Thymulin Restrains Age-Associated Myeloid Inflammation and Enhances Cancer Immunotherapy in Preclinical Models
Key Insights
A thymus-derived peptide, thymulin, suppressed pro-inflammatory cytokine production in aged myeloid cells by inhibiting NF-κB (search) signaling in mouse models and human PBMCs.
Aged mice treated with thymulin showed delayed breast tumor growth, increased tumor-infiltrating T-cell IFN-γ production, and restored responsiveness to anti-PD-L1 (search) therapy.
Age-associated expansion of pro-inflammatory myeloid cells was confirmed in both aged mice and older healthy human donors, with similar inflammatory signatures found in human breast tumors.
A research team at the University of Southern California has identified thymulin, a thymus-derived nonapeptide, as a key circulating factor capable of restraining age-associated myeloid inflammation and enhancing the efficacy of anti-PD-L1 (search) immunotherapy in aged mice. The findings, published in Nature Communications, illuminate a previously underappreciated role for the thymus in regulating innate immune inflammation during aging and suggest a potential strategy to address age-related barriers to cancer treatment.
The study demonstrates that pro-inflammatory cytokine-producing myeloid cells — including monocytes, granulocytes, macrophages, and dendritic cells — are markedly expanded in aged mice compared with young counterparts. These cells, characterized by elevated production of IL-1α, IL-1β (search), IL-6, and TNF-α (search), were further enriched in the tumor microenvironment and associated with accelerated tumor progression in syngeneic orthotopic breast cancer (search) models (AT-3 and E0771). Aged female mice exhibited faster tumor growth and reduced survival relative to young mice.
Age-associated myeloid inflammation is conserved in humans
The translational relevance of these findings was reinforced by analysis of peripheral blood mononuclear cells from 93 healthy donors. The frequency of CD33⁺CD11b⁺ myeloid cells producing pro-inflammatory cytokines positively correlated with age and was significantly elevated in older individuals (60–87 years) compared with younger individuals (21–33 years).
Analysis of a publicly available single-cell RNA sequencing dataset comprising 26 primary breast tumors from patients aged 35–88 years revealed a relative increase in intratumoral myeloid cell frequency in tumors from aged patients. Myeloid cells from older patients exhibited increased expression of pro-inflammatory cytokines (IL1B and TNF), NF-κB (search) pathway regulators (NFKBIA, NFKBIZ), DAMP molecules (S100A8, S100A9, S100A12), and inflammasome components (NLRP3, PYCARD). In contrast, myeloid cells from younger patients were enriched for genes associated with antigen presentation and maturation, including HLA-DRB5, HLA-DPA1, and CD83. Pathway analysis confirmed enrichment of IL-1, IL-6, and TNF signaling, as well as downstream NF-κB and AP-1 activation, in aged patient myeloid cells.
Youthful systemic environment reverses myeloid inflammation
Using heterochronic parabiosis — surgically joining aged and young mice to share a common circulatory system — the researchers demonstrated that exposure to a youthful systemic milieu reduced the frequency of circulating pro-inflammatory myeloid cells in aged parabionts to levels comparable to those in young-young pairs. This was accompanied by delayed tumor progression and improved survival. Congenic CD45.1/CD45.2 parabiosis experiments confirmed that aged-derived myeloid cells in heterochronic pairs exhibited substantially reduced expression of IL-1α, IL-1β (search), IL-6, and TNF-α (search) compared with aged-aged controls.
Bone marrow chimera experiments further revealed that the suppressive factors originate from non-bone marrow-derived sources. In aged-to-young (A→Y) chimeras, expression of pro-inflammatory cytokines in aged bone marrow-derived CD11b⁺ myeloid cells was markedly reduced compared with aged-to-aged (A→A) controls, reaching levels comparable to young bone marrow-derived cells in young-to-young (Y→Y) chimeras. Heterochronic mixed bone marrow chimeras in Rag2-deficient mice confirmed that these effects are independent of adaptive immunity.
Thymulin suppresses inflammation via NF-κB (search) inhibition
Ingenuity Pathway Analysis identified FOXO3, LMNA, and SIRT family genes as candidate upstream regulators of inflammaging. Notably, thymic atrophy is a shared phenotype in knockout models of these genes, prompting the hypothesis that thymus-derived circulating factors may suppress myeloid inflammation. Among candidate thymic factors, thymulin was selected for further investigation based on prior reports that its circulating activity declines with age and that it suppresses pro-inflammatory cytokine production in PBMCs in vitro.
Thymulin treatment of human PBMCs ex vivo reduced expression of IL-1α, IL-1β (search), IL-6, and TNF-α (search) in an age-dependent manner. In aged mice, one week of thymulin treatment significantly reduced the frequency of circulating myeloid cells producing pro-inflammatory cytokines, whereas no significant effect was observed in young mice. Thymulin also attenuated pro-inflammatory cytokine production from aged, but not young, bone marrow-derived macrophages following LPS stimulation.
Mechanistically, thymulin inhibited NF-κB (search) p65 DNA-binding activity in nuclear extracts from LPS-activated bone marrow-derived macrophages of aged mice and decreased LPS-induced phosphorylation of IκBα. In luciferase reporter assays, thymulin inhibited LPS-induced NF-κB-dependent reporter activity, while mutation of the NF-κB binding site abolished this inhibitory effect.
Thymulin enhances antitumor immunity and anti-PD-L1 (search) response
In tumor-bearing aged mice, thymulin treatment markedly delayed the growth of AT-3 and E0771 tumors and improved survival, while having minimal effect in young mice. Similar age-dependent therapeutic effects were observed in BALB/c mice bearing EMT6 mammary adenocarcinoma. Thymulin reduced the frequency of tumor-infiltrating myeloid cells producing IL-1α, IL-1β (search), and IL-6 in an age-dependent manner and increased IFN-γ production by tumor-infiltrating CD4⁺ and CD8⁺ T cells in aged mice, with enhanced cytotoxic T lymphocyte activity measured by target-specific lysis assays.
Critically, while anti-PD-L1 (search) monotherapy had no impact on tumor growth and survival in aged mice, thymulin treatment rendered tumors responsive to anti-PD-L1 therapy, resulting in enhanced tumor control and prolonged survival in an age-dependent manner.
The authors note that their findings should be interpreted within the context of breast cancer (search) models, and further validation in additional cancer types and human studies is warranted. Nonetheless, the study identifies thymulin as a key regulator of age-associated inflammation that enhances antitumor immunity and improves responsiveness to immune checkpoint blockade in aged hosts, supporting the concept that targeting inflammatory pathways may improve antitumor immunity in older patients.