Seattle Treatment Education Project (STEP) Perspective, Vol. 7, No. 1 - Spring 1995; A Publication of the Seattle Treat- ment Education Project, 127 Broadway E. Ste. 200, Seattle, WA 98102 * (206) 329-4857. Published 3 times a year
Anne Marie Drexler, ARNP

Cytokines

Cytokine is a generic term for peptides (small proteins) which mediate signaling between between cells involved in inflammatory and immune responses. Cytokines act somewhat like hormones in that when they are released, they can affect the cells around them or travel to a different part of the body and influence the function of other cells. Unlike hormones, cytokines are produced by nearly every cell and primarily in the presence of disease.

TNF is a cytokine secreted by macrophages (TNF-alpha) and T- cells (TNF-beta). TNF is also referred to as a monokine as it is primarily produced by mononuclear phagocytes (monocytes and macrophages). Other mediators released by macrophages include: interferon, interleukans, leukotrienes, prostaglandins, proteases, complement proteins, oxygen-derived free radicals, plasminogen activators, coagulation, colony-stimulating and platelet-activating factors. Many of these cytokine mediators induce or inhibit various cell to cell interactions through feedback loops and many of their actions are redundant. When communication between cytokines becomes disrupted_such as during blood infections (sepsis)_loss of cytokine regulation can lead to uncontrolled inflammatory resposes harming the host.

Inflammation and Immune Response

Inflammation is a nonspecific response to tissue injury triggered by chemical, mechanical or infectious stimulation. Inflammation enhances the movement of nutrients and inflammatory and immune cells and mediators to the site of injury, preventing infection and extension of injury. The major functions of monocytes and macrophages (large white blood cells in blood and tissue) in the inflammatory and immune response are phagocytosis, removal of antigen and debris, and antigen processing and presentation. Once the macrophage has phagocytized (consumed) the antigen (bacterial or viral) it is processed into fragments and presented to lymphocytes (T and B white blood cells) to induce activation and proliferation of helper and killer T cells and antibodies. By presenting the antigen to lymphocytes, macrophages link the inflammatory and immune response.

TNF stimulates a variety of responses, both locally and systemically. TNF attracts and activates neutrophils (white blood cells) causing them to migrate from the vascular system to the area of injury and enhance phagocytic function of both neutrophils and macrophages. TNF is also thought to render liver cells more resistant to the damaging effects of parasites. TNF stimulates the endothelium (cells that line the vascular system) to become more permeable, allowing plasma and blood cells into the site of injury, resulting in edema (swelling). TNF aids in wound healing by stimulating the growth of new blood vessels and tissue. TNF may also enhance lymphocytic activity by stimulating interleukin, platelet activating factors and interferon release from immune cells. TNF and interleukin (IL) produce endogenous pyrogens or proteins that induce fever, anorexia (loss of appetite), lethargy and sleep. Although fever is not usually thought of as adaptive, it may be helpful in suppressing bacterial replication, while anorexia, lethargy and sleep may be important in energy consevation while the host is battling the invading agent or antigen.

Sepsis can be initiated by gram-negative or gram-positive bacteria, viruses, fungi and protozoa. Bacterial infection and the consequent host response can induce secretion of multiple cytokines that affect vascular tone and increase vascular permeability in an inflammatory response. This response produces discrete increased blood flow and capillary permeability to deliver white blood cells to the area of infection. Diffuse activation of this inflammatory response during sepsis is associated with a cascade of physiologic and clinical effects. If the process is not interrupted, maldistribution of blood flow, altered organ perfusion (blood flow to organs), multisystem organ dysfunction and death may occur.

Infection with gram-negative bacteria such as escherichia coli, psuedomonas aeruginosa, klebsiella pneumoniae and staphyococcus aureus, have typically been associated with high fever and hypotension (dangerously low blood pressure) leading to shock secondary to the effects of endotoxin. Endotoxin is a lipopolysaccharide (LPS) that is a component of cell walls of gram- negative bacteria. Endotoxin or LPS is released during infection or following bacterial lysis (death) and stimulates macrophages, monocytes and neutrophils to produce their cytokines and perpetuate the septic cascade.

Gram-negative infections have also been noted to induce hemorrhagic necrosis (bleeding into tissues causing death) of tumors. That observation stimulated the search for the mediator of LPS and it was named "tumor necrosis factor" because when injected into animals with tumors, necrosis of the cancer soon followed. Purified or recombinant TNF has shown antitumor activity against a variety of tumors and is partcularly damaging to capillary endothelial cells, the presumed origin of Kaposi's sarcoma (KS). Early clinical trials sought to capitalize on this finding by treating people with KS with purified TNF.

In infection, TNF mediates many effects of LPS. When TNF was injected into noninfected animals they exhibited many of the signs and symptoms commonly found in sepsis: low blood pressure, increased heart rate, respiration and glucose, metabolic acidosis, fluid shifts, decreased blood supply to the gastrointestinal tract and kidneys. Other evidence implicating TNF as a mediator in infection is the finding that treatment with anti-TNF antibodies appears to offer protection from the sequelae of shock when animals were given lethal doses of LPS and parasites.

Metabolic and Nutritional Alterations

Infection, inflammation, trauma and some malignant diseases also induce a constellation of host responses referred to as the acute phase response (APR), of which TNF and IL appear to be the main mediators. The APR response is associated with metabolic changes in liver protein and fat synthesis. While there is an increase in some proteins, others essential for health and efficient wound healing, including albumin and transferrin, are decreased. Activity of lipoprotein lipase (LPL), an enzyme produced in the liver and essential for regulating fat metabolism is decreased during APR, resulting in reduced fat uptake and improper fat storage. TNF and IL stimulate synthesis of fatty acids and serve to augment the hypertriglyceridemia observed in APR. This may be an explanation for the paradoxical finding of increased serum triglyceride levels sometimes seen in patients with the wasting syndrome.

