In the vast nanomedicine landscape, the design and development of nanocarriers (NCs) for precise drug delivery are a pivotal innovation. NCs address significant pharmacological challenges, such as enhancing drug solubility, ensuring specific distribution, and facilitating the crossing of biological barriers . Tailoring NCs to transport drugs, mRNAs, and other therapeutic agents directly to the site of pathology represents a significant advancement in medical treatment modalities. This approach can significantly support the shift towards more targeted and efficient therapeutic strategies. This progress results from decades of technological evolution, during which NCs have become indispensable components of drug delivery systems, known for their adaptability and efficiency .

The “family” of nanoparticles (NPs) includes a broad range of materials such as lipids, polymers, proteins, dextran, silica , and metals such as iron and gold . Each material is chosen for its unique properties, such as size, hydrophilicity, and charge, that make it suitable for acting as a drug carrier. NCs can be functionalized on their surface to improve the stability and solubility of high-payload encapsulated cargos, promote transport across membranes, and extend circulation times. These advantages could reduce the negative effects of off-target drug accumulation and improve the release to the disease sites compared to current delivery systems .

Despite the expected applications in the biomedical field, the journey of NPs from research to clinical application faces significant hurdles, primarily due to interactions with the mononuclear phagocyte system (MPS). After administration in host bodies, NCs encounter systems of phagocytic cells, predominantly resident macrophages such as Kupffer cells (KCs) in the liver and macrophages in the spleen and lymph nodes, that sequester them. This occurs often independent of their design and structure . Although a significant challenge, this interaction presents a unique clinical application opportunity.

Macrophages are involved in the pathogenesis of several diseases and thus can be considered a therapeutic target, exploiting their natural ability to phagocyte external agents such as NCs. Both monocytes and macrophages perpetuate tissue damage during chronic inflammatory disorders. They are implicated in preventing and resolving inflammation and wound-healing response . Strategies for manipulating macrophage activation and function are diverse, ranging from depleting macrophages in diseased tissues, such as in cancer immunotherapy , to employing non-surgical treatments like extracorporeal shock wave therapy in various rheumatic diseases to promote resolution and healing .

The interplay between NCs and the immune system, especially macrophages, presents a complex scenario of challenges and insights pivotal for developing effective treatment options. Macrophages exhibit extraordinary versatility by adopting various functional phenotypes, such as classically activated (M1-like behavior) and alternatively activated (M2-like behavior) macrophages. M1 macrophages are involved in pro-inflammatory responses crucial for defending against pathogens. M2 macrophages mediate anti-inflammatory effects and may promote tumor growth and metastasis through their pro-tumor characteristics .

This dynamic and complex spectrum of macrophage activity features nuanced challenges and opportunities in leveraging macrophage responses to enhance the therapeutic potential of NCs. Recent research has highlighted the dual role of macrophages in the context of nanomedicine. While their ability to recognize and engulf NCs can impede the delivery of therapeutic agents to target tissues, it also opens avenues for novel strategies that exploit macrophage behavior for benefits, like targeted drug delivery and immunomodulation . This review will explore the physiological functions of macrophages and the challenges of NC filtering by the MPS and conclude with innovative strategies to exploit these interactions for therapeutic benefit.

Macrophages are immune cells derived from the yolk sac, fetal liver in mice, or differentiated by circulating monocytes . They act as the first line of defense in tissue by recognizing and engulfing pathogens and cellular debris via phagocytosis. This process is facilitated by detecting pathogen-associated molecular patterns (PAMPs) through pattern recognition receptors (PRRs) and the following degradation in lysosomes using hydrolytic enzymes and reactive oxygen species (ROS). Additionally, macrophages present antigen fragments through major histocompatibility complex (MHC) molecules, activating adaptive immune responses .

Macrophages are involved in tissue repair and homeostasis, regulating inflammation and its resolution by adopting different functional states, simplified in classically activated (M1) and alternatively activated (M2). This activation occurs on a spectrum, with various intermediate states influenced by microenvironmental signals. These states can exhibit overlapping functions and markers, demonstrating the plasticity and adaptability of macrophages in different physiological and pathological contexts. Within the M2 phenotype, macrophages can be further classified into four subgroups based on their specific activating stimuli, that is, M2a, M2b, M2c, and M2d . Each subgroup plays distinct roles, such as tissue repair, immune regulation, or tumor progression, emphasizing the complexity of macrophage activation. While the M1/M2 classification provides a valuable framework for understanding macrophage polarization, it does not encompass the full complexity of macrophage biology . Further research is needed to explore the diverse activation states of macrophages and their specific roles in health and disease. Therefore, in this review, we will utilize the commonly used M1/M2 dichotomy.

