The current role of dendritic cells in the progression and treatment of colorectal cancer

DCs induce specific immune responses

Upon detection of CRC tumor antigens, DCs undergo rapid activation to present these antigens on MHC molecules. DCs recognize damage signals in the TME through pattern recognition receptors and other mechanisms, as occurs in other malignancies^50,51. This activation leads to heightened expression of MHC class I and II antigens, the release of effector cytokines, such as proinflammatory cytokines [IL-12, tumor necrosis factor-alpha (TNF-α), and IL-17] and co-stimulatory molecules, and migration of DCs to TDLNs. As a result, this process induces adaptive CD8⁺ and CD4⁺ T-cell responses against tumors⁵². Mature DCs in the TME follow chemokine gradients toward TDLNs, thereby initiating activation of naive T cells and attracting various immune cells (Figure 2). DCs also promote B-cell proliferation and differentiation by releasing B-cell-activating factor^53,54, which drives effective antitumor cytotoxic T-cell immune responses and modulates local antitumor cytotoxic T-cell function to inhibit tumor growth⁵⁵. Furthermore, DCs stimulate NK cells, thereby enhancing the immune response. The release of chemokines by DCs draws new DC precursors and stimulates NK cells, which in turn recruit naive T cells to the tumor and elicits a protective CD8⁺ T-cell response. Activated NK cells secrete cytokines that promote DCs maturation, further amplifying T-cell-mediated antitumor immunity⁵⁶. Current research indicates that an increase in activated and mature DCs infiltration in CRC could be linked to enhanced overall survival and disease-free survival rates given that CRC patients with higher DC counts have increased 5-year overall survival rates. This finding is possibly attributed to DCs secretion of IFN-γ, which activates cytotoxic T cells⁵⁷.

• Download figure
• Open in new tab
• Download powerpoint

Figure 2

Summary of the mechanisms by which DCs influence CRC progression. I. Antitumor mechanisms: When the body finds the presence of CRC tumor antigens, DCs are rapidly activated, present CRC antigens to MHC molecules, and sense damage signals in the TME through special receptors, such as pattern recognition receptors, to induce antitumor adaptive CD8⁺ and CD4⁺ T-cell responses. Mature DCs are translocated from the TME to the TDLN along a chemokine gradient, priming naive T cells and attracting T cells and other immune cells. II. Tumor-promoting mechanisms: a. After DC differentiation is blocked, an obvious immunosuppressive cytokine spectrum is formed that inhibits the activation and proliferation of T cells and weakens the immune attack on colorectal cancer cells to induce immune tolerance; b. Impaired DC differentiation promotes the accumulation of other cell subsets with immunosuppressive functions, such as MDSCs, inducing the development of Tregs, hindering DC maturation, and promoting tumor progression; c. Immunosuppressive regulation of DCs induces proliferation of Treg lymphocytes and autoreactive T cells, thereby promoting immune tolerance.

Complex relationship between DCs and tumor angiogenesis

A sophisticated interplay exists between tumor-related angiogenesis and DCs infiltration. Angiogenesis supplies tumors with essential and sufficient oxygen and nutrients, which facilitates tumor progression⁵⁸. However, angiogenesis can also increase infiltration of immune cells into tumor tissues. Elevated DCs infiltration bolsters antitumor immunity through antigen exposure⁵⁹.

In some instances, DCs may incite endothelial cell proliferation, migration, and tubule formation, which contributes to remodeling of the vascular endothelium and indirectly facilitates CRC proliferation. Moreover, DCs extracted from CRC patients exhibit high expression of CXCL1, which is known to enhance angiogenesis, cancer stemness, and epithelial–mesenchymal transition, and thereby promotes tumor cell migration⁶⁰.

Moreover, research has shown that CRC patients infected with human papillomavirus (HPV) display reduced levels of isthmin 2 (ISM2). Low ISM2 expression is associated with increased tumor angiogenesis, resulting in CRC immune cell infiltration enhancement and a more robust immune response^61–63. Low ISM2 expression combined with the antiviral response contributes to a more favorable prognosis for HPV-infected patients with CRC⁶⁴. Additionally, DCs indirectly control the tumor blood supply and inhibit tumor development by releasing anti-angiogenic substances and inhibiting DCs maturation.

It is important to recognize that among patients with different types of CRC, DCs and tumor angiogenesis are intertwined and function together. Future research endeavors should further explore the specific functions and regulatory role of DCs in the initiation and progression of different types of CRC to provide novel strategies and insight for immunotherapy.

