2.1 Origin and formation of T_RM cells

T_RM cells originate from the activation of naïve T cells, which, upon encountering antigens, differentiate into various subsets, including effector and memory T cells.²⁶ The differentiation into T_RM cells is a critical process that involves a series of cell fate decisions governed by a complex interplay of transcriptional regulators and environmental cues. Memory T cells further diversify into T_EM cells, T_CM cells, and T_RM cells, which are distinguished by their residence in peripheral tissues. Recent epigenetic studies have shown that long-lived memory CD8⁺ T cells originate from a subset of effector CD8⁺ T cells that are capable of re-expressing genes typically associated with a naïve state, facilitated by an open chromatin state that allows rapid effector function upon re-encountering an antigen.^{21, 27}

The formation of T_RM cells is intimately linked to the expression of CD127 and killer cell lectin-like receptor G1 (KLRG1) on effector T cells.²⁸ CD127^high effector T cells, identified as memory precursor effector cells (MPECs), are particularly adept at giving rise to both resident and circulating memory T cells.^{29, 30} These cells express high levels of antiapoptotic molecules, enabling them to persist and provide long-term protective immunity. In contrast, KLRG1^highCD127^low cells give rise to short-lived effector cells (SLECs), which are important for immediate immune responses but do not contribute to long-term immunity.³⁰ Longitudinal tracking of T cells has revealed the marked developmental plasticity of KLRG1^highCD8⁺ effector T cells. These cells have the capacity to downregulate KLRG1 expression in a Bach2-dependent manner, allowing them to differentiate efficiently into a variety of memory T-cell lineages.³¹ This adaptive response endows them with versatility that makes them highly effective in both antiviral and antitumor immunity. A pivotal role in T_RM cells’ formation is played by the transcription factors Hobit and Blimp1, whose expression suppresses the expression of genes related to tissue egress, thereby promoting a transcriptional program conducive to tissue residency.³² Additionally, microenvironmental cues, such as transforming growth factor β (TGF-β) and interleukin 15 (IL-15) expression, are essential for the formation of long-lived memory T cells and their specific localization within tissues.^{33, 34}

In essence, the emergence of T_RM cells is a tightly controlled process shaped by intrinsic cellular programs and extrinsic environmental factors, ultimately producing a subset of T cells uniquely adapted for tissue-specific immunity.

2.2 Cellular biomarkers of T_RM cells

T_RM cells exhibit unique surface markers that are critical for their tissue-specific homing and retention. Key markers include CD103, CD69, CD49a, and CXCR6 expression, which enable T_RM cells to inhabit peripheral tissues such as the intestine, skin, and liver, where they perform immunosurveillance and exhibit enhanced antigen recall capacity. Although T_RM cells possess a set of fundamental transcriptional markers, tissue-specific microenvironments induce organ-specific genetic responses, allowing T_RM cells to adapt to their tissues of residence.

2.3 Maintenance and activation of T_RM cells

The persistence of T_RM cells in peripheral tissues is fundamental to their role in long-term immunological memory and protection against recurrent infections. T_RM cells persist in peripheral tissues by adapting to local cues and resisting dispersal signals.⁶² One key mechanism involves interactions with epithelial cells via the integrin αEβ7, which binds E-cadherin, potentially anchoring T_RM cells and promoting their survival through the expression of prosurvival proteins such as Bcl-2.^{33, 40, 63} However, the role of αEβ7 is context-dependent, and this molecule may not be necessary for T_RM cells maintenance in all tissues. Another critical factor is the regulation of S1PR1, which mediates T-cell egress from the lymph nodes.⁶¹ T_RM cells downregulate S1PR1 and upregulate CD69 expression to prevent egress and promote tissue residency. The expression of the transcription factor KLF2, which influences S1PR1 expression, is downregulated in T_RM precursors upon tissue entry. Additionally, other S1P receptors, such as S1PR5, may play tissue-specific roles in T_RM cell behavior.

