The mucosal chemokine CXCL17 was first characterized in 2006 by Pisabarro et al. (Genentech Inc., San Francisco, CA) as a monocyte-attracting chemokine (Table 1) (37). CXCL17 is also referred to as Vascular Endothelial Correlated Chemokine (VCC-1) or VEGF Co-regulated Chemokine 1, and as Dendritic Cell and Monocyte Chemokine-like Protein (DMC) (37, 38). The CXCL17 gene is located on chromosome 19 at position 19q13.2 (Figure 1A). It consists of four exons that encode a 119-amino-acid propeptide, with an approximate molecular mass of ~13.8 kDa in humans and ~13.6 kDa in mice (37, 38). CXCL17 is constitutively expressed in the oral mucosal tissues, as well as the gastrointestinal, respiratory, and genital tracts (Figure 1B) (22, 55, 56). Additionally, CXCL17 has been implicated in regulating infection and inflammation in the gastrointestinal, respiratory, and female reproductive systems (23). Notably, it exhibits broad antimicrobial activity at mucosal surfaces (24). Nevertheless, the precise mechanisms by which CXCL17 mediates mucosal protection against invading pathogens remain largely unclear.

In 2022, Shabgah et al. reviewed the role of the chemokine CXCL17 in infection and inflammation (23). CXCL17 plays a crucial role in maintaining homeostasis across various mucosal tissues by regulating myeloid cell recruitment, promoting angiogenesis, and controlling microbial populations (23). Their findings confirmed that CXCL17: (1) under homeostatic conditions, exhibits anti-inflammatory, antibacterial, and chemotactic activities; (2) under pathological conditions such as malignancies, promotes angiogenesis, metastasis and cellular proliferation; (3) possesses anti-tumor properties; (3) is implicated in the pathogenesis of diseases including idiopathic pulmonary fibrosis, multiple sclerosis, asthma, and systemic sclerosis; and (5) may serve as a biomarker for disease diagnosis and prognosis via its dysregulation.

In 2024, Dominguez-Lopez et al. reported significant upregulation of CXCL17 expression in conjunctival and corneal mucosal epithelial cells in patients with dry eye disease (57). These findings suggest that corneal and conjunctival epithelial cells within the ocular mucosal immune system (OMIS) may serve as a source of CXCL17 during ocular infections and inflammatory responses (57–59).

Back in 2010, Fallarini et al. were the first to report on the expression of the orphan G protein-coupled receptor (GPR35) on human iNKT cells (60). Later, in 2015, Maravillas-Montero et al. confirmed that CXCL17 signals through GPR35, which was later renamed CXCR8 (61). Several studies have suggested that CXCR8 may have both pro-inflammatory and anti-inflammatory effects (60, 61). More recently, GPR25 has been identified as a receptor for CXCL17 by multiple groups (62, 63).

In 2018, Pease et al. reported the existence of another CXCL17 receptor, CXCR4 (20). Similarly, in 2018, Hill et al. confirmed that CXCR4 is the other receptor for CXCL17, in addition to CXCR8 (20, 35, 64). In 2023, Peace et al. demonstrated that the C-terminal fragments of CXCL17 readily bound glycosaminoglycans (GAGs) and may serve as prototypic inhibitors of CXCR4 function (20, 41, 65). Later, in 2024, Hill et al. confirmed that CXCL17 was an allosteric endogenous inhibitor of CXCR4, an effect that was mimicked by GAGs, surfen, and protamine sulfate (20, 35, 64). Disruption of putative GAG binding domains in CXCL17 prevented CXCR4 binding (20, 35, 64).

A characteristic feature of chemokines is their similar three-dimensional structure, four conserved cysteines that form two disulfide bonds, and binding to glycosaminoglycans (GAGs). Both disulfides are essential for receptor activation, and GAG binding is critical for the directed migration of leukocytes. However, CXCL17 has only three conserved cysteines (instead of four) and thus may not adopt the typical chemokine structure. It also exhibits a distinct distribution of positively charged residues, suggesting a novel GAG-binding mechanism. Another unusual feature of CXCL17 is that it is active only at high concentrations, with optimal activity at concentrations up to 1000-fold higher than those typically observed for most chemokines (µM vs. nM). Below, we review and discuss how CXCL17 regulates the homing of neutrophils, monocytes/macrophages, dendritic cells, and T cells to mucosal tissues (20, 35–39).

Identifying the underlying mechanism of CXCL17’s activity in homeostatic, inflammatory, and pathological situations holds promise for the development of novel treatment strategies. CXCL17 has been implicated in several human pathologies and inflammatory diseases (39, 41, 56, 66–68). More recently, in 2024, Lowry et al. demonstrated that, compared to their littermate control wild-type (WT) mice, CXCL17^(-/-) deficient mice were significantly impaired in peritoneal neutrophil recruitment following lipopolysaccharide (LPS) challenge (69). The CXCL17^(-/-) deficient mice showed: (1) dysregulated levels of inflammatory mediators CXCL1, CXCR2, and IL6, all of which directly impact neutrophil recruitment (69); and (2) no difference in monocyte recruitment following LPS-challenge (69). The study confirmed that the CXCL17 pro-inflammatory chemokine plays a crucial role in early inflammatory responses by promoting neutrophil recruitment to the lungs (69).

