution of acute inflammation; ⁴ tuning of inflammation by cytokines and decoy receptors; ⁵ a cellular perspective on the regulation of T-cell-mediated immune responses by antigen-presenting cells (APCs) that express indoleamine 2,3-dioxygenase (IDO); ⁶ the role of regulatory T cells in modulating immune responses; ⁷ and a clinical perspective on approaches to unanswered questions whose answers could suggest a means to dampening the inflammatory response. ⁸

This Dermatology Express Report^TM reviews the important concepts discussed during these presentations to provide a thorough appreciation of the complex interplay of mediators and cells that contribute to chronic inflammation.

Acute Inflammation - Initiation and Resolution

"As it pertains to medicine, the term 'resolve' means the subsiding or termination of an abnormal condition," advised Derek W. Gilroy, PhD, Wellcome Trust Fellow, University College London, London, United Kingdom. ⁴ "With respect to inflammation, resolution should be viewed as an active and highly controlled process through endogenous pro-resolution chemical mediators that are synthesized by inflammatory leukocytes or local parenchymal tissue," indicated Dr. Gilroy. Mediators of inflammation resolution include fatty acids, chemokines, and cytokines, representing substances involved in cell signaling.

According to Dr. Gilroy, the role of fatty acids in the resolution of inflammation is illustrated by the activity surrounding the release and metabolism of arachidonic acid. Arachidonic acid is taken up by cyclooxygenase (two isoforms, COX-1 or COX-2) and converted into prostaglandin H₂. The hematopoietic prostaglandin D₂ synthase converts COX-derived prostaglandin H₂ to prostaglandin D₂, which is further dehydrated to cyclopentenone prostaglandins.

Using a mouse model of peritonitis to illustrate this process, ⁹ Dr. Gilroy and associates found paradoxical production of normally pro-inflammatory COX-2 that produced prostaglandin D₂, which has been associated with recruitment of macrophages. Inhibiting COX-2 at the site of inflammation blocked the onset of resolution. Moreover, when COX-2 metabolites were introduced at the site of inflammation concurrently with a COX-2 inhibitor (eg, indomethacin), resolution was restored.

"Quite clearly, we demonstrated that cyclooxygenase-2-derived prostaglandin D₂ and cyclopentenone prostaglandins play an important role in bringing about spontaneous resolution of acute inflammation," stated Dr. Gilroy.

More recently, Dr. Gilroy examined the role of hematopoietic prostaglandin D₂ synthase in controlling delayed type hypersensitivity reactions in hematopoietic prostaglandin D₂ synthase knockout and transgenic mice. ¹⁰ Compared with their respective controls, hematopoietic prostaglandin D₂ synthase knockouts displayed a more severe inflammatory response that failed to resolve, characterized histologically as persistent acute inflammation, whereas control transgenic mice had little detectable chronic inflammation. The results of this study suggest that hematopoietic prostaglandin D₂ synthase may act as an internal braking signal essential for resolution of delayed type hypersensitivity reactions.

Dr. Gilroy reviewed the role of lipoxins, epi-lipoxins, and resolvins in the inflammatory response. Actions attributable to lipoxins and epi-lipoxins in the inflammatory response include:

• ↑ phagocytosis of apoptotic polymorphonuclear cells
• ↑ monocyte adhesion
• ↓ polymorphonuclear and eosinophil migration
• ↓ adhesion molecule activation and pro-inflammatory gene expression
• ↓ interleukin(IL)-8 release by epithelial cells
• ↓ superoxide production by polymorphonuclear cells

Dr. Gilroy then discussed practical applications that result from understanding specific mechanisms involved in the inflammatory response. For example, in a murine model of asthma, ¹¹ allergen challenge initiated airway biosynthesis of lipoxin A₄ and increased expression of its receptor. Administering a stable analog of lipoxin A₄ blocked both airway hyper-responsiveness and pulmonary inflammation, as shown by decreased leukocytes and mediators. Thus, those findings suggest that lipoxin and related pathways offer novel multi-pronged therapeutic approaches for managing asthma. Another study found that a β-oxidation-resistant lipoxin A₄ analog effectively treated hapten-induced colitis by attenuating inflammation and immune dysfunction; ¹² as did resolvin E1. ¹³ After reviewing the roles of cytokines and chemokines in the inflammatory response, Dr. Gilroy concluded his presentation by emphasizing the importance of further understanding of the pathophysiology of host-response to injury. By understanding the etiology of chronic inflammation, therapeutic agents could be developed that mimic the actions of mediators essential to resolution of inflammation, ¹⁴ thus providing effective treatment options for chronic inflammatory diseases.

