Effects of neurokinin receptor antagonists in virus-infected airways

Substance P—Friend or Foe

Substance P (SP), a neuropeptide and pain transmitter has multiple roles and is involved in various processes in the body [...]

Eosinophil and airway nerve interactions in asthma

Airway eosinophils are increased in asthma and are especially abundant around airway nerves. Nerves control bronchoconstiction and in asthma, airway hyperreactivity (where airways contract excessively to inhaled stimuli) develops when eosinophils alter both parasympathetic and sensory nerve function. Eosinophils release major basic protein, which is an antagonist of inhibitory M₂ muscarinic receptors on parasympathetic nerves. Loss of M₂ receptor inhibition potentiates parasympathetic nerve-mediated bronchoconstriction. Eosinophils also increase sensory nerve responsiveness by lowering neurons' activation threshold, stimulating nerve growth, and altering neuropeptide expression. Since sensory nerves activate parasympathetic nerves via a central neuronal reflex, eosinophils' effects on both sensory and parasympathetic nerves potentiate bronchoconstriction. This review explores recent insights into mechanisms and effects of eosinophil and airway nerve interactions in asthma.

Role of Substance P Neuropeptide in Inflammation, Wound Healing, and Tissue Homeostasis

Substance P (SP) is an undecapeptide present in the CNS and the peripheral nervous system. SP released from the peripheral nerves exerts its biological and immunological activity via high-affinity neurokinin 1 receptor (NK1R). SP is also produced by immune cells and acts as an autocrine or paracrine fashion to regulate the function of immune cells. In addition to its proinflammatory role, SP and its metabolites in combination with insulin-like growth factor-1 are shown to promote the corneal epithelial wound healing. Recently, we showed an altered ocular surface homeostasis in unmanipulated NK1R^-/- mice, suggesting the role of SP-NK1R signaling in ocular surface homeostasis under steady-state. This review summarizes the immunobiology of SP and its effect on immune cells and immunity to microbial infection. In addition, the effect of SP in inflammation, wound healing, and corneal epithelial homeostasis in the eye is discussed.

Role of TNF-α in virus-induced airway hyperresponsiveness and neuronal M₂ muscarinic receptor dysfunction

BACKGROUND AND PURPOSE

Infections with respiratory viruses induce exacerbations of asthma, increase acetylcholine release and potentiate vagally mediated bronchoconstriction by blocking inhibitory M₂ muscarinic receptors on parasympathetic neurons. Here we test whether virus-induced M₂ receptor dysfunction and airway hyperresponsiveness are tumour necrosis factor-alpha (TNF-α) dependent.

EXPERIMENTAL APPROACH

Guinea pigs were pretreated with etanercept or phosphate-buffered saline 24 h before intranasal infection with parainfluenza. Four days later, pulmonary inflation pressure, heart rate and blood pressure were measured. M₂ receptor function was assessed by the potentiation by gallamine (an M₂ receptor antagonist) of bronchoconstriction caused by electrical stimulation of the vagus nerves and measured as increased pulmonary inflation pressure. Human airway epithelial cells were infected with influenza and TNF-α concentration in supernatant was measured before supernatant was applied to human neuroblastoma cells. M₂ receptor expression in these neuroblastoma cells was measured by qRT-PCR.

KEY RESULTS

Influenza-infected animals were hyperresponsive to vagal stimulation but not to intravenous ACh. Gallamine did not potentiate vagally induced bronchoconstriction in virus-infected animals, indicating M₂ receptor dysfunction. Etanercept prevented virus-induced airway hyperresponsiveness and M₂ receptor dysfunction, without changing lung viral titres. Etanercept caused a non-significant decrease in total cells, macrophages and neutrophils in bronchoalveolar lavage. Influenza infection significantly increased TNF-α release from isolated epithelial cells, sufficient to decrease M₂ receptors in neuroblastoma cells. This ability of supernatants from infected epithelial cells to inhibit M₂ receptor expression was blocked by etanercept.