Previous investigations have revealed that complex and contradictory molecular and cellular factors in the TME significantly change the phenotype of DCs. Moreover, CRC cells can evade the local tumor-specific immune response induced by DCs, ultimately resulting in immune dysfunction within the TME.

Impaired differentiation of DCs promotes tumor progression

Compromised DC precursor differentiation programs may lead to the accumulation of other cell subpopulations with immunosuppressive functions, such as myeloid-derived suppressor cells (MDSCs), which are vital immune regulators that facilitate direct tolerance of antitumor T lymphocytes and the development of Tregs. This process inhibits the maturation of DCs, thereby aiding in tumor progression (Figure 2)^65,66.

Recent investigations have shown that the differentiation and maturation of DCs from CD14⁺ monocyte precursors are compromised in patients with CRC, which manifests as the accumulation of an immature phenotype with low levels of NF-κB, MHC class II, and co-stimulatory molecule expression. Consequently, these immature DCs are unable to provide adequate co-stimulation and cytokine signals to T cells, hindering the proliferation of tumor-specific CD4⁺ and CD8⁺ T cells and weakening the immune response. The production of immunosuppressive cytokines, such as IL-10 and TGF-β is increased, while the levels of immunostimulatory cytokines, such as IL-12 and TNF-α, are decreased, resulting in a pronounced immunosuppressive cytokine profile that hampers T-cell activation and proliferation, diminishes immune attack against CRC, and promotes immune tolerance^67–73. Among the tumor-derived cytokines that impede DCs maturation, IL-10 permanently inhibits the differentiation of monocytes into DCs and hinders the directed differentiation into the macrophage lineage (Figure 2). Conversely, IL-6 and M-CSF hinder the differentiation of CD34⁺ progenitor cells into DCs but fail to induce allogeneic T-cell proliferation. Elevated levels of tumor-derived vascular endothelial growth factor (VEGF) have also been correlated with an increased number of circulating immature DCs displaying an inhibitory phenotype in CRC patients⁶⁶.

Tumors can also secrete other factors that impede DCs differentiation, including gangliosides, prostaglandins, and polyamines. These factors facilitate the recruitment and accumulation of immature DCs. Studies have also revealed that COX-2 is prominently expressed in CRC and via the COX-2 downstream signaling molecules [prostaglandin E2 (PGE2) and PGE2 receptors (EP2/EP4)] which suppresses DCs differentiation and maturation, dampening host antitumor immunity⁷⁴. Together, these tumor factors impede DCs differentiation, interfere with the development and maturation of DCs, limit the initiation of antitumor immune responses, and strongly facilitate the evasion of tumors from the immune system in CRC patients.

Impaired antigen presentation function of DCs promotes tumor progression

The antigen-presenting function of DCs becomes compromised in patients with CRC owing to the influence of the TME, resulting in a diminished capacity for antigen uptake, processing, and presentation. This impairment negatively impacts the generation of antitumor immune responses. Dysfunction in DC cross-presentation ability is primarily linked to defects in transporting peptide-MHC class I complexes to the cell surface. Furthermore, oxidized lipids within tumor tissues have the potential to hinder antigen cross-presentation and T-cell activation through interactions with the molecular chaperone, heat shock 70 kDa protein (HSP70)⁷⁵. Recent studies have elucidated the detailed mechanisms underlying the deficiency in antigen-presenting capabilities, highlighting HSP70 targeting as an attractive therapeutic strategy for CRC given the stress-induced overexpression in cancer cells compared to healthy cells⁷⁶. A study by Huang et al.⁷⁷ revealed that circulating levels of DCs in CRC patients were reduced to approximately 60% of control levels and total resection of the colorectal tumor restored these levels to normal. The apparent decrease in circulating DC levels was correlated with enhanced infiltration of LCs into the colorectal mucosa and a notable increase in serum TGF-β1 levels⁷⁷. Analysis of DC subsets indicated that the decrease in the number of circulating DCs was predominantly attributed to alterations in the number of pDCs⁷⁸. A decrease in circulating DCs could influence cytokine production and antigen presentation to T cells, thereby facilitating cancer cell evasion from host immune surveillance⁷⁹.

Numerous subsets of DCs can transition into a functionally suppressive state that is referred to as regulatory DCs (DCregs), although this categorization lacks the precise definition of a defined DC subset. DCregs exhibit decreased antigen presentation and effector T-cell activation abilities. However, DCregs stimulate the proliferation of Tregs and autoreactive T cells, thereby promoting immune tolerance⁶⁶. Several molecules, including IL-10, TGF-β, and indoleamine 2,3 dioxygenase (IDO), are implicated in the functional regulation of DCregs. Factors derived from tumors are also able to promote the generation of immunosuppressive regulatory DCs, thus inhibiting T-cell activity via various mechanisms and promoting tumor immune evasion.

Impaired migratory capacity of DCs contributes to tumor advancement

Direct migration of DCs to tumor sites is hindered by the reduced expression of co-stimulatory signals in patients with CRC, resulting in ineffective chemotaxis that is driven by special chemokine gradients. This impediment prevents DC migration to TDLNs, enabling the tumor to evade immune surveillance and attack, and ultimately inducing immune tolerance^67,68,71,80. Chemokines, such as CCL19 and CCL21, are key factors in the DC-to-lymph node migration. CCR7 is essential for DC-migrated CCL21 and CCL19 chemokine receptors. However, tumor cells within the TME produce factors, such as VEGF, that interfere with the expression and distribution of chemokines. As a result, the chemotactic gradient leading to DC migration and migration ability are impaired⁸¹. Studies have confirmed that tumor-produced TGF-β immobilize DCs, inhibiting migration from the tumor-to-TDLNs and therefore facilitating tumor metastasis. At the same time, increased hyaluronic acid (HA) in the ECM also inhibits DCs migration⁸².

Increased DCs apoptosis promotes tumor progression

Apoptosis is responsible for DCs loss in the TME. Studies have indicated that high mobility group box-1 protein produced by CRC contributes to the induction of DCs apoptosis, thereby impeding the generation of lymph node DCs^71,83. Esche et al.⁸⁴ reported that factors derived from tumors trigger DCs apoptosis and accelerate DCs turnover. Other factors, such as gangliosides, neuropeptides, NO, and other molecules, also shorten the lifespan of DCs, with tumor-secreted gangliosides identified as inducers of DCs apoptosis⁸⁵. Studies have also confirmed that purified mucin-2 (MUC2) from CRC cell lines augments DCs apoptosis, which is partly mediated by the relationship between MUC2 mucins and Siglec-3 on DCs⁸⁶. Given that the role of DCs in programmed cell death is crucial for regulating the duration and intensity of immune responses, substantial depletion of DCs from the TME can lead to ineffective antitumor immunity and immune evasion.

Preparation and use of DC vaccines

DC vaccines involving cultivation of mature DCs in vitro reintroduce antigen-stimulated mature DCs back into the body to restore the ability of DCs to present tumor-related antigens and activate T cells, thereby eliciting a host immune response against tumor antigens. That is, DC vaccines induce the production of T lymphocytes with specific killing effects to control tumors by presenting antigens and these vaccines have been applied in some CRC patients.

There are four primary types of DC vaccines that have been developed, as follows: tumor lysate-loaded DC vaccines; tumor antigen-loaded DC vaccines; mRNA-pulsed DC vaccines; and DC-tumor fusion cell vaccines⁸⁷.

In the context of CRC, the primary focus has been on tumor lysate- and tumor antigen-loaded DC vaccines. Despite notable achievements with DC vaccines, the immunosuppression induced by tumors and the functional limitations of commonly used MoDC vaccines remain as obstacles to the adoption of these vaccines. To address these obstacles, numerous novel studies have been conducted. Recently, validated research has indicated that MoDCs can load multiple tumor antigens onto MHC I and II molecules, which activate CD8⁺ and CD4⁺ T cells, respectively, thereby enhancing the antitumor immune response to MoDC vaccines and overcoming tumor antigen escape⁸⁸. Considering the crucial role of pDCs in immune responses, autologous tumor antigen-loaded pDC vaccines have been investigated. This subgroup of DC vaccines overcomes the functional limitations of MoDCs, thereby better initiating or enhancing anticancer immune responses³⁹. Research has also identified DC-derived exosomes (DCexos) as inert vesicles that are resistant to tumor-induced immune suppression and capable of targeting cancer cells⁸⁹.

Vaccines loaded with pDCs or carrying DCexos are promising novel DC vaccines that can effectively overcome tumor-induced immune suppression⁹⁰. According to preclinical and clinical studies, most vaccine formulations relying on DCs are not directly purified from natural DCs in the blood but rather utilize DC substitutes. In contrast to DC substitutes, natural DC subsets exhibit elevated levels of MHC molecule expression and specialized functions, highlighting the need to develop next-generation vaccines⁹¹. Clinical testing of DC vaccines in conjunction with alternative strategies, including the administration of chimeric antigen receptor T cells, cytokine-induced killer cells or NK cells, as well as recombinant cytokines or immunostimulants, holds promise for future therapeutic advances (Table 1)¹⁰².

