Thymosin Alpha-1 vs LL-37
Thymosin alpha-1 is a 28-amino-acid peptide derived from prothymosin alpha, registered as a prescription immunomodulator in a number of countries under the name thymalfasin and studied in more than thirty controlled human trials. LL-37 is the sole human cathelicidin, a 37-residue amphipathic alpha-helical peptide released from the hCAP18 precursor by proteolytic cleavage, whose literature is dominated by in vitro and animal work alongside a small number of topical and oral clinical trials. The two are habitually paired because both are described as immune peptides, but they sit on opposite sides of the innate and adaptive divide and their evidence bases were assembled for different purposes. No published study has administered thymosin alpha-1 and LL-37 as separate agents within one experiment. What exists instead is a series of papers in which fragments of the two sequences were fused into a single chimeric peptide, together with reviews that catalogue both among host-defence and immunomodulatory peptides.
No study has compared them directly.
Everything below is drawn from separate studies that used different models, endpoints and species. That is a real limit on what can be concluded, not a formality.
No located study administered thymosin alpha-1 and LL-37 as separate agents in the same design, so no direct comparison of the two peptides exists in the published literature. The papers that engage both are of two kinds. The first is a chimeric-peptide programme from a group at China Agricultural University: eight hybrids were built by joining the active centres of antimicrobial peptides, LL-37 residues 13-36 among them, to thymopentin or to the thymosin alpha-1 active centre at residues 17-24, and the winning construct, designated LL-37-Talpha1 or LTA, was carried into cell assays and a lipopolysaccharide-induced murine intestinal inflammation model. Companion papers varied which segment of the thymosin alpha-1 sequence was used and expressed the fusion recombinantly in Pichia pastoris. In none of these were the two parent peptides dosed separately as comparator arms; the fragments only ever appeared inside the chimera. The second kind is review literature: narrative and catalogue reviews of host-defence peptides, sepsis immunotherapy, COVID-19 trial landscapes and antimicrobial peptide pharmacology that describe thymalfasin and LL-37 in adjacent sections without generating comparative data.
The 11 publications that cover both
- 01Animal in vivo2019
Design and Development of a Novel Peptide for Treating Intestinal Inflammation
Zhang L, et al. · Frontiers in Immunology · peptide design study: eight hybrids combining the active centres of LL-37 (13-36), YW12D, innate defence regulator 1 and cathelicidin 2 (1-13) with thymopentin or the thymosin alpha-1 (17-24) active centre, screened by molecular docking and cell assays, then tested in a lipopolysaccharide-induced murine intestinal inflammation model
The hybrid LL-37-Talpha1, abbreviated LTA, was selected as the construct with the strongest anti-inflammatory activity and minimal cytotoxicity. In lipopolysaccharide-treated mice, LTA was associated with less jejunal epithelial injury and leukocyte infiltration, lower tumour necrosis factor-alpha, interferon-gamma, interleukin-6 and interleukin-1beta, higher zonula occludens-1 and occludin expression, and reduced permeability and apoptosis. The activity was attributed to lipopolysaccharide neutralisation, binding to the TLR4/MD-2 complex and modulation of nuclear factor-kappa B signalling. Neither LL-37 nor thymosin alpha-1 was administered as a separate comparator arm.
- 02Animal in vivo2019
Development of a Highly Efficient Hybrid Peptide That Increases Immunomodulatory Activity Via the TLR4-Mediated Nuclear Factor-κB Signaling Pathway
Zhang L, et al. · International Journal of Molecular Sciences · three hybrid peptides (LTAa, LTAb, LTAc) joining a characteristic fragment of LL-37 to the thymosin alpha-1 centres 17-24, 20-25 and 20-27; molecular docking, cell assays and a cyclophosphamide-immunosuppressed murine model
LTAa was identified as the most active of the three constructs and showed immunoregulatory activity and stability with minimal cytotoxicity. In cyclophosphamide-treated mice, LTAa was associated with greater immune organ development, activation of peritoneal macrophage phagocytosis, altered T lymphocyte subsets and higher levels of tumour necrosis factor-alpha, interleukin-6, interleukin-1beta and immunoglobulins A, G and M. The effects were attributed to binding of the TLR4/MD-2 complex. The parent peptides were cited as the source of the fragments rather than tested as standalone arms.
