Thymosin alpha-1 (Ta1) is a 28-amino-acid, N-acetylated peptide first isolated from calf thymus in the 1970s and later shown to be the amino-terminal fragment of the precursor protein prothymosin alpha. In the published literature it is studied primarily as a signaling molecule in dendritic cell and T-cell biology, with a research history that spans in vitro immunology, rodent infection models, and human clinical trials of the synthetic drug thymalfasin. This article covers the peptide's discovery, its sequence and origin, what the toll-like receptor literature reports, and its relationship to the thymic extract product thymalin.
| Attribute | Detail |
|---|---|
| Name | Thymosin alpha-1 (Ta1) |
| Class | Thymic peptide, N-acetylated acidic polypeptide |
| Sequence length | 28 amino acid residues |
| Molecular formula / weight | C129H215N33O55, approximately 3,108 g/mol |
| Sequence | Ac-Ser-Asp-Ala-Ala-Val-Asp-Thr-Ser-Ser-Glu-Ile-Thr-Thr-Lys-Asp-Leu-Lys-Glu-Lys-Lys-Glu-Val-Val-Glu-Glu-Ala-Glu-Asn |
| Biological origin | Amino-terminal cleavage product of prothymosin alpha (PTMA gene product) |
| What studies examine | Toll-like receptor signaling, dendritic cell maturation, T-cell priming, indoleamine 2,3-dioxygenase (IDO) induction |
| Approved drug form | Thymalfasin, approved as a prescription medicine in a number of countries outside the United States; not FDA-approved in the US |
| Form supplied (research) | Lyophilized powder, research use only |
What is the discovery history of thymosin alpha-1?
Thymosin alpha-1 was characterized in 1977 by Goldstein and colleagues, who isolated and sequenced it from thymosin fraction 5, a partially purified extract of calf thymus tissue that was being investigated for its effects on thymus-dependent lymphocyte (T-cell) function. The 1977 paper describes Ta1 as "a heat stable, highly acidic molecule composed of 28 amino acid residues," one of several distinct polypeptides present in fraction 5, and proposes a nomenclature for the fraction 5 peptide family (Goldstein et al., 1977). That original isolation and sequencing work is the basis for the peptide's name and its defined 28-residue structure, distinguishing it from the many other peptides that were present in the same crude thymic extract.
What is the amino acid sequence of thymosin alpha-1?
Ta1 has the sequence Ac-Ser-Asp-Ala-Ala-Val-Asp-Thr-Ser-Ser-Glu-Ile-Thr-Thr-Lys-Asp-Leu-Lys-Glu-Lys-Lys-Glu-Val-Val-Glu-Glu-Ala-Glu-Asn, with a molecular formula of C129H215N33O55 and a molecular weight of approximately 3,108 g/mol (U.S. Food and Drug Administration, 2024). The "Ac-" prefix denotes N-alpha-terminal acetylation on the serine residue at the start of the chain, a post-translational modification that is not optional decoration: mass spectrometry and HPLC comparisons show that recombinant Ta1 produced without this acetylation step does not match the chemically synthesized, clinically studied peptide (Chen et al., 2013). Because the sequence carries a high proportion of acidic glutamate and aspartate residues, the peptide is strongly acidic, a property noted in the original 1977 characterization.
How is thymosin alpha-1 related to prothymosin alpha?
Circulating and tissue Ta1 is not synthesized as a standalone 28-residue peptide. It is generated by proteolytic cleavage of prothymosin alpha (ProTalpha), a roughly 12 kDa, 109 to 111 residue protein encoded by the PTMA gene (UniProt, PTMA entry, human). ProTalpha itself has broader roles in chromatin biology that are distinct from the immune signaling attributed to the cleaved Ta1 fragment. Studies using recombinant human ProTalpha have shown that the lysosomal asparaginyl endopeptidase legumain cleaves ProTalpha at a specific site to release the amino-terminal Ta1 sequence, and that Ta1 generated this way carries the same N-alpha-acetylated serine found in the synthetic peptide used in clinical studies (Chen et al., 2013). This precursor-to-fragment relationship is why Ta1 is sometimes described in the literature as a proteolytic product rather than a hormone secreted in its final form.
What does the literature report about thymosin alpha-1 and Toll-like receptor signaling?