The role of macrophages in disease pathogenesis is closely tied to their activation states.

M1 macrophages – drivers of inflammation: M1 macrophages are induced by pro-inflammatory stimuli like lipopolysaccharide (LPS) and interferon gamma (IFN-γ), which trigger a strong pro-inflammatory response. These cells release cytokines such as tumor necrosis factor-α (TNF-α), interleukin (IL)-6, and IL-1β, essential for pathogen clearance and initiating immune defense mechanisms . However, if an inflammation remains active for extended periods, it can contribute to tissue damage and chronic inflammation. This prolonged M1 activity is a hallmark of diseases like rheumatoid arthritis (RA) and inflammatory bowel disease (IBD).

The activation of M1 macrophages is primarily mediated by the nuclear factor-κB (NF-κB), which is triggered by microbial ligands binding to toll-like receptor 4 (TLR4) . This interaction leads to the expression and release of type-1 interferons (IFN-α and IFN-β), which further amplify the immune response through the Janus kinase/signal transducers and activator of transcription (JAK/STAT) signaling cascade .

M2 macrophages – pro-resolving functions: In contrast, M2 macrophages are activated by anti-inflammatory stimuli, such as IL-4 and IL-13, and are primarily involved in resolving inflammation and promoting tissue repair. They secrete anti-inflammatory cytokines such as IL-10 and transforming growth factor β (TGF-β) , as well as vascular endothelial growth factor (VEGF) and platelet-derived growth factor (PDGF), which promote extracellular matrix (ECM) deposition and angiogenesis, playing a key role in conditions like fibrosis and wound healing .

The M2 activation state is mediated by the STAT6 pathway via the IL-4Rα1 receptor, triggered by the binding with IL-4 or IL-13. This process inhibits the M1 response by blocking NF-κB and activator protein 1 (AP-1) by activating the peroxisome proliferators-activated receptor-γ (PPARγ) .

Within this broader M2 category, macrophages can be further classified into four subtypes, that is, M2a, M2b, M2c, and M2d, based on their specific activation signals, cytokine production, and functional roles. The already mentioned M2 macrophages driven by IL-4 and IL-13, are known as M2a. M2b macrophages, induced by immune complexes and TLR ligands, regulate inflammation through the secretion of IL-10 and low levels of TNF-α. M2c macrophages, stimulated by IL-10 and glucocorticoids, resolve inflammation and promote tissue regeneration. Last, M2d macrophages, known as tumor-associated macrophages (TAMs), are activated by adenosine and IL-6 and characterized by their pro-angiogenic role. They produce VEGF and matrix metalloproteinases (MMPs), which are particularly relevant in tumor progression .

Therapeutic implications of macrophage polarization: Based on the aforementioned mechanisms, it is evident that macrophage dysregulation can be implicated in various clinical conditions. For instance, TAMs, which often exhibit an M2-like phenotype, contribute to tumor progression by promoting angiogenesis and suppressing antitumor immune responses . Reprogramming these macrophages towards an M1 phenotype has shown promise in improving antitumor immunity and enhancing cancer immunotherapy outcomes. Additionally, natural compounds such as berberine and quercetin can modulate macrophage polarization by inhibiting M1 pathways or promoting M2 activity, highlighting the therapeutic potential of targeting macrophage states in inflammatory and degenerative diseases .

To exert their therapeutic effects, NCs must efficiently reach and accumulate in the target tissues. However, numerous biological barriers hinder this process, which vary depending on the administration route, as well as the type and stage of the patient’s disease, thus restricting precise delivery .

