Photodynamic therapy involves administration of a tumor-localizing photosensitizing agent, which may require metabolic synthesis (i.e., a prodrug), followed by activation of the agent by light of a specific wavelength. This therapy results in a sequence of photochemical and photobiologic processes that cause irreversible photodamage to tumor tissues. Results from preclinical and clinical studies conducted worldwide over a 25-year period have established photodynamic therapy as a useful treatment approach for some cancers. Since 1993, regulatory approval for photodynamic therapy involving use of a partially purified, commercially available hematoporphyrin derivative compound (Photofrin) in patients with early and advanced stage cancer of the lung, digestive tract, and genitourinary tract has been obtained in Canada, The Netherlands, France, Germany, Japan, and the United States. We have attempted to conduct and present a comprehensive review of this rapidly expanding field. Mechanisms of subcellular and tumor localization of photosensitizing agents, as well as of molecular, cellular, and tumor responses associated with photodynamic therapy, are discussed. Technical issues regarding light dosimetry are also considered.

Photodynamic Therapy

Sepsis is characterized by a severe inflammatory response to infection, and its complications, including acute kidney injury, can be fatal. Animal models that correctly mimic human disease are extremely valuable because they hasten the development of clinically useful therapeutics. Too often, however, animal models do not properly mimic human disease. In this Review, we outline a bedside-to-bench-to-bedside approach that has resulted in improved animal models for the study of sepsis — a complex disease for which preventive and therapeutic strategies are unfortunately lacking. We also highlight a few of the promising avenues for therapeutic advances and biomarkers for sepsis and sepsis-induced acute kidney injury. Finally, we review how the study of drug targets and biomarkers are affected by and in turn have influenced these evolving animal models.

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Animal models of sepsis and sepsis-induced kidney injury

BACKGROUND Sepsis is a deadly immunological disorder and its pathophysiology is still poorly understood. We aimed to determine if specific pro-inflammatory and anti-inflammatory cytokines can be used as diagnostic and therapeutic targets for sepsis. MATERIALS AND METHODS Recent publications in the MEDLINE database were searched for articles regarding the clinical significance of inflammatory cytokines in sepsis. RESULTS In response to pathogen infection, pro-inflammatory cytokines [interleukin-6 (IL-6), IL-8, IL-18 and tumor necrosis factor-α (TNF-α)] and anti-inflammatory cytokine (IL-10) increased in patients with sepsis. Importantly, a decrease in IL-6 was associated with a better prognosis and overproduction of IL-10 was found to be the main predictor of severity and fatal outcome. CONCLUSION Both pro-inflammatory and anti-inflammatory cytokines constitute a double-edged sword in sepsis; on one hand they are critical to eliminate the infection while on the other, excessive production can cause tissue and organ damage. Increase in cytokines such as IL-6, Il-8, IL-10, IL-18 and TNF-α may have implications in diagnosis and treatment of sepsis.

Role of cytokines as a double-edged sword in sepsis.

The aldo-keto reductase (AKR) superfamily comprises enzymes that catalyze redox transformations involved in biosynthesis, intermediary metabolism, and detoxification. Substrates of AKRs include glucose, steroids, glycosylation end-products, lipid peroxidation products, and environmental pollutants. These proteins adopt a (β /α )8 barrel structural motif interrupted by a number of extraneous loops and helixes that vary between proteins and bring structural identity to individual families. The human AKR family differs from the rodent families. Due to their broad substrate specificity, AKRs play an important role in the phase II detoxification of a large number of pharmaceuticals, drugs, and xenobiotics.

The aldo-keto reductase superfamily and its role in drug metabolism and detoxification.

miR-146a Is Critical for Endotoxin-induced Tolerance IMPLICATION IN INNATE IMMUNITY

Effects of aging on mortality, hypothermia, and cytokine induction in mice with endotoxemia or sepsis.

Gamma interferon (IFN-γ) is an important mediator of endotoxin (lipopolysaccharide [LPS])-induced immune responses. However, the specific cell types that produce IFN-γ in response to LPS and the cellular factors that regulate LPS-induced IFN-γ production have not been fully determined. The present studies were undertaken to characterize the cell populations that produce IFN-γ after LPS challenge in the spleens of mice and to determine the regulatory factors that modulate LPS-induced production of IFN-γ. Our studies show that the levels of splenic IFN-γ mRNA and protein production peak at 6 and 8 h, respectively, after systemic LPS challenge. Approximately 60% of IFN-γ-producing cells are natural killer (NK) cells (CD3−DX5+) and 25% are NKT cells (CD3+DX5+). Most of the remaining IFN-γ-producing cells are T cells (CD3+DX5−), macrophages, and dendritic cells. Functionally, interleukin-12 (IL-12) is the major IFN-γ-stimulating factor after LPS challenge, with costimulation provided by IL-15, IL-18, and B7 proteins. IL-10 is a major inhibitor of LPS-induced IFN-γ production. Unlike intact heat-killed gram-negative and gram-positive bacteria, the class II major histocompatibility complex did not play a functional role in LPS-induced IFN-γ production. LPS is a potent stimulus for splenic IL-10, IL-12 p40, and IL-15 mRNA expression, whereas IL-12 p35 and IL-18 mRNAs, as well as B7 proteins, are constitutively expressed in the mouse spleen. Of the factors studied, IL-18 serves as the most potent costimulus with IL-12 for IFN-γ production, followed by IL-15 and B7 proteins. These data demonstrate that NK cells and NKT cells are the most abundant IFN-γ-producing cells in the mouse spleen after LPS challenge and that IL-10 and IL-12 are key functional regulators of LPS-induced IFN-γ production.

https://cvi.asm.org/content/cdli/9/3/530.full.pdf

Endotoxin-induced gamma interferon production: contributing cell types and key regulatory factors.

Monophosphoryl lipid A (MPLA) is a Toll-like receptor 4 (TLR4) agonist that is currently used as a vaccine adjuvant in humans. In this study, we evaluated the effect of MPLA treatment on the innate immune response to systemic bacterial infections in mice. Mice treated with MPLA after burn injury showed improved survival and less local and systemic dissemination of bacteria in a model of Pseudomonas aeruginosa burn wound infection. Prophylactic treatment with MPLA significantly enhanced bacterial clearance at the site of infection and reduced systemic dissemination of bacteria despite causing attenuation of proinflammatory cytokine production during acute intra-abdominal infection caused by cecal ligation and puncture. Administration of MPLA at 1 h after CLP also improved bacterial clearance but did not alter cytokine production. MPLA treatment increased the numbers of granulocytes, double-positive myeloid cells, and macrophages at sites of infection and increased the percentage and total numbers of myeloid cells mediating phagocytosis of bacteria. Depletion of Ly6G+ neutrophils, but not macrophages, eliminated the ability of MPLA treatment to improve bacterial clearance. The immunomodulatory effects of MPLA were absent in TLR4-deficient mice. In conclusion, these studies show that MPLA treatment significantly augments the innate immune response to bacterial infection by enhancing bacterial clearance despite the attenuation of proinflammatory cytokine production. The enhanced bacterial clearance is mediated, in part, by increased numbers of myeloid cells with effective phagocytic functions at sites of infection and is TLR4 dependent.

The Toll-Like Receptor 4 Agonist Monophosphoryl Lipid A Augments Innate Host Resistance to Systemic Bacterial Infection

Endotoxin (lipopolysaccharide [LPS]) tolerance is a state of altered immunity characterized, in part, by suppression of LPS-induced gamma interferon (IFN-γ) expression. However, the cellular mediators regulating LPS-induced production of IFN-γ in normal mice and the effect of LPS tolerance on these mediators has not been well characterized. Our studies show that macrophage dysfunction is the primary factor causing suppressed IFN-γ expression in LPS-tolerant mice. Specifically, LPS-tolerant macrophages have a markedly impaired ability to induce IFN-γ secretion by T cells and NK cells obtained from either control or LPS-tolerant mice. However, T cells and NK cells isolated from LPS-tolerant mice produce normal levels of IFN-γ when cocultured with control macrophages or exogenous IFN-γ-inducing factors. Assessment of important IFN-γ-regulating factors showed that interleukin-12 (IL-12) and costimulatory signals provided by IL-15, IL-18, and CD86 are largely responsible for LPS-induced IFN-γ expression in control mice. IL-10 is an inhibitor of IFN-γ production in both the control and LPS-tolerant groups. Expression of IL-12 and the IL-12 receptor β1 (IL-12Rβ1) and IL-12Rβ2 subunits are suppressed in the spleens of LPS-tolerant mice. LPS-tolerant splenocytes also exhibit decreased production of IL-15 and IL-15Rα. However, expression of IL-18 and the B7 proteins CD80 and CD86 are unchanged or increased compared to controls after induction of LPS tolerance. CD28, a major receptor for B7 proteins, is also increased in the spleens of LPS-tolerant mice. Expression of the inhibitory cytokine IL-10 and the IL-10R are sustained after induction of LPS tolerance. These data show that suppression of IFN-γ production in LPS-tolerant mice is largely due to macrophage dysfunction and provide insight into the cellular alterations that occur in LPS tolerance. This study also better defines the factors that mediate LPS-induced IFN-γ production in normal mice.

Cellular Mechanisms That Cause Suppressed Gamma Interferon Secretion in Endotoxin-Tolerant Mice

Thermal injury to 40% or more of the total body surface area poses a significant risk for the development of opportunistic infections that increase complications and mortality. Altered cytokine induction profiles, including suppression of the Th1 cytokines IFN-gamma and IL-12 and elevations in the anti-inflammatory cytokine IL-10, are believed to contribute to burn-associated immunosuppression and the development of sepsis. The specific changes that lead to altered cytokine production following major burns are not known. We examined the effects of burn injuries to 40% of the mouse body surface on IFN-gamma induction in the major IFN-gamma-producing cell types of the spleen. Additionally, effects on key IFN-gamma-regulatory cytokines were examined after bacterial challenge. We report that in vivo induction of IFN-gamma in natural killer lymphocytes is suppressed in burned mice. Splenic IFN-gamma was suppressed at both the mRNA and protein levels. Early suppression was associated with impairments in both the macrophage/dendritic cell and lymphocyte populations, whereas persistent suppression was associated with impaired lymphocyte function and decreased responsiveness to IFN-gamma-inducing factors. IFN-gamma production could be restored by neutralization of the upregulated cytokine IL-10. Induction of the IFN-gamma-inducers IL-15, IL-12, and IL-2 was also impaired after burn injury, whereas IL-18 levels remained unaffected. Exogenous application of these suppressed cytokines to isolated splenocytes did not restore IFN-gamma to sham levels, indicating a loss of responsiveness to these factors. Expression of the IL-2, IL-12, and IL-15 receptors was suppressed after thermal injury. We conclude that burn-associated suppression of IFN-gamma is due to deficient production of inducing factors and their receptors, leading to severe impairments in cellular IFN-gamma induction pathways.

Tushar K. Varma

Papers

Effects of aging on mortality, hypothermia, and cytokine induction in mice with endotoxemia or sepsis.

Endotoxin-induced gamma interferon production: contributing cell types and key regulatory factors.

The Toll-Like Receptor 4 Agonist Monophosphoryl Lipid A Augments Innate Host Resistance to Systemic Bacterial Infection

Cellular Mechanisms That Cause Suppressed Gamma Interferon Secretion in Endotoxin-Tolerant Mice

Interferon-γ production is suppressed in thermally injured mice: Decreased production of regulatory cytokines and corresponding receptors