Memory T_RM cells maintain long-term immune protection through selective homing and positioning facilitated by adhesion molecules and chemokine receptors. Their retention and metabolic adaptability are supported by the stable expression of markers such as CD69 and CD103. Epigenetic modifications, including DNA methylation and histone modifications, play crucial roles in maintaining T_RM cell-specific gene expression profiles.⁶⁴ Self-renewal mechanisms and interactions with tissue-resident cells provide survival signals and nutritional support, further increasing T_RM cells’ persistence.⁶⁵

Upon re-encounter with pathogens or, upon tumor recurrence, T_RM cells swiftly initiate an immune response within infected tissues. They maintain their tissue residency through the expression of specific transcription factors such as Hobit, which collaborates with Blimp-1 to suppress pathways that facilitate tissue egress while promoting tissue retention.²² They recognize antigens, expand rapidly and locally, and produce proinflammatory cytokines such as IFN-γ and TNF-α to eradicate pathogens. Some T_RM cells differentiate into circulating T_EM cells that patrol the body for new threats, whereas others migrate to draining lymph nodes to form secondary T_RM cells.⁶⁶ These processes collectively position T_RM cells as crucial components of early immune defense, providing effective protection during recurrent infections.

Understanding the nuanced regulatory mechanisms of T_RM cell maintenance and activation is pivotal for developing strategies to enhance their protective functions while minimizing potential adverse effects in chronic inflammatory conditions. This knowledge has important implications for vaccine development and targeted immunotherapies.

2.4 Differentiation pathways of T_RM cells

The differentiation of T_RM cells into tissue-specific subsets integrates intrinsic cellular programs with environmental signals, shaping the roles of these cells in local immune responses against infections and tumors. This section summarizes key insights from recent studies on T_RM cell differentiation.

The initial differentiation of T_RM cells is influenced by the priming of naïve T cells in lymph nodes by DCs, which present pathogen-derived peptides, with cytokines such as TGFβ playing a pivotal role.⁶⁷ Research by Carbone emphasized the role of RUNX3 as a key transcription factor that, in conjunction with TGFβ signaling, directs the development and maintenance of CD8⁺ T_RM cells, particularly in nonlymphoid tissues.⁷ RUNX3 expression orchestrates the expression of genes involved in tissue retention and immune function, and TGFβ expression is a critical upstream signal. This signaling axis is essential for suppressing the expression of tissue egress receptors such as S1PR1 and promoting the expression of tissue retention molecules such as CD103.⁶⁸ T_RM cells diversify into subsets adapted to their tissue microenvironments. For example, in the urogenital tract, T_RM cells must adapt to a unique set of challenges, including sexually transmitted infections and tumor surveillance, which require specific differentiation influenced by local TGFβ and cytokine expression.⁸ In the lungs, the transient nature of T_RM cell residency may be a result of organ-specific features that prioritize the delicate balance between immune protection and tissue integrity.⁶⁹

Understanding the differentiation process of T_RM cells is crucial for leveraging their potential in immunotherapy and vaccine development. Future research should explore the nuances of T_RM cell differentiation to enhance their protective functions while minimizing adverse effects in chronic inflammatory conditions.

3.1 Immune surveillance

T_RM cells play a crucial role in immune surveillance by maintaining a constant presence in tissues to quickly detect and respond to infections and tumor development. T_RM cells can secrete effector molecules such as IFN-γ, TNF, and IL-2, which not only activate local immune responses but also recruit additional immune cells to bolster defense.⁷⁰ Because they are equipped with cytotoxic molecules such as perforin and granzyme, T_RM cells possess the unique ability to directly eliminate tumor cells. By binding to E-cadherin on tumor cells through CD103 expression, T_RM cells stabilize their immunological synapses and consequently increase their cytotoxic efficiency.⁷¹ Furthermore, T_RM cells contribute to maintaining tumor–immune equilibrium, thereby curbing tumor growth and spread. Their memory function provides long-term protection, making them vital for counteracting recurring threats. The unique metabolic features and rapid response capabilities of T_RM cells underscore their essential role in adaptive immunity as a first line of defense against various pathogens and tumors. Delving into the dynamics of T_RM cells activity reveals that their role clearly transcends mere surveillance. In the elimination phase, T_RM cells actively engage and destroy tumor cells, either directly through cytotoxic mechanisms or indirectly by modulating the immune microenvironment. Despite potential exhaustion, they persist in the equilibrium phase, maintaining a balance that hinders tumor advancement. However, if T_RM cells succumb to senescence, the escape phase becomes a reality as the tumor maneuvers to evade weakened immune control, which underscorse the imperative need to understand and bolster T_RM cells’ function in cancer immunotherapy strategies.⁷²