One of the primary challenges of NC therapy is overcoming biological barriers following systemic administration, which remains the most common delivery route despite the potential advantages of local approaches . Once in the bloodstream, NCs are exposed to a wide range of forces, such as fluid shear stress, blood flow, opsonization, excretion, and interaction with the MPS, all of which influence NC stability and delivery. This challenging and complex scenario is furthermore amplified when NCs are administered upon pathological conditions, especially in inflamed tissues where immune cells such as macrophages are highly activated (as described in Section 2) . Interestingly, studies have shown that both macrophage phenotypes significantly affect NP internalization. They actively engulf NCs and accelerate their clearance, acting differentially in a time-dependent manner and altering the fate of nanomaterials .

In addition to immune-related barriers, the physicochemical properties of the nanomaterial itself can impair the NCs’ ability to protect their cargo, promote extravasation, and reach the target tissue effectively. Fluid forces can strip NCs of their surface coatings, reducing their ability to adhere to vessel walls, an essential step for extravasation into the parenchyma of target tissues. Particles larger than 200 nm and shapes like ellipsoids, discoids, and nanorods with higher aspect ratios are more effectively localized close to blood vessel walls, enhancing their internalization into endothelial cells and potentially improving their therapeutic delivery .

After systemic administration, NCs tend to accumulate in hepatic tissue because of its large blood volume, supplied by both the portal vein and hepatic artery. This allows the NCs to extravasate towards the liver’s sinusoidal capillary walls, where they interact with various hepatic cells, particularly KCs and mononuclear phagocytic cells. These cells, which account for 80–90% of the body’s total macrophage population, play a central role in immune surveillance and clearance of foreign entities. KCs utilize scavenger receptors (SRs), a superfamily of transmembrane glycoproteins expressed by myeloid cells, to detect and eliminate unwanted substances such as NPs, pathogens, and oxidized lipoproteins . These SRs function also as PRRs identifying both endogenous (e.g., damaged cells) and exogenous molecules (e.g., pathogens), activating intracellular signal transduction, and maintaining hepatic homeostatic functions . Notably, SR-A1, expressed in KCs, is essential for clearing infections caused by the Gram-positive bacterium Listeria monocytogenes .

In nanomedicine, SRs are also responsible for clearing negatively charged NPs such as those composed of silica . This interaction leads to the internalization of NCs via endocytosis, reinforcing the role of the MPS in NC clearance.

Initially classified as a distinct cell lineage, the MPS consists of phagocytic cells, predominantly monocytes, and macrophages that can rapidly sequester NCs after injection. This clearance process begins with opsonization in the bloodstream, mediated by opsonins that recognize plasma proteins (serum albumin, apolipoproteins, complement components, and immunoglobulins) adsorbed onto the surface of circulating NPs. This forms the so-called “protein corona” (PC), a layer of more than 300 proteins that effectively masks the functionalization of groups coated on the NC surface. The formation of this corona acts as a clearance signal, prompting macrophages to recognize and engulf NCs . The denser the proteins adsorbed onto the NC surface, the faster the uptake into the liver and spleen .

Table 1: Summary of NC types and their physicochemical properties.

To improve the efficacy of NC-based drug delivery systems, it is crucial to develop strategies that reduce macrophage uptake and extend NC circulation time. This could be achieved by acting on NCs exploiting alternative administration routes or physicochemical modifications, or directly on the macrophages with immune evasion techniques.

As discussed in Section 3.1, systemic administration remains the most common route in nanomedicine. However, undesired macrophage clearance is minimized through alternative delivery methods. Intranasal delivery has emerged as a promising strategy for targeting the central nervous system by bypassing the blood–brain barrier (BBB). This approach was demonstrated by the nose-to-brain administration of D6-cholestrol-loaded liposomes, which led to an accumulation of D6-cholesterol in the brain in healthy mice and in a murine model of Huntington's disease . Similarly, inhalation for lung targeting , subcutaneous injection for reaching lymph nodes , or oral administration for gastrointestinal tract disorders are important alternative routes.