CONCLUSIONS AND IMPLICATIONS

TNF-α is a key mediator of virus-induced M₂ muscarinic receptor dysfunction and airway hyperresponsiveness.

Acute and chronic cough

Individuals are generally content to self-medicate for acute cough. It is only when the cough becomes persistent that they seek medical assistance. It is not known why patients cough in association with an acute upper respiratory tract infection (URTI), although interest has focused on how viruses may influence airway sensory nerve function and contribute to heightened cough reflex sensitivity. Why some patients develop a persistent cough following a viral URTI is also unclear. Much more is known about the causes and aggravants of chronic cough although there is no broad agreement as to the best way to manage such patients. Asthma, upper gastrointestinal dysfunction and rhinitis are frequently associated with chronic cough, although the impact of cough in suppurative lung disease and interstitial lung fibrosis is rarely considered. The development of effective treatments for cough remains a challenge and will require co-operation between clinicians, scientists and the pharmaceutical industry.

The role of substance P in inflammatory disease

The diffuse neuroendocrine system consists of specialised endocrine cells and peptidergic nerves and is present in all organs of the body. Substance P (SP) is secreted by nerves and inflammatory cells such as macrophages, eosinophils, lymphocytes, and dendritic cells and acts by binding to the neurokinin-1 receptor (NK-1R). SP has proinflammatory effects in immune and epithelial cells and participates in inflammatory diseases of the respiratory, gastrointestinal, and musculoskeletal systems. Many substances induce neuropeptide release from sensory nerves in the lung, including allergen, histamine, prostaglandins, and leukotrienes. Patients with asthma are hyperresponsive to SP and NK-1R expression is increased in their bronchi. Neurogenic inflammation also participates in virus-associated respiratory infection, non-productive cough, allergic rhinitis, and sarcoidosis. SP regulates smooth muscle contractility, epithelial ion transport, vascular permeability, and immune function in the gastrointestinal tract. Elevated levels of SP and upregulated NK-1R expression have been reported in the rectum and colon of patients with inflammatory bowel disease (IBD), and correlate with disease activity. Increased levels of SP are found in the synovial fluid and serum of patients with rheumatoid arthritis (RA) and NK-1R mRNA is upregulated in RA synoviocytes. Glucocorticoids may attenuate neurogenic inflammation by decreasing NK-1R expression in epithelial and inflammatory cells and increasing production of neutral endopeptidase (NEP), an enzyme that degrades SP. Preventing the proinflammatory effects of SP using tachykinin receptor antagonists may have therapeutic potential in inflammatory diseases such as asthma, sarcoidosis, chronic bronchitis, IBD, and RA. In this paper, we review the role that SP plays in inflammatory disease.

Virus-induced asthma attacks

Viral infections are responsible for the majority of asthma attacks in both children and adults. Vagally mediated reflex bronchoconstriction is potentiated due to loss of function of inhibitory M2 muscarinic receptors on the airway parasympathetic nerves. Multiple mechanisms are involved. Production of interferons may down regulate the expression of the M2 receptor gene. This effect is reversed by steroids. In allergic animals (and perhaps in atopic humans) eosinophils are recruited to the airway nerves, where they are activated, releasing major basic protein, which binds to the M2 receptors, blocking their function. Despite these negative physiological consequences of eosinophil activation, eosinophils are capable of exerting a potent antiviral effect. Thus, the inflammatory response to viral infections may have positive, as well as negative, consequences.