Synthesis of fat by the liver and conversion of carbohydrates and protein into fat during a period of negative energy balance wastes energy and is referred to as "futile cycling", in which there is shuttling of fatty acids from adipose tissue to the liver and back to adipose tissue. The main source of fuel during this phase is derived from muscle protein, resulting in loss of lean body mass. Fever also increases oxygen and caloric demands stimulating further breakdown of muscle tissue for energy.

The observation of increased triglycerides secondary to decreased LPL and its relationship to cachexia or wasting led a different group of investigators to search for the mediator they named "cachectin", that could induce these changes. Substances from activated macrophages were found to decrease synthesis of LPL in cultured fat cells and cause wasting when given to animals. These researchers purified the "cachectin factor" and found it to be identical to TNF. It is now appreciated that other inflammatory mediators such as IL and several interferons have the same ability to decrease production of LPL and may also be involved in the wasting syndrome seen in cancer and AIDS.

The administration of TNF (cachectin) is not always accompanied by a shock state. When infused slowly over a period of days, large doses of cachectin produced anorexia and wasting similar to cachexia observed in chronic disease states. Though the mechanism by which anorexia is produced is unclear, TNF has been shown to delay gastric emptying and affect the hypothalamus in relation to feeding. Whether the effect is induced by changes in the brain or more peripheral receptors, the outcome is diminished food intake. More recently, studies in animals using injections of recombinant TNF led to acute weight loss secondary to anorexia and negative fluid balance. However, with repeated injections, the animals recovered and regained the weight that was lost, though hypertriglyceridemia (increase in triglycerides) remained.

TNF Levels and AIDS

Serum TNF levels in people with HIV/AIDS have been variable, but probably increase with disease progression. The reason for this discrepancy may be related to the short half-life of TNF (6-18 minutes) making it possible to miss peak TNF production during studies. Factors such as duration of TNF response, presence of other cytokines, site of infection, number of activated macrophages near the infection and whether TNF is confined to the site or systemic are all variables involved in TNF production and levels.

Macrophages, the main source of TNF, are important cellular targets in HIV infection. Macrophages appear to be relaviely resistant to the damaging effects of HIV infection and may act as a reservoir for the virus, serving as a vehicle for viral dissemination. Several laboratory studies indicate TNF selectively kills HIV-infected cells, providing rationale for early studies in AIDS patients where recombinant TNF was used to "boost" immune function. Other in vitro studies have shown it can also stimulate HIV messenger RNA production. The effects of TNF on viral replication may be dose and time dependent. Elevated blood levels of TNF have also been associated with progressive encephalopathy in persons with AIDS. One theory suggests that the central nervous system equivalent of the macrophage, the microglial cell, produces TNF in response to HIV infection and contributes to the development of AIDS-related dementia.

Treatment

Although the mediators of abnormal metabolism in HIV infection_sepsis and wasting_have not been clearly defined, recent research implicates cytokines, with TNF and IL-1 being cited most often. While no definitive treatment for these problems has emerged, the possibility that anti-cytokine intervention might be useful in the clinical setting is encouraging to researchers and practitioners who treat people with AIDS.

One study used dietary N-3 fatty acid (fish oil) supplementation in people with advanced AIDS. It has been shown in vitro that large doses of fish oil reduced production of TNF and IL-1 by monocytes and macrophages stimulated by endotoxin (bacterial infection). Another study showed that a fish oil based diet blocked IL-1 induced anorexia in rats. However, clinical trial results were disappointing as the fish oil supplementation was unable to prevent further weight loss in subjects who developed new AIDS complications.

Additional research is concentrating on cytokine manipulation for people with wasting syndrome. Pentoxifylline (Trental) has been shown to diminish TNF production and when given as an adjunct with antiretroviral therapy may also decrease viral load. Further study is needed in HIV-infected people to ascertain the benefit of pentoxifylline as an additional antiretroviral treatment.

Other strategies for TNF antagonism include anti-TNF monoclonal antibodies and soluble TNF receptors. Animal studies have shown infusions of neutralizing anti-TNF antibodies prevent shock and death due to sepsis or endotoxemia. Based on these studies, humans have received mouse derived anti-TNF antibodies. A study in non-HIV infected patients suffering from sepsis documented a slight improved survival rate. Although monoclonal antibodies are in various clinical trials, soluble TNF receptors are likely to be equally effective. These molecules bind and neutralize TNF in a manner similar to anti-TNF antibodies, but may require frequent infusions whereas antibody treatment in shock only requires a single injection.

Similar inhibitory molecules have been developed to neutralize gamma interferon and other proteins participating in the inflammatory response. The utility of these molecules in the treatment of shock, wasting, and other disorders is not established, yet it seems reasonable that combined administration of several such inhibitors may prevent cachexia or wasting.

In conclusion, the etiology of AIDS-associated wasting is a complex and dynamic problem impacting greatly on the quality of life of people living with AIDS. As our understanding of the immune system grows, so should our ability to manipulate it to our advantage. Current laboratory and clinical studies on the role of TNF, IL-1, and other mediators of the inflammatory and immune response provide important clues to reversing the metabolic derangements of AIDS-related wasting syndrome. With continued research in this area and the development of new anabolic therapies, it is hoped that the loss of lean body mass can soon be removed from the list of AIDS complications.

Anne Marie Drexler, ARNP, is the Nurse Practitioner for Bailey Boushay House References

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