Targeting the Tumor Microenvironment in Solid Tumors: Emerging Immunotherapeutic and Nanotechnology-Based Approaches

About 90% of all human cancers originate from solid tumours, which can be found in a variety of tissues and organs [1]. The diagnosis for patients with advanced-stage solid tumours remains poor despite advancements in surgery, chemotherapy, radiotherapy, and targeted treatments because of the complex and mutable nature of the tumour microenvironment (TME) [1,2]. The tumor's growth, immune evasion, metastasis, and drug resistance all depend on the TME, a dynamic, diverse network of cells and tissues. As a result, the TME has also been recognised as a possible target for enhancing solid tumour treatment outcomes [3–5]. Together, the cellular and non-cellular components of the TME contribute to immunological resistance and tumour growth. The foremost cellular components of the TME comprise cancer-associated fibroblasts (CAFs), tumor-associated macrophages (TAMs), myeloid-derived suppressor cells (MDSCs), and regulatory T cells (Tregs). The role of CAFs embraces remodelling ECM, growing the tumour’s stiffness, and secreting growth factors that support tumor survival and invasion. Transformed M2-like macrophages (TAMs) increase immune suppression, tissue remodelling, and angiogenesis along with M2-like macrophages, thus driving the pro-tumor microenvironment [5–7]. MDSCs mediate immune evasion by reactive oxygen species and immunosuppressive cytokine production that suppresses T-cell activity. Tregs will control cytotoxic T-cell activity and promote immune tolerance, thus enabling tumor immune evasion [1,3]. In addition to the cellular components, non-cellular TME constituents play a significant role in tumor progression and therapy resistance. The ECM provides mechanical support and creates a barrier for drug access, while also mediating cell signalling and movement [1,3]. Tumor vasculature dysregulation leads to hypoxia, which activates hypoxia-inducible factor 1-alpha (HIF-1α). This protein turns on metabolic remodelling, angiogenesis, and apoptosis resistance. Increased glycolysis (the Warburg effect) by tumor cells leads to lactate accumulation and acidification of the TME, further inhibiting immune responses. Furthermore, inflammatory signalling in the TME, facilitated by cytokines like interleukin-6 (IL-6) and tumor necrosis factor-alpha (TNF-α), promotes tumor growth, metastasis, and immune avoidance. This adaptive and intricate nature of the TME poses a main challenge to traditional treatments and highlights the need for advanced approaches that act on both tumor cells and the microenvironment [3,6].  While therapies such as chemotherapy, targeted therapies, and radiation aim to destroy cancer cells, they do not provide long-term remission due to the emergence of resistance mechanisms facilitated by the immunosuppressive and protective tumor microenvironment (TME) [2]. TME is a physiological and structural barrier that not only enhances immune escape but also makes drug delivery ineffective. The TME is also a physical as well as biological barrier that hampers drug entrance and boosts immune evasion. Henceforth, effective cancer therapy demands a two-pronged approach against both tumor cells and the TME [3,5,8]. Immunotherapy has arisen as a newer approach by boosting the immune system's capacity to recognize and kill tumor cells. Immune checkpoint inhibitors (e.g., pembrolizumab, nivolumab), chimeric antigen receptor (CAR) T-cell therapy, and cancer vaccines have proven successful in some malignancies [9]. Yet, the protective character of the TME hinders the optimal potential of immunotherapy. For example, immune checkpoint inhibitors cannot enter the deep tumor areas as a result of inappropriate drug penetration, and immune cells also cannot be activated by the immunosuppressive signals in the TME. Thus, the TME must be modified or disrupted to increase the effectiveness of immunotherapy [1,10,11]. The interaction between nanotechnology and immunotherapy is an extremely promising approach to counteract TME's impediments in solid tumors [6,8,12]. Immunotherapy stimulates the immune system against cancer cells, but its efficacy is restricted by low drug delivery, immune resistance, and off-target toxicity. Drug delivery systems based on nanotechnology resolve these by enhancing drug targeting, augmenting drug tumor accumulation, and