3.2 Cellular interactions

3.2.1 Interaction between T_RM cells and macrophages

T_RM cells and lung-resident macrophages (MLRs) engage in interactions that are essential for immune surveillance and response in both homeostatic and disease contexts (Figure 1). Using ex vivo lung perfusion (EVLP) technology, researchers have discovered that human lung T_RM cells and MLRs colocalize, particularly around airways, where they participate in intricate immunological interactions. Mechanistically, MLRs interact with highly PD-1-expressing T_RM (PD-1^high T_RM) cells by providing essential costimulatory signals.⁷³ This interaction is pivotal, as it promotes the expression of the activation marker CD107a on the surface of PD-1^high T_RM cells, thereby priming these cells for a more effective immune response. Furthermore, this interplay between MLRs and PD-1^high T_RM cells promotes the release of crucial cytokines, such as IFNγ and TNFα. These cytokines play pivotal roles in recruiting and activating other immune cells, reinforcing the local immune response.

In the non-small-cell lung cancer (NSCLC) TME, a subset of tumor-associated macrophages (TAMs), characterized by an M1-like phenotype (M1^hot TAMs), has been found to significantly increase T_RM cell infiltration and survival.⁷⁴ These M1^hot TAMs express high levels of chemokines, including CXCL9, which are critical for attracting activated TH1 T cells and T_RM cells expressing the chemokine receptor CXCR3. Moreover, M1^hot TAMs provide the essential fatty acids for the sustenance of T_RM cells, facilitating their infiltration and function within the tumor. The presence of M1^hot TAMs is correlated with a greater density of CD8⁺ T_RM cells within the tumor and is associated with improved patient outcomes. However, the TME is highly heterogeneous, and the presence of M2-like TAMs can present a significant challenge. M2-like TAMs, which are typically associated with immunosuppression, compete with T_RM cells for critical nutrients such as fatty acids, potentially impairing the long-term survival and antitumor efficacy of T_RM cells.⁷⁵ This competitive dynamic underscores the importance of the TAM phenotype in shaping the overall immune response against cancer. Indeed, M2-like TAMs contribute to a tumor-promoting microenvironment by secreting anti-inflammatory cytokines such as IL-10 and TGF-β, which can dampen the activity of T_RM cells and hinder their ability to control tumor progression. Furthermore, recent studies have suggested that the polarization of macrophages toward the M1 or M2 phenotype is not static but can be dynamically regulated by environmental cues within the TME. Therefore, therapeutic strategies that target the modulation of TAM polarization, particularly those that promote the M1 hot phenotype, could be pivotal in enhancing T_RM cell-mediated antitumor immunity. For example, therapies designed to increase CXCL9 expression or fatty acid availability within the TME could improve T_RM cells’ function and survival, providing a new avenue for cancer immunotherapy.

In summary, the interaction between T_RM cells and macrophages is a key determinant of immune efficacy in both homeostasis and cancer. By understanding the molecular mechanisms that govern these interactions, we can develop targeted therapeutic strategies that promote the beneficial aspects of TRM–macrophage crosstalk while mitigating the suppressive influences of M2-like TAMs. These findings highlight the potential of modulating the T_RM–macrophage axis as a promising approach for improving immunotherapeutic outcomes in cancer.

4.1 T_RM cells in cancer

The TME exhibits highly complex heterogeneity among different types of tumors, and a variety of immune cells are present in tumors, including T cells, B cells, TAMs, neutrophils, DCs, myeloid-derived suppressor cells, and natural killer (NK) cells.^{94, 95} T_RM cells include CD4⁺ T_RM cells and CD8⁺ T_RM cells. CD4⁺ T_RM cells respond mainly to viruses, parasites, and fungal pathogens, whereas CD8⁺ T_RM cells play crucial roles in tumor immune surveillance and immunotherapy.^{96, 97} In addition, the intratumoral infiltration of CD8⁺ T_RM cells is closely related to favorable prognostic outcomes in patients with cancer.⁹⁸ Tumor-localized CD8⁺ T_RM cells in solid tumors are frequently defined by the expression of CD103, CD69, and/or VLA-1 (CD49).^{99, 100} Over the past decade, T_RM cells expressing CD69 or CD103 have been identified in multiple human solid tumors, including glioblastoma (GBM), hepatocellular carcinoma (HCC), esophageal squamous cell carcinoma (ESCC), gastric cancer (GC), colorectal cancer (CRC), intrahepatic cholangiocarcinoma (ICC), pancreatic ductal adenocarcinoma (PDAC), NSCLC, nasopharyngeal carcinoma (NPC), laryngeal squamous cell carcinoma (LSCC), triple-negative breast cancer (TNBC), bladder cancer, cervical cancer, endometrial cancer, head and neck cancer (HNSC), melanoma, and urogenital tumors.^101-116

