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{"project":"LitCovid-PD-HP","denotations":[{"id":"T5","span":{"begin":1186,"end":1191},"obj":"Phenotype"},{"id":"T6","span":{"begin":1336,"end":1342},"obj":"Phenotype"},{"id":"T7","span":{"begin":1793,"end":1810},"obj":"Phenotype"},{"id":"T8","span":{"begin":1820,"end":1837},"obj":"Phenotype"},{"id":"T9","span":{"begin":2028,"end":2034},"obj":"Phenotype"},{"id":"T10","span":{"begin":2461,"end":2469},"obj":"Phenotype"},{"id":"T11","span":{"begin":2721,"end":2728},"obj":"Phenotype"},{"id":"T12","span":{"begin":2910,"end":2916},"obj":"Phenotype"},{"id":"T13","span":{"begin":3167,"end":3173},"obj":"Phenotype"},{"id":"T14","span":{"begin":3605,"end":3620},"obj":"Phenotype"}],"attributes":[{"id":"A5","pred":"hp_id","subj":"T5","obj":"http://purl.obolibrary.org/obo/HP_0002664"},{"id":"A6","pred":"hp_id","subj":"T6","obj":"http://purl.obolibrary.org/obo/HP_0002664"},{"id":"A7","pred":"hp_id","subj":"T7","obj":"http://purl.obolibrary.org/obo/HP_0200123"},{"id":"A8","pred":"hp_id","subj":"T8","obj":"http://purl.obolibrary.org/obo/HP_0200123"},{"id":"A9","pred":"hp_id","subj":"T9","obj":"http://purl.obolibrary.org/obo/HP_0002664"},{"id":"A10","pred":"hp_id","subj":"T10","obj":"http://purl.obolibrary.org/obo/HP_0002861"},{"id":"A11","pred":"hp_id","subj":"T11","obj":"http://purl.obolibrary.org/obo/HP_0002583"},{"id":"A12","pred":"hp_id","subj":"T12","obj":"http://purl.obolibrary.org/obo/HP_0100806"},{"id":"A13","pred":"hp_id","subj":"T13","obj":"http://purl.obolibrary.org/obo/HP_0100806"},{"id":"A14","pred":"hp_id","subj":"T14","obj":"http://purl.obolibrary.org/obo/HP_0001888"}],"text":"1. Introduction\nThymosin α1 (Tα1) is an immunostimulatory peptide initially isolated from calf thymus [1] and also abundant in humans. Tα1 is synthesized as the N-terminal moiety of the highly acidic (pI = 3.7) prothymosin α (ProTα), a peculiar cytoplasmic protein due to the absence of any sulfur-containing as well as aromatic side chains and, in particular, its random coil structure under physiological conditions [2,3]. Cleaved by the lysosomal asparaginyl endopeptidase (legumain; δ-secretase), the N-terminally acetylated 28-residue peptide Tα1 gets released [4]. Tα1 plays a significant role in activating and regulating various cells of the immune system [5], e.g., by stimulating toll-like receptor (TLR)-2 and TLR-9 on myeloid and plasmacytoid dendritic cells (DCs), which results in the secretion of immune-related cytokines [6,7]. Furthermore, Tα1 increases the number of activated T helper (Th) cells and provokes a shift towards the Th1 subclass, thus promoting the cell-mediated immune response. In addition, Tα1 was reported to reduce apoptosis of immune cells [8] and to upregulate the expression of major histocompatibility complex I (MHC I) molecules [9] as well as tumor antigens [10]. Interestingly, Tα1 mediates increased intracellular glutathione (GSH) levels [11], which not only inhibits the growth of certain cancer cells in vitro [7,12], but also blocks the assembly of virus particles by hindering disulfide bond formation that is required for envelope glycoprotein oligomerization [13]. These features offer a broad range of clinical applications for Tα1 [14].\nIn fact, a chemically synthesized Tα1 peptide drug has been marketed for more than 20 years under the trade name Zadaxin™ in more than 30 countries. Zadaxin™ is clinically approved for the treatment of chronic hepatitis B (HBV), chronic hepatitis C (HCV), as a vaccine adjuvant, and as adjuvant therapy for chemotherapy-induced immune suppression [7]. Of note, the use of Tα1 has recently undergone reassessment in the context of modern cancer immunotherapies, as there is indication that this peptide may support immune checkpoint inhibition. Preclinical data provide evidence that Tα1 transforms so called cold