In 2014, Burkhardt et al. demonstrated in a mouse model that CXCL17 is a central chemotactic mediator for macrophage recruitment into the lungs (70). Compared with WT mice, Cxcl17^(-/-)mice displayed a significant reduction in the frequency of macrophages in their lungs (70). In addition, the authors detected a concurrent increase in a new population of macrophage-like cells in the lungs of CXCL17^(-/-) deficient mice, characterized by an F4/80⁽⁺⁾CD11c^(mid) phenotype (70). Later, in 2019, Hernandez-Ruiz et al. confirmed that CXCL17^(-/-) deficient mice displayed lower frequencies of macrophages and dendritic cells in the lungs. In 2021, Choreño-Parra et al. reported that CXCL17 production increased in the lungs of mice infected with M. tuberculosis, and that mice treated with recombinant CXCL17 showed increased accumulation of lung myeloid cells (71). Taken together, these results suggest that CXCL17 is a chemoattractant for both macrophages and dendritic cells in mucosal tissues of the respiratory tract.

Since CXCL17 facilitates the recruitment of both macrophages and dendritic cells, two primary antigen-presenting cells (APCs), into mucosal tissues (20, 35–39), we will review how CXCL17 regulates the trafficking and localization of CD4+ and CD8+ T cells to combat invading mucosal pathogens (Figure 3).

Studies have shown that GPR35 activation can stimulate the production of pro-inflammatory cytokines and facilitate the movement of immune cells, including effector CD4⁺ and CD8⁺ T_RM cells, towards inflammatory and infected mucosal tissues of gastrointestinal tract, respiratory tract, and female genital tract (28, 60, 72–80). Conversely, other investigations have suggested that GPR35 possesses anti-inflammatory properties in mucosal tissues by inhibiting the generation of inflammatory mediators and promoting the differentiation of regulatory T cells (T_reg cells) (28, 60, 72–82).

Later, in 2018, we reported that CXCL17 is involved in the recruitment of CD8⁺ T cells that mediate protective immunity against sexually transmitted herpes infection (4, 83). We used herpes simplex virus type 1 (HSV-1) as a model of a genital viral pathogen. We selected HSV-1 because (i) it is widely present in more than 3.7 billion people worldwide and (ii) it is becoming an increasingly common cause of genital infection, mainly in developed countries (84). We found that intravaginal HSV-1 infection of WT B6 mice increased the frequency of HSV-specific IFN-γ-producing cytotoxic CXCR8⁺CD8⁺ T cells in the female genital tract, and that this recruitment was associated with protection against genital herpes infection and disease. In contrast, when compared to WT B6 mice, following HSV-1 infection, the female genital tract of CXCL17^(-/-) deficient mice developed significantly fewer HSV-specific functional IFN-gamma-producing cytotoxic CXCR8⁺CD8⁺ T_RM cells. It generates more exhausted VISTA⁺TIGIT⁺CD8⁺ T cells that fail to control genital herpes infection (4, 83). These observations strongly suggest that the CXCL17/CXCR8 axis is a major regulator of female genital tract-resident CD103^highCD8⁺ T_RM cell responses, which lead to the clearance of genital herpes infection (4, 83). Whether the increase in the number of CXCR8⁺CD8⁺ T cells and CCR10⁺CD8⁺ T cells is due to increased recruitment, formation, retention, and/or expansion of these cells within the female genital tract remains to be determined.

In 2019, Hernandez-Ruiz et al. reported that CXCL17 is also a chemoattractant that suppresses myeloid cell chemoattraction and recruits regulatory T cells (85). Compared with WT littermate mice, CXCL17^(-/-) mice developed more severe disease in a T-cell-dependent model of experimental autoimmune encephalomyelitis (EAE) (85). After immunization with myelin oligodendrocyte peptide, only 44% of CXCL17^(-/-) deficient mice were still alive vs. 90% for WT mice. During EAE, CXCL17^(-/-) mice exhibited reduced myeloid cell infiltration into the CNS and higher serum levels of inflammatory cytokines (85). In 2021, Choreño-Parra et al. reported that, in humans, lower serum CXCL17 levels are observed among active pulmonary TB patients than among subjects with latent TB infection and healthy controls, suggesting a protective role (71).

These reports suggest that CXCL17 may regulate the development of protective mucosal CD4⁺ and CD8⁺ T_RM cells at steady state and during inflammatory and infectious conditions (Figure 3).