4.2 T_RM cells in infections

4.3 T_RM cells in chronic inflammatory diseases

Chronic inflammatory diseases can affect any organ or tissue in the body, including the central nervous system, gut, skin, joints, and muscles. These conditions include chronic virus infection, autoimmune conditions, and neurodegenerative disorders.^{177, 178} CD4⁺ and CD8⁺ T_RM (sub) cell populations are enriched in inflammatory diseases, and T_RM cell infiltration in tissues positively correlates with chronic inflammatory disease activity.⁶⁰

CD4⁺ T cells play a critical role in maintaining the long-term presence of CD8⁺ T_RM cells in the CNS by supporting their cytotoxic functions and contributing to progressive autoimmune-driven neuronal damage.^{169, 170} A population of CD8⁺ T lymphocytes (CD69⁺CD122⁻PD1⁺CD44⁺CD62L⁻) uniquely accumulates in the CNSs of neuropsychiatric lupus-prone mice.¹⁷⁹ In humans, the population of T_RM cells (CD69⁺CD103⁺CD8⁺) is increased in the cerebrospinal fluid of patients with chronic inflammatory diseases, including multiple sclerosis, as well as neurodegenerative diseases, such as Alzheimer's disease and Parkinson's disease, compared with controls.¹⁸⁰ T_RM cells from aged patients promote inflammation following ischemic stroke.¹⁸¹ These findings indicate that T_RM cells significantly influence the immunopathology of various chronic inflammatory processes in the CNS, potentially contributing to neurological decline. CD103⁺CD161⁺CCR5⁺CD4⁺ T_RM cells, which are specific to patients with Crohn's disease, produce significant amounts of type 1 inflammatory cytokines, thereby orchestrating the local inflammatory response.¹⁸² Additionally, CD4⁺ T_RM cells with a Th17 signature (as well as CD8⁺ T_RM cells) contribute to disease pathogenesis through IFN-γ induction and subsequent chemokine production in myeloid cells.¹⁸³ Highly activated CD8⁺ T_RM cells, which mediate bile duct damage and disease progression by producing IL-17 and IL-22, play important roles in primary sclerosing cholangitis and primary biliary cholangitis.^{184, 185} In chronic pancreatitis patients, the surface expression of PD-1 was lower on CD8⁺ T_RM cells than on control pancreatic cells while T-bet expression was increased, and the degree of T-bet expression upregulation was significantly associated with PD-1 expression downregulation.¹⁸⁶ In chronic liver diseases caused by viral or parasitic infections, autoimmune hepatitis, and nonalcoholic steatohepatitis, the expansion of T_RM cells can result in heightened liver inflammation.^{177, 187-190} Hopefully, the expansion of these T_RM cells can be directly inhibited by glucocorticoids, thereby reducing hepatic inflammation.¹⁹⁰ Additionally, CD103 expression is observed on renal T cells, particularly on CD8⁺ T cells, in patients with systemic lupus erythematosus (SLE) and in SLE-prone mice.^{191, 192} Infection with S. aureus or C. albicans causes kidney CD4⁺ T_RM cells to adopt an inflammatory Th17 phenotype, exacerbating the disease.¹⁹¹ The activation of kidney T_RM cells by cytokines such as IL-1β, IL-6, and IL-23 through the JAK/STAT pathway amplifies the inflammatory response. The presence of these T_RM cells in chronic inflammatory diseases suggests a role for microbiota-driven inflammation in the promotion of autoimmunity. In psoriasis patients with psoriatic arthritis and atopic dermatitis, T_RM cells, particularly IL-17-producing CD8⁺ T cells, contribute to the chronicity and relapse of the disease by producing cytokines such as IL-4, IL-13, IL-17, and IL-22.^{193, 194} T_RM cells are also involved in other chronic inflammatory skin diseases, including systemic sclerosis, cutaneous lupus erythematosus, frontal fibrosing alopecia, alopecia areata, polymorphic light eruption, allergic contact dermatitis, delayed-type drug hypersensitivity, and fixed drug eruption.^195-201 These cells contribute to the chronicity and recurrence of these diseases through similar mechanisms involving cytokine production and interactions with the skin barrier. Targeting T_RM cells and their proinflammatory cytokines, such as IL-17 and IFN-γ, may offer novel therapeutic approaches to prevent disease flares and promote long-term remission.