minimizing systemic toxicity [13,14]. To target the TME more efficiently, nanoparticles can be designed to attach to some receptors that are overexpressed on tumors or immune cells. Nanoparticle accumulation in tumors due to the permeable vasculature and dysfunctional lymphatics is referred to as the EPR effect, which is considered one of the basic mechanisms through which nanoparticles are able to selectively concentrate in the tumor area [6,15]. Immunomodulatory drug delivery, such as TLR agonists and cytokines, is also likely in nanoparticles to relieve immune suppression in the TME. For example, in preclinical models, anti-PD-L1 antibody-conjugated nanoparticles have proved improved tumor penetration and immune cell activation [16]. Furthermore, cytokine and chemokine delivery via nanoparticles has shown the possibility to convert tumor-associated macrophages (TAMs) from an M2 to an M1 pro-tumor phenotype, so augmenting immune responses [6,15]. Numerous successful approaches have been documented that intricately fuse immunotherapy and nanotechnology. An example of such a technique is liposomal doxorubicin (Doxil), which is a form of doxorubicin encapsulated in a liposome. Doxil is a well-studied example of employing nanotechnology that increases the accumulation of chemotherapeutics in the tumor while markedly decreasing cardiotoxic side effects [17]. In addition, immune checkpoint inhibitors are nanoparticles designed for immunotherapy that have become increasingly effective in preclinical and early clinical phases due to enhanced drug retention and decreased off-target effects. The integration of CAR-T cell therapy with nanoparticle-mediated cytosine and immune agent delivery has also demonstrated greater T-cell infiltration and activity in solid tumors [14]. Proposals in nanotechnology suggest benefits related to controlled and sustained drug release, which improves drug delivery, therapeutic efficacy, and reduces toxicity. The specificity and efficacy for cancer treatment are further enhanced by nanoparticles that can release their payloads to the target tissues using specific triggers such as pH, enzymes, or temperature within the tumor microenvironment (TME) [8,14,18].  The origin of integrating immunotherapy and nanotechnology lies in the mechanisms of action of individual components. In a complementary manner, immunotherapy helps to further stimulate and strengthen the activity of the immune system to increase the recognition and destruction of tumor cells, while nanotechnology facilitates the delivery of drugs and modulation of the immune system to the tumor microenvironment (TME) [14,15,19]. This collective method addresses the most significant confines of current cancer treatments, which include poor drug delivery, immune suppression, off-target toxicity, and has the prospect to progress, prolong survival, and decrease recurrence in patients with solid tumors. The incorporation of these two modalities is a pioneering development in the treatment of cancer and offers renewed anticipation for patients with difficult-to-treat solid tumors. This review describes a significant problem in cancer therapy by assessing the combination of immunotherapy with nanotechnology focused on the tumor microenvironment (TME). Despite the advancements made in immunotherapy, its efficacy is typically limited by the TME’s immunosuppressive qualities. On the other hand, while using nanotechnology to improve drug delivery, reduce toxicity, and protect the TME, it fails to reshape the TME itself. This paper analyzes the intricate biology of the TME, recent developments in immune-based therapies, and emerging strategies in nanotech. It also discusses preclinical and clinical studies that demonstrate the integrated approach’s benefits. Primary focal points include versatile muscle nano systems beyond drug capsules to evade capture, design sophisticated, advanced, programmable nanocarriers, devise methods for overcoming resistance, and tackle immune disguise. Moreover, the review introduces the epitome of bioengineering technologies to task-specific stimuli-responsive biomaterials, fail-proof smart nanoparticles, and engineered cells tailored for immune recognition, response, and modulation. All these ideas intend to form the foundation of next-generation cancer therapies and therefore have great significance for oncology researchers and clinicians.