The genital tract mucosal surface represents one entry route of herpes simplex virus type 1 and/or type 2 (HSV-1 and HSV-2) (86, 87). Herpes infection is widespread in human populations, with a higher incidence in women than in men (84, 88–91). HSV viruses cause initial infections in the mucocutaneous tissues of the mouth, lips, genital tract, nose, or eyes (92). Most HSV-seropositive women are asymptomatic (ASYMP) (93–96). They do not experience any recurrent herpetic genital disease even though the virus spontaneously reactivates from latency and sheds multiple times each year in their vaginal secretions (89, 90, 97, 98). In contrast, a small proportion of HSV-seropositive women are symptomatic (SYMP) and experience endless recurrences of herpetic disease, usually multiple times a year (86, 87), often requiring continuous antiviral therapy (i.e., acyclovir and derivatives).

Mucosa-resident memory T_RM cell subsets are phenotypically and functionally distinct from conventional T_EM and T_CM cell subsets that circulate through the blood to access lymphoid and non-lymphoid tissues (4–6, 99–101). The mechanisms that regulate the generation, retention, and expansion of genital tract mucosal CD4⁺ and CD8⁺ T_RM cells are distinct from those regulating conventional circulating T_EM and T_CM cells (7–10). Direct experiments in animal models (102–104) and indirect evidence in humans (105, 106) suggest that the induction of robust, polyfunctional T_RM cells residing within the vaginal submucosal tissues contributes to the successful control of herpes infection in the female genital tract (107). However, the female genital tract tissues appear to be immunologically restricted and remain “a closed immunological compartment, “ resistant to the homing of peripheral T cells that may originate from the draining lymph nodes and circulation (108, 109). Genital herpes infection of vaginal epithelial cells (VECs) likely triggered the production of inflammatory cytokines, such as IFN-gamma TGF-beta, IL-1beta, and IL-6, which promotes VECs: (1) to produce mucosal chemokines, such as CXCL17 and CCL28, and (2) increased CXCR8 expression on T cells contributing to homing and retention of antiviral T_RM cells within the female genital tract tissues (Figure 3) (108, 109).

Specifically, CD8⁺ T_RM cells mediate protection against genital herpes in both human and mouse models (90, 103, 110–114). CD8⁺ T_RM cells are maintained in the dermal-epidermal junction of the female genital tract (32, 115). We recently demonstrated the role of CXCL17, a mucosal chemokine, in the development of anti-herpes CD8⁺ T cell memory (T_RM) cells within the female genital tract (4). We showed that following intravaginal HSV-1 infection of WT B6 mice, the CXCL17 production is strongly induced in the female genital tract, and CXCL17 local expression paralleled an increase in both the number and effector function (IFN-gamma-production and CD107^a/b degranulation) of HSV-specific CXCR8⁺CD8⁺ T_RM cells associated with protection against genital herpes infection and disease (4). In contrast, HSV-1 infection of CXCL17^(-/-) deficient mice resulted in an increased number of female genital tract-resident exhausted VISTA⁺TIGIT⁺CD8⁺ T_RM cells, associated with a failure to contain genital herpes infection and disease (4). These results were the first to suggest a crucial role of the CXCL17/CXCR8 chemokine axis in the development of protective female genital tract mucosal CD8⁺ T_RM cells (4). Besides effector CD8⁺ T cells, CD4⁺ T helper cells appeared to indirectly control the migration of CD8⁺ T cells through the secretion of IFN-gamma and local induction of chemokine secretion in the infected female genital tract tissue (34). We recently reported that the CXCL17 chemokine plays a role in CD4⁺ T cell immunity in the female genital tract (4, 83, 116). A recent study also showed that chemokines secreted by a local macrophage network maintained vaginal T_RM cells in memory lymphocyte clusters (MLCs) independently of circulating memory T cells and dendritic cells (51).

In 2018, we reported that CXCR8 is constitutively expressed in female genital tract-derived B cells, T cells, dendritic cells (DCs), and monocytes/macrophages (4). CXCR8 expression on these cells increased following HSV-2 genital herpes infection (4). Moreover, both the frequency of female genital tract-resident CXCR8⁺CD8⁺ T cells and the expression level of CXCR8 were significantly increased following the intravaginal administration of exogenous CXCL10 in HSV-2-infected mice (4). These results suggest that, although the CXCL10/CXCR3 axis contributes to the recruitment of CD8⁺ T cells into the female genital tract, their retention within the mucosal epithelium for an extended period is controlled by the CXCL17:CXCR8 axis (4).

Therapeutic manipulation of the immune system, referred to as immunotherapy, is an attractive strategy for addressing genital herpes disease, HSV-1 and HSV-2 shedding, and ultimately reducing HSV-1 and HSV-2 transmission in the community (93–96). For successful herpes immunotherapy to occur, one must first identify the immune mechanisms underlying the apparent protection observed in seropositive asymptomatic women, who appear to contain infection and disease immunologically. Understanding the mechanisms by which CD8⁺ T_RM cells develop, maintain, and expand in the female genital tract tissue will have a significant impact not only on genital herpes but also on other sexually transmitted infectious pathogens (STIs).