- 03In vitro2019
Expression and Purification of Hybrid LL-37Tα1 Peptide in Pichia pastoris and Evaluation of Its Immunomodulatory and Anti-inflammatory Activities by LPS Neutralization
Ahmad B, et al. · Frontiers in Immunology · recombinant expression of the LL-37-Talpha1 fusion in Pichia pastoris with nickel-affinity and reverse-phase HPLC purification; limulus amebocyte lysate endotoxin assay, lactate dehydrogenase assay and cytokine measurement in mouse RAW264.7 macrophages
The fusion peptide was expressed and purified at an estimated mass of about 3.9 kDa and bound endotoxin directly, neutralising lipopolysaccharide in a concentration-dependent manner. In lipopolysaccharide-stimulated RAW264.7 macrophages it was associated with reduced cytotoxicity, lower nitric oxide, tumour necrosis factor-alpha, interleukin-6 and interleukin-1beta, and fewer apoptotic cells. The two parent peptides were described as the sequences the construct was derived from and were not evaluated separately.
- 04Review2022
Antimicrobial peptides could antagonize uncontrolled inflammation via Toll-like 4 receptor
Buccini DF, et al. · Frontiers in Bioengineering and Biotechnology · narrative review of antimicrobial peptides acting on Toll-like receptor 4
The review described LL-37 as a cathelicidin that binds the TLR4/MD-2 complex and prevents lipopolysaccharide binding in dendritic cells, and tabulated its sequence alongside the sequence of LTAa, the hybrid formed from a characteristic fragment of LL-37 and a thymosin alpha-1 centre. It presented the hybridisation work as an attempt to extend half-life and immunoregulatory activity while reducing cytotoxicity, and reported no experiment comparing the two parent peptides.
- 05Review2025
Host defense peptides as a new drug lead to a strategy for inflammatory bowel disease
Rodrigues JM, et al. · Drug Discovery Today · short review of host defence peptides as candidates for inflammatory bowel disease
The review listed the LL-37-Talpha1 construct among the cathelicidin-derived host defence peptides considered for inflammatory bowel disease, alongside KR-12 and other short cathelicidin fragments, defensins, cecropins and alpha-melanocyte-stimulating hormone derivatives. It described the shared immunoregulatory mechanisms of the class as downregulation of nuclear factor-kappa B signalling and modulation of cytokine release, and framed the whole group as candidates rather than established treatments.
- 06Review2025
A Comprehensive Overview of Antimicrobial Peptides: Broad-Spectrum Activity, Computational Approaches, and Applications
Marciano CL, et al. · Antibiotics (Basel) · narrative review cataloguing antimicrobial and host-defence peptides, their sequences, mechanisms and clinical or commercial status
The review tabulated both peptides in its catalogue of clinically used or investigated peptides: thymalfasin as a synthetic antiviral immunomodulator marketed as Zadaxin for hepatitis B and C, and LL-37 as a human antibacterial peptide acting by membrane disruption and used by topical and inhaled routes. LL-37 was separately discussed as an immune regulator that binds lipopolysaccharide and limits macrophage activation. The two entries appear in the same table without any comparative experiment.
- 07Review2025
Targeting the immuno-inflammatory-microbial network: a key strategy for sepsis treatment
Xu Y, et al. · Frontiers in Immunology · narrative review of immunomodulatory and anti-inflammatory strategies in sepsis
The review tabulated thymosin alpha-1, under the trade name thymalfasin, as an immunomodulator that activates dendritic cells, natural killer cells and macrophages and increases T-cell numbers through Toll-like receptor and nuclear factor-kappa B signalling. In the same table, LL-37 appeared as one of the mediators through which mesenchymal stromal cell therapy improves bacterial clearance, and the cathelicidin sepsis literature was cited separately. The two agents were placed in different rows of the same treatment landscape rather than compared.
- 08Review2022
Sepsis-induced immunosuppression: mechanisms, diagnosis and current treatment options
Liu D, et al. · Military Medical Research · narrative review of the mechanisms, diagnosis and treatment of sepsis-induced immunosuppression
The review gave thymosin alpha-1 its own treatment section, describing it as a thymic peptide that activates dendritic cells, natural killer cells and macrophages, increases T-cell numbers and enhances the antibacterial action of T helper 1 cells, and summarising the clinical trials conducted in sepsis. LL-37 appeared in the mesenchymal stromal cell section as the antimicrobial peptide whose increased release was reported to mediate part of the antibacterial effect of those cells. No comparison between the two was attempted.
- 09Review2021
Immunotherapies and immunomodulatory approaches in clinical trials - a mini review
Iqbal Yatoo M, et al. · Human Vaccines & Immunotherapeutics · mini review of immunotherapies and immunomodulatory agents under clinical investigation, with a COVID-19 focus
The review described thymosin alpha-1 as a 28-amino-acid thymic peptide investigated for COVID-19 immunotherapy on the basis of reversing lymphopenia and exhausted T-cell phenotypes, and noted that the thymalfasin product Zadaxin had been approved for hepatitis B. In the same survey of immunomodulatory options it listed CSA0001, described as an LL-37 antiviral peptide, among the agents then in COVID-19 development. Both appeared as separate entries in a landscape summary.
- 10Review2020
Novel insights into the treatment of SARS-CoV-2 infection: An overview of current clinical trials
Oroojalian F, et al. · International Journal of Biological Macromolecules · overview of registered clinical trials and investigational agents for SARS-CoV-2 infection
The overview tabulated a registered phase 3 trial combining recombinant human interferon alpha-1b with thymosin alpha-1 among the immunomodulatory studies then recruiting. Separately it described LL-37 as one of the antimicrobial mediators, alongside lipocalin-2 and beta-defensin-2, through which mesenchymal stromal cells were reported to act via Toll-like receptor 4 signalling. The two peptides were catalogued within the same trial landscape without being compared.
- 11Review2009
Therapeutic approaches using host defence peptides to tackle herpes virus infections
Jenssen H · Viruses · narrative review of host defence peptides in antiviral drug development, with a herpesvirus focus
The review listed Zadaxin, thymosin alpha-1 or thymalfasin, in its table of host defence peptide drugs, giving the 28-residue sequence and describing approved and phase 3 use in chronic hepatitis B and C together with effects on MHC class I expression, interleukin-2 and interferon-gamma. LL-37 was discussed among the alpha-helical peptides that were reported to cause minimal or no inactivation of herpes simplex virus, in contrast to magainins, dermaseptin and melittin. The two were treated as unrelated entries in one drug class.
Side by side.
| Thymosin Alpha-1 | LL-37 | |
|---|---|---|
| Evidence maturity | Approved drug | Early clinical |
| Studies cited here | 16 | 17 |
| Published 2023 or later | 11 | 13 |
| Newest paper | 2026 | 2026 |
| Sequence and origin | A 28-amino-acid acetylated peptide corresponding to the N-terminal region of prothymosin alpha, originally isolated from thymic tissue and produced synthetically for clinical use. | A 37-residue amphipathic alpha-helical peptide, the sole human cathelicidin, released by proteolytic cleavage of the hCAP18 precursor encoded by the CAMP gene and expressed by neutrophils, monocytes and epithelial cells. |
| Mechanism as described in the literature | Described as acting largely through Toll-like receptor signalling on dendritic cells and monocytes, influencing T-cell differentiation, thymic output and cytokine balance, which places it on the adaptive side of the immune response. | Described as acting by direct membrane permeabilisation of microbes, neutralisation of lipopolysaccharide, and receptor-mediated chemotaxis through FPR2/FPRL1, with transcription of the CAMP gene induced by 1,25-dihydroxyvitamin D3. |
| Size of the evidence base | Sixteen verified studies in this library, weighted toward controlled human research: nine human clinical studies and five systematic reviews. | Seventeen verified studies in this library, weighted toward laboratory work: five human clinical studies against five in vitro and four animal in vivo reports, plus reviews. |
| Best-characterised finding | Modulation of lymphocyte subsets and reduction of infectious complications in critically ill patients, reported repeatedly across small and mid-sized trials in sepsis, pancreatitis and hepatitis B, although the largest trial found no mortality benefit. | Concentration-dependent bactericidal and antibiofilm activity, including disruption of pre-grown Pseudomonas aeruginosa biofilms far below the minimum inhibitory concentration, together with receptor-mediated chemotaxis of neutrophils, monocytes and T cells. |
| Human data | More than thirty controlled trials, including the phase 3 TESTS sepsis trial published in the BMJ in 2025, the earlier ETASS sepsis trial, a multicentre necrotising pancreatitis trial and randomised oncology and hepatitis studies. | A small number of controlled trials by topical and oral routes only: a phase IIb venous leg ulcer trial that was negative on its primary analysis, a diabetic foot ulcer trial, and a randomised trial of an oral recombinant formulation in COVID-19. No human data exist for systemic or injectable administration. |
| Evidence maturity | Approved drug. Registered as a prescription immunomodulator in a number of countries, principally for chronic hepatitis B and as an adjuvant in sepsis and oncology, though not approved by the FDA or EMA. | Early clinical. Controlled trials exist but only for topical and oral formulations, and no LL-37 product has been approved in any jurisdiction. |
| Direction of the open safety question | The unresolved question concerns efficacy rather than harm: the large trials reported acceptable tolerability while failing to show mortality benefit, and the pooled meta-analytic signal weakens when analysis is restricted to high-quality multicentre trials. | The unresolved question is mechanistic. LL-37 is documented as a pathogenic driver in rosacea, psoriasis and atherosclerosis as well as a host-defence effector, and its switch between protective and inflammatory behaviour with concentration and context is not adequately characterised. |
| Basis of most marketing claims | Extrapolation from approved infectious-disease and oncology indications to general immune support and healthy ageing, for which no controlled human trial has been conducted. | Extrapolation from in vitro antimicrobial and antibiofilm data, and from vitamin D-driven expression biology, to injectable use in humans, a route for which no clinical data have been published. |
And what it does not.