Another approach to reducing macrophage uptake of NCs is to modulate their activity, thereby decreasing their overall presence in the target organs. KCs play a role in maintaining an inflammatory state in various liver disorders. Clodronate, a bisphosphonate, interferes with cell metabolism by inhibiting ADP/ATP translocase in the mitochondria, ultimately leading to KCs apoptosis. When encapsulated in liposome in combination with nintedanib, a triple tyrosine kinase inhibitor, there is also a reduction in the secretion of inflammatory cytokines, which enhances the antifibrotic effects. This has been demonstrated by Ji and colleagues in a mouse model of carbon tetrachloride (CCl4)-induced fibrosis, where they inhibited the proliferation of fibroblasts . An alternative to depletion is the inhibition of KCs through chloroquine, an antimalaria agent that inhibits macrophage-specific endocytosis, or saturation of the uptake with other non-toxic NPs administered as a pre-treatment. The disruption of lysosome and endocytotic processes causes the engulfment of KCs and the temporary blockade of the MPS. Although the induced modulation of innate immunity is effective, these strategies may lead to an increased susceptibility to infection, toxicity, and other liver disorders because of the suppression of essential physiological roles of KCs .

At a subcellular level, the interaction between liver macrophages and NCs can be prevented by masking the NC surface with hydrophilic polymers such as PEG. PEGylation is widely used for its “stealth” effect, hindering protein adsorption on the hydrophobic polymer surface by steric repulsion . However, the long-term use of PEGylated NCs for treating chronic diseases can lead to side effects, such as activation of the complement system, due to the accumulation of PEG in the body as a non-biodegradable polymer. Therefore, PEGylation must be carefully considered when designing NC-based therapies .

A strategy to avoid possible immunoreactions is to mask NPs by marking them as “self” and biomimetic. Coating NCs with membranes from red blood cells or neutrophils or decorating them with peptides can camouflage the NCs and prevent macrophage ingestion .

After reaching the target site, as discussed in the previous paragraphs, NCs should release their cargo to exert their final effect. Endocytosis poses a significant challenge for delivering drugs and nucleic acids to the cytosol as most remain trapped in endosomes and subsequently degrade. Efficient delivery requires the payload to be released before lysosomal maturation, a crucial stage known as endosomal escape . NCs enhance the delivery of biological therapeutics, and endosomal escape can be controlled by tuning their structure and physicochemical properties. For example, NCs designed with “proton sponge” capability contain materials that adsorb and buffer protons under acidic conditions and, typically, have high buffering capacities in the acidic pH range of endosomes (pH 5–6). Lipid nanoparticles (LNPs), which include cationic and ionizable materials, exhibit such intracellularly triggered delivery mechanisms and are often used to carry nucleic acids into cells. In this case, the endosomal escape is influenced by the molar ratio between ionizable lipids and mRNA nucleotides; thus, it protects the nucleic acid and promotes efficient in vivo delivery .

In summary, combining physicochemical modifications, surface coatings, and immune evasion techniques can significantly enhance the therapeutic potential of NCs by reducing macrophage uptake and extending circulation time. These strategies pave the way for more effective and targeted drug delivery systems.

Recent advancements in NC technologies have highlighted their remarkable potential in targeting macrophages for therapeutic applications, capitalizing on the unique characteristics of these immune cells. Macrophages are highly versatile agents for drug delivery because of their ability to evade immune surveillance, perform phagocytosis, and home to inflamed or diseased tissues . Additionally, their large size (≈25 μm) facilitates the efficient loading of diverse drugs (e.g., hydrophilic or hydrophobic) . As detailed in Section 2, macrophages persist throughout acute and chronic inflammatory phases, broadening their therapeutic applicability across various pathological conditions.

This chapter examines strategies that position macrophages as direct biological targets of NPs, aiming to modulate their activity as a therapeutic intervention for various pathological conditions, rather than merely using them as biomimetic drug carriers.

As described previously, NCs provide an innovative approach for delivering therapeutics to macrophages, particularly by targeting specific subsets, such as M2-like macrophages in tumor microenvironments. One widely studied approach involves sugars like mannose and hyaluronic acid, which naturally bind to macrophage-specific receptors. Mannose-decorated NCs, for example, leverage the overexpression of mannose receptors (CD206) on polarized M2 cells. Hatami et al. demonstrated the efficacy of self-assembling Pluronic® F127 polymer and tannic acid cores decorated with mannose in enhancing macrophage uptake . The unique physicochemical properties of these mannose-decorated hybrid NPs, such as controlled particle size (≈265 nm) and stability ensured by negative zeta potential, make them highly effective for receptor-mediated endocytosis and intracellular drug delivery . Similarly, hyaluronic acid-coated NCs target CD44 receptors on macrophages, improving drug delivery in inflammatory disease models and highlighting their potential in treating chronic inflammation and autoimmune conditions .