Herpes simplex virus (HSV) can infect the gastrointestinal tract, leading to conditions such as herpes gastritis, herpes proctitis, herpes esophagitis, and herpes colitis, which are rare but serious health concerns in immunocompromised patients (27, 117, 118). While the CCL25 mucosal chemokine is highly expressed in gut-associated lymphoid tissues (GALT), its role in gut immunity and immunopathology in response to herpes gastritis, herpes proctitis, herpes esophagitis, and herpes colitis remains to be fully determined.

The mucosal chemokine CCL25 belongs to the CC chemokine family, which is also known as TECK (Thymus-Expressed Chemokine). Vicari et al. first reported in 1997 that CCL25 is a thymic dendritic cell-specific CC chemokine involved in T-cell development (Table 1) (119). The CCL25 chemokine was first identified in the thymus of mice and mapped to chromosome 8. The primary source of CCL25 in the thymus was thymic dendritic cells and mucosal epithelial cells (Figure 1B) (120). In contrast, bone marrow-derived dendritic cells do not express CCL25. Later, in 1998, the human CCL25 gene (SCYA25) was mapped to chromosome 19 (Figure 1A) (121). Human CCL25 is produced as a 151-amino acid protein precursor (121). In humans, mice, and pigs, CCL25 plays a crucial role in the segregation and compartmentalization of the mucosal immune system by recruiting B and T cells to specific mucosal locations (121, 122). CCL25 attracts an overlapping subpopulation of IgA-secreting B cells, which are concentrated in the small intestine and its draining lymphoid tissues (33, 123–126).

Meurens et al. reported the cloning and the sequencing of ovine CCL25 and the subsequent assessment of its mRNA expression by quantitative PCR in several tissues, including thymus, gut-associated lymphoid tissue, and mammary gland, from young and adult sheep and in the fetal lamb during the development of the immune system (127–129). CCL25 mRNA is highly expressed in the thymus and gut. These results are consistent with observations in mice, humans, and other species (127–129). In fetuses, CCL25 mRNA is expressed early in the thymus, small intestine, and nasal mucosa. Furthermore, their expression increased towards the end of gestation (127–129). Consequently, CCL25 may play a crucial role in the lymphocyte colonization of fetal tissues, thereby facilitating the development of a functional immune system (127–129).

The CCR9 is a G protein-coupled receptor and the main receptor for CCL25 chemokine. Zaballos et al. first reported that the receptor for the CCL25 ligand is C-C chemokine receptor type 9 (CCR9) (130). In 2015, Kim et al. discovered CCR9-mediated signaling through beta-catenin and confirmed CCL25 as a CCR9 antagonist (131). The CCR9 receptor for CCL25 was later confirmed in 2000 by Gosling et al., who reported that it is expressed on dendritic cells (DCs) and T cells (132). CCR9 was also expressed on subsets of mucosal CD4⁺ and CD8⁺ T cells, gamma-delta T cells, plasmacytoid dendritic cells (pDCs), IgA plasma blasts, IgA plasma cells, and intraepithelial lymphocytes (IELs) from the mucosal tissues cited above (133, 134). CCR9 is specifically co-expressed on a subset of gut-homing B and T cells expressing the integrin α4β7, immunoglobulin A (IgA)- secreting B cells from the gastrointestinal tract. CCR9 drives the migration of these B and T cell subsets to gradients of its cognate ligand, the CCL25 chemokine (133, 134). CCR9 expression defines a subset of peripheral blood T cells with a mucosal phenotype and a Th₁ or T-regulatory 1 cytokine profile (135–139). The CCR9 receptor equips these T cells to respond to CCL25 (140). Notably, CCR9 appears to be predominantly expressed on CD8⁺ T cells but less so on CD4⁺ T cell subsets, thereby imposing distinct tissue tropisms on CD4⁺ and CD8⁺ T cells in various mucosal tissues.

Naive CD4⁺ and CD8⁺ T cells require activation in lymphoid tissues before differentiating into effector or memory T cells capable of trafficking to nonlymphoid mucosal tissues (141, 142). Distinct non-recirculating T_RM cell subsets are developed and retained within peripheral non-lymphoid mucosal tissues (4–6, 99–101, 143). These mucosa-resident T_RM cells are phenotypically and functionally distinct from conventional memory T cells, which are defined as T_EM and T_CM cells that circulate in the blood and access lymphoid and non-lymphoid tissues (4–6, 99–101, 143). The mechanisms that regulate the generation, retention, and expansion of mucosal CD4⁺ and CD8⁺ T_RM cells within the mucosal tissues are distinct from those regulating conventional circulating T_EM and T_CM cells (7–10). While there are well-established T-cell-attracting chemokines (i.e., CCL5, CXCL9, CXCL10, CXCL11, and CCL18) (45–49), several mucosal chemokines also play a role in shaping mucosal T cell immunity (92, 144–159). Tissue-selective trafficking of memory and effector CD4⁺ and CD8⁺ T cells is mediated by distinct combinations of adhesion molecules and chemokines (20, 35–39). The role of CCR9-CCL25 in immune cell migration, including effector and regulatory T cells, is well studied in both homeostasis and disease (133).