However, the persistence of autoimmune diseases in the CNS and the specific role of T_RM cells in driving chronic autoimmunity remain underexplored. While CD8⁺CD103⁺ T_RM cells are clearly abundant in the CNS of patients with autoimmune conditions such as multiple sclerosis (MS), their precise contributions to lesion formation and chronic inflammation are still not fully understood.¹⁷¹ MS, for example, is a chronic inflammatory and immune-mediated disease of the CNS characterized by the interaction of T and B cells, and both EBV-infected B cells and EBV-specific CD8⁺ T cells have been found in MS lesions.²⁰² A subset of these CD8⁺ T cells, which resemble T_RM cells, express markers such as CD103, CD69, GZMB, and PD-1. These T_RM cells may exhibit impaired control over EBV infection, potentially leading to sustained inflammation.²⁰³ In CNS autoimmune diseases, such as MS, T_RM cells not only persist but may amplify local inflammation. The inability of TRM cells to exit the tissue, combined with their enhanced proliferative capacity (indicated by markers such as CXCR6, Ki67, and GPR56), suggests that T_RM cells may contribute to the chronicity of inflammation in CNS autoimmunity. For example, an increase in CD69⁺CD103⁺CD49a⁺PD-1⁺ T_RM cells has been observed in MS lesions, and this increase correlates with active inflammation and ongoing demyelination.¹⁰ Furthermore, T_RM cells may act as local reservoirs of inflammation, reactivating upon exposure to antigens or inflammatory signals in the tissue microenvironment. This is not only observed in MS but also may extend to other CNS autoimmune diseases, such as neuromyelitis optica and autoimmune encephalitis, emphasizing the potential pathogenic role of T_RM cells across various neuroinflammatory conditions.^{204, 205}

4.4 T_RM cells in other diseases

A longitudinal analysis of blood and bronchoalveolar lavage fluid samples from more than 20 lung transplant recipients demonstrated that the long-term persistence of donor lung T_RM cells (expressing CD69, CD103, and CD49a) is associated with a reduced incidence of clinical events that precipitate lung injury, such as primary graft dysfunction and acute cellular rejection.²⁰⁶ Kidney allografts contain donor-derived T cells that predominantly express tissue residency-associated markers such as CD69 and CD103, but it is still unclear how long donor-derived cells persist in the graft. In kidney transplant rejection, allospecific T cells in recipients are recruited to the graft, where they develop into T_RM cells and damage the kidney through the production of cytokines (such as IFN-γ, TNF-α, and GZMB) and cytotoxic molecules.²⁰⁷ On the basis of these findings, we can conclude that a higher number of donor-derived organ T_RM cells is associated with lower rates of organ transplant rejection.

In addition, skin-resident regulatory T cells (Tregs) initially promote inflammation at the keratinocyte layer following barrier breach.²⁰⁸ In burn injuries, skin T-cell populations shift from a resident phenotype to a circulating homing marker profile. The later stages of epithelial injury involve skin-resident γδ-T cells and BATF⁺CCR8⁺ skin Tregs, which are thought to promote physiological wound healing by participating in a tightly regulated response that includes pro- and anti-inflammatory signals and growth factors.^{209, 210}