A body of published research, much of it from groups studying fungal and viral infection models, has examined how Ta1 interacts with dendritic cells (DCs) through Toll-like receptor (TLR) pathways. In a mouse model of Aspergillus fumigatus infection, Ta1 was reported to induce functional maturation and interleukin-12 production in fungus-exposed dendritic cells through a p38 MAPK/NF-kappaB-dependent pathway, signaling through the MyD88-dependent branch of TLR signaling, and to activate T-helper type 1 (Th1) antifungal immunity in vivo (Romani et al., 2004). A follow-up study using human and murine dendritic cells reported that Ta1 also engages TLR9 signaling to induce indoleamine 2,3-dioxygenase (IDO) activity, interleukin-10 production, and the generation of regulatory T cells, describing a dendritic cell response that combines Th1 priming with a parallel regulatory, tolerance-associated program (Romani et al., 2006). In a murine cytomegalovirus infection model, Ta1 administration was reported to activate plasmacytoid dendritic cells through the TLR9/MyD88/IRF7-dependent viral sensing pathway, promoting an interferon-alpha and interferon-gamma dependent antiviral response and protecting susceptible and resistant mice from infection (Bozza et al., 2007). Across these papers, the consistent finding is that Ta1 acts upstream of T-cell responses by modulating dendritic cell signaling through TLR pathways, rather than acting directly on T cells themselves; all of the in vivo work cited here was conducted in mice, and the in vitro work used isolated human or murine dendritic cell cultures.
Is thymosin alpha-1 an FDA-approved medicine?
The synthetic drug version of this peptide, thymalfasin, has been marketed under trade names including Zadaxin in a number of countries in the Asia-Pacific region, Latin America, Eastern Europe, and the Middle East, where manufacturer reporting describes regulatory approval for indications such as hepatitis B and hepatitis C. According to a 2024 FDA Pharmacy Compounding Advisory Committee briefing document, however, thymosin alpha-1 "is not approved in the United States, Japan, or Europe (except Italy)," and the agency noted it was unable to independently verify all of the approval claims made for other countries (U.S. Food and Drug Administration, 2024). That same document records that a 2023 nomination to add Ta1-related substances to the FDA's list of bulk drug substances eligible for compounding under section 503A of the Federal Food, Drug, and Cosmetic Act was withdrawn, and that the agency was evaluating the substance at its own discretion. In short: thymalfasin is an approved prescription medicine in some jurisdictions outside the United States, it has no FDA-approved drug application in the US, and the research-grade material sold for laboratory use is a distinct, non-clinical-grade article that is not equivalent to, and is not sold as, that medicine.
How does thymosin alpha-1 differ from thymalin?
Ta1 is frequently confused with thymalin, a different thymic material developed by Khavinson and colleagues, and the two are worth separating clearly. Ta1 is a single, chemically defined 28-residue peptide with a fixed sequence, isolated and later synthesized to match that exact structure. Thymalin, by contrast, originates from the same general tradition of thymic extract research but is described in the literature as a low-molecular-weight polypeptide complex extracted from calf thymus, a mixture rather than one defined sequence, and it sits within the broader Khavinson "bioregulator" peptide family alongside compounds such as epithalon and vilon. Readers comparing the two, or trying to place thymalin within that wider family of short regulatory peptides, can find more detail on our thymalin research page and in our overview of bioregulator peptides. Ta1 and thymalin are studied in overlapping fields (thymic and immune biology) but are chemically and historically distinct materials, and findings for one should not be assumed to apply to the other.
How is thymosin alpha-1 supplied and handled in the lab?
Research-grade Ta1 is supplied as a lyophilized (freeze-dried) powder, which is the standard format for peptides of this size because it is far more stable in storage than a solution. Reconstitution is bench chemistry: bringing a known mass of lyophilized peptide into solution at a known concentration using an appropriate diluent, which is a matter of accurate weighing, diluent volume, and mixing technique rather than a use protocol. Before reconstituting or running any assay, researchers should confirm identity and purity against the lot's certificate of analysis; our guide to reading a peptide certificate of analysis covers what each figure on that document means and how it was generated. As with other acidic, highly charged peptides, avoid repeated freeze-thaw cycles once a solution has been prepared and keep lyophilized stock protected from moisture and light until use. Our current lot of research-grade thymosin alpha-1 ships with its corresponding certificate of analysis.
Frequently asked questions
Is thymosin alpha-1 the same molecule as thymalfasin?