Early in 2012, Kathuria et al. demonstrated the generation of antigen-specific T cells following systemic immunization with a DNA vaccine encoding CCL25 chemokine as immunoadjuvant (172).

In 2020, using a Chemokine-Adjuvanted Plasmid DNA expressing CCL25, Aldon et al. demonstrated in mice enhanced both splenic and intestinal, vaginal Ag-specific antibodies and T-cells primed by intramuscular immunization (173–176). This indicates that CCL25, a genetic chemokine immunoadjuvant, enhances vaccine Antigen-Specific humoral and cellular responses and induces homing to the gastrointestinal and female genital tract mucosae (173–176).

Similarly, in 2020, Hsu et al. demonstrated that parenteral administration of a virus-like particle-based vaccine formulated with CCL25 chemokine as an immunoadjuvant induced protective systemic and mucosal immune responses (144). This suggests that VLPs formulated with the CCL25 immunoadjuvant may serve as a potential vaccine strategy to protect against enteric viral infections.

More recently, in 2020, McKay et al. investigated the programming of T and B cells to home to the gastrointestinal and female genital tracts using genetic chemokine adjuvants (173). BALB/c mice were primed intramuscularly with plasmid DNA encoding a model Ag HIV-1 Env gp140 and selected chemokines/cytokines and boosted intravaginally with gp140 recombinant protein (173). CCL25 enhanced splenic and vaginal Ag-specific T cell responses, whereas CCL28 increased the levels of specific T cells only in the female genital tract (173). The levels of Ab were modulated in the systemic circulation, as well as the vaginal vault and intestinal lumen, with CCL20 playing a central role (173).

CCL28 is a β- or CC chemokine that is involved in host immunity through the interactions with its receptors CCR10 and CCR3. CCL28 chemokine is expressed by columnar epithelial cells in the gut, lung, breast, and salivary glands. One or both receptors are expressed by B and T cells, eosinophils, mast cells, and inflammatory cells, and drive their mucosal homing (33, 124, 125, 190, 198–201). The CCR10 Receptor, originally called orphan receptor GPR2, is a receptor for CCL28 (124, 125, 132, 190, 200, 202). CCR10 is most highly expressed in eosinophils and basophils but is also expressed in Th1 and Th2 cells and airway epithelial cells. The CCR10 receptor has been implicated in skin inflammation and is involved in the recruitment of regulatory T cells (T_regs) into mucosal layers (188, 203–215). The CCR10/CCL28 axis influences the initiation and progression of mucosal inflammation in asthma by regulating the recruitment and retention of leukocyte populations in healthy and inflamed genital tracts.

The CCR receptor CCR3 is also a receptor for CCL28 that attracts B and T cells to mucosal tissues (26, 190, 216–226). Thus, CCR3 contributes to allergic reactions. CCR3 is also known as CD193 (26, 190, 219–226).

The role of CCL28 chemokine in T cell trafficking and intestinal immunity was reported back in 2002 by Kunkel, Campbell, and Butcher (166). The CCL28 chemokine regulates the chemotaxis of cells that express the chemokine receptors CCR3 and CCR10. CCL28 is expressed by columnar epithelial cells in the gut, lung, breast, and salivary glands. It drives the mucosal homing of T cells expressing CCR10 and the migration of eosinophils expressing CCR3 (33, 125, 190). This chemokine is constitutively expressed in the colon. Still, its levels can be increased by pro-inflammatory cytokines and certain bacterial products, implying a role in effector cell recruitment to sites of epithelial injury (227–231). CCL28 has also been implicated in the migration of IgA-expressing cells to the mammary gland, salivary gland, intestine, and other mucosal tissues (227–231). It has also been shown to be a potential antimicrobial agent effective against specific pathogens, including Gram-negative and Gram-positive bacteria, as well as the fungus Candida albicans (227–231).

In humans, mice, and pigs, CCL28 plays a crucial role in the segregation and compartmentalization of the mucosal immune system by recruiting B and T cells expressing CCR10 to specific locations (125, 147, 188, 232–236), facilitating the migration of eosinophils expressing CCR3 (190, 237). CCL28 plays a crucial role in the migration of IgA-expressing cells to the mammary glands (238), salivary glands, intestines (123), and other mucosal tissues (33). In 2006, Eksteen et al. demonstrated that epithelial inflammation is associated with CCL28 production and the recruitment of regulatory T cells expressing CCR10. The authors suggested that CXCR3 promotes the recruitment of T_regs to inflamed tissues, and CCR10 allows them to respond to CCL28 secreted by epithelial cells, resulting in the accumulation of tissue-resident CCR10⁺ T_regs at mucosal surfaces. An indispensable role for CCL28 and its receptor, CCR10, in the accumulation of IgA antibody-secreting B cells in the lactating mammary gland was reported by Wilson and Butcher in 2004 (238, 239). In 2008, Morteau et al. reported an essential role for the chemokine receptor CCR10 in the accumulation of IgA antibody-secreting cells (239).

