Chronic spontaneous urticaria (CSU) is defined as the appearance of itchy wheals and/or angioedema for longer than 6 weeks. The cell‒cell interactions among mast cells, basophils, and eosinophils/T cells regulate their function and may involve the CSU pathomechanism. In addition to the second generation of antihistamines (46), the latest therapies based on pathological mechanisms include omalizumab, dupilumab, and Bruton's tyrosine kinase inhibitors (47). According to the pathological mechanism, it is necessary to explore the development of new drugs to relieve patients' pain since there is no way to achieve a radical cure for CSU. Gal-9 plays a role in type 2 cells mediated eosinophilic allergic diseases, mainly by interacting with eosinophils (35). The polarization toward type 2 immunity of Gal-9 is usually accompanied by increased production of IgG1 and IgE and the activation of eosinophils and mast cells to release cytokines and chemokines, which further exacerbates disease activity in patients with refractory CSU (48). Recently, we reported that the number of circulating Gal-9⁺ eosinophils and basophils is significantly increased in CSU patients and that the number of Gal-9⁺ cells in skin lesions is increased. Upregulation of Gal-9 is associated with disease activity, IgE levels, and negative basophil activation test (BAT) results in CSU patients. TNF-α upregulated the level of Gal-9 in eosinophils via PI38K in the CSU. Gal-9 levels are increased in patients who respond to omalizumab treatment, and omalizumab reduces Gal-9 levels in CSU patients (49). The Gal-9⁺ eosinophil/basophil ratio can be used to predict individual reactivity to omalizumab in CSU patients. Omalizumab inhibits mast-cell degranulation by reducing FcεRI expression by binding free IgE, but Gal-9 can also bind IgE directly. Whether there is dual regulation here requires further study. Patients who responded to omalizumab treatment instead had higher Gal-9 before treatment, suggesting the potential presence of a feedback loop—a high IgE environment induces upregulation of Gal-9 as a compensatory mechanism. It is necessary to distinguish between membrane-bound Gal-9 (which directly inhibits effector cells) and soluble Gal-9 (biomarker).

The negative correlation between Gal-9⁺ eosinophils/basophils and Tim-3⁺ Th17 cells in the CSU supports the above Gal-9/Tim-3 inhibition of Th17 cell generation. In addition, the serum levels of soluble Gal-9 were similar in CSU patients and HCs in our study (49). In another study, although the serum Gal-9 level was greater than that in HCs, it was not associated with sex, age, disease duration, Chronic Urticaria Quality of Life Questionnaire score, CSU activity (Urticaria Activity Score Over 7 Days, UAS7 and Sabroe's grade), or autologous serum skin test (ASST) activity (50). These findings suggest that soluble Gal-9 may be a bystander molecule. Notably, this study did not stratify patients by ASST status or autoantibody profiles, potentially diluting subtype-specific associations. This heterogeneity underscores the need for endotype-driven biomarker analysis in future studies. Thus, different forms of Gal-9 are involved in the disease process of CSU in diverse manners: positive and bystander. However, the role of Gal-9 in CSU development requires further research stratified by autoimmune status, IgE autoantibody levels, and response to omalizumab.

Systemic lupus erythematosus (SLE) is a multisystem chronic autoimmune disease that is more common in women of childbearing age. T cells secrete proinflammatory cytokines, induce autoantibody production by B cells, and maintain disease via autoreactive memory T cells, leading to abnormal proportions and functions of some T-cell subsets in SLE patients (51). For example, decreased IL-2 in SLE patients leads to impaired Treg development and function and reduces the restriction of the proinflammatory factor IL-17. The pathological activation pathways linked to B-lymphocyte dysregulation encompass Toll-like receptor (TLR) signaling, β-cell activating factor (BAFF) stimulation, and B-cell receptor (BCR)-dependent activation. This multifaceted signaling dysregulation culminates in systemic loss of immune tolerance in SLE pathogenesis (52). Mehta et al. measured serum and urine Gal-9 and C-X-C motif chemokine ligand 10 (CXCL-10) levels in 97 SLE patients via ELISA. Serum Gal-9 can be used to determine the activity of SLE but has little significance in distinguishing active renal disease from active nonrenal disease. Gal-9 performed slightly better than the validated marker CXCL-10 (53, 54). The expression of Tim-3 and the level of Gal-9 in the CD4⁺ and CD8⁺ T cells of SLE patients were also greater than those in healthy controls (54–56). Gal-9 suppresses functional maturation of plasmacytoid dendritic cells (pDCs) and B lymphocytes, impairing their TLR7/TLR9 ligand-induced cytokine responses. Mechanistically, it inhibits both immune complex-triggered and neutrophil extracellular trap-mediated pDC activation (57). Furthermore, Gal-9 demonstrates pathway inhibition through mTOR/p70S6K signaling- a critical regulator of TLR-dependent IFN secretion in pDCs and autoantibody generation in B cells- via CD44 interaction-mediated suppression of these cellular populations. Gal-9 is a good indicator of disease activity in children with SLE, and larger samples are needed for further experimental confirmation (57). Enhancing coinhibitory signaling to block immune responses may be helpful in the treatment of autoimmune diseases (58). Enhancing co-inhibitory signaling to block immune responses may be helpful in the treatment of autoimmune diseases (59). Further studies revealed that monocyte immune responses in the peripheral blood of SLE patients were inhibited by a Gal-9-blocking antigen. Tim-3 may be a new target for the treatment of SLE (60). In the future, different biomarkers will be combined to evaluate SLE more effectively (61).

