Tumor Necrosis Factor: Tumor necrosis factor (TNF-α) is one of the most studied cytokines. Initially called cachectin, TNF-α was first described for its remarkable antitumor activity and association with cachexia in chronic disease states.¹²⁸ Activated macrophages are a major source of TNF-α; however, other activated cell types will upregulate TNF-α production.¹²⁸ TNF-α is produced as a membrane-bound surface protein, cleaved by metalloproteases and released in soluble form. TNF-α interacts with two known receptors, TNFR1 and TNFR2,¹²⁸ which are found on numerous cell types, suggesting that TNF-α mediates an array of effects.

In response to a stimulus, TNF-α concentrations peak quickly.²⁰⁰ Consequently, TNF-α may go undetected in some studies of inflammation, even after major surgical interventions.¹⁴ After release, TNF-α initiates production of proinflammatory cytokines (i.e., IL-6), reactive oxygen intermediates,¹⁷² chemotaxins, and endothelial adhesion molecules, resulting in invasion of cells at the site of inflammation.^43,72 TNF-α causes a wide range of additional effects, including activation of natural killer cells,²³⁹ proliferation of cytotoxic T-lymphocytes,¹²³ and T-cell apoptosis.²³⁵ These effects are inherently counterregulated in vivo by the release of tumor necrosis factor receptors from the cell surface. These solubilized receptors bind to TNF-α and effectively reduce the cytokine’s activity. Tumor necrosis factor soluble receptors are constitutively released in low numbers but increase in inflammatory conditions such as sepsis.

TNF-α release has both beneficial and deleterious consequences.⁹⁷ Administered experimentally, TNF-α results in classic signs of endotoxic shock, including hypotension, metabolic acidosis, and death.²²⁹ Inhibition of TNF-α activity is protective in endotoxic shock.²¹ Conversely, TNF-α is necessary for protection from mycobacterial infection,⁸¹ and blocking its activity increases mortality in septic human patients.⁸⁰ As a major initiator of inflammation, TNF-α has been directly linked to a number of diseases, leading to interest in TNF-α as a therapeutic target. Although steroids are known to inhibit production of TNF-α, a more focused approach has been applied to specific diseases.²⁰⁰ Commercially available anti-tumor necrosis factor monoclonal antibodies and recombinant tumor necrosis factor soluble receptors have proven efficacious in human patients with Crohn’s disease and rheumatoid arthritis.^12,165 It is interesting to note that therapeutic inhibition of TNF-α for rheumatoid arthritis has been associated with recrudescence of pulmonary mycobacterial infection and infectious complications after orthopedic surgeries.^23,81,92 In spite of these reports, the success of anticytokine therapy is believed to outweigh the risks.

Interleukin-1: The term interleukin-1(IL-1) denotes several cytokines produced by macrophages and other cell types.²⁰⁰ IL-1β is secreted as an inactive proform, which is cleaved by IL-1 converting enzyme, also known as caspase-1.²²⁶ However, genetically engineered mice deficient in IL-1 converting enzyme remain responsive to endotoxin, suggesting that redundancy exists between IL-1β and other interleukins. IL-1β complexes with a functional receptor called IL-1RI and a third component, the IL-1 receptor accessory protein (IL-1RAcP), to initiate cellular signaling pathways. Another member of the interleukin -1 family of cytokines, IL-1 receptor antagonist (IL-1ra), serves a counterregulatory function. IL-1ra is actually an antiinflammatory cytokine that competes with IL-1 for receptor sites. Genetically manipulated experimental mice deficient in IL-1ra show exaggerated inflammatory responses, illustrating its importance in IL-1 regulation. IL-1 demonstrates the intricacies of cytokine regulation that may involve several layers of control, including production, processing, receptor availability, and accessory proteins.

The proinflammatory functions of IL-1 are similar to those of TNF-α, and these cytokines often work synergistically to further enhance inflammation.172,179 In response to inflammatory stimuli, IL-1 mediates increases in production of proinflammatory cytokines, prostaglandins, and nitric oxide. These changes are manifest in host responses, including hypotension, fever, decreased white blood cell counts, hemorrhage, and pulmonary edema.^93,179 Competitive inhibition of the IL-1 receptor improves survival after experimental administration of endotoxin. As with tumor necrosis factor, IL-1 has been implicated in a number of inflammatory diseases, including sepsis, Crohn’s disease, and rheumatoid arthritis.

Interleukin-6: Interleukin-6 (IL-6) increases in virtually all inflammatory conditions. IL-6 is produced by macrophages, T-cells, epithelial cells, and enterocytes. It plays a pivotal role in initiating hepatic synthesis of the acute phase proteins185,200 and influences the proliferation of lymphocytes. In addition, IL-6 has a contradictory role in initiating compensatory responses by inducing antiinflammatory responses and downregulating proinflammatory cytokine production.^5,243 In inflammatory states, plasma IL-6 increases proportionately with the duration²¹⁶ and severity of the condition. After surgical trauma, plasma levels are higher with invasive procedures^58,86 as compared with laparoscopy.^126,232 IL-6 levels have been used to predict postoperative infection¹⁶¹ and mortality associated with sepsis.¹⁹⁷ IL-6 may also predict the possibility of recurrent abdominal adhesions.⁵⁰ Consequently, IL-6 is considered to be not only a mediator but also a diagnostic and prognostic biomarker of inflammation.