Strategies to Sense Pathogens - Cytokines and Decoy Receptors

An emerging concept in the resolution of inflammation relates to the potential use of decoy receptors to regulate pro-inflammatory primary cytokines and chemokines. ¹⁵ As canonically conceptualized, a receptor has specific ligand recognition, characterized by high affinity and specificity, and signaling. In contrast, decoy receptors recognize inflammatory cytokines with high affinity and specificity but lack the capability for signaling or presenting the agonist to signaling receptor complexes. Instead, a decoy receptor acts as a molecular trap for the agonist and for signaling receptor components, according to Alberto Mantovani, MD, Professor of General Pathology, School of Medicine, State University of Milan; Scientific Director of Instituto Clinico Humanitas; Milan, Italy. ⁵

Dr. Mantovani noted that the first pure decoy receptor to be identified was IL-1 type II receptor. ¹⁶ Subsequently, Toll-like receptors were identified and characterized. ^17,18 Together, IL-1 receptors and Toll-like receptors form a conserved sequence called the Toll/IL-1R (TIR) domain, which is involved in activating a stereotypical signaling pathway. TIR8 has been shown to inhibit signaling from the IL-1R complex. ¹⁹

The D6 decoy receptor, a scavenger decoy receptor for inflammatory CC chemokines, ¹⁵ has been of particular interest, Dr. Mantovani indicated. D6 is prominently expressed in lymphatic endothelium in the skin and is non-redundant for controlling skin inflammation in response to diverse inflammatory signals.

Dr. Mantovani reviewed several known properties of D6²⁰:

• Engagement of D6 by inflammatory CC chemokines does not elicit a calcium response or chemotaxis, but results in efficient agonist internalization and degradation
• In lymphatic endothelium, D6 does not elicit cellular responses other than ligand internalization and degradation
• No evidence has been obtained for D6-mediated transcytosis of chemokines in the apical-to-basal or basal-to-apical directions

D6 has unique functional and structural features, which make D6 ideally adapted to act as a chemokine decoy and scavenger receptor, being strategically located on lymphatic endothelium to dampen inflammation in tissues and draining lymph nodes, summarized Dr. Mantovani.

Regulation of T-cell-mediated Immune Responses - APCs and IDO

APCs can induce tolerance or immunity. ²¹ IDO is a highly conserved, intracellular heme-enzyme that catalyzes oxidative catabolism of compounds with indole rings, such as the essential amino acid tryptophan. ²² The concept that cells expressing IDO can suppress T-cell response and promote tolerance is a relatively new paradigm in immunology, according to Andrew L. Mellor, PhD, Professor & Georgia Research Alliance Eminent Scholar in Immunogenetics; Director, Medical College of Georgia Immunotherapy Center; Associate Director and Chief, Program in Molecular Immunology, Institute of Molecular Medicine and Genetics; Augusta, Georgia. ⁶

"The current hypothesis guiding research is that inducible IDO expression converts T-cell stimulatory dendritic cells into T-cell regulatory dendritic cells," commented Dr. Mellor. "IDO is expressed at the maternal-fetal interface, the eye, the gut-associated lymphoid tissue, and the lung, while induced by inflammation, tumor growth, and infection," added Dr. Mellor. Dr. Mellor reviewed several established properties of IDO activity:

• IDO activity suppresses undesirable T-cell immunity and tissue pathology in autoimmune diseases (eg, colitis, rheumatoid arthritis), allergy, pregnancy, and transplantation
• IDO activity suppresses desirable T-cell immunity targeted to tumors and infect cells (eg, HIV-NOD/SCID)
• IDO-competent dendritic cell subsets acquire potent and dominant T-cell regulatory functions when induced to express functional IDO
• IDO inducers include immunosuppressive (CTLA4-Ig) and immunostimulatory reagents (CpG-ODNS, interferon types 1 and 2)
• IDO-GCN2 signaling blocks naïve T-cell activation and enhances FoxP3 expression in CD4+/CD25+ regulatory T cells

According to Dr. Mellor, IDO activity has important implications in immunotherapy. For example, IDO inhibitors (eg, 1methyl-[D]- tryptophan) are being investigated in clinical syndromes caused by suppressed T-cell immunity (eg, cancer, chronic infections) while IDO inducers (eg, CTLA4-Ig, CpG-ODNS, CD200, interferon types 1 and 2) are being investigated in clinical syndromes caused by excessive T-cell immunity (eg, autoimmune diseases, allergy, asthma, and transplantation rejection). The National Cancer Institute has planned a phase I clinical trial of 1methyl-[D]-tryptophan as a cancer vaccine adjuvant expected to begin next year, noted Dr. Mellor.

T-cell Depletion as a Therapeutic Strategy

Strategies aimed at depletion of regulatory T cells offer another potential approach to therapeutic regulation of inflammatory and immune-mediated responses. The rationale for such an approach includes the existence of CD4+/CD25+ regulatory T cells in humans that suppress immune responses to self and tumor antigens; increased levels of regulatory T cells in cancer patients; antibody-mediated elimination of regulatory T cells shown to elicit antitumor immunity in tumor-bearing mice; and enhancement of the therapeutic effects of tumor vaccines by administration of an anti-CD25 monoclonal antibody, according to Johannes W. Vieweg, MD, Associate Professor of Urology and Immunology, Duke University Medical Center, Durham, North Carolina. ⁷ One therapeutic strategy under investigation involves using denileukin diftitox, ²³ a recombinant fusion protein that contains the catalytical and membrane translocation domain of diphtheria toxin fused to human IL-2, which allows targeting of CD25+ cells.