Beyond surface modifications, innovative strategies have used macrophages as “Trojan horse”-like carriers for drug-loaded NPs into injured or diseased sites. This strategy takes advantage of the macrophages’ innate ability to infiltrate diseased tissues, including hypoxic tumor regions that are otherwise difficult to access. Evans et al. demonstrated that macrophages loaded with hypoxia-activated prodrug NPs, such as tirapazamine (TPZ), significantly enhanced drug accumulation in hypoxic regions of solid tumors. Macrophage-mediated delivery achieved a 3.7-fold greater tumor weight reduction than free TPZ alone or in NP form . This method underscores the dual utility of macrophages as both therapeutic targets and delivery vehicles.

Antibody-functionalized NCs are a powerful method for selective drug delivery. For example, anti-CD163 antibodies can be conjugated to NCs to target M2 macrophages specifically. This strategy has shown significant promise in modulating TAMs by delivering agents that either reprogram or deplete these cells, effectively inhibiting tumor growth and metastasis. Such selective targeting reduces off-target effects and minimizes damage to healthy tissues .

While these strategies highlight the versatility of NCs, their clinical translation requires careful attention to safety and sterility. For instance, endotoxins or LPS remaining on the NC surface after synthesis can enhance macrophage uptake but may also cause unwanted immune activation . Consequently, ensuring sterility is a critical prerequisite for developing nanomedical devices.

The modulation of macrophage polarization has shown significant potential in addressing specific conditions. By leveraging the plasticity of macrophages, these approaches aim to dynamically repolarize macrophages between a pro-inflammatory M1 state and an anti-inflammatory M2 state, depending on the context. In cancer therapy, reprogramming M2 macrophages into M1 enhances antitumor immunity, while in chronic inflammatory diseases, shifting from M1 to M2 facilitates inflammation resolution and tissue repair. This modulation can be achieved using small molecules, cytokines, or nanotechnology to target key signaling pathways .

M1 polarization in cancer therapy: Inducing or sustaining M1 polarization has proven effective in enhancing antitumor immunity. For example, chimeric antigen receptor macrophages (CAR-Ms), engineered by Klichinsky et al., sustained a robust M1 phenotype for over 40 days, secreting pro-inflammatory cytokines that reprogrammed surrounding M2 macrophages into M1-like cells. This approach not only eliminated tumor cells but also enhanced the activation of antitumor T-cells .

Huo et al. further explored the impact of M1 polarization on CAR-Ms in models of HER2-positive ovarian cancer and lung metastases. They demonstrated that pre-polarizing CAR-Ms to an M1 phenotype significantly enhanced their antitumor efficacy, in vitro and in vivo, using murine models of intraperitoneal ovarian cancer and lung metastases. M1-polarized CAR-Ms reduced tumor burden, prolonged survival, and improved immune responses by increasing secretion of pro-inflammatory cytokines such as IL-12 and TNF-α . These findings highlight the enhanced therapeutic potential of combining M1 polarization with CAR-M therapy, particularly in treating solid tumors with a challenging immunosuppressive microenvironment.

An innovative approach involves “tail-flipping” nanoliposomes incorporating carboxylated phospholipids, such as 1-palmitoyl-2-azelaoyl-sn-glycero-3-phosphocholine (PAPC), designed to selectively target M2 TAMs via scavenger receptors, such as SR-B1. When loaded with therapeutic agents such as the STAT6 inhibitor AS1517499, zoledronic acid, or muramyl tripeptide (MTP), these liposomes inhibited M2 polarization. In preclinical breast cancer models, PAPC nanoliposomes reduced tumor growth, inhibited the M2 phenotype, and prevented pre-metastatic niche formation, achieving up to a 70% reduction in tumor burden without inducing toxicity .

M2 polarization in restoring inflammatory diseases: Polarizing macrophages to the M2 phenotype has shown considerable promise in treating inflammatory diseases such as IBD and RA. M2 macrophages secrete anti-inflammatory cytokines like IL-10 and TGF-β, aiding inflammation resolution and tissue repair.

