Differential effect on polyamine metabolism in mitogen- and superantigen-activated human T-cells

Arginase Signalling as a Key Player in Chronic Wound Pathophysiology and Healing

Arginase (ARG) represents an important evolutionarily conserved enzyme that is expressed by multiple cell types in the skin. Arg acts as the mediator of the last step of the urea cycle, thus providing protection against excessive ammonia under homeostatic conditions through the production of L-ornithine and urea. L-ornithine represents the intersection point between the ARG-dependent pathways and the urea cycle, therefore contributing to cell detoxification, proliferation and collagen production. The ARG pathways help balance pro- and anti-inflammatory responses in the context of wound healing. However, local and systemic dysfunctionalities of the ARG pathways have been shown to contribute to the hindrance of the healing process and the occurrence of chronic wounds. This review discusses the functions of ARG in macrophages and fibroblasts while detailing the deleterious implications of a malfunctioning ARG enzyme in chronic skin conditions such as leg ulcers. The review also highlights how ARG links with the microbiota and how this impacts on infected chronic wounds. Lastly, the review depicts chronic wound treatments targeting the ARG pathway, alongside future diagnosis and treatment perspectives.

T cell metabolism in metabolic disease-associated autoimmunity

This review discusses the relevant metabolic pathways and their regulators which show potential for T cell metabolism-based immunotherapy in diseases hallmarked by both metabolic disease and autoimmunity. Multiple therapeutic approaches using existing pharmaceuticals are possible from a rationale in which T cell metabolism forms the hub in dampening the T cell component of autoimmunity in metabolic diseases. Future research into the effects of a metabolically aberrant micro-environment on T cell metabolism and its potential as a therapeutic target for immunomodulation could lead to novel treatment strategies for metabolic disease-associated autoimmunity.

Arginine Supplementation Does Not Enhance Serum Nitric Oxide Levels in Elderly Nursing Home Residents With Pressure Ulcers

The purpose of this study was to determine whether arginine supplementation enhances in vitro (neutrophil burst and mitogen-induced lymphocyte proliferation) and in vivo (delayed-type hypersensitivity [DTH] and serum nitric oxide) measures of immune function in nursing home elders with pressure ulcers. Twenty-six elders, 65 years of age or older, with one or more pressure ulcers, were randomized to receive 8.5 g of arginine or an isonitrogenous supplement for 4 weeks. Immune function studies and serum arginine, ornithine, citrulline, and nitric oxide were measured at baseline, 4 weeks postsupplementation (Week 4) and after a 6-week washout (Week 10). At Week 4, serum ornithine increased (p = .01) and arginine trended to increase (p = .055), but there was no increase in citrulline or nitric oxide with arginine supplementation. There were no differences in neutrophil burst or DTH responses between groups. Whole blood mitogen–induced proliferation decreased significantly atWeek 10 in the isonitrogenous but not in the arginine-supplemented group. There is mounting concern that arginine supplementation during an inflammatory state could be detrimental due to overwhelming nitric oxide production. A key finding of this study is that arginine supplementation did not increase serum nitric oxide levels over that observed in elders with pressure ulcers given an isonitrogenous supplement.

Metabolism of activated T lymphocytes

Activated T cells undergo metabolic reprogramming which promotes glycolytic flux and lactate production as well as elevated production of lipids, proteins, nucleic acids and other carbohydrates (i.e. induction of biomass) even in the presence of oxygen. Activated T cells show induced expression of, among other things, Glucose Transporter 1 and several glycolytic enzymes, including ADP-Dependent Glucokinase and the low affinity isoform Pyruvate Kinase-M2 (which promote glycolytic flux), as well Glutamine Transporters and Glycerol-3-phosphate Dehydrogenase 2 which make available glutamate and glycerol-3-phosphate as mitochondrial energy sources. Intracellular leucine concentrations critically regulate mammalian target of rapamycin (mTOR) signaling to promote Th1, Th2, and Th17 CD4(+) T effector cell differentiation. In contrast, T regulatory (Treg) cells are generated when AMP-Activating Protein Kinase signaling is activated and mTOR activation is suppressed. Unlike effector CD4(+) and CD8(+) T cells, Tregs and memory T cells oxidize fatty acids for fuel. Effector and memory T cells perform different functions and thus show distinct metabolic profiles which are exquisitely controlled by cellular signaling. Upon activation, T cells express the insulin and leptin receptors on their surface and become sensitive to insulin signaling and nutrient availability and show changes in differentiation. Thus, metabolism and nutrient availability influence T cell activation and function.

Arginine supplementation and wound healing

Modern advances in nutritional therapies have led to the specific use of arginine supplementation for protein synthesis, cell signaling through the production of nitric oxide, and cell proliferation through its metabolism to ornithine and to polyamines. Arginine is classified as a nonessential amino acid that becomes a conditionally essential substrate in stressed adults. Arginine has been shown to enhance wound strength and collagen deposition in artificial incisional wounds in rodents and humans. A role for dietary intervention in the form of arginine supplementation has been proposed to normalize or enhance wound healing in humans. Although this hypothesis is frequently discussed, the therapeutic effect of arginine supplementation on chronic wound healing in humans is still undetermined and requires further objective evidence. Well-designed clinical trials are required to determine whether arginine supplementation is effective in enhancing healing of acute and chronic wounds in humans and how much arginine is recommended to meet metabolic needs during the phases of wound healing.

Arginine and immunity: a unique perspective

Arginine functions in the body as a free amino acid, a component of most proteins, and the substrate for several non-protein, nitrogen-containing compounds, many of which function in immunity. Although arginine is synthesized in the body, it is not made in sufficient quantities to support growth or meet metabolic requirements during periods of stress. Based on the biochemical and physiological role of arginine in maintaining health and immunity, arginine is being added at pharmacologic concentrations to enteral formulas to boost immune function. Unfortunately, animal and human studies that investigate enteral arginine supplementation as the single variable do not show clear immunologic benefit. The inconsistent effects of arginine supplementation on immune function are due to numerous factors, such as the amount and timing of arginine supplementation, the animal species or strain of species, and the experimental model. Systematic study is required to determine whether a basal dietary intake of arginine is required to maintain immune function during health and how much arginine is required to meet metabolic requirements during periods of growth or stress.

Overexpression of CRIP in transgenic mice alters cytokine patterns and the immune response

Cysteine-rich intestinal protein (CRIP), which contains a double zinc finger motif, is a member of the Group 2 LIM protein family. Our results showed that the developmental regulation of CRIP in neonates was not influenced by conventional vs. specific pathogen-free housing conditions. Thymic and splenic CRIP expression was not developmentally regulated. A line of transgenic (Tg) mice that overexpress the rat CRIP gene was created. When challenged with lipopolysaccharide, the Tg mice lost more weight, exhibited increased mortality, experienced greater diarrhea incidence, and had less serum interferon-gamma (IFN-gamma) and more interleukin (IL)-6 and IL-10. Similarly, splenocytes from the Tg mice produced less IFN-gamma and IL-2 and more IL-10 and IL-6 upon mitogen stimulation. Delayed-type hypersensitivity response was less in the Tg mice. Influenza virus infection produced greater weight loss in the Tg mice, which also showed delayed viral clearance. The observed responses to overexpression of the CRIP gene are consistent with a role for this LIM protein in a cellular pathway that produces an imbalance in cytokine pattern favoring Th2 cytokines.

Effects of L-arginine on the proliferation of T lymphocyte subpopulations

BACKGROUND

Dietary supplementation of L-arginine as a mechanism to enhance cellular immune response (T lymphocytes), has slowly gained approval, and appears especially important during critical illness. Despite its clinical use, little is known as to the direct effects of L-arginine on the different T lymphocyte subpopulations.

METHODS

Lymphocytes were harvested from spleens of C57 B1/6 mice, and proliferation was induced with anti-CD3 in the presence of different concentrations of L-arginine ranging from 0 to 1000 micromol/L. Flow cytometry was used to evaluate the effect of L-arginine on T lymphocyte subpopulations. Interleukin-2 production was measured by ELISA and gene expression by RT-PCR.

RESULTS

L-Arginine at or greater than 100 micromol/L significantly enhanced anti-CD3 stimulated T lymphocyte proliferation (p = .01). L-Arginine was essential for adequate T lymphocyte (CD3+) cellular maturation (p = .01). Proliferation of Helper T cells (CD4+) was not dependent on L-arginine. In contrast, Cytotoxic T cells (CD8+) showed a dose dependent proliferation in response to L-arginine (p = .01). Of the CD8+ cells, an increase in the CD45RA negative CD8 positive (memory) T cell subpopulation was observed with the addition of L-arginine. In addition, the number of cell surface CD8 receptors (CD8R) and CD3 receptors (CD3R) increased in the presence of L-arginine (p = .01, p = .04). Interleukin-2 receptor (IL-2R) expression was not up-regulated by L-arginine. L-Arginine modestly increased IL-2 production and had pronounced effects on its disappearance from the culture media (p < .0001). Interleukin-2 mRNA expression was not dependent on L-arginine.

CONCLUSIONS

The requirements for L-arginine for the proliferation of CD3 stimulated T lymphocytes vary widely, and have to be taken into account when studying the mechanism of how L-arginine enhances cellular proliferation. L-Arginine may increase cellular proliferation by increasing specific receptor expression and the utilization of interleukin-2.