Novel DC-related therapies

At present, increasing attention has been paid to the use of various DC-related adjuvants and new drugs. Studies have confirmed that the interaction between DCs and Tregs enhance immunosuppression, which is the main obstacle to immunotherapy. Targeting these suppressor cell groups, such as Tregs, is a promising strategy for chemical and immunologic interventions. Researchers have shown that the use of specific adjuvants, such as TLR ligands or cytokines, fully activates DCs while impeding the expansion and recruitment of Tregs. TLR ligands act as adjuvants with the potential to increase immune potency against a given antigen. For example, CRC cells treated with chemotherapeutic drugs, such as oxaliplatin and/or 5-fluorouracil, can induce DC activation and maturation in vaccines through TLR4, therefore enhancing the generation of antitumor T-cell immune responses in laboratory settings and in living organisms⁹². Gao et al.⁹³ reported that combining DC vaccines with NK cells produced ex vivo through the release of cytokines, such as IL-2, IL-1b, and IFN-g, enhance disease-free survival in patients with CRC.

Other adjuvants, such as the oncolytic herpes simplex virus (type 1 talimogene laherparepvec) expressing GM-CSF⁹⁴, the TLR7 agonist imiquimod¹⁰³, and the tuberculosis Bacillus Calmette–Guérin (BCG) vaccine⁹⁵, can assist DC vaccines by enhancing DC immunogenic activity, thereby inducing antitumor immune responses. Evidence from animal studies indicated that specific immune responses triggered by DCs can be enhanced by directly administering tumor antigens. Apart from adjuvant utilization, novel materials can also be used to directly deliver tumor antigens to DCs and induce tumor-specific immune responses. This approach includes strategies, such as antigen conjugation to DC-targeting antibodies and the selective targeting of DCs using nanoparticles^96,97. For example, lipid nanoparticles containing DNA-encoding tumor antigens have been functionalized with mannose mimics to enhance uptake by mannose receptor-expressing DCs⁹⁸. Additionally, chemotherapeutic drugs have been shown to induce apoptosis in tumor cells by releasing numerous antigens for DCs, directly enhancing DC function, and promoting DC maturation or activation, thereby inducing strong antitumor responses⁹⁹.

Recent studies have shown that the chemotherapeutic drug, gemcitabine, modulates DCs, thereby reactivating HA-specific T cells at the effector location and increasing tumor-resident DCs and cross-presenting tumor-infiltrating CD8⁺ T cells for full effector function¹⁰⁰. Antibiotics can also be combined with chemotherapeutic drugs, such as vancomycin and chemotherapeutic drugs, enhancing DC cross-presentation of tumor antigens and thereby activating cytotoxic CD8⁺ T cells¹⁰⁴. Increasingly, non-cancer drugs that have been shown to effectively enhance DC antitumor immunity are emerging as new options for cancer treatment. Sitagliptin, a drug effective in treating type II diabetes, has been shown to effectively promote cDC1-mediated antigen presentation while inhibiting DPP4 and prevent the degradation of chemokines and cytokines that activate DCs, thereby enhancing the activation of tumor-specific CD8⁺ T cells and controlling tumor progression. Sitagliptin has also been reported to reduce the risk of cancer recurrence after curative surgery in CRC patients, making sitagliptin a potential anti-cancer drug¹⁰¹. In the future, more drugs capable of effectively enhancing DC antitumor immunity may be used in clinical settings.

A new type of tumor immune DC checkpoint inhibitor therapy has become an important treatment in recent years. Recent advances in cancer treatment have been driven by the emergence of ICB therapies, which have significantly impacted the CRC treatment. These therapies not only affect T cells but also exert direct or indirect influences on the antitumor abilities of DCs, which lays the foundation for some ICB treatments⁴⁹. While approved ICB therapies, such as programmed death-1 (PD-1), programmed death-ligand 1 (PD-L1), and cytotoxic T-lymphocyte-associated protein 4 (CTLA4), have shown success in treating mismatch repair-deficient or microsatellite instability-high CRC, most advanced CRC patients with stable microsatellites do not respond well to ICB therapy¹⁰⁵. Studies have shown that targeting multiple immune checkpoints enhances antigen uptake and interactions with effector T-cell mregDCs, suggesting that novel ICBs that integrate various traditional ICB therapies could have greater therapeutic potential. Moreover, moDCs have shown promise in ICB therapy with moDC-derived CXCL10 enhancing CD8⁺ T-cell migration in a PD-1 blockade setting. The attributes of moDCs have considerable predictive significance in ICB therapy and offer potential options for enhancing treatment effectiveness³⁰.

Recent studies have presented novel strategies, such as utilizing liposomal nanoparticles to transport STING agonists to CD103⁺ DCs via clec9a-binding peptides. This method has been shown to augment the efficacy of anti-PD-L1 antibodies in combating tumors in mouse models¹⁰⁶. Additionally, a new type of nanoscale coordination polymer nanoparticle, OxPt/BP, has been created to improve immunotherapy by decreasing PD-L1 expression on cancer cells and cDCs. This nanoparticle facilitates the migration of immunologically active CTLs and diminishes immunosuppressive Tregs, leading to the successful suppression of tumor growth in mouse models without negative side effects. This development presents a promising direction for future investigations of ICB therapy (Table 1)¹⁰⁷.

Combination therapy with DC-based ICB

Combining ICB with various treatment modalities can further enhance therapeutic effects. Amplifying the number of DCs is one of the future directions. Research has confirmed that Flt3L, a growth factor involved in the proliferation of DC progenitors, improves cancer immune surveillance in mice and enhances the response to anti-PD-L1 treatment for CRC lesions^108,109. Consequently, Flt3L combined with ICB treatment significantly improves survival rates in mice and outperforms Flt3L treatment alone. This combination therapy represents a future direction for treating human CRC.

Promoting the maturation of DCs, improving the activity of DCs, and enhancing DC antigen presentation function synergistically enhances the therapeutic effect. The activity of cDC1s has a critical role in initiating and sustaining antitumor T-cell responses, often determining the effectiveness of immunotherapy with ICB. To facilitate the interaction between PD-1⁺ T cells and cDC1s, scientists have designed a new PD-1 × CLEC9A bispecific antibody (bsAb) in mouse models that has been proven to increase antitumor immune activity and mitigate tumor progression. Compared to monospecific anti-PD-L1 antibodies, bispecific antibodies that combine PD-L1 blockade with T-cell recruitment potential, such as CD3 × PD-L1 or LAG3 × PD-L1, exhibit superior antitumor effects¹¹⁰. Moreover, photothermal therapy (PTT) has recently been shown to increase systemic antitumor immunity and eliminate metastatic tumors. Recent studies have demonstrated that PTT priming prior to in vivo administration of mature DCs, followed by the application of an anti-PD-L1 antibody to disrupt the interaction between tumor PD-1 and PD-L1, enhances CTL function⁵⁷. Thus, combining PTT with anti-PD-L1 treatment significantly improves CRC treatment outcomes, although corollary studies are needed regarding the optimal treatment timing. Cu₂O@Au serves as an effective inducer of immunogenic cell death (ICD), which enhances tumor antigen presentation and results in optimal DC maturation and the strongest in vivo immune response. This Cu₂O@Au effect triggers a robust antitumor immune response. Studies have shown that combining Cu₂O@Au with PD-L1 treatment initiates an effective immune response and more effectively suppresses tumor growth¹¹¹. Some gut bacteria also synergize with ICB to enhance therapeutic efficacy. For example, members of the genus Bifidobacterium can stimulate DCs to produce IL-12, thereby activating adaptive immune antitumor immunity and enhancing the response to ICB. Moreover, the inosine produced by Bifidobacteria also enhance the response to ICB treatment. Therefore, supplementing inosine may promote T-cell-induced tumor killing and increase the effectiveness of ICB treatment, although this has not been formally tested in humans¹⁰⁴. Additionally, allogeneic NK cell products have been demonstrated to activate DCs, provoking a proinflammatory response, even under immunosuppressive conditions associated with cancer. This effect has a pivotal role in the success of T-cell-based therapies and allogeneic NK cell-based cancer immunotherapies, potentially offering a promising combination therapy to improve the clinical outcomes of ICB treatment¹¹². Oleuropein has been identified as an effective immunomodulator capable of reprogramming immunosuppressive myeloid cells into immunostimulatory subgroups, demonstrating strong antitumor effects in colon cancer models and enhancing anti-PD-1 therapy. Therefore, oleuropein has great potential in cancer treatment. Oncolytic virus therapy (OVT) enhances the immunogenic activity of DCs or delivers tumor antigens to DCs and is recognized as an effective immune therapy for CRC treatment. Notably, a new multifunctional oncolytic adenovirus developed using bimetallic ions of copper and manganese, OA@CuMnC, can induce DC maturation, enhance NK cell activity and cytotoxicity, significantly infiltrate CD4⁺ and CD8⁺ T cells into tumors, and inhibit PD-L1 expression in tumor cells. Therefore, OA@CuMnC has strong potential for use in CRC therapy (Table 1)¹¹³.