In IBD, sustaining the M2 phenotype is particularly challenging because of the pro-ferroptotic microenvironment, which undermines macrophage survival. Zhao et al. addressed this issue by developing calcium carbonate (CaCO3)-mineralized liposomes (CLF) loaded with the ferroptosis inhibitor Fer-1. These liposomes promoted M2 polarization via the CaSR/AKT/β-catenin pathway while protecting macrophages from ferroptosis. In murine models of IBD, CLF reduced oxidative stress, inhibited ferroptosis, and restored intestinal homeostasis by increasing the M2/M1 macrophage ratio .

In RA, the predominance of M1 macrophages in inflamed joints drives synovitis and cartilage destruction. Yang and colleagues developed folic acid-modified silver nanoparticles (FA-AgNPs) to target M1 macrophages via folate receptor-mediated endocytosis. Once internalized, these NPs scavenged ROS, induced M1 apoptosis, and facilitated M1 to M2 polarization. In murine models of collagen-induced arthritis, FA-AgNPs significantly reduced joint swelling, improved cartilage integrity, and outperformed standard treatments like methotrexate .

Stabilizing macrophage polarization for long-lasting therapeutic effects remains a significant challenge, as their activation states are dynamic and often characterized by mixed or transitional phenotypes. This underscores the need for refined approaches to enhance efficacy while minimizing risks such as excessive immune activation. RNA-based therapeutics offer a promising solution by precisely targeting genes that regulate macrophage polarization, paving the way for more adaptable treatments.

Developing RNA-based therapies that target macrophage polarization presents significant challenges, particularly in ensuring specific and compelling modulation without causing adverse side effects. However, therapeutic molecules such as miRNAs, siRNAs, and mRNAs show promising potential in modulating macrophage behavior.

MicroRNA (miRNA) therapeutics: miRNAs are small non-coding RNA molecules (≈22 nucleotides) that regulate gene expression post-transcriptionally by binding to the 3′-UTR of target mRNAs, thereby inhibiting their translation. Their ability to promote specific macrophage phenotypes has made miRNAs a powerful tool for macrophage modulation. For instance, miR-155 promotes the M1 phenotype by suppressing anti-inflammatory pathways, while miR-146a enhances endotoxin tolerance by modulating TLR signaling through Notch1 inhibition . miR-221-3p drives M2 macrophages towards the M1 phenotype by inhibiting the JAK3/STAT3 signaling pathway. Conversely, miR-1246 promotes M2 polarization by targeting TERF2IP, activating STAT3, and inhibiting NF-κB .

Innovative delivery systems for miRNAs have further advanced their therapeutic application. Liu et al. developed a hybrid nanovector with dual redox/pH-responsive properties for targeted miRNA delivery in cancer therapy. This system, comprising galactose-functionalized polypeptides (GLC) coated with PEG-PLL copolymers (sPEG), was designed to release miR-155 specifically in the acidic tumor microenvironment. At neutral pH, the sPEG coating masked the cationic core, minimizing off-target effects, while at acidic pH, the coating detached, exposing GLC and enhancing miRNA uptake by TAMs. This nanovector, sPEG/GLC/155, increased miR-155 expression in TAMs 100–400-fold, leading to a robust M1-like polarization characterized by upregulated IL-12, iNOS, and MHC II, along with reduced M2 markers such as Arg1 and Msr2. The therapy also stimulated the activation of T-cells and natural killer cells, resulting in significant tumor regression .

Small interfering RNA (siRNA) therapeutics: siRNAs are double-stranded RNA molecules that induce the degradation of specific mRNAs, effectively silencing their expression. This approach has shown promise in mitigating inflammation and promoting tumoricidal macrophage polarization.

For inflammatory diseases like IBD, targeting pro-inflammatory cytokines with siRNA has proven effective. Laroui et al. developed polymeric NPs made of poly(lactic acid)–poly(ethylene glycol) block copolymer (PLA-PEG) grafted with the Fab’ fragment of F4/80 antibodies for specific macrophage targeting. These Fab’-bearing NPs delivered TNF-α siRNA to the colonic macrophages of mice, attenuating colitis more efficiently than non-targeted systems .

In cancer therapy, siRNAs have been used to reprogram TAMs to the M1 phenotype. For example, the co-delivery of a STAT6 inhibitor and IKKβ siRNA successfully repolarized M2-like TAