Role of activin in bacterial infections: a potential target for immunointervention?

Activin A levels are raised during human tuberculosis and blockade of the activin signaling axis influences murine responses to M. tuberculosis infection

Activin A strongly influences immune responses; yet, few studies have examined its role in infectious diseases. We measured serum activin A levels in two independent tuberculosis (TB) patient cohorts and in patients with pneumonia and sarcoidosis. Serum activin A levels were increased in TB patients compared to healthy controls, including those with positive tuberculin skin tests, and paralleled severity of disease, assessed by X-ray scores. In pneumonia patients, serum activin A levels were also raised, but in sarcoidosis patients, levels were lower. To determine whether blockade of the activin A signaling axis could play a functional role in TB, we harnessed a soluble activin type IIB receptor fused to human IgG1 Fc, ActRIIB-Fc, as a ligand trap in a murine TB model. The administration of ActRIIB-Fc to Mycobacterium tuberculosis-infected mice resulted in decreased bacterial loads and increased numbers of CD4 effector T cells and tissue-resident memory T cells in the lung. Increased frequencies of tissue-resident memory T cells corresponded with downregulated T-bet expression in lung CD4 and CD8 T cells. Altogether, the results suggest a disease-exacerbating role of ActRIIB signaling pathways. Serum activin A may be useful as a biomarker for diagnostic triage of active TB or monitoring of anti-tuberculosis therapy.

IMPORTANCE

Tuberculosis remains the leading cause of death by a bacterial pathogen. The etiologic agent of tuberculosis, Mycobacterium tuberculosis, can remain dormant in the infected host for years before causing disease. Significant effort has been made to identify biomarkers that can discriminate between latently infected and actively diseased individuals. We found that serum levels of the cytokine activin A were associated with increased lung pathology and could discriminate between active tuberculosis and tuberculin skin-test-positive healthy controls. Activin A signals through the ActRIIB receptor, which can be blocked by administration of the ligand trap ActRIIB-Fc, a soluble activin type IIB receptor fused to human IgG1 Fc. In a murine model of tuberculosis, we found that ActRIIB-Fc treatment reduced mycobacterial loads. Strikingly, ActRIIB-Fc treatment significantly increased the number of tissue-resident memory T cells. These results suggest a role for ActRIIB signaling pathways in host responses to Mycobacterium tuberculosis and activin A as a biomarker of ongoing disease.

Activin A increases phagocytosis of Escherichia coli K1 by primary murine microglial cells activated by toll-like receptor agonists

Background

Bacterial meningitis is associated with high mortality and long-term neurological sequelae. Increasing the phagocytic activity of microglia could improve the resistance of the CNS against infections. We studied the influence of activin A, a member of the TGF-β family with known immunoregulatory and neuroprotective effects, on the functions of microglial cells in vitro.

Methods

Primary murine microglial cells were treated with activin A (0.13 ng/ml–13 μg/ml) alone or in combination with agonists of TLR2, 4, and 9. Phagocytosis of Escherichia coli K1 as well as release of TNF-α, IL-6, CXCL1, and NO was assessed.

Results

Activin A dose-dependently enhanced the phagocytosis of Escherichia coli K1 by microglial cells activated by agonists of TLR2, 4, and 9 without further increasing NO and proinflammatory cytokine release. Cell viability of microglial cells was not affected by activin A.

Conclusions

Priming of microglial cells with activin A could increase the elimination of bacteria in bacterial CNS infections. This preventive strategy could improve the resistance of the brain to infections, particularly in elderly and immunocompromised patients.

Activin A regulates activation of mouse neutrophils by Smad3 signalling

Activin A, a member of the transforming growth factor beta superfamily, acts as a pro-inflammatory factor in acute phase response, and influences the pathological progress of neutrophil-mediated disease. However, whether activin A can exert an effect on the activities of neutrophils remains unclear. In this study, we found that the release of activin A was enhanced from neutrophils of mouse when stimulated with lipopolysaccharide. Furthermore, neutrophils were not only the source of activin A but also the target cells in response to activin A, in which canonical activin signalling components existed, and levels of ACTRIIA, SMAD3 and p-SMAD3 proteins were elevated in activin A-treated neutrophils. Next, the role of activin A was determined in regulation of neutrophils activities. Our data revealed that activin A induced O₂⁻ release and reactive oxygen species production, promoted IL-6 release, and enhanced phagocytosis, but failed to attract neutrophils migrating across the trans-well membrane. Moreover, we found that effect of activin A on IL-6 release from the peritoneal neutrophils of mouse was significantly attenuated by in vivo Smad3 knockdown. In summary, these data demonstrate that activin A can exert an effect on neutrophils activation in an autocrine/paracrine manner through Smad3 signalling, suggesting that activin A is an important regulator of neutrophils.

