Effect of nitric oxide synthase inhibition during group B streptococcal sepsis in neonatal piglets

The Neonatal Innate Immune Response to Sepsis: Checkpoint Proteins as Novel Mediators of This Response and as Possible Therapeutic/Diagnostic Levers

Sepsis, a dysfunctional immune response to infection leading to life-threatening organ injury, represents a significant global health issue. Neonatal sepsis is disproportionately prevalent and has a cost burden of 2-3 times that of adult patients. Despite this, no widely accepted definition for neonatal sepsis or recommendations for management exist and those created for pediatric patients are significantly limited in their applicability to this unique population. This is in part due to neonates' reliance on an innate immune response (which is developmentally more prominent in the neonate than the immature adaptive immune response) carried out by dysfunctional immune cells, including neutrophils, antigen-presenting cells such as macrophages/monocytes, dendritic cells, etc., natural killer cells, and innate lymphoid regulatory cell sub-sets like iNKT cells, γδ T-cells, etc. Immune checkpoint inhibitors are a family of proteins with primarily suppressive/inhibitory effects on immune and tumor cells and allow for the maintenance of self-tolerance. During sepsis, these proteins are often upregulated and are thought to contribute to the long-term immunosuppression seen in adult patients. Several drugs targeting checkpoint inhibitors, including PD-1 and PD-L1, have been developed and approved for the treatment of various cancers, but no such therapeutics have been approved for the management of sepsis. In this review, we will comparatively discuss the role of several checkpoint inhibitor proteins, including PD-1, PD-L1, VISTA, and HVEM, in the immune response to sepsis in both adults and neonates, as well as posit how they may uniquely propagate their actions through the neonatal innate immune response. We will also consider the possibility of leveraging these proteins in the clinical setting as potential therapeutics/diagnostics that might aid in mitigating neonatal septic morbidity/mortality.

Exploring Clinically-Relevant Experimental Models of Neonatal Shock and Necrotizing Enterocolitis

Neonatal shock and necrotizing enterocolitis (NEC) are leading causes of morbidity and mortality in premature infants. NEC is a life-threatening gastrointestinal illness, the precise etiology of which is not well understood, but is characterized by an immaturity of the intestinal barrier, altered function of the adaptive immune system, and intestinal dysbiosis. The complexities of NEC and shock in the neonatal population necessitate relevant clinical modeling using newborn animals that mimic the disease in human neonates to better elucidate the pathogenesis and provide an opportunity for the discovery of potential therapeutics. A wide variety of animal species-including rats, mice, piglets, and primates-have been used in developing experimental models of neonatal diseases such as NEC and shock. This review aims to highlight the immunologic differences in neonates compared with adults and provide an assessment of the advantages and drawbacks of established animal models of both NEC and shock using enteral or intraperitoneal induction of bacterial pathogens. The selection of a model has benefits unique to each type of animal species and provides individual opportunities for the development of targeted therapies. This review discusses the clinical and physiologic relevance of animal models and the insight they contribute to the complexities of the specific neonatal diseases: NEC and shock.

An Unsettled Promise: The Newborn Piglet Model of Neonatal Acute Respiratory Distress Syndrome (NARDS). Physiologic Data and Systematic Review

Despite great advances in mechanical ventilation and surfactant administration for the newborn infant with life-threatening respiratory failure no specific therapies are currently established to tackle major pro-inflammatory pathways. The susceptibility of the newborn infant with neonatal acute respiratory distress syndrome (NARDS) to exogenous surfactant is linked with a suppression of most of the immunologic responses by the innate immune system, however, additional corticosteroids applied in any severe pediatric lung disease with inflammatory background do not reduce morbidity or mortality and may even cause harm. Thus, the neonatal piglet model of acute lung injury serves as an excellent model to study respiratory failure and is the preferred animal model for reasons of availability, body size, similarities of porcine and human lung, robustness, and costs. In addition, similarities to the human toll-like receptor 4, the existence of intraalveolar macrophages, the sensitivity to lipopolysaccharide, and the production of nitric oxide make the piglet indispensable in anti-inflammatory research. Here we present the physiologic and immunologic data of newborn piglets from three trials involving acute lung injury secondary to repeated airway lavage (and others), mechanical ventilation, and a specific anti-inflammatory intervention via the intratracheal route using surfactant as a carrier substance. The physiologic data from many organ systems of the newborn piglet—but with preference on the lung—are presented here differentiating between baseline data from the uninjured piglet, the impact of acute lung injury on various parameters (24 h), and the follow up data after 72 h of mechanical ventilation. Data from the control group and the intervention groups are listed separately or combined. A systematic review of the newborn piglet meconium aspiration model and the repeated airway lavage model is finally presented. While many studies assessed lung injury scores, leukocyte infiltration, and protein/cytokine concentrations in bronchoalveolar fluid, a systematic approach to tackle major upstream pro-inflammatory pathways of the innate immune system is still in the fledgling stages. For the sake of newborn infants with life-threatening NARDS the newborn piglet model still is an unsettled promise offering many options to conquer neonatal physiology/immunology and to establish potent treatment modalities.

Effects of endothelin and nitric oxide on cardiac muscle functions in experimental septic shock model

We aimed to investigate the possible roles of nitric oxide (NO) and endothelin on the changes of cardiac muscle function in both hyper- and hypodynamic septic shock periods. Cecal ligation and puncture was performed in 50 Wistar albino rats to induce septic shock. Changes in atrium and right ventricle papillary muscle contractions, atrium beat rate, adrenergic and cholinergic responses in these tissues were evaluated in vitro. Atrium beat rate increased in hypodynamic period ( p < 0.001) that was reversed by bosentan ( p < 0.001) and NG-nitro-l-arginine methylester (l-NAME; p < 0.05). Atrium contractions decreased in both hyper- and hypodynamic periods ( p < 0.001) that were partially ameliorated by bosentan in both periods ( p < 0.01) and only in hypodynamic period by l-NAME ( p < 0.001). l-NAME increased papillary muscle contractions in both periods ( p < 0.01), but bosentan increased it only in hyperdynamic period ( p < 0.01). Bosentan and l-NAME increased potency of isoproterenol on atrium beat rate in both periods and increased carbachol potency on atrium beat rate and atrium contraction amplitude only in hypodynamic period. Bosentan increased atrium contraction response to isoproterenol in hypodynamic period ( p < 0.05). Papillary muscle contraction response to isoproterenol increased in hypodynamic period ( p < 0.05). l-NAME increased papillary muscle contraction response to carbachol in both periods ( p < 0.01, p < 0.05, respectively). These results show that NO and endothelin may play a role in positive inotropic and negative chronotropic effects for atrium in septic shock. Bosentan and l-NAME may change potency and efficacy of isoproterenol and carbachol via upregulation of adrenergic and cholinergic receptors and/or through post receptor factors.

DEVELOPMENT AND CHARACTERIZATION OF A NOVEL PORCINE MODEL OF NEONATAL SEPSIS

Sepsis and its sequela remain a major source of morbidity and mortality in neonates despite advances in antimicrobials and aggressive supportive care. Many models of neonatal sepsis have been developed for investigating the pathophysiology of this disease and application of therapy, and a model with an infectious focus is closer to clinical reality. To establish an animal model that mimics the clinical characteristics of neonatal sepsis, the cecal devascularization and perforation procedure was implemented on 15 mixed-strain newborn piglets, which produced an infectious focus that acted as a continuous source of microorganisms to the peritoneal cavity. The mean survival time in animals with sepsis was 10.4 h (range 5.5-17.9 h), whereas all of the sham-operated control animals survived more than 24 h. Animals with sepsis showed a gradual significant decrease in the mean systemic blood pressure (mSBP; 71 +/- 3 mmHg in sepsis vs. 64 +/- 3 mmHg in control at 3 h, 38 +/- 7 mmHg in sepsis vs. 59 +/- 4 mmHg in control at 6 h, mean +/- SEM). They also showed an increase of serum levels of endotoxin (5.6 x 10 +/- 4.5 x 10 pg/mL in sepsis vs. 6.0 x 10 +/- 3.8 x 10 pg/mL in control at 6 h). Serum levels of TNF-alpha in the animals with sepsis became significantly higher than the control animals at 0 h (96 +/- 31 pg/mL in sepsis vs. 12 +/- 1 pg/mL in control) and remained significantly higher than all through the experiment. Serum levels of IL-6 in animals with sepsis showed a gradual increase (484 +/- 231 pg/mL in sepsis in its peak at 6 h vs. 24 +/- 5 pg/mL in control), however, there were no significant differences in serum IL-10 levels between the groups. Microorganisms detected in the blood of animals with sepsis were gram-negative enteric and anaerobic organisms. These results suggested that this model mimics the clinical state of neonatal sepsis and hence may have significant implications for the treatment of sepsis, including its use as a model in further investigations.

Nitric oxide inhalation and nitric oxide synthase inhibitor supplement for endotoxin-induced hypotension

BACKGROUND

This study was performed to determine whether a combined therapy of nitric oxide (NO) inhalation and nitric oxide synthase (NOS) inhibitor is effective in experimental animals with endotoxin-induced refractive hypotension accompanied by pulmonary hypertension.

METHODS

Escherichia coli lipopolysaccharide (1 mg/kg) was administered to 10 newborn piglets to induce endotoxemia. The experiment then began 60 min later, when the systemic arterial pressure dropped. The inhalation of 20 p.p.m. NO at 60 and 120 min of endotoxemia created a control group. Another group was also administered N w-nitro-L-arginine (L-NNA; 5 mg) after the first NO inhalation at 60 min of endotoxemia (the L-NNA group). Pulmonary arterial pressure, systemic arterial pressure and cardiac output were measured and compared among the groups.

RESULTS

Three of the 5 piglets in the control group died of hypotensive shock, while in the L-NNA group the systemic arterial pressure recovered to pre-endotoxin administration levels. The L-NNA group produced a further increase in pulmonary arterial pressure against which NO inhalation was effective.

CONCLUSION

Nitric oxide inhalation alone carries a potential risk of further lowering systemic arterial pressure in a piglet with hypotension induced by endotoxin, whereas the combined therapy resulted in the recovery of the blood pressure to pre-endotoxin levels. The combined therapy was simultaneously effective against pulmonary hypertension.

The hemodynamic effects of inhaled nitric oxide and endogenous nitric oxide synthesis blockade in newborn piglets during infusion of heat-killed group B streptococci

OBJECTIVE

To determine the effects of therapy with inhaled nitric oxide (NO) gas and partial or complete blockade of endogenous NO synthesis with N(omega)nitro-L-arginine (L-NA) on the hemodynamic responses to group B streptococci infusion in newborn piglets.

DESIGN

Randomized, acute intervention study.

SETTING

Animal research laboratory.

SUBJECTS

Twenty-five anesthetized piglets younger than 3 days of age divided into five groups.

INTERVENTIONS

Heat-killed group B streptococci (GBS) were infused systemically until a 50% increase in pulmonary artery pressure (PAP) was obtained, and the infusion was continued for another 2 hrs. The five groups were designed as follows: group 1, sepsis control: continuous GBS infusion, with two brief trials (10 mins) of inhaled NO given after the initial development of pulmonary hypertension and again 2 hrs later; group 2, continuous inhaled NO: NO was given at 40 ppm for 2 hrs during GBS infusion; group 3, high-dose L-NA pretreatment: 10 mg/kg L-NA bolus followed by 1 mg/kg/min before, and continuing throughout, GBS infusion; group 4, high-dose L-NA: same dose as in group 3, but given after the start of the GBS infusion with continuous inhaled NO at 40 ppm; and group 5, low-dose L-NA: 3 mg/kg bolus given after start of GBS infusion with continuous inhaled NO at 40 ppm.

MEASUREMENTS AND MAIN RESULTS

The sepsis controls, group 1, had an increase in PAP, which took 15-45 mins to develop, from a mean of 3.4 (SD 0.7) to 5.9 (1.9) kPa (p < .05), at which time the cardiac index had decreased from 169 (28) to 146 (46) mL/kg/min (p < .05). Brief inhaled NO during the early phase decreased PAP to normal. Two hours later, PAP had increased to 6.1 (0.2) kPa and cardiac index had decreased to 88 (31) mL/kg/min. Inhaled NO after 2 hrs decreased PAP to 3.2 (0.5) kPa and increased cardiac index to 106 (44) ml/kg/min (p < .05). Continuous inhaled NO (group 2) ameliorated the deterioration in cardiac index, which at 2 hrs was 140 (30) mL/kg/min (significantly greater than in the sepsis controls) (p < .05). The L-NA-pretreated animals (group 3) had a greater increase in PAP and pulmonary vascular resistance index when GBS infusion was started. PAP increased from 3.0 (0.7) to 7.3 (1.5) kPa within 15 mins, and cardiac index simultaneously decreased to 68 (20) mL/kg/min. Cardiac index subsequently rapidly deteriorated to 48 (21) mL/kg/min, and only one of five animals survived for 2 hrs. Group 4 animals also developed a rapid deterioration in cardiac output, and only two of five survived for 2 hrs. Group 5 animals had results indistinguishable from group 2 animals.

CONCLUSION

Pulmonary hypertension and shock resulting from GBS infusion in newborn piglets are much worse if endogenous NO production is completely inhibited. Continuous inhaled NO with or without low-dose L-NA inhibits the decrease in cardiac output.

Models of persistent pulmonary hypertension of the newborn (PPHN) and the role of cyclic guanosine monophosphate (GMP) in pulmonary vasorelaxation

At birth, a marked decrease in pulmonary vascular resistance allows the lung to establish gas exchange. Persistent pulmonary hypertension of the newborn (PPHN) occurs when this normal adaptation of gas exchange does not occur. We review animal models used to study the pathogenesis and treatment of PPHN. Both acute models, such as acute hypoxia and infusion of vasoconstrictors, and chronic models of PPHN created both before and immediately after birth are described. Inhaled nitric oxide is an important emerging therapy for PPHN. We review nitric oxide receptor mechanisms, including soluble guanylate cyclase, which produces cGMP when stimulated by nitric oxide, and phosphodiesterases, which control the intensity and duration of cGMP signal transduction. A better understanding of these mechanisms of regulation of vascular tone may lead to safer use of nitric oxide and improved clinical outcomes.

Haemodynamic effects of dopexamine and nitric oxide synthase inhibition in healthy and endotoxaemic sheep

Chronically instrumented awake healthy sheep (n = 6) received the synthetic catecholamine, dopexamine, during or without a background infusion of the nitric oxide synthase inhibitor. L-nitro-arginine-methylester (L-NAME). Three days later, hypotensive-hyperdynamic circulation was induced and maintained by continuous infusion of Salmonella typhosa endotoxin (10 ng/kg per min). After 24 h of continuous endotoxin infusion, the dopexamine L-NAME protocol was repeated. In healthy and endotoxaemic animals with and without nitric oxide synthase inhibition dopexamine caused the same haemodynamic changes: heart rate and cardiac output increased, mean arterial pressure and systemic vascular resistance decreased. L-NAME infusion induced normalisation of the hypotonic-hyperdynamic circulation in endotoxaemic animals. Dopexamine reduced some adverse effects of L-NAME treatment, like increased pulmonary vascular resistance and decreased oxygen delivery. In conclusion the haemodynamic effects of dopexamine are independent of the amount of nitric oxide production. Dopexamine may attenuate some of the adverse effects of nitric oxide synthase inhibition.

Nitric oxide contributes to surfactant-induced vasodilation in surfactant-depleted newborn piglets

To investigate whether nitric oxide (NO) is involved in surfactant-induced systemic and pulmonary vasodilatation in newborn piglets with surfactant deficiency, 2-6-d-old piglets were subjected to repeated saline lung lavages. They were then randomly assigned to one of two groups (seven in each group): the N(omega)-nitro-L-arginine methyl ester (L-NAME) group received 3 mg/kg L-NAME i.v. 45 min before endotracheal instillation of 200 mg/kg porcine surfactant; the saline group received saline i.v. at the same time point, and instillation of 200 mg/kg surfactant. Mean arterial blood pressure, systemic vascular resistance, pulmonary arterial pressure, and pulmonary vascular resistance increased significantly after injection of L-NAME (all p < 0.01), whereas the cardiac index decreased significantly (p < 0.05). Saline injection did not change any variable. Significant decreases in mean arterial blood pressure (from a mean +/- SD of 66 +/- 10 to 53 +/- 9 mm Hg, p < 0.01), pulmonary arterial pressure (from 29 +/- 6 to 23 +/- 6 mm Hg, p < 0.01), and systemic vascular resistance (from 0.40 +/- 0.13 to 0.33 +/- 0.12 mm Hg/mL/min/kg, p < 0.05) were observed only in the saline group after surfactant instillation, whereas the decrease in pulmonary vascular resistance was not significant after surfactant instillation (p = 0.06). In contrast to the saline group, these variables were not modified in the L-NAME group after surfactant instillation. We conclude that the vasodilatory effect of porcine surfactant instillation in newborn piglets with surfactant deficiency is associated with activation of NO synthase.

Effects of group B Streptococcus on the responses to U46619, endothelin-1, and noradrenaline in isolated pulmonary and mesenteric arteries of piglets

The release of endogenous vasoconstrictors together with changes in the vascular responses are central to the pathophysiology of sepsis. The effects of in vitro incubation for 20 h with heat-killed group B Streptococcus (GBS, 3 x 10(7) colony-forming units mL-1) on the vasoconstrictor responses to noradrenaline (NA, 10(-8) to 10(-4) M), the thromboxane A2 analog 9,11-dideoxy-11 alpha, 9 alpha-epoxymethanoprostaglandin F2 alpha (U46619; 10(-10) M to 10(-6) M) and endothelin-1 (ET-1, 10(-11) to 3 x 10(-9) M) were evaluated on isolated intrapulmonary and mesenteric arteries from 10-17-d-old piglets. The incubation with GBS reduced the maximal contractile response to NA and ET-1 (p < 0.01) in both arteries. The nitric oxide (NO) synthase (NOS) inhibitor N omega-nitro-L-arginine methyl ester (L-NAME; 10(-4) M) completely reversed this hyporesponsiveness. GBS-treated mesenteric arteries also showed a significant reduction of the maximal contractions induced by U46619 (p < 0.05) and this effect was inhibited by 10(-4) M L-NAME. In contrast, the maximal contractile responses to U46619 were similar in control and in GBS-treated pulmonary arteries. Addition of L-NAME did not modify the contractile responses to U46619 in GBS-treated pulmonary arteries. In conclusion, GBS-treated systemic arteries from neonatal piglets showed decreased responses to NA, U46619, and ET-1 due to enhanced NO release. GBS-treated pulmonary arteries also exhibited decreased responses to NA and ET-1 but not to U46619. Induction of NOS in vascular smooth muscle may play a key role in the hypotension and loss of systemic vascular responsiveness that occurs in GBS sepsis. The absence of pulmonary hyporesponsiveness to U46619 may partially explain the coexistence during sepsis of pulmonary hypertension and lung NOS induction.

Effect of methylene blue on refractory neonatal hypotension

Excess nitric oxide is a mediator of the hypotension in septic shock. Nitric oxide dilates vascular smooth muscle through activation of soluble guanylate cyclase. We report the increase in blood pressure caused by methylene blue (MB), a soluble guanylate cyclase inhibitor, in five neonates with presumed septic shock unresponsive to colloids, inotropic agents, and corticosteroids. MB was given intravenously at a dose of 1 mg/kg during a 1-hour period. MB increased blood pressure in each patient (average, 33% +/- 20%). Blood pressure subsequently decreased to near baseline values in three patients, who then received a second infusion of MB. Blood pressure again increased in these patients. Three of five patients were weaned from inotropic support within 72 hours. Three of five patients survived and were discharged home. We suggest that MB increased blood pressure in these neonates with refractory hypotension.

Interaction between ATP-sensitive K+ channels and nitric oxide on pial arterioles in piglets

The interaction between ATP-sensitive K+ channels (KATP) and nitric oxide (NO) was studied in pial arterioles of piglets. We examined the effects of N omega-nitro-L-arginine methyl ester (L-NAME), a general inhibitor of nitric oxide synthase (NOS), and 7-nitroindazole (7-NI), a selective inhibitor of neuronal NOS, on aprikalim-induced cerebral vasodilation. Topically applied, aprikalim, a selective activator of KATP, dilated arterioles by 11 +/- 7% at 10(-8) M and 17 +/- 6% at 10(-6) M. After L-NAME treatment (15 mg/kg, i.v.), the response was reduced (4 +/- 4% and 12 +/- 7%, respectively; n = 8, p < 0.05). Administration of 7-NI (50 mg/kg, i.p.) did not change pial arteriolar responsiveness to aprikalim. However, both L-NAME and 7-NI reduced the vasodilator responses to 10(-4) M N-methyl-D-aspartate (NMDA) (by 73% and by 36%, respectively). Furthermore, 7-NI treatment abolished the glutamate-induced dilatation of pial arterioles. Administration of L-NAME reduced the NOS activity in the cerebral cortex by 88%, whereas the reduction after the 7-NI treatment was 44%. Pre-treatment and coadministration of 10(-5) M glibenclaminde, a specific inhibitor of KATP or L-NAME administration, did not change the dilatory response to sodium nitroprusside. We conclude that NO may be involved in aprikalim-induced dilation of pial arterioles.

Production of pulmonary vasodilation by tolazoline, independent of nitric oxide production in neonatal lambs

OBJECTIVE

To determine whether tolazoline reduces pulmonary vascular resistance (PVR) by means of endogenous nitric oxide production.

DESIGN

Thirty newborn lambs (2 to 7 days of age) were anesthetized with pentobarbital, and their lungs were ventilated through an endotracheal tube. Intravascular catheters were placed in the left ventricle, descending aorta, right atrium, and pulmonary artery for continuous monitoring of intravascular pressures. Cardiac output was measured with radiolabeled microspheres. Arterial carbon dioxide pressure and pH were maintained in a normal range throughout the experiments. Animals were randomly assigned to the following groups: group 1, lungs ventilated with a hypoxic gas mixture and administered tolazoline; group 2, given N omega-nitro-L-arginine (L-NA) (5 mg/min intravenously for 60 minutes) and tolazoline; group 3, given L-NA with hypoxia and tolazoline. Acetylcholine (0.5 microgram/kg) was injected into the right atrium to assess pulmonary nitric oxide synthase activity before and after the L-NA infusion. Data were analyzed by analysis of variance.

RESULTS

L-NA inhibited the acetylcholine-induced reduction in mean pulmonary artery pressure (MPAP) by more than 75%. Hypoxia and L-NA increased both MPAP and PVR. Tolazoline produced immediate reductions in both MPAP and PVR in all three groups (group 1, 27% +/- 3% and 50% +/- 5%; group 2, 34% +/- 5% and 50% +/- 6%; and group 3, 31% +/- 4% and 46% +/- 5%, respectively).

CONCLUSIONS

These results suggest that tolazoline produces vasodilation independent of nitric oxide production. Understanding the mechanism by which tolazoline produces pulmonary vasodilation may provide insight into the clinical use of this drug and information regarding other potential endogenous mediators of pulmonary vasomotor tone in the neonate.