5.1 Neoadjuvant therapy

Neoadjuvant cancer therapy refers to systemic treatment, including neoadjuvant chemotherapy (NAC) and neoadjuvant immunotherapy (NAI), administered before a definitive surgical operation in treatment-naïve patients. NACs for cancer include anthracyclines (epirubicin and pirarubicin), taxanes (paclitaxel), and platinum agents (carboplatin, cisplatin, and oxaliplatin). Platinum drugs are cytotoxic DNA-damaging compounds that can cause DNA chain breaks, potentially leading to cell apoptosis.²³⁶ This mechanism of action makes them particularly effective in cancer cells with DNA repair defects, such as TNBC cells carrying BRCA mutations.²³⁷ For patients with enriched CD8⁺CD103⁺ T_RM cells, platinum-containing neoadjuvant chemotherapy agents can significantly improve the pathologic complete response (pCR) rate.^{111, 238, 239} In the context of resectable NSCLC, treatment with NAC was associated with increased numbers of CD8⁺CD103⁺ and CD4⁺CD103⁺PD-1⁺TIM3⁻ T_RM cells in the TME and promoted antitumor immunity.²¹⁸ After NAC treatment in ESCC patients, many patients exhibit increased CD103 expression within tumors, and T_RM cells in chemotherapy-experienced patients exhibit improved survival compared with those in chemotherapy-naïve patients.¹¹⁹

Immune checkpoint inhibitor (ICI) therapy has demonstrated marked clinical efficacy. Several monoclonal antibody drugs targeting CTLA-4 and PD-1/PD-L1 have been developed and approved for an increasing number of indications.²⁴⁰ NAIs using ICIs have achieved clinical success in multiple tumor types, which are closely related to intratumoral T_RM cells.²⁴¹ In a clinical study, preventive anti-PD-1 antibody treatment increased CD8⁺ T_RM cell infiltration into the immune microenvironment for an extended period and provided long-term benefits to patients with ESCC.^{135, 242} Treatment with anti-PD-1 and anti-CTLA-4 therapy led to the expansion of intratumoral CD69⁺CD103⁺CD8⁺ T_RM cells, significantly increasing their cytotoxic capacity in TNBC patients and providing local immune protection against tumor rechallenge.²⁴³ In a preclinical murine model, compared with adoptive T_CM cell therapy alone, the combination of adoptive T_CM cells and anti-PD-1 antibody administration delayed the development of intradermal B16-OVA tumors and subcutaneously injected MC38-OVA tumors.¹¹⁹ These results showed that anti-PD-1 treatment expands populations of intratumoral T_RM cells. Similar results have also been reported for metastatic melanoma, MIBC, HNSC, NSCLC, lung adenocarcinoma, cervical cancer, gastrointestinal cancers, and nonmetastatic clear cell renal cell carcinoma.^{138, 230, 244-250} Interestingly, Rainey MA noted that NAIs could facilitate the exit of T_RM cells from tumors into the circulation, thereby promoting systemic tumor immunity via TGF-β signaling.²⁵¹

Unfortunately, ICI therapy can effectively inhibit tumor progression only in “hot” tumors (those with high immune cell infiltration). A lack of infiltrating T cells in the TME limits the antitumor effect of ICIs.²⁵² “Cold” tumors (those lacking immune cells) need to be addressed with other therapies to increase immune cell infiltration in the TME, thereby transforming them into “hot” tumors and improving sensitivity to ICIs.²⁵³ Combining chemotherapy, radiotherapy, or chemoradiotherapy (CRT) with ICIs is a promising therapeutic strategy and is currently being used and evaluated in the clinic.^254-257 In patients with early-stage TNBC, the presence of T_RM cells in the breast tissue is associated with improved long-term survival following treatment with chemotherapy combined with anti-PD-1 therapy.²⁴³ CRT-exposed TMEs are highly enriched in CD103⁺CD8⁺ T_RM cells in both patients with colorectal cancer and CT26 tumor-bearing mice.²⁵⁷

5.2 Radiotherapy

High infiltration of CD103⁺ T_RM cells was strongly associated with improved prognosis in cervical tumor patients receiving radiotherapy. In a preclinical mouse model, combining HPV E6/E7-targeted therapeutic vaccination with radiotherapy increased the number of intratumoral CD103⁺CD8⁺ T_RM cells.¹¹³ The most recent research revealed that sublethal thorax-targeted radiation led to a rapid and sustained decline in the number of IAV-specific lung T_RM cells in mice, which was associated with decreased heterosubtypic immunity. Importantly, boosting with an mRNA vaccine regenerated lung T_RM cells.²⁵⁸