In 2022, Terefe et al. confirmed that CCR10 is involved in the trafficking, recruitment, and infiltration of T cells into epithelia, such as the skin, through its interactions with the chemokines CCL27 and CCL28 (232). Furthermore, CCL28 binds to the chemokine receptor CCR3. CCR3 and/or CCR10 receptors are expressed by T cells that are implicated in the pathogenesis of asthma.

Lazarus et al. demonstrate that the mucosae-associated CCL28 chemokine is selectively chemotactic for IgA-secreting B cells (ASC): CCL28 attracts IgA- but not IgG- or IgM-producing ASC from both intestinal and non-intestinal lymphoid and effector tissues, including the intestines, lungs, and lymph nodes draining the bronchopulmonary tree and oral cavity (33, 123–126). These findings suggest a broad and unifying role for CCL28 in the physiology of the mucosal IgA immune system (33, 123–126).

Indirect evidence in humans (105, 106) and direct experiments in animal models (102–104) suggest that successful control of herpes infection is associated with induction of robust and polyfunctional T_RM cells that reside within the vaginal sub-mucosal tissues (107). However, the female genital tract tissues appear to be immunologically restricted and remain “a closed immunological compartment, “ resistant to the homing of T cells that may originate from the draining lymph nodes and circulation (108, 109). Genital herpes infection of VECs likely triggered the production of inflammatory cytokines, such as IFN-gamma, TGF-beta, IL-1beta, and IL-6, which promote VECs: (1) to produce CCL28, and (2) increased CCR10 expression on local T cells contributing to homing and their retention of even more T_RM cells within the female genital tract tissues (Figure 3; Table 2) (108, 109). Moreover, following genital herpes infection, increased T-cell production, which attracts CXCL9 and CXCL10 chemokines, likely contributes to the recruitment of additional T_RM cells within the female genital tract tissues (Figure 4) (108, 109).

We recently performed bulk RNA sequencing of herpes-specific CD8⁺ T cells isolated from the peripheral blood mononuclear cells (PBMC) of women infected with HSV who were either symptomatic (SYMP) or asymptomatic (ASYMP) (83). Our analysis revealed distinct regulation of the chemokine pathway and significantly increased expression of the CC28 chemokine receptor, CCR10, in ASYMP compared with SYMP herpes-infected women. We further confirmed this result by flow cytometry analysis of immune cells from PBMCs of SYMP and ASYMP HSV-infected women (83). Thus, we detected increased CCR10 expression on HSV-specific CD8⁺ T cells in ASYMP compared with SYMP women, both at the transcriptional and translational levels (83). We also explored the role of the mucosal chemokine CCL28/CCR10 axis in protection against genital herpes infection and disease in mouse models (83). We used SYMP and ASYMP mice infected intravaginally with HSV. We found that an increased expression of CCL28 chemokine in the VM was associated with protection in ASYMP mice but not in SYMP mice. We further confirmed increased expression of the chemokine CCL28 in the VM of HSV-2-infected ASYMP mice using Western blotting and immunohistochemistry (83). Moreover, we found that the number of female genital tract CCR10⁺CD8⁺ T cells increased following intravaginal administration of the chemokine CXCL10 in HSV-2-infected mice (83). The corresponding increase in CCR10-expressing genital tract mucosal-resident memory T cells was demonstrated by flow cytometry, further suggesting a critical role for the mucosal chemokine CCL28/CCR10 axis in protective T-cell immunity against genital herpes (83).

The immune profile of cells in the VM of infected mice showed that CCL28^(-/-) mice had fewer CCR10-expressing CD8⁺ and CD4⁺ T cells, as well as a lower frequency of CCR10⁺CD44⁺ memory CD8⁺ T cells, compared with WT mice (83). The role of the CCL28/CCR10 chemokine axis in the mobilization of IgA-secreting cells in the mucosa is well established in the literature (83). To further elucidate the role of CCL28 and its receptor in humoral immunity during genital herpes infection, we investigated CCR10 expression on B cells in the VM. Interestingly, most memory B cells in the VM of these mice expressed the chemokine receptor CCR10. There was also a decrease in the frequency of CD27⁺B220⁺ memory B cells in these CCL28^(-/-) mice (83). The increased frequency of CCR10-expressing CD8⁺ T cells in asymptomatic individuals with herpes may suggest an association between the mucosal chemokine CCL28 and protection against herpes infection. Thus, the mucosal chemokine CCL28 mediates protection from disease severity by mobilizing both CCR10⁺CD44⁺ memory CD8⁺ T cells and CCR10⁺B220⁺CD27⁺ memory B cells to the VM (83) (Table 2).

We recently generated novel knockout (KO) mice for the CCL28 chemokine and the CCR10 receptor and are currently studying their phenotypes upon infection with virulent and non-virulent strains of HSV-1 and HSV-2.