Nothing in the published record establishes how these two peptides compare, because no experiment has administered them side by side. The papers that engage both did something else: short segments were cut from each sequence and fused into a single chimeric peptide, and even in those studies the parent peptides were not run as comparator arms, so the chimera work says nothing about how intact thymosin alpha-1 and intact LL-37 measure against one another. The separate literatures are asymmetric rather than contradictory. Thymosin alpha-1 has the stronger clinical record by design tier, with more than thirty controlled trials and registration as a prescription immunomodulator in several countries, but its largest and most rigorous trial found no mortality benefit in sepsis, and no trial has tested it as a longevity or general immune-support intervention. LL-37 has a much larger mechanistic literature and clearly documented antimicrobial biology, but its controlled human evidence is confined to topical and oral formulations, the largest of those was negative on its primary analysis, and the same peptide is described in the literature as a driver of inflammatory skin and vascular disease.
Common questions.
- Has any study given both thymosin alpha-1 and LL-37 to the same subjects?
No such study was located in PubMed or Europe PMC. The publications that engage both peptides fall into two groups. One is a chimeric-peptide programme in which short segments of the two sequences were joined into a single molecule, and only that molecule was tested. The other consists of reviews of host-defence peptides, sepsis immunotherapy, antimicrobial peptide pharmacology and COVID-19 trial landscapes that describe the two agents in adjacent sections. Neither group produced comparative efficacy data for the intact peptides.
- What is the LL-37-Talpha1 hybrid, and does it say anything about either parent peptide?
It is a designed fusion peptide, abbreviated LTA or LTAa depending on the paper, built by joining a fragment of LL-37 to the active centre of thymosin alpha-1. It was reported to neutralise lipopolysaccharide, bind the TLR4/MD-2 complex and lower inflammatory cytokines in cell assays and in mouse models of intestinal inflammation and cyclophosphamide-induced immunosuppression. Because the parent peptides were not tested as separate arms, those results describe the properties of a new molecule rather than the relative activity of the two peptides it was derived from.
- Which of the two has the stronger record in controlled human trials?
Thymosin alpha-1, by design tier and by volume. It has been studied in more than thirty controlled trials, including a phase 3 randomised placebo-controlled sepsis trial, a multicentre pancreatitis trial and randomised hepatitis and oncology studies, and it is registered as a prescription drug in a number of countries. LL-37 has only a small number of controlled trials, all using topical or oral formulations, and the largest of them, a phase IIb venous leg ulcer study, found no significant improvement in healing on its primary analysis.
- Why is LL-37 described as both protective and harmful in the literature?
Because its published behaviour depends on concentration, receptor binding, disease stage and local environment. The peptide that kills bacteria, disrupts biofilms and recruits immune cells is also documented as a pathogenic driver in rosacea, where injection reproduced rosacea-like inflammation in mice, in psoriasis, and in atherosclerosis, where it acts as an autoantigen and its complex with apolipoprotein B-100 tracked with coronary disease severity. Reviews of LL-37 in intestinal disease set out the conditions under which its activity converts from protective to pathogenic without resolving where that threshold sits.
- Do the two peptides act on the same part of the immune system?
Not as the published mechanisms describe them. Thymosin alpha-1 is characterised as acting through Toll-like receptor signalling on dendritic cells and monocytes to influence T-cell differentiation, thymic output and cytokine balance, which is adaptive-side biology. LL-37 is characterised as an innate effector that permeabilises microbial membranes, neutralises lipopolysaccharide and chemoattracts neutrophils, monocytes and T cells through FPR2/FPRL1. The overlap the review literature identifies sits at the level of Toll-like receptor and nuclear factor-kappa B signalling rather than in shared molecular targets.
For research purposes only. Not for human consumption, diagnosis, treatment, or prevention of any condition. Nothing on this page is medical advice, and no comparison here should be read as a recommendation of either compound.