It is important to note that the proposed therapeutic blockade of the Gal-9/Tim-3 axis in SLE appears to contradict its anti-inflammatory roles observed in conditions like psoriasis or HSV infection (sections below). This discrepancy highlights the context-dependent functionality of Gal-9. In SLE, Gal-9's predominant interaction within the dysregulated immune milieu, particularly its potent suppression of pDC-derived IFN-α via mTOR/p70S6K signaling and potential engagement with other receptors like VISTA (Table 1), contributes to the breakdown of tolerance and perpetuates autoimmunity. Therefore, in this specific pathological context characterized by aberrant innate immune activation and interferon signatures, inhibiting Gal-9 signaling is proposed as a strategy to restore immune balance.

Atopic dermatitis (AD), is the most common form of skin inflammation and is characterized by itching, peeling, and sometimes damp red skin. Owing to damage to cutin formation from the terminal differentiation of cells, AD barrier dysfunction in skin lesions leads to a high incidence of bacterial, fungal, or viral infection (62). AD, a type 2-skewed inflammatory dermatosis, manifests peripheral eosinophilia and mast cell hyperactivation (63). Elevated Gal-9 levels in AD microenvironments regulate inflammatory cascades and keratinocyte hyperproliferation (28). Serological profiling revealed Gal-9/Tim-3 axis-mediated suppression of type 1/Th17 responses with concomitant type 2/Th22 polarization (64). Notably, Gal-9 modulates keratinocyte immunoreactivity by attenuating IL-6/IL-17 release from IL-4-activated cells and IL-6 production in TNF-α/IFN-γ-stimulated models (65). Clinical investigations demonstrate Gal-9 serum elevation positively correlating with SCORAD indices and lesional surface area in AD (9). The serum level of Gal-9 is higher in patients with AD and positively correlated with the severity index and eczema area (63, 66). Gal-9 expression was observed in mast cells and epidermal keratinocytes in skin lesions (65). Immunohistochemical analyses detect Gal-9 expression in cutaneous mast cells and activated keratinocytes, supporting its biomarker potential (3, 66). Post-therapeutic reduction of epidermal and circulating Gal-9 levels implies pro-inflammatory activity (63). Kim et al. reported that probiotic-induced Gal-9 could alleviate dinitrochlorobenzene-induced AD in mice, demonstrating the great potential of Gal-9 in the treatment of AD (67). Murine studies indicate exogenous Gal-9 exerts anti-inflammatory properties through selective suppression of IFN-γ/IL-17 production without altering type 2 cytokine profiles (68). This evidence suggests that Gal-9 is a relevant therapeutic target.