During the last 20 years, only a single vaccine strategy (adjuvanted recombinant HSV glycoprotein D (gD), with or without gB) has been tested and retested in clinical trials (91). Despite inducing potent HSV-specific neutralizing antibodies, this strategy failed to meet the primary endpoint of reducing herpes disease (240). These failures emphasize the need to induce T-cell-mediated immunity (241). Following the resolution of viral infections, a long-lived memory CD8⁺ T cell subset that protects against secondary (2°) infections is generated (242–246). This memory CD8⁺ T cell subset is heterogeneous but can be divided into three major subsets: (1) effector memory CD8⁺ T cells (CD8⁺ T_EM cells); (2) central memory CD8⁺ T cells (CD8⁺ T_CM cells); and (3) CD8⁺ T cells (CD8⁺ T_RM cells) (108). The three significant memory subpopulations of T cells differ in their phenotypes, functions, and anatomical distributions. T_CM cells are CD62L^highCCR7^highCD103^low. T_EM cells are CD62L^lowCCR7^lowCD103^low. T_RM cells are CD62L^lowCCR7^lowCD103^highCD11a^highCD69^high (108, 247, 248). CD8⁺ T_RM cells are found in the female genital tract and offer protection in mouse models of genital herpes (113). CD8⁺ T_EM cells are also found in the dermal-epidermal junction in the female genital tract (32, 115). Once formed, T_RM cells do not re-enter the circulation and play an essential role in locally guarding mucosal tissues against secondary (2°) infections. However, the precise mechanisms by which non-circulating mucosa-resident memory CD8⁺ T_RM cells are formed, maintained, and expanded remain incompletely understood. In a recent study, we found that a high frequency of CD8⁺ T_RM cells is retained in the female genital tract of HSV-infected asymptomatic mice compared to symptomatic mice and that this is associated with CCL28 mucosal chemokine production. Specifically, we demonstrated that higher frequencies of CCL28-dependent antiviral CD8⁺ T_RM cells in the female genital tract are a key mediator of protection against genital herpes, consistent with previous reports (90, 103, 110–112, 115, 249). Since the primary cell target of HSV-2 is VECs, the key to achieving anti-herpes mucosal immunity is likely to boost the frequencies of HSV-specific CD8⁺ T_RM cells in the female genital tract that can expand locally and persist in the long term. CD8⁺ T_RM cells persist long-term in tissues and are often located at the epithelial borders of mucosal tissues (29, 250–253). However, little information exists on the mechanisms regulating the formation, retention, and expansion of vaginal-mucosa-resident CD8⁺ T_RM cells. This report is the first to show that CCL28/CCR10 chemokine axis-mediated signals may be required for high frequencies of female genital tract antiviral CD8⁺ T_RM cells. It remains to determine the mechanism of expansion and long-term retention of these CD8⁺ T_RM cells within the female genital tract. Such knowledge would inform the design of innovative vaccines that induce CD8⁺ T_RM cell-mediated protection against genital herpes. Collectively, this knowledge could significantly enhance our understanding of mucosal immunity and provide a unique opportunity to develop a robust, long-lasting genital herpes vaccine, with a substantial impact on the epidemiology of this disease. To our knowledge, our study represents the first in-depth analysis of the role of the CCL28/CCR10 chemokine axis in anti-herpes T- and B-cell responses in the VM during HSV-2 infection (83) (Table 2). We demonstrated that following intravaginal HSV-2 re-infection of B6 mice, high production of the CCL28 chemokine in the VM was associated with increased infiltration of CCR10⁺CD44⁺ memory CD8⁺ T cells and CD27⁺B220⁺ memory B cells in the VM (83) (Table 2). Our findings could further aid in immunotherapeutic approaches for genital herpes. In 2020, Aldon et al. demonstrated that the use of Chemokine-Adjuvanted Plasmid DNA expressing CCL28 increased the levels of specific T cells only in the female genital tract (173–176). This indicates that CCL28, a genetic chemokine immunoadjuvant, enhances vaccine Antigen-Specific humoral and cellular responses and induces homing to the female genital mucosa (173–176).

In 2021, Hu et al. demonstrated that CCL28 facilitated HSV-2 gD-induced protective systemic immunity against genital herpes (233). They showed that CCL28 enhanced robust antibody responses to the HSV-2 gD ectodomain (gD-306aa) and to Th1- and Th2-like responses that were recalled after the HSV-2 challenge. Interestingly, as expected, the mucosal chemokine CCL28 appeared to be more effective than CCL19 in promoting gD-specific immune responses and in promoting T-cell migration to secondary lymphoid tissues. Notably, both CCL19 and CCL28 significantly facilitated gD-induced protective mucosal immune responses in the genital tract. The findings collectively highlight the potential of the mucosal chemokine CCL28, in combination with gD, as a strategy for controlling HSV-2 infection. Recently, Huppler et al. demonstrated that CCL28 is also a potent therapeutic agent for Oropharyngeal Candidiasis (254).