Role of macrophages in virus-induced airway hyperresponsiveness and neuronal M2 muscarinic receptor dysfunction

Viral infections exacerbate asthma. One of the pathways by which viruses trigger bronchoconstriction and hyperresponsiveness is by causing dysfunction of inhibitory M(2) muscarinic receptors on the airway parasympathetic nerves. These receptors normally limit acetylcholine (ACh) release from the parasympathetic nerves. Loss of M(2) receptor function increases ACh release, thereby increasing vagally mediated bronchoconstriction. Because viral infection causes an influx of macrophages into the lungs, we tested the role of macrophages in virus-induced airway hyperresponsiveness and M(2) receptor dysfunction. Guinea pigs infected with parainfluenza virus were hyperresponsive to electrical stimulation of the vagus nerves but not to intravenous ACh, indicating that hyperresponsiveness was due to increased release of ACh from the nerves. In addition, the muscarinic agonist pilocarpine no longer inhibited vagally induced bronchoconstriction, indicating M(2) receptor dysfunction. Treating animals with liposome-encapsulated dichloromethylene-diphosphonate depleted macrophages as assessed histologically. In these animals, viral infection did not cause airway hyperresponsiveness or M(2) receptor dysfunction. These data suggest that macrophages mediate virus-induced M(2) receptor dysfunction and airway hyperresponsiveness.

Dexamethasone prevents virus-induced hyperresponsiveness via multiple mechanisms

In the lungs, neuronal M2 muscarinic receptors inhibit acetylcholine release from the parasympathetic nerves. Parainfluenza virus infection causes loss of M2 receptor function, which increases acetylcholine release and vagally mediated bronchoconstriction. Because glucocorticoids are known to inhibit airway hyperresponsiveness, we tested whether dexamethasone (6.5 or 65 microg/kg i.p.) prevents virus-induced hyperresponsiveness and M2 receptor dysfunction in guinea pigs. In controls, pilocarpine, a muscarinic agonist, inhibited vagally induced bronchoconstriction, demonstrating functional M2 receptors. However, in virus-infected animals, pilocarpine failed to inhibit vagally induced bronchoconstriction, demonstrating M2 receptor dysfunction. Frequency-dependent bronchoconstriction was greater in virus-infected animals than in controls, indicating airway hyperresponsiveness. Low-dose dexamethasone (6.5 microg/kg i.p.) treatment prevented virus-induced airway hyperresponsiveness, ameliorated M2 receptor dysfunction, and decreased viral content in the lungs without inhibiting virus induced inflammation. High-dose dexamethasone (65 microg/kg i.p.) prevented virus-induced hyperresponsiveness, completely reversed M2 receptor dysfunction, decreased viral titers, and decreased virus-induced inflammation. This high-dose dexamethasone also increased M2 receptor function in uninfected animals. In conclusion, dexamethasone prevented virus-induced hyperresponsiveness and M2 receptor dysfunction via multiple mechanisms.

Reduced CTL response and increased viral burden in substance P receptor-deficient mice infected with murine gamma-herpesvirus 68

One component of the protective host response against mucosal pathogens includes the local production and increased expression of certain neuropeptides and their receptors. The present study further demonstrates this fact by investigating the contribution that substance P receptor expression makes toward immunity against a gamma-herpesvirus infection. Following intragastric inoculation with murine gamma-herpesvirus 68 (gamma HV-68), expression of substance P and its receptor was increased in mucosal and peripheral lymphoid organs in wild-type strains of mice. These results suggested that this receptor/ligand pair might be an important component of the host response against this viral infection. Such a hypothesis was supported by the demonstration that mice, genetically deficient in substance P receptor expression, showed an increased viral burden when compared with syngeneic C57BL/6 mice. Furthermore, substance P receptor-deficient mice showed a reduced CTL response against gamma HV-68, suggesting a mechanism to explain this increased viral burden. Such limitations in the Ag-specific CTL response in substance P receptor-deficient mice could result from lowered expression of IL-12 during viral infection. Consistent with this hypothesis, increases in mRNA encoding IL-12 and secretion of this cytokine into sera of infected, wild-type animals were markedly reduced in substance P receptor-deficient mice. These studies demonstrate that genetic elimination of substance P receptors in mice results in an increased gamma-herpesvirus burden and an altered host response.