2. The Biology and Complexity of the Tumor Microenvironment

The TME is a very dynamic and complex ecosystem that arranges the onset, development, metastasis, and even the therapeutic evasion of solid tumors. It proves a complex interplay between various components that are cellular and acellular and interrelate with the tumor cells in question, prompting their proliferative capacity and treatment resistance [5,20]. Understanding the biological intricacy of TME is important for developing appropriate treatment strategies. The TME is composed of numerous and different cellular and acellular components that interact to influence tumor growth (Figure 1).

Figure 1: The TME is made up of diverse cellular and non-cellular elements that dynamically interact to sustain tumor growth and immune evasion

2.1 Cellular Components of the TME

2.1.1 Cancer-Associated Fibroblasts (CAFs): Cancer-Associated Fibroblasts (CAFs): CAFs are among the most documented stromal cell populations in the TME. They are involved with extracellular matrix remodelling; they also actively participate in the secretion of growth factors, and in the production of cytokines and chemokines that support tumor progression CAFs heavily mediate CD8 T cell suppression and therefore augment tumor cell growth, increased metastasis, and immune evasion through the secretion of TGF-β and FGF which aids angiogenesis and immunosuppression [5,7,20].

2.1.2 Tumor-Associated Macrophages (TAMs): TAMs are very plastic immune cells that are capable of acquiring a pro-tumorigenic (M2) or anti-tumorigenic (M1) phenotype based on the TME signal. M2-polarized TAMs support tissue remodelling, angiogenesis, and immune suppression by secreting vascular endothelial growth factor (VEGF), interleukin-10 (IL-10), and prostaglandin E2 (PGE2). They suppress cytotoxic T-cell activity and enhance tumor cell invasion and metastasis [5,7,20].

2.1.3 Myeloid-Derived Suppressor Cells (MDSCs): MDSCs are immature myeloid cells that reside in the TME and are immune suppressive. They impair T-cell activation and proliferation by the generation of reactive oxygen species (ROS), nitric oxide (NO), and arginase. MDSCs further support the amplification of Tregs and help with immune evasion, leading to dismal therapeutic outcomes [5,7,20].

2.1.4 Regulatory T Cells (Tregs): Tregs are a population of CD4+ T cells that ensure immune tolerance and dampen anti-tumor immune responses. They suppress the activation and activity of cytotoxic T cells and natural killer (NK) cells by secreting immunosuppressive cytokines like IL-10 and TGF-β. The presence of Tregs in the TME is associated with unfavourable prognosis and resistance to immunotherapy [5,20].

2.2 Non-Cellular Components of the TME

2.2.1 Extracellular Matrix (ECM): ECM is structural support for the tumor, controls cell motility and signalling, and presents a dense physical impediment that prevents drug diffusion and immune cell extravasation. ECM stiffening due to up-regulation of collagen cross-linking and lysyl oxidase (LOX) activity boosts tumor cell motility and immune evasion [5,7,20].

2.2.2 Hypoxia and Metabolic Alterations: Accelerated tumor growth establishes a disparity between oxygen demand and supply, resulting in hypoxia. Hypoxia stabilizes HIF-1α, which enhances metabolic reprogramming (glycolysis increase), angiogenesis, and apoptosis resistance. Hypoxia-mediated lactate buildup decreases the pH of the TME, which inhibits immune responses and enhances drug resistance. The Warburg effect, which involves enhanced glucose consumption and lactate production, also increases tumor cell survival and immune evasion [5,20].

2.2.3 Inflammatory Signalling Pathways: Chronic inflammation in the TME is induced by cytokines like interleukin-6 (IL-6), tumor necrosis factor-alpha (TNF-α), and interleukin-1 beta (IL-1β). These cytokines induce nuclear factor-kappa B (NF-κB) and signal transducer and activator of transcription 3 (STAT3), which induce tumor cell proliferation, immune suppression, and resistance to apoptosis. Sustained inflammation establishes a pro-tumorigenic environment favouring metastasis and immune evasion [5,20].

3. Immunotherapeutic Strategies Targeting the TME

By enhancing the ability of the immune system to recognize and destroy cancerous cells, immunotherapy has been identified as a prospective treatment for cancer. However, the implementation of solid tumor immunotherapy faces challenges due to the suppressive nature of the tumor microenvironment (TME), which facilitates immune evasion and resistance to treatment [3,21]. Efforts to modify the TME through immunotherapy have been effective in improving response rates for resistance in solid tumors. Several major strategies have been developed to enhance antitumor activity by altering TME composition and restoring immune system activity.