In 2022, Terefe et al. confirmed that CCR10 is implicated in the trafficking, recruitment, and infiltration of T cells into epithelia via ligation by the chemokines CCL27 and CCL28 (232).

The above reports support the use of mucosal chemokines as an immune adjuvant to augment parenteral subunit vaccine-induced B- and T-cell immunity at mucosal surfaces (Table 2).

Although CXCL14 induces chemotaxis of monocytic cells at high concentrations, its physiological role in leukocyte trafficking remains elusive (276, 277). CXCL14 primarily regulates T cell migration but also plays a distinct role in antimicrobial immunity and is proposed to combat bacteria at the earliest stage of infection, well before the establishment of inflammation (31).

In 2016, Pyeon et al. demonstrated the protective role of the CXCL14 chemokine in preventing human papillomavirus (HPV) infection, which can lead to cancer, through a mechanism that induces NK, CD4^+,and CD8⁺ T cells (278–283). Conversely, immune evasion is demonstrated by HPVs, which suppress antitumor immune responses through epigenetic downregulation of CXCL14 expression (282). Pyeon et al. then analyzed gene-expression changes across all known chemokines and their receptors using our global gene-expression datasets from human HPV-positive and -negative head and neck cancer and cervical tissue specimens at different disease stages (282). While many proinflammatory chemokines are upregulated throughout cancer progression, CXCL14 is markedly downregulated in HPV-positive cancers in humans (282). Restoration of CXCL14 expression in HPV-positive mouse oropharyngeal carcinoma cells clears tumors in immunocompetent syngeneic mice but not in Rag1-deficient mice (282).

Furthermore, CXCL14 re-expression significantly increases the infiltration of natural killer (NK) cells, CD4^{+ T cells}, and CD8⁺ T cells into the tumor-draining lymph nodes in vivo (282). In vitro transwell migration assays show that CXCL14 re-expression induces chemotaxis of NK, CD4⁺, and CD8⁺ T cells (282). Later, in 2019, the Pyeon et al. group demonstrated that CXCL14 suppresses human papillomavirus-associated head and neck cancer through antigen-specific CD8⁺ T-cell responses by upregulating MHC-I expression (279, 280). In 2024, based on the above strong results, Pyeon et al. proposed targeting the CXCL14 chemokine signaling pathway as a practical approach for cancer immunotherapy to halt HPV-cancer progression (278).

Given its roles in NK, CD4⁺, and CD8⁺ T-cell homing to tissues, CXCL14 stands out as a promising candidate for novel cell immunotherapies (Figure 3). However, it remains to be determined: the molecular mechanism that regulates CXCL14-mediated activity; how to deliver CXCL14; the appropriate dose; and which combinations with existing approved therapies may enhance NK, CD4⁺, and CD8⁺ T cell responses. We recently generated knockout (KO) mice for the CXCL14 chemokine and are currently studying their phenotype upon infection with HSV-1 and HSV-2. Preliminary, unpublished data suggest that CXCL14 plays a significant role in the homing of CD4+ and CD8+ T cells to peripheral tissues. In CXCL14^(⁻/⁻) mice intravaginally infected with HSV-2, we observed increased disease severity and mortality, accompanied by reduced infiltration of CD4⁺ and CD8⁺ T cells at the site of infection and elevated viral loads.

In addition to CXCL14, in 2023-2024, Han et al. reported that CXCL13, primarily produced by CD4⁺ T cells, is an important chemokine involved in the recruitment of CXCR5-expressing naïve B and T follicular helper cells to the lungs ( (284) and Abstract AAI, 2024).

In summary, the initial differentiation of T_RM cells is primarily driven by local tissue signals, including cytokines such as TGF-β, IL-15, and IL-33, as well as transcription factors like Hobit and BLIMP-1 (285, 286). The chemokines listed here do not primarily generate the T_RM lineage itself but create the necessary microenvironment for their homing and subsequent survival/retention. (285, 286). (1) Homing: CCL25 and CCL28 act as crucial “pull” signals, expressed constitutively by epithelial cells, guiding circulating T cell precursors to specific mucosal sites (e.g., small intestine for CCL25) where they can differentiate into T_RM cells (285, 286). CXCL17 also functions as a chemoattractant for T cells and antigen-presenting cells (APCs) to mucosal tissues. (2) Maintenance: Once established, these chemokines, along with adhesion molecules such as CD103 (which binds to E-cadherin) and CD69 (which prevents S1P-mediated egress), contribute to the long-term residency and survival of T_RM cells in their respective niches (285, 286). The continuous presence of the chemokines acts as a “keep” signal, retaining cells locally. (3) Reactivation: Upon local antigen re-encounter, pre-existing T_RM cells are rapidly reactivated (e.g., secreting IFN-γ \ and TNF-α) to control infection and recruit circulating immune cells, but the specific role of these four chemokines in the reactivation process itself is less defined compared to their roles in homing and maintenance (285). The expression profile during inflammation may change (e.g., elevated CCL28/CXCL17 in dry eye or infection), but their primary role remains homing/retention (285, 286).