Imbalance of Amniotic Fluid Activin-A and Follistatin in Intraamniotic Infection, Inflammation, and Preterm Birth

CONTEXT

Microbial invasion of the amniotic fluid (AF) cavity stimulates an inflammatory response that involves activin-A, a pleiotropic mediator member of the TGFβ superfamily involved in connective tissue remodeling. The role of AF follistatin, a natural inhibitor of activin-A, in inflammation-induced preterm birth (PTB), has yet to be determined.

OBJECTIVE

The objective of the study was to investigate the relationships between AF activin-A and follistatin in physiological gestation and in pregnancies complicated by PTB and to evaluate a possible role played by the activin-A-follistatin balance in processes leading to PTB and preterm premature rupture of membranes (PPROM).

STUDY DESIGN

The AF levels of total activin-A and follistatin were immunoassayed in 168 women with a normal pregnancy outcome or PTB with and without intraamniotic inflammation or PPROM. The impact of the activin-A-follistatin imbalance on PTB terminal effector pathways (prostaglandins [prostaglandin E2, prostaglandin F2α] and matrix metalloproteinases [MMP-1, MMP-2, MMP-3, and MMP-9]) was investigated in an amniochorion explant system challenged with lipopolysaccharide (LPS) to mimic inflammation.

RESULTS

AF follistatin and the activin-A to follistatin ratio varied with gestational age, both decreasing toward term (P < .001). Activin-A was up-regulated in AF infection (>2-fold elevation in activin-A to follistatin ratio) correlating directly with severity of inflammation (both P < .001). Activin-A increased prostaglandins, MMP-1, and MMP-9 released by amniochorion (P < .05) to LPS-equivalent levels. Follistatin effectively blunted the prostaglandin response to activin-A and LPS and that of MMPs after activin-A but not after LPS challenge.

CONCLUSION

Activin-A and follistatin are part of the complex inflammatory response of the gestational sac to infection and modulate effector pathways leading to PTB. The activin-A to follistatin ratio may play a role in determining the clinical phenotype of PTB as preterm labor or PPROM.

Differential transcription of fathead minnow immune-related genes following infection with frog virus 3, an emerging pathogen of ectothermic vertebrates

Frog virus 3 (FV3) and other ranaviruses are responsible for die-offs involving wild, farmed, and captive amphibians, fish, and reptiles. To ascertain which elements of the immune system respond to infection, we explored transcriptional responses following infection of fathead minnow cells with either wild type (wt) FV3 or a knock out (KO) mutant targeting the 18 kDa immediate early gene (18K). At 8h post infection we observed marked upregulation of multiple transcripts encoding proteins affecting innate and acquired immunity. Sequences expressed 4-fold or higher in wt-infected cells included transcripts encoding interferon (IFN), IFN regulatory factors (IRFs), IFN stimulated genes (ISGs) such as Mx and MHC class I, and interleukins IL-1β, IL-8, IL-17C and IL-12. Cells infected with the 18K KO mutant (∆18K) showed qualitative differences and lower levels of induction. Collectively, these results indicate that ranavirus infection induced expression of multiple cellular genes affecting both innate and acquired immunity.

Aptamer-based proteomic signature of intensive phase treatment response in pulmonary tuberculosis

BACKGROUND

New drug regimens of greater efficacy and shorter duration are needed for tuberculosis (TB) treatment. The identification of accurate, quantitative, non-culture based markers of treatment response would improve the efficiency of Phase 2 TB drug testing.

METHODS

In an unbiased biomarker discovery approach, we applied a highly multiplexed, aptamer-based, proteomic technology to analyze serum samples collected at baseline and after 8 weeks of treatment from 39 patients with pulmonary TB from Kampala, Uganda enrolled in a Centers for Disease Control and Prevention (CDC) TB Trials Consortium Phase 2B treatment trial.