Chemokines: In acute inflammation, chemokines peak shortly after TNF-α and IL-1, along with IL-6. Chemokines are the chemotactic cytokines responsible for attraction of cells across a concentration gradient. More than 40 known chemokines are secreted by macrophages and endothelial and other cell types to recruit cells during embryonic development, wound healing, angiogenesis, and inflammatory responses.²⁰⁰ As with all cytokines, redundancy in cell specificity, receptor affinity, and function is noted among the chemokines.¹³ Therefore, chemokines are categorized into families according to structural placement of conserved cysteine residues (e.g., CXC chemokines have one amino acid separating two cysteine residues). Of the four chemokine families, CXC and CC chemokines contain members most actively involved in the proinflammatory response to trauma or infection. Within the CXC family, a subgroup carries an ELR moiety (glutamine-leucine-arginine), conferring the ability to attract neutrophils, while an ELR negative subgroup attracts mononuclear cells. Interleukin-8 (IL-8) is the archetypical neutrophil chemoattractant in the majority of mammals and, under the most recent nomenclature, is referred to as CXCL8.¹ It is noteworthy that rodents commonly used in inflammation research do not express IL-8/CXCL8 but have several functional counterparts. IL-8/CXCL8 attracts neutrophils, upregulates surface expression of adhesion molecules, triggers degranulation of proteases, and promotes production of other inflammatory mediators. As the inflammatory response continues, additional chemokines, such as monocyte chemoattractant protein-1 (MCP-1/CCL2) and macrophage inflammatory protein (MIP-1α/CCL3), participate in the recruitment of monocytes, promoting a transition from active to chronic phases of inflammation. Over time, cellular recruitment slows as chemokines are degraded by enzymes and further production slows.

Interleukin-10: Although antiinflammatory cytokines are numerous, interleukin-10 (IL-10) is the archetype. IL-10 is produced primarily by CD4+ Th-2 cells, monocytes, and B-cells.¹⁸⁰ It depresses the production of several proinflammatory cytokines and chemokines, including TNF-α, IL-1, IL-6, and IL-8, by inhibiting translocation of nuclear factor κB (NF-κB) and promoting degradation of messenger RNAs.^54,180 IL-10 downregulates Th-1 cytokines, which are protective during microbial infection,⁸ and plays a role in limiting inflammatory responses to normal gut-associated bacteria.¹⁸⁰ In addition, IL-10 promotes shedding of tumor necrosis factor receptors into the systemic circulation.¹²⁰ It also inhibits antigen presentation by macrophages and dendritic cells.¹⁸⁰ In a balanced immune response, IL-10 levels would be low during acute phase inflammation and would increase over time. IL-10 deficiencies have been reported in chronic inflammatory, autoimmune diseases and after transplantation surgeries, which may contribute to poor outcomes.¹⁸⁰ Conversely, exogenous IL-10 has been used to reduce intestinal inflammation in human patients with Crohn’s disease.⁸ However, excess IL-10 can increase susceptibility to microbial infection and may influence survival.¹³⁶ This illustrates that a fine balance of cytokines is necessary to ensure appropriate inflammatory responses.

Prostaglandins: Prostaglandins are produced in the cyclooxygenase pathway, where arachidonic acid metabolism is catalyzed by the enzymes cyclooxygenase (COX)-1 and COX-2. COX-1 is a constitutively expressed enzyme involved in homeostasis and present in the majority of mature cells. Expression of COX-2 is induced by trauma, growth factors, proinflammatory cytokines, and other mediators.88,195 Prostaglandins mediate many inflammatory responses primarily through G protein–coupled receptors on a number of cell types (Table 1-2).^29,166 Prostaglandins are chemotactic agents that cause recruitment of leukocytes. Prostaglandins also induce vasodilation and contribute to the pathogenesis of pain and fever during inflammation.²⁴¹ Aspirin and nonsteroidal antiinflammatory drugs (e.g., carprofen, indomethacin) inhibit the cyclooxygenase enzymes. Selective inhibition of the inducible COX-2 while sparing the constitutively produced COX-1 has received a great deal of attention. It was initially believed that inhibition of COX-1 caused gastric ulceration and thus should be spared. However, in clinical trials in humans, inhibition of COX-2 alone increased the risk of cardiovascular and cerebrovascular events,¹⁹⁵ probably through the as yet ill-defined role of COX-2 in vascular homeostasis.^103,124,228 In addition, COX-2 may actually help resolve acute inflammation and heal gastric ulcers. Thus, the use of selective COX-2 inhibitors for treating chronic inflammation has gone out of favor in human medicine.^27,195 However, no compelling evidence suggests that dogs develop cardiovascular events with COX-2 inhibitor use. This, combined with the decreased incidence of gastric ulceration, makes COX-2 selective agents a good option in dogs.⁵⁴

Leukotrienes: Leukotrienes are produced in the lipoxygenase pathway, where lipoxygenase enzymes act on arachidonic acid to form the major types of leukotrienes, LTB₄, and the peptidoleukotrienes (LTC₄, LTD₄, and LTE₄), which are proinflammatory modulators of leukocyte trafficking and blood flow (see Figure 1-3). Leukotrienes are primarily secreted by leukocytes but are also produced by platelets and endothelial cells.^29,82 LTB₄ is a potent chemotactic agent and an activator of neutrophils, potentiating their extravasation, degranulation, and production of free radicals.¹⁸⁹ The autocrine activity of LTB₄ on leukocytes results in cyclic production of leukotrienes during acute inflammation. In addition, the peptidoleukotrienes provoke vasoconstriction, bronchoconstriction, and increased venule permeability.^107,189 In general, leukotrienes are more potently vasoactive than histamine.¹⁰⁶ Consequently, agents that block leukotriene production or antagonize leukotriene receptors have been used to treat both inflammation and airway responsiveness associated with asthma.¹⁰⁷