Melanoma, also known as skin cancer, is a malignant tumor derived from melanin cells (69). Early diagnosis can improve the survival rate of melanoma patients, so more sensitive biomarkers and targeted combined immunotherapy targets need to be developed. Studies have shown that in patients with stage IV melanoma, Gal-9 expression is associated with a relatively high survival rate (70). Although the assessment of Gal-9 expression is not currently part of routine clinical practice for melanoma staging or prognostication, this association highlights its potential as a novel prognostic biomarker. High expression of Gal-9 promotes the growth of tumors, including promoting tumor angiogenesis (20). Gal-9 may promote NK cell-mediated antitumor effects via amplification of macrophages (71). Tumor immunology studies reveal CD44-associated clonal expansion of Gal-9⁺ NK cells in melanoma-bearing mice (72). Mechanistically, Gal-9 impedes melanoma-endothelial adhesion and extracellular matrix remodeling (75). Gal-9 expression inversely correlates with metastatic potential: high levels in primary lesions/nevi vs. low expression in metastases (42), demonstrating direct pro-apoptotic effects on melanoma cells. It has been reported that Gal-9 can directly promote the apoptosis of melanoma cells (73). The expression of Gal-9 is high in primary melanoma lesions and nevi, and low in metastatic melanoma lesions (42). This suggests that the Gal-9 overall may be a tumor suppressor. The Gal-9/Tim-3 axis critically mediates cytotoxic T lymphocyte (CTL) exhaustion in melanoma immunity. Experimental Gal-9 knockdown combined with Tim-3 blockade enhances antitumor responses by preventing CTL depletion (74). Complementary to molecular inhibition, Ren et al. recently demonstrated that a tumor-targeted nanodrug (FSGG/siGal-9) combined with photothermal therapy enhances Gal-9 blockade efficacy, promoting cytotoxic T-cell recruitment and melanoma regression in vivo. The dual role of Gal-9 (pro-tumorigenic in the microenvironment vs. tumor-suppressive via direct apoptosis and immune modulation) and its association with survival underscore its complexity and the need for further investigation to determine its precise clinical utility. Notably, solid tumors exhibit predominant Gal-9 expression among immune checkpoints. Preferentially expressed antigen in melanoma (PRAME)—a poor-prognosis tumor antigen—co-regulates multiple checkpoint molecules (PD-1/PD-L1/Gal-9) through epigenetic silencing effects (76). Further validation in larger cohorts and the development of standardized assays for Gal-9 detection in tumor tissue or liquid biopsies are necessary steps towards evaluating its potential integration into clinical decision-making algorithms or as a target for novel immunotherapies.

Systemic sclerosis (SSc) represents a multi-organ autoimmune disorder marked by progressive cutaneous/organ fibrosis and microvascular pathology (77). Current therapeutic strategies involving cyclophosphamide, mycophenolate mofetil, and hematopoietic stem cell transplantation show efficacy in dermal sclerosis improvement. Nintedanib and tocilizumab were temporarily not found to effectively relieve skin hardening effects (78). As a biomarker, Gal-9 is inferior in sensitivity and specificity to chemokine cytokine ligand 18 (CCL18) (79). The combination of multiple biomarkers can improve the efficiency of disease detection. The cytotoxic T lymphocyte-associated antigen 4 fusion protein demonstrates antifibrotic activity in SSc models (80). Pathological Gal-9 overproduction in SSc patients suppresses type 1 cytokine generation through CD4⁺ T cell modulation (81). Dermal fibroblasts facilitate Treg-to-type 2 cells conversion while secreting Gal-9 to inhibit IFN-γ expression in infiltrating lymphocytes, thereby promoting fibrotic progression (81). Elevated serum Gal-9 levels in diffuse/limited SSc subtypes correlate with erythrocyte sedimentation rate, mortality risk, and visceral involvement (82). Both Tim-3 overexpression and serum elevation exhibit positive associations with cutaneous induration severity, while PD-1/Tim-3 co-expression participates in SSc pathogenesis (83, 84).

Herpes simplex virus (HSV) infections predominantly manifest as mucocutaneous lesions, with atypical presentations including eczema herpeticum in atopic dermatitis, herpes gladiatorum in athletes, and disseminated eruptions in Darier/Sézary syndromes (85). Erythema multiforme is common in HSV infection. Currently, the main treatment for herpes simplex virus is acyclovir, and drug resistance is relatively low (86). However, HSV vaccine development, gene therapy, and various pathological mechanisms are still under further study. Some studies have shown that Gal-9 promotes recovery from HSV infection by reducing type 1 and CD4⁺ T cells, increasing the expression of anti-inflammatory factors such as TGF-β and IL-10, and downregulating the expression of proinflammatory factors such as IFN-γ and IL-6 (87). Gal-9 can stabilize 4-1BB and control innate immunity to the virus (17). In an animal model of HSV infection, Gal-9-deficient mice presented a stronger CD8⁺ cell response. Moreover, blockade of Gal-9 also reduced the response of Tregs. Intraperitoneal injection of Gal-9 reduces CD8⁺ cell responses (88, 89). Gal-9 participates in blocking CD8⁺ T cells in HSV infection after incubation (89). These findings suggest that blocking Gal-9 may facilitate better and faster clearance of HSV.

Bullous pemphigoid (BP) is an autoimmune disease with antibodies against the dermal‒epidermal adhesion complex, and type 2 cytokines and IL-17 are highly expressed in the peripheral blood and skin (90–92). In autoimmune bullous dermatoses (AIBDs), Immunophenotypic analysis reveals upregulated PD-1/Tim-3 expression in lesional skin, particularly within CD8⁺ T lymphocytes and macrophages (93). Beyond eosinophil cationic protein (ECP) (94), Gal-9 emerges as a prognostic biomarker for BP severity, potentially mediating eosinophil chemotaxis critical to blister formation (95). Monocyte-derived Gal-9 facilitates granulocyte recruitment in BP pathogenesis, with elevated expression observed in epidermal keratinocytes of BP lesions and intestinal epithelia in type 2 immunity-associated food allergies (96). Gal-9 expression was significantly observed in epidermal keratinocytes of BP-affected skin and intestinal epithelial cells of patients with type 2 immunity-associated food allergy (96). The main goal of BP treatment is to control the development of new skin lesions, and a treatment plan needs to be formulated according to different patients (97). Gal-9 and its interaction with mononuclear cells remain to be elucidated.

Psoriasis vulgaris, a chronic immune-mediated dermatosis, features hyperproliferative keratinocytes and psoriasiform plaques (98). Dysregulated Gal-9/Tim-3 signaling contributes to type 1 cells/Th17 imbalance, with experimental Gal-9 administration reducing pathogenic T cell populations (IFN-γ⁺/IL-17⁺) and attenuating epidermal hyperplasia (68, 99). Although lesional Tim-3⁺ CD8⁺ T cells/macrophages are prevalent, the Gal-9/PD-L1 dissociation suggests predominant type 2 immunity pathway involvement (93). While some studies propose Gal-9 as a therapeutic target, clinical data from Nofal et al. show no significant correlation between serum Gal-9 levels and Psoriasis Area Severity Index, though associations with leukocyte/eosinophil counts and hepatic enzymes persist (66, 100). Gal-9 is found to be elevated in patients with psoriasis and may become a new therapeutic target (68, 101). Plasma Gal-9 level was not correlated with psoriasis severity score but was correlated with white blood cell, eosinophil, and ALT levels, indicating that its Gal-9 can reflect the severity of the disease (101). At present, there are still some patients who do not respond to biological agents or have secondary treatment failure; that is, the response to the original effective biological agents has decreased. Moreover, the current price of biological agents is high, and the development of new therapeutic targets is still very promising (102).

The loss of functional melanocytes in the skin or hair and the appearance of white macules. Vitiligo is a common acquired pigmentary disease (103). Vitiligo is fully reversible by suppressing autoimmunity and by promoting the regeneration of a stem-cell niche for melanocytes in hair follicles. Considering the high recurrence rate, more effective targeted therapies are needed (104). Vitiligo pathogenesis involves CD8⁺ T cell-mediated melanocyte destruction through IFN-γ-chemokine axis activation (105). Elevated Tim-3/Gal-9 expression in peripheral blood and peri-lesional skin correlates with depigmentation extent, suggesting their involvement in melanocyte targeting. Co-upregulation of Tim-3/PD-1 on cytotoxic T lymphocytes indicates checkpoint-mediated immune dysregulation, positioning these molecules as potential biomarkers and immunotherapeutic targets (106). CXCL9, a T cell chemoattractant, reliably reflects vitiligo disease activity (105).

Maculopapular exanthema (MPE) is involved in immediate allergic drug reactions, and type 1 cytokines and CD4⁺ T cells play important roles (107). Immunophenotypic analysis revealed type 1 lymphocyte Tim-3 downregulation in malignant pleural effusion (MPE) patients. Exogenous recombinant Gal-9 administration induced Treg expansion concomitant with type 1 population contraction. Type 1 cell levels were restored after Tim-3 blockade (26).

Acute allograft rejection pathophysiology involves proinflammatory type 1/Th17 cell axis activation. The expression of endogenous Gal-9 is associated with the severity of rejection, but exogenous Gal-9 has the opposite effect on graft rejection (108, 109). Gal-9/Tim-3 pathway engagement exerts dual immunoregulatory effects: suppressing CD4⁺/CD8⁺ alloreactive T cell responses through IFN-γ suppression, thereby enhancing allogeneic skin graft survival (110). Importantly, Tim-3 antibody blockade neutralizes donor-derived Treg-mediated graft tolerance in preclinical models (111).