Emerging evidence suggests that T_RM cells exhibit greater resistance to radiation than their circulating counterparts do. Longitudinal in vivo imaging and functional analyses have shown that a significant proportion of intratumoral T cells, including T_RM cells, survive clinically relevant doses of radiation. These surviving T cells display increased motility and increased production of IFN-γ, thereby contributing to effective tumor control without the need for newly infiltrating T cells.^{259, 260} The interaction between T_RM cells and radiotherapy involves complex molecular mechanisms, including the firm adhesion of CD103 to E-cadherin, which triggers phosphorylation cascades involving the expression of Pyk2 protein tyrosine kinase and paxillin adaptor protein. This outside-in signaling pathway promotes the migratory behavior and effector functions of CD8⁺ T_RM cells, facilitating their survival and functionality postirradiation.²⁶¹

The radioresistance of T_RM cells has significant implications for the design and optimization of radioimmunotherapy trials. Since T_RM cells are relatively resilient to radiation, local irradiation may not inherently suppress the immune response within the tumor. Instead, irradiating multiple tumors can potentially enhance systemic effects, leveraging the immunostimulatory effects of radiotherapy to amplify antitumor immunity. Additionally, transcriptomic analyses suggest that TGF-β is a key regulator of the reprogramming of T cells within the TME, contributing to the radioresistance of T_RM cells.²⁵⁹ This finding opens avenues for targeting TGF-β signaling pathways to further modulate T_RM cell responses and improve therapeutic outcomes.

Moreover, radiotherapy can modulate the immune microenvironment by releasing protein DAMPs from dying tumor cells, which can stimulate the sequential recruitment of neutrophils and monocytes, facilitating antitumor immune priming.²⁶² Local irradiation is not inherently immunosuppressive; rather, irradiating multiple tumors might optimize the systemic effects of radiotherapy, suggesting implications for the design of radioimmunotherapy trials.

5.3 Vaccines

T_RM cells play a critical role in vaccine-induced protective immunity and have attracted great attention in vaccine development for infection prevention and tumor immunotherapy. The relationship between vaccines and T_RM cells has been extensively studied in small animal and nonhuman primate models.²⁶³ The route of vaccine immunization has been shown to play a crucial role in the formation of T_RM cells. Existing data indicate that intramuscular immunization results in weaker T_RM cell responses in mice than mucosal or epidermal immunization does, as muscle tissue contains relatively few DCs.²⁶⁴ Accordingly, we discuss mucosal and epidermal immunization vaccines, which can generate a broad and robust T-cell response.

A mucosal vaccine is a promising strategy for increasing the ability of mucosal T_RM cells to respond effectively to infections. Zens KD reported that intranasal administration of live attenuated influenza virus led to the development of CD4⁺ and CD8⁺ T_RM cells in the lungs, whereas systemic immunization did not achieve this in mice.²⁶⁵ Importantly, the coexpression of IL-1β significantly increased the mucosal immunogenicity of adenoviral vector influenza vaccines.²⁶⁶ In addition, antibody-targeted vaccination strategies can deliver antigens specifically to respiratory DCs, leading to the formation of lung CD8⁺ T_RM cells expressing high concentrations of GZMB and IFN-γ, which provide strong protection against lethal influenza infections.²⁶⁷ Intranasal immunization, but not intramuscular immunization, with a chimpanzee adenoviral vector vaccine targeting the SARS-CoV-2 spike protein induced CD103⁺CD69⁺ T_RM cells activity in the lungs, effectively preventing both upper and lower respiratory tract infections caused by SARS-CoV-2.²⁶⁸ Intranasal vaccination with the mucosal vector B subunit of Shiga toxin generated local T_RM cells and suppressed HNSC growth in a mouse model.²⁶⁹ Administration of the H. pylori vaccine can induce the generation of antigen-specific EGFP⁺CD4⁺ T_RM cells (CD69⁺CD103⁻) in the gastric subserous layer, which can reduce H. pylori colonization. These T_RM cells proliferate and differentiate in situ, enhancing local immunity during the recall response.¹⁶⁶ In summary, these data suggest that mucosal vaccination against pathogens can elicit pathogen-specific T_RM cells and prevent or eliminate infection at the site of entry.