tumors, which poorly respond to immune checkpoint inhibitors, into highly lymphocyte-infiltrated tumors and, thus, boosts therapeutic efficacy [15]. Indeed, a retrospective study evaluating clinical phase II data reported an increase in overall survival of melanoma patients receiving sequentially Tα1 and the immune checkpoint inhibitor ipilimumab, a CTLA-4-blocking antibody, compared to ipilimumab therapy alone [16]. Moreover, Tα1 was shown to prevent intestinal toxicity in a murine model of anti-CTLA-4-induced colitis, hence contributing to an improved safety profile of immune checkpoint inhibitors [17]. Beyond that, emerging indications such as cystic fibrosis [18], multiple sclerosis [19], and sepsis [20] further emphasize the huge potential of this small therapeutic peptide. Finally, with the outbreak of the COVID-19 pandemic, Tα1 recently gained new attention in virology. Clinical symptoms of COVID-19 to some extent resemble a pathogen-induced sepsis, which justifies immunomodulatory Tα1 treatment [21]. Along these lines, Chinese medical staff members at high risk of COVID-19 infection already received a weekly Tα1 injection in combination with human interferon 1 (hIFN-α) nasal drops as a prophylactic measure [22]. Furthermore, a recent retrospective study in treated Chinese patients revealed that Tα1 significantly reduced the mortality of severe COVID-19 by restoration of lymphocytopenia and reversion of exhausted T cells [23].\nHowever, there are two major drawbacks of the currently marketed Tα1, both of which are related to its peptidic nature. First, the biopharmaceutical production of Tα1 is challenging. Apart from the posttranslational modification via N-terminal acetylation, chemical synthesis of Tα1 is complex, and obstacles comprise the large number of required protecting groups as well as the aggregation tendency of intermediates during synthesis [24]. Furthermore, the overall yield of the solid-phase synthesis is low, typically reaching only around 25% [25]. On the other hand, the biotechnological production as a recombinant peptide in an economic manner has failed so far. Short peptides, in general, are quickly degraded in the bacterial cytoplasm; thus, efficient one-step production of mature Tα1 in E. coli is not feasible. An alternative would be expression as part of a larger protein such as natural ProTα [26], as artificial concatamers [27], in fusion with an intein [28] or in fusion with a highly expressed bacterial protein like thioredoxin [29]. In each case, enzymatic or chemical cleavage is necessary to liberate the mature peptide, which complicates the biopharmaceutical downstream process. Second, after administration in vivo, the small peptide Tα1 is quickly eliminated via renal filtration with a terminal plasma half-life in humans of less than 3 h [30]. This limits its clinical efficacy and, to maintain viable drug levels, would require twice daily dosing.\nIn the present study, we applied the PASylation® technology in order to overcome both of these obstacles of the currently available Tα1 drug and to create a long-lasting N-terminally acetylated therapeutic peptide, also offering cheap and efficient biotechnological production in E. coli. To this end, we combined fusion with a 600-residue polypeptide comprising the small natural L-amino acids Pro, Ala, and Ser [31] with in situ N-acetylation by overexpressing the host cell N-acetyltransferase RimJ [32]. The genetically encoded uncharged “PAS” sequence is highly soluble and structurally disordered, with an expanded hydrodynamic volume, thus showing a biophysical behavior very similar to the chemical polymer polyethylene glycol (PEG), which has been utilized for plasma half-life extension of a series of other therapeutic peptides and proteins [33,34,35]."}

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