From thymus to cystic fibrosis: the amazing life of thymosin alpha 1

1. Editorial

Thymosin alpha 1 (Tα1) is a naturally occurring thymic peptide first described and characterized by Goldstein and Goldstein [1]. Although the peptide is produced in small amounts in several peripheral lymphoid and nonlymphoid tissues, the highest concentrations of Tα1 are found in the thymus. The mechanism of action of the synthetic polypeptide is thought to be related to its immunomodulating activities, centered primarily on the augmentation of T-cell function. However, mechanistically, Tα1 has shown an action beyond its effect on T lymphocytes to include an ability to act as an endogenous regulator of both the innate and adaptive immune systems. The peptide activates Toll-like receptors 2 and 9, thus enhancing phagocytes and dendritic cells functions, promotes T cell and antibody responses and modulates cytokine and chemokine production [2,3]. Prepared as a 28mer synthetic amino-terminal acylated peptide, Tα1 (ZADAXIN®) is approved in 35 countries for treating viral infections, immunodeficiencies, malignancies, and HIV/AIDS [4,5] (Figure 1). The clinical potentials for allergy have recently been reported [6]. ZADAXIN has shown an excellent safety profile and does not induce the side effects and toxicities commonly associated with most immunomodulatory agents.

Figure 1. The multitasking activities of thymosin α1.

Despite the pleiotropic effects of Tα1 on a variety of immune cells, the ability of Tα1 to activate the indoleamine 2,3-dioxygenase (IDO)1 enzyme – which confers immune tolerance during transplantation and restrains the vicious circle of chronic inflammation – has been a turning point, suggesting specific function in immunity [3]. Accordingly, Tα1 has recently been shown to foster immune reconstitution and improve survival of recipients of HLA-matched sibling T cell-depleted stem cell transplants in a phase I/II clinical trial [7].

These observations opened novel perspectives in the field of anti-inflammatory drugs and especially in chronic lung inflammation like that observed in patients with cystic fibrosis (CF). CF is indeed a complex genetic disease [8,9] that affects approximately 80,000 patients worldwide and approximately 30,000 patients in the United States. CF is a chronic disease that affects the lungs, the digestive system, and other organs, such as the pancreas and the reproductive tract [8]. CF patients, with significantly impaired quality of life, have an average life span approximately shorter than the population average. CF patients require lifelong treatment with multiple daily medications, frequent hospitalizations, and ultimately lung transplant, which is life extending but not curative. There currently is no cure for CF, but a range of treatments with correctors and potentiators can now improve lung function and nutritional measurements and reduce the risk of pulmonary exacerbations, hospitalization, transplantation, and death [10].

CF is caused by a mutation in the gene for the CF transmembrane conductance regulator (CFTR) protein, which results in abnormal transport of chloride across cell membranes [11]. Transport of chloride is required for effective hydration of epithelial surfaces in many organs of the body. Normal CFTR channel allows movements of chloride ions to outside of the cells in ATP-dependent manner [12]. Mutant CFTR channel does not allow the move of chloride ions, causing sticky mucous to build up on the outside of the cell. CFTR dysfunction results in dehydration of dependent epithelial surfaces, leading to damage of the affected tissues and subsequent disease, such as lung chronic inflammation and infections, malabsorption in the intestinal tract, and pancreatic insufficiency [13].

Potentiator medications increase the open probability of the CFTR channel, thus improving ion transport, in Class III and Class IV mutations. However, Pseudomonas airway infection persists, suggesting that a combination of controlling infection and improving CFTR function may be necessary in targeting CF treatments [14]. The Class II mutations include the most common CFTR mutation, phe508del. This mutation produces a misfolded CFTR protein that is degraded in the endoplasmic reticulum before it can be chaperoned to the airway surface. However, even this abnormal protein partially functions as a chloride channel once it reaches the luminal surface of the airway. Therefore, medications like lumacaftor, referred to as correctors, prevent degradation of the protein in the endoplasmic reticulum, allowing it to be transported from the Golgi to the airway surface where a potentiator like ivacaftor can increase the probability that this remains open [15,16].

Correcting mutated CFTR trafficking and intracellular localization constitutes the most efficient approach to address CF treatment [17]. However, because of multiple defects caused by the phe508del mutation, it is probable that correction of the mutant protein will require combined treatment with drugs having different mechanisms of action [10]. In parallel, there is also active search for therapy to treat the chronic inflammatory state eventually leading to tissue remodeling and bronchiectasis.

As previously documented, the CFTR mutation seems to directly affect the function of cells from innate immunity. In neutrophils, the mutation in CFTR has been shown to affect phagosome formation [18]. In macrophages, the high intraphagolysosomal pH prevents lysosomal proteases and lipases from digesting phagocytosed bacteria, and this may explain the high rate of bacterial infections in the lungs of CF patients [19]. The ensuing runway innate immunity leads to a dysregulated adaptive immunity, with the prevalence of inflammatory Th17/Th2 responses over regulatory T cell responses [20–22]. Due to its ability to activate the IDO1 [3], Tα1 was tested for its ability to promote lung immune tolerance and surprisingly not only did it and restrained inflammation but also restored the defective CFTR activity by promoting CFTR maturation, stability and function [23].

The loss of phenylalanine at position 508 in the CFTR protein in the Class II mutation results in protein misfolding and rapid degradation by the ubiquitin-proteasome system through a process referred to as endoplasmic reticulum-associated degradation [9] leading to a loss-of-function phenotype. Thus, increasing attention has been devoted to identifying pathways governing cellular proteostasis, such as authophagy [24,25]. Expectedly, the correcting effect of Tα1 was found to be dependent on its ability to activate autophagy via IDO1. By providing a multipronged attack against CF, i.e. restraining inflammation and correcting the basic defect, Tα1 favorably opposed CF symptomatology in preclinical relevant disease settings, thus suggesting its possible exploitation for ‘real-life’ clinical efficacy in CF. However, a recent paper questioned about the corrector activity of Tα1 in CF [26]. This prompted us to go through the paper and to find out that the improper and incorrect use of Tα1 – mainly its solubilization in DMSO instead of water ‒ fully explains the opposing results obtained by Tomati et al. (Romani et al., Reply letter to Nat. Med. in press). Therefore, the corrector activity of Tα1 in CF still holds true in vitro. Further in vivo studies are required to establish whether a drug with the unique capability to correct CFTR defects through regulation of inflammation could represent a major conceptual advance in the CF field and, more in general, in several inflammatory lung diseases where restoration of autophagy has been considered a promising therapeutic option [27].

Declaration of interest

The authors have no relevant affiliations or financial involvement with any organization or entity with a financial interest in or financial conflict with the subject matter or materials discussed in the manuscript. This includes employment, consultancies, honoraria, stock ownership or options, expert testimony, grants or patents received or pending, or royalties.

Reviewer disclosures

Peer reviewers on this manuscript have no relevant financial or other relationships to disclose.

Additional information

Funding

This paper has been published as part of a supplement issue covering the proceedings of the Fifth International Symposium on Thymosins in Health and Disease and is funded by SciClone Pharmaceuticals.