Evidence has shown that epidermal disruption is the best method for eliciting a T-cell response (including T_RM cells and T_CM cells) to vaccinia virus (VACV), which contributes to the success of the smallpox vaccine.^{264, 270} The modified vaccinia virus Ankara (MVA), a highly attenuated VACV, was recently approved as a smallpox vaccine. When administered via skin scarification, MVA immunization produced greater numbers of lung OVA-specific CD8⁺ T_RM cells and protected mice against a lethal respiratory challenge with VACV_OVA.²⁷¹

In addition to vaccines, vaccine adjuvants can enhance the T_RM cell response generated by vaccines. The carbomer-based nanoemulsion adjuvant system is a classic example that includes either the Toll-like receptor 9 agonist CpG or the TLR4 agonist glucopyranosyl lipid A. Adjuvant spike protein-based vaccines induce systemic or mucosal lung CD4⁺ T_RM cells and helped CD8⁺ T cells develop effective immunity to the South African B1.351 β-variant, even without detectable mucosal or circulating virus-neutralizing antibodies.²⁷² Topical application of CpG oligodeoxynucleotides as adjuvants during subcutaneous immunization can enhance vaccine formulations by generating T_RM cells.²⁷³ In addition, the administration of a neoantigen peptide vaccine (NeoVAC) combined with α-PD-1 can elicit a strong neoantigen-specific antitumor response and establish long-term tumor-specific immune memory in HCC by increasing CD8⁺ T_RM infiltration.²⁷⁴

While T_RM cells are crucial for tumor therapy and defense against local reinfections, they are undesirable in autoimmune diseases such as vitiligo and psoriasis, in which they contribute to pathology. Therefore, strategies to prevent the formation of new T_RM cells and reduce the survival of established T_RM cells could be beneficial in managing autoimmune diseases. Short-term treatment with anti-CD122 (an IL-15 signaling inhibitor) suppresses the production of IFN-γ by T_RM cells, whereas long-term treatment eliminates T_RM cells from skin lesions in patients with vitiligo.²⁷⁵

5.4 Adoptive immunotherapy

Adoptive T-cell therapy (ACT) is the direct infusion of cancer neoantigen-experienced T_RM cells into patients suffering from cancer. In a mouse model of ACT for melanoma, CD8⁺ TILs lacking Runx3 (key regulatory factors of T_RM cells) expression did not accumulate within tumors, leading to increased tumor growth and heightened mortality rates. Conversely, Runx3 overexpression increased the number of T_RM cells, slowed tumor growth, and extended survival.³⁵

A more recent branch of adoptive cell transfer is chimeric antigen receptor (CAR)-T-cell therapy. T_RM cells from patients are collected and designed to recognize and eliminate target cells by expressing a specific antigen on the cell surface.²⁷⁶ Virus-specific CD8⁺ T_RM cells are present in some tumors, and these nontumor-responsive T cells have been shown to recognize viruses.²⁷⁷ Local injection of viral peptides can reactivate these intratumoral virus-specific CD8⁺ T cells and activate the immunosuppressive microenvironment, resulting in limited tumor growth.¹²³ This finding suggests that activating tumor-residing T_RM cells can contribute to tumor clearance.²⁷⁸ To date, CAR-T-cell therapy has been used to treat hematological malignancies but not solid tumors because it has difficulty reaching and infiltrating tumor sites.²⁷⁹ Hopefully, CAR-T-cell therapy can be used to increase and maintain the number of functional T_RM cells in solid tumors by inducing a T_RM cells phenotype with TGF-β and IL-15.²⁸⁰ In the treatment of other diseases, PD-1-targeting CAR-T cells selectively deplete liver CD8⁺ T_RM cells and alleviate autoimmune cholangitis in an autoimmune cholangitis model.¹⁸⁴ Thus, ACT and CAR-T-cell therapy using T_RM cells seem to be promising strategies for cancer therapy.

In addition to the aforementioned strategies, numerous clinical trials have been conducted in recent years to explore therapeutic approaches based on T_RM cells. These trials encompass a variety of treatments, including different drug combinations, immune checkpoint inhibitors, radiotherapy, and vaccines. To help readers better understand these potential therapeutic strategies and their clinical applications, we have summarized the current research progress. The key clinical trials and research findings are presented in Table 3.