3.1. Immune Checkpoint Inhibitors (ICIs) Targeting PD-1/PD-L1 and CTLA-4

The advent of immune checkpoint inhibitors (ICIs) marked a drastic change in cancer immunotherapy. It works by lifting the brakes placed on T-cell acceleration by the tumor. Terminally differentiated Effector CD8 T cells express, among other surface markers for positive immunological signal integration, programmed cell death receptor one (PD-1), which, after binding to PD-L1, gets downregulated along with its functions [16]. The most effective strategy to boost their immune response to the tumor is by affording them the capacity to bind targets, the tumor. Hence, get rid of the blockade that is put there by some antibodies against programmed cell death one. This blockade can be removed by administering monoclonal antibodies against PD-1, like nivolumab, pembrolizumab, or anti-PD-L1 antibodies, atezolizumab, or durvalumab [9]. Similarly, the cytotoxic T-lymphocyte-associated protein 4 (CTLA-4) receptor is a T-cell activation-negative regulator. Ipilimumab, an anti-CTLA-4 antibody, inhibits T-cell suppression, thus increasing T-cell growth and tumor invasion. Both PD-1/PD-L1 and CTLA-4 have proved improved efficacy in melanoma and non-small cell lung cancer (NSCLC) [16].

2. Cancer Vaccines and Neoantigen-Targeted Therapies

Cancer vaccines intend to induce the immune system to see and destroy tumor-specific antigens. They are made to display tumor-associated antigens (TAAs) or neoantigens (mutation-related antigens) to dendritic cells, which then activate cytotoxic T cells [2]. Sipuleucel-T is a FDA-approved dendritic cell-based prostate cancer vaccine directed against prostatic acid phosphatase (PAP) [22]. Neoantigen-targeted therapies are gaining traction due to the highly specific nature of neoantigens, which are not present in normal tissues [23]. Personalized cancer vaccines, designed based on individual tumor mutational profiles, have demonstrated enhanced immunogenicity and clinical responses in melanoma and other solid tumors.

3. Chimeric Antigen Receptor (CAR) T-Cell Therapy and T-Cell Engineering

CAR T-cell therapy involves the genetic modification of a patient’s T cells to express a synthetic receptor (CAR) that recognizes specific tumor antigens. CAR T cells are engineered to bypass major histocompatibility complex (MHC) restrictions, allowing them to target tumor cells directly. While CAR T-cell therapy has shown remarkable success in hematologic malignancies, its efficacy in solid tumors has been limited by poor tumor infiltration, antigen heterogeneity, and the immunosuppressive TME [24,25]. To overcome these barriers, second- and third-generation CAR T cells have been engineered to secrete pro-inflammatory cytokines (e.g., IL-12) and express dominant-negative receptors that resist immune checkpoint signalling. Recent advancements include dual-targeting CAR T cells that recognize multiple tumor antigens and armoured CAR T cells that are resistant to TME-induced immunosuppression [24,25].

4. Cytokine-Based Therapies

Cytokines are critical modulators of immune cell function and signalling. Pro-inflammatory cytokines, including interleukin-2 (IL-2), interferon-alpha (IFN-α), and interleukin-15 (IL-15), have been employed to activate immune responses in cancer treatment [2]. Durable responses have been observed with high-dose IL-2 therapy in metastatic melanoma and renal cell carcinoma, but its application is constrained by systemic toxicity. Understanding the IL-15's role in enhancing the survival and proliferation of NK and CD8+ T cells suggests its potential use, alongside checkpoint blockade inhibitors, to amplify anti-tumor activity. Efforts are also being made to create cytokines to are less toxic but more specific to the TME [26,27].

5. Tumor-Infiltrating Lymphocytes (TILs) and Adoptive Cell Transfer (ACT)

Ex vivo expansion and reinfusion of ex vivo expanded TILs after lymphodepletion is referred to as TIL therapy. TILs are naturally occurring T cells that are capable of invading tumor tissue. The technique of TIL therapy does not require modification of the patient’s T cells. The patient’s TILs are harvested from the tumor, expanded in culture, and then reinfused into the patient after lymphodepletion. The therapy augments the ability of T cells to eradicate tumor cells in situ without needing genetic modifications. TIL therapy has also shown excellent response rates in melanoma because of the high tumor mutational burden and abundance of neoantigens. The synergistic effect of TIL therapy with secretion of immune checkpoint inhibitors can enhance the activity and persistence of T cells within the TME, which has been documented [28]. ACT also includes an approach referred to as T-cell receptor (TCR) therapy, which involves the genetic modification of T cells to express receptors that specifically target tumor antigens presented by MHC molecules [29].