Expression of tachykinins in nonnociceptive vagal afferent neurons during respiratory viral infection in guinea pigs

Immunohistochemistry was combined with retrograde labeling to characterize the effect of respiratory infection with Sendai virus on the number of Substance P/Neurokinin A-containing vagal afferent neurons whose cell bodies resided in the nodose ganglia and whose receptive fields were located in guinea pig trachea. Of the neurons labeled from the trachea of vehicle-inoculated guinea pigs, few stained positively for Substance P/Neurokinin A (approximately 3% of total labeled neurons). These neurons had small diameter cell bodies (mode = 16-20 microm), a feature of nociceptive-like C-fibers. Viral infection (Day 4 after inoculation) was associated with a significantly greater number of labeled neurons containing Substance P/Neurokinin A (approximately 20% of total labeled neurons). The majority of these had a relatively large cell body diameter (mode = 36- 40 microm), a feature of nonnociceptive afferent neurons. This induction appeared to be reversible as there were significantly fewer Substance P/Neurokinin A positive neurons in nodose ganglia from virus-inoculated guinea pigs at Day 28 after inoculation, a time point when virus-induced airway inflammation had all but resolved. These findings support the hypothesis that viral infection leads to a qualitative change in the vagal afferent innervation of guinea pig airways such that both small diameter nociceptive-like neurons and large diameter nonnociceptive neurons express tachykinins.

Decreased pulmonary and tracheal smooth muscle expression and activity of type 1 nitric oxide synthase (nNOS) after ovalbumin immunization and multiple aerosol challenge in guinea pigs

Pharmacological evidence supports a role of a transient decreased endogenous nitric oxide (NO) synthesis in ovalbumin (OVA)-induced early airway hyperresponsiveness in guinea pigs. However, no data are available regarding the expression and activity of the constitutive NO synthases (cNOS; NOS1 and NOS3, nNOS and eNOS, respectively) in this model. Therefore, we evaluated cNOS activity (conversion of L-[3H]arginine to L-[3H]citrulline in the presence of Ca2+ and calmodulin), nitrate and nitrite (NOx) concentration (modified Griess method), and NOS1 and NOS3 protein expression (Western blot) in lung homogenates and in the tracheal smooth muscle from OVA-immunized and multiple aerosol-challenged guinea pigs (six challenges, once daily). The expression and activity of the inducible NOS isoform (NOS2), the levels of exhaled NO, and the in vivo airway reactivity were also determined. Constitutive NOS activity and NO(x) concentration were significantly lower 6 h after the last OVA challenge as compared with saline exposure, being similar at 24 h. Expression of NOS1 paralleled cNOS activity, which was reduced 6, but not 24 h after OVA challenge. The decrease in NOS1 expression was accompanied by a significant decrease in the amounts of exhaled NO and by a maximal airway hyperresponsiveness to histamine. The levels of NOS3 were not modified at the two time points evaluated, and no NOS2 expression and activity were found at any time point. Similar modifications were observed in the tracheal smooth muscle. We conclude that OVA stimulation in immunized guinea pigs induced a transient reduction in NOS1 protein expression and activity in the respiratory system, which probably participates in airway hyperresponsiveness.

Tachykinin receptor antagonists: potential in airways diseases

Several lines of evidence indicate a role for the tachykinin peptides in airways diseases. For instance, elevated levels of tachykinins have been recovered from the airways of patients with asthma and chronic obstructive pulmonary disease (COPD), and airway inflammation leads to an upregulation of the tachykinin NK1 and NK2 receptors. Recent advances in tachykinin receptor pharmacology have allowed a more detailed analysis of this system and preclinical animal studies have indicated a role for the NK1 and NK2 receptors in bronchoconstriction, airway hyperresponsiveness and airway inflammation caused by allergic and nonallergic stimuli. In the past three years, work has entered the clinic and selective or dual-selective NK1/NK2 receptor antagonists appear to have the potential to affect the different aspects of asthma and COPD.