Thymalfasin is the international nonproprietary name for the synthetic drug form of the same 28-residue sequence. The chemistry is the same peptide sequence; the distinction that matters is regulatory and manufacturing status, not structure. Thymalfasin as a marketed drug is approved in certain countries outside the United States and is manufactured, tested, and labeled to pharmaceutical standards for that use. Research-grade thymosin alpha-1 sold for laboratory work is not manufactured or labeled as that drug and is not FDA-approved.
Why is thymosin alpha-1 described as "N-acetylated"?
N-acetylation refers to an acetyl group attached to the free amino group at the start (N-terminus) of the peptide chain, on the serine residue in Ta1's case. Published comparisons using mass spectrometry and HPLC show that this modification is present on both the originally isolated natural peptide and on synthetic material studied in the literature, and that removing it changes the molecule's identity from a chromatography and mass standpoint (Chen et al., 2013).
What cell types are most studied in connection with thymosin alpha-1?
Dendritic cells are the most frequently studied cell type in the TLR-signaling literature on Ta1, with T cells studied downstream as the cells whose priming and differentiation dendritic cells influence. Several of the papers cited in this article used human or murine dendritic cell cultures in vitro, paired with murine in vivo infection models to test the same signaling pathway in an intact immune system.
Is prothymosin alpha the same protein as thymosin alpha-1?
No. Prothymosin alpha (ProTalpha) is a larger, roughly 12 kDa protein of about 109 to 111 residues, encoded by the PTMA gene, with functions in chromatin biology that extend beyond the immune signaling described for Ta1. Thymosin alpha-1 is the 28-residue amino-terminal fragment released from ProTalpha by proteolytic cleavage.
Does the FDA recognize thymosin alpha-1 as an approved drug ingredient in the United States?
No. FDA materials reviewed for this article state that thymosin alpha-1 is not approved in the United States, and that a nomination to add it to the list of bulk drug substances eligible for pharmacy compounding under section 503A was withdrawn, with the agency evaluating the substance at its own discretion (U.S. Food and Drug Administration, 2024).
How does thymosin alpha-1 differ from thymosin beta-4 or TB-500?
They belong to different thymosin families entirely. Thymosin alpha-1 is derived from prothymosin alpha and is studied mainly for dendritic cell and TLR signaling, while thymosin beta-4 and its synthetic fragment TB-500 belong to the beta-thymosin family and are studied for actin-binding and cell-motility pathways. They share a naming convention from their shared origin in thymic tissue research, not a common sequence or mechanism.
References
- Goldstein AL, Low TL, McAdoo M, et al. Thymosin alpha1: isolation and sequence analysis of an immunologically active thymic polypeptide. Proc Natl Acad Sci U S A. 1977. https://pubmed.ncbi.nlm.nih.gov/265536/
- Chen SY, et al. Generation of mature N-alpha-terminal acetylated thymosin alpha1 by cleavage of recombinant prothymosin alpha. PMC. 2013. https://pmc.ncbi.nlm.nih.gov/articles/PMC3830889/
- Romani L, et al. Thymosin alpha 1 activates dendritic cells for antifungal Th1 resistance through toll-like receptor signaling. PubMed. 2004. https://pubmed.ncbi.nlm.nih.gov/14982877/
- Romani L, et al. Thymosin alpha1 activates dendritic cell tryptophan catabolism and establishes a regulatory environment for balance of inflammation and tolerance. PubMed. 2006. https://pubmed.ncbi.nlm.nih.gov/16741252/
- Bozza S, et al. Thymosin alpha1 activates the TLR9/MyD88/IRF7-dependent murine cytomegalovirus sensing for induction of anti-viral responses in vivo. PubMed. 2007. https://pubmed.ncbi.nlm.nih.gov/17804687/
- UniProt Consortium. PTMA - Prothymosin alpha - Homo sapiens (Human). UniProtKB P06454. https://www.uniprot.org/uniprotkb/P06454/entry
- U.S. Food and Drug Administration. Thymosin Alpha-1 (Ta1) Related Bulk Drug Substances. Pharmacy Compounding Advisory Committee Meeting briefing document. December 4, 2024. https://www.fda.gov/media/183892/download
Research use only. Homegrown Peptides products are for laboratory research and are not for human or animal use. Nothing in this article is medical advice.