4. Role of Nanotechnology in Modulating the TME

Nanotechnology has emerged as a powerful tool for improving cancer therapy by enhancing drug delivery, increasing tumor penetration, and modulating the tumor microenvironment (TME) [30]. Nanoparticles engineered for tumor penetration and immune modulation are actively reshaping therapeutic paradigms by targeting specific TME components (Figure 2). The unique properties of nanoparticles, such as their small size, high surface-to-volume ratio, and functionalization potential, allow them to bypass biological barriers and deliver therapeutic agents directly to tumor sites. Nanotechnology-based approaches aim not only to improve drug accumulation within tumors but also to remodel the TME to overcome immune suppression and resistance. Stimuli-Responsive Nanoparticles and Their Role in Tumor Microenvironment Modulation are shown in Table 1.

4.1 Nanocarriers for Targeted Drug Delivery

Nanocarriers, including liposomes, polymeric nanoparticles, dendrimers, and micelles, have been widely used for targeted drug delivery in cancer therapy. The enhanced permeability and retention (EPR) effect allows nanoparticles to accumulate preferentially in tumors due to the leaky vasculature and poor lymphatic drainage of the TME [14,19]. Functionalized nanoparticles can be designed to target specific receptors overexpressed on tumor cells and immune cells, such as folate receptors, integrins, and transferrin receptors. For example, liposomal doxorubicin (Doxil) has shown improved tumor targeting and reduced cardiotoxicity compared to conventional doxorubicin [31]. Polyethylene glycol (PEG) coating enhances nanoparticle stability and prolongs circulation time, improving drug bioavailability and accumulation within the TME [32].

4.2 Tumor-Penetrating Nanoparticles

Effective drug delivery to the TME is often hampered by dense extracellular matrix (ECM) and high interstitial pressure. Tumor-penetrating nanoparticles are engineered to overcome these barriers by incorporating matrix-degrading enzymes (e.g., hyaluronidase) or using size-switchable designs that shrink upon exposure to specific stimuli in the TME (e.g., acidic pH or enzymatic activity). Nanoparticles conjugated with tumor-homing peptides, such as iRGD (internalizing arginine-glycine-aspartic acid), enhance penetration through the ECM and facilitate deeper tumor infiltration, increasing drug availability at the tumor site [33].

Figure 2: Nanotechnology-driven strategies have emerged as promising approaches to remodel the TME and enhance immunotherapy efficacy.

4.3 Nanoparticle-Based Immune Activation and Modulation

Nanoparticles are able to target immunostimulatory agents to the TME directly, where they enhance the activation of immune cells and restore immune suppression. For instance, nanoparticles containing TLR agonists (e.g., CpG oligonucleotides) have been able to activate dendritic cell maturation and improve antigen presentation. Nanoparticles attached to anti-PD-L1 antibodies have shown enhanced immune checkpoint blockade through enhanced drug residence at the tumor site and augmented immune cell infiltration. Moreover, cytokine-loaded nanoparticles like interleukin-2 (IL-2) and interferon-gamma (IFN-γ) have been employed to recondition the immunosuppressive TME and increase cytotoxic T-cell function [13,19,33].

4.4 Nanotechnology for Reprogramming the Hypoxic TME

Hypoxia is a signature of the TME that induces immune suppression and resistance to treatment. Nanotechnology-based oxygen delivery platforms, including perfluorocarbon-based nanoparticles, have been engineered to enhance oxygen levels in tumors, thus inhibiting hypoxia-induced resistance. Nanoparticles delivering HIF-1α inhibitors or reactive oxygen species (ROS)-producing agents have also been demonstrated to interrupt hypoxia-induced signalling and improve the sensitivity of tumor cells to immunotherapy [34].

Table 1: Stimuli-Responsive Nanoparticles and Their Role in Tumor Microenvironment Modulation

4.5 Controlled Release Systems for Immunomodulation

Controlled release systems based on pH-sensitive, enzyme-sensitive, or thermos-responsive nanoparticles enable the targeted delivery of immunomodulatory agents in the TME [37]. For instance, nanoparticles designed to release anti-PD-1 antibodies upon acidic pH in tumors have shown enhanced drug retention and increased T-cell infiltration [16]. Polymeric micelles encapsulating IL-12 have exhibited controlled cytokine release, minimized systemic toxicity while maximizing local immune responses. These stimulus-responsive systems increase the therapeutic window and minimize off-target effects.