"The action of denileukin diftitox occurs after binding to the high affinity interleukin-2 receptor [CD25], internalization, and intracellular activation, leading to enzymatic inhibition of protein synthesis and cell death," advised Dr. Vieweg.

Dr. Vieweg and colleagues have demonstrated that denileukin diftitox selectively eliminates CD4+/CD25+ regulatory T cells from the peripheral blood mononuclear cells of patients with metastatic renal cell carcinoma without inducing toxicity in cellular subsets with low or intermediate expression of CD25. ²⁴ Moreover, denileukin diftitox-mediated regulatory T-cell depletion resulted in enhanced stimulation of proliferative and cytotoxic T-cell responses in vitro and abrogated regulatory T-cell-mediated immunosuppressive activity in vivo. Importantly, denileukin diftitox-mediated elimination of regulatory T cells followed by vaccination with RNA-transfected dendritic cells significantly improved the stimulation of tumor-specific T-cell responses when compared with vaccination alone.

Although the data with denileukin diftitox have not been consistent (eg, denileukin diftitox failed to eliminate regulatory T lymphocytes in metastatic melanoma), ²⁵ Dr. Vieweg suggested that therapeutic strategies targeting depletion of regulatory T cells (eg, denileukin diftitox) could have broad implications for the future design of active and passive immune-based treatment protocols.

Dampening Inflammation - A Delicate Balance

Clinical and laboratory evidence accumulated to date show that dampening of the inflammatory response requires a therapeutic approach that is sensitive to the delicate balance of pro- and anti-inflammatory factors involved, according to Frank O. Nestle, MD, Mary Dunhill Chair of Cutaneous Medicine and Immunotherapy, St. John's Institute of Dermatology, Division of Genetics and Molecular Medicine, King's College London School of Medicine at Guy's King's College and St. Thomas' Hospitals, London, United Kingdom. ⁸ However, the involvement of multiple counterbalancing factors in inflammation suggests a variety of potential therapeutic targets, including T-cell trafficking, T-cell activation, cytokine inhibitors, and counteroffensive strategies, ³ Dr. Nestle noted.

Dr. Nestle reviewed several examples of anti-cytokine therapies. Infliximab, a tumor necrosis factor (TNF) inhibitor, has demonstrated efficacy as initial and maintenance therapy for moderate-to-severe psoriasis. ²⁶ The IL-12 family of cytokines offers another potential strategy for regulating inflammation, indicated Dr. Nestle. IL-12 is a pro-inflammatory cytokine that induces production of interferon-g, favors differentiation of T helper 1 cells, and links innate resistance and adaptive immunity. ²⁷ Dendritic cells and phagocytes produce IL-12 in response to pathogens during infection. IL-23, a member of the IL-12 family, has emerged as a potentially important contributor to the pathogenesis of psoriasis. The p19 and p40 subunits of IL-23 are increased in psoriatic lesions. ²⁸ According to Dr. Nestle, an initial clinical trial of anti-p40 antibody demonstrated between 60% and 90% of patients with psoriasis had at least 50% improvement in the Psoriasis Area and Severity Index (PASI) at 16 weeks. At the highest dose (400 mg), approximately 60% of patients had an improvement in PASI ≥90%. However, Dr. Nestle emphasized that much more clinical investigation is needed to fully elucidate the therapeutic potential of anti-IL-12 treatment. Therapy directed against interferon-α was another area of active study, noted Dr. Nestle.

References

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Disclosure

Brian J. Nickoloff, MD, PhD
No significant relationships to disclose

Amy S. Paller, MD
Grant/Research Support-Astellas Pharma US, Inc., Novartis; Consultant-Astellas Pharma US, Inc., Novartis; Speakers Bureau-Astellas Pharma US, Inc., Novartis

This report contains information on commercial products that are unlabeled for use or investigational uses of products not yet approved.

The opinions expressed in this publication are those of the participating faculty and do not necessarily reflect the opinions or the recommendations of their affiliated institutions; Millennium CME Institute, Inc.; or any other persons. Any procedures, medications, or other courses of diagnosis or treatment discussed or suggested in this publication should not be used by clinicians without evaluation of their patients’ conditions, assessment of possible contraindications or dangers in use, review of any applicable manufacturer’s product information, and comparison with the recommendation of other authorities. This Dermatology Express Report^TM was made possible through an educational grant from Centocor, Inc.

© 2006 Millennium CME Institute, Inc.

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