RESULTS

We identified protein expression differences associated with 8-week culture status, including Coagulation Factor V, SAA, XPNPEP1, PSME1, IL-11 Rα, HSP70, Galectin-8, α2-Antiplasmin, ECM1, YES, IGFBP-1, CATZ, BGN, LYNB, and IL-7. Markers noted to have differential changes between responders and slow-responders included nectin-like protein 2, EphA1 (Ephrin type-A receptor 1), gp130, CNDP1, TGF-b RIII, MRC2, ADAM9, and CDON. A logistic regression model combining markers associated with 8-week culture status revealed an ROC curve with AUC = 0.96, sensitivity = 0.95 and specificity = 0.90. Additional markers showed differential changes between responders and slow-responders (nectin-like protein), or correlated with time-to-culture-conversion (KLRK1).

CONCLUSIONS

Serum proteins involved in the coagulation cascade, neutrophil activity, immunity, inflammation, and tissue remodeling were found to be associated with TB treatment response. A quantitative, non-culture based, five-marker signature predictive of 8-week culture status was identified in this pilot study.

Gram-negative bacterial lipopolysaccharide stimulates activin a secretion from human amniotic epithelial cells

Activin A is involved in inflammation. The present study was performed to clarify if lipopolysaccharide, a component of Gram-negative bacteria, stimulates activin A secretion from human amniotic epithelial cells and to determine if activin A plays a role in amnionitis. Fetal membranes were obtained during elective cesarean sections performed in full-term pregnancies of patients without systemic disease, signs of premature delivery, or fetal complications. Amniotic epithelial cells were isolated by trypsinization. The activin A concentrations in the culture media were measured by enzyme-linked immunosorbent assay, and cell proliferation was assessed by 5-bromo-2'-deoxyuridine incorporation. Amniotic epithelial cells secreted activin A in a cell density-dependent manner, and lipopolysaccharide (10  μ g/mL) enhanced the secretion at each cell density. Lipopolysaccharide (10-50  μ g/mL) also stimulated activin A secretion in a dose-dependent manner. Contrary to the effect of activin A secretion, lipopolysaccharide inhibited cell proliferation in amniotic epithelial cells. The present study suggests that lipopolysaccharide stimulation of activin A secretion may be a mechanism in the pathogenesis of amnionitis.

Gut microbiota, immune development and function

The microbiota of Westerners is significantly reduced in comparison to rural individuals living a similar lifestyle to our Paleolithic forefathers but also to that of other free-living primates such as the chimpanzee. The great majority of ingredients in the industrially produced foods consumed in the West are absorbed in the upper part of small intestine and thus of limited benefit to the microbiota. Lack of proper nutrition for microbiota is a major factor under-pinning dysfunctional microbiota, dysbiosis, chronically elevated inflammation, and the production and leakage of endotoxins through the various tissue barriers. Furthermore, the over-comsumption of insulinogenic foods and proteotoxins, such as advanced glycation and lipoxidation molecules, gluten and zein, and a reduced intake of fruit and vegetables, are key factors behind the commonly observed elevated inflammation and the endemic of obesity and chronic diseases, factors which are also likely to be detrimental to microbiota. As a consequence of this lifestyle and the associated eating habits, most barriers, including the gut, the airways, the skin, the oral cavity, the vagina, the placenta, the blood-brain barrier, etc., are increasingly permeable. Attempts to recondition these barriers through the use of so called 'probiotics', normally applied to the gut, are rarely successful, and sometimes fail, as they are usually applied as adjunctive treatments, e.g. in parallel with heavy pharmaceutical treatment, not rarely consisting in antibiotics and chemotherapy. It is increasingly observed that the majority of pharmaceutical drugs, even those believed to have minimal adverse effects, such as proton pump inhibitors and anti-hypertensives, in fact adversely affect immune development and functions and are most likely also deleterious to microbiota. Equally, it appears that probiotic treatment is not compatible with pharmacological treatments. Eco-biological treatments, with plant-derived substances, or phytochemicals, e.g. curcumin and resveratrol, and pre-, pro- and syn-biotics offers similar effects as use of biologicals, although milder but also free from adverse effects. Such treatments should be tried as alternative therapies; mainly, to begin with, for disease prevention but also in early cases of chronic diseases. Pharmaceutical treatment has, thus far, failed to inhibit the tsunami of endemic diseases spreading around the world, and no new tools are in sight. Dramatic alterations, in direction of a paleolithic-like lifestyle and food habits, seem to be the only alternatives with the potential to control the present escalating crisis. The present review focuses on human studies, especially those of clinical relevance.