5. Rationale for Combining Nanotechnology and Immunotherapy

Reason for Synergism of Nanotechnology and Immunotherapy

Immunotherapy has shown impressive efficacy in melanoma and lung cancer, but in solid tumors, its effectiveness is scarce due to compromised immune cell infiltration, immune suppression, and off-target toxicity. Nanotechnology comes as a solution to enhance drug delivery, activate the immune system, and modulate the TME. By synergizing immunotherapy with nanotechnology, therapeutic drugs can be targeted more efficiently to the tumor site while, at the same time, amplifying immune responses and evading resistance mechanisms [14,19,38].

5.1 Nanoparticle-Based Delivery of Immune Checkpoint Inhibitors

Nanoparticles designed for delivering immune checkpoint inhibitors (ICIs) have exhibited increased tumor accumulation and extended retention of drugs. For instance, PEGylated liposomes with anti-PD-1 antibody conjugation have exhibited enhanced tumor penetration as well as immune cell activation. Combination strategies involving the use of nanoparticles to deliver PD-1 and CTLA-4 inhibitors have exhibited synergistic outcomes in preclinical models [14,19,38,39].

5.2 Synergistic Effects of Combining CAR-T Cells with Nanocarrier Delivery

CAR-T cell therapy has achieved limited success against solid tumors as a result of inadequate tumor infiltration and immune suppression caused by TME. Chemokines (e.g., CXCL9 and CXCL10) delivered by nanoparticles have been used to enhance CAR-T cell recruitment into tumors. Nanoparticles also loaded with cytokines (e.g., IL-15) have been used to improve CAR-T cell expansion and persistence within the TME, enhancing anti-tumor effects [24,25].

5.3 Nano vaccines and Their Role in TME Remodelling

Nano vaccines are made up of tumor antigens and adjuvants presented by nanoparticles to trigger antigen-specific immune responses. Liposomal and polymeric nano-vaccines have been reported to promote dendritic cell activation and T-cell priming. Patient-specific nano vaccines directed against patient-specific neoantigens have exhibited improved tumor-specific immune responses and less immune escape [14,40].

6. Challenges and Limitations

Immune Resistance and Tumor Heterogeneity

Tumor heterogeneity remains a major challenge, as antigenic variability within tumors can lead to immune escape and therapy resistance. Strategies to target multiple antigens simultaneously or use bispecific nanocarriers are being explored to overcome this challenge [20,41].

Drug Delivery Challenges and Off-Target Effects

Nanoparticles face challenges in penetrating deep into the tumor due to high interstitial pressure and ECM barriers. Off-target effects and systemic toxicity remain significant issues, particularly for nanoparticles carrying immunostimulatory agents [11,33].

Immune-Related Adverse Events

Overactivation of the immune system can lead to autoimmune reactions, cytokine release syndrome, and tissue damage. Strategies to incorporate controlled release and tumor-specific targeting are being explored to reduce these risks [42,43].

Regulatory and Manufacturing Challenges for Nanomedicines

The complexity of nanomedicine manufacturing and the need for reproducibility and scalability present significant regulatory hurdles. Standardization of production methods and comprehensive safety testing are critical for clinical translation [4,20,27].

7. Clinical Translation and Ongoing Trials

Several clinical trials are currently evaluating the combination of ICIs with nanoparticle-based drug delivery. For example, a phase II trial combining nivolumab with a nanoparticle-based IL-12 delivery system has shown promising immune activation in melanoma [44]. Key Clinical Trials Involving Immunotherapy and Nanotechnology are shown in Table 2.

FDA-Approved Therapies Targeting the TME

Liposomal doxorubicin (Doxil) and nanoparticle albumin-bound paclitaxel (Abraxane) have been FDA-approved for the treatment of solid tumors [31,45]. Combination therapies with immune checkpoint inhibitors are currently under investigation.

Biomarker-Based Patient Selection Strategies

Biomarkers such as PD-L1 expression, tumor mutational burden, and immune cell infiltration are being explored to identify patients most likely to benefit from combination therapies [46].

Table:2 Key Clinical Trials Involving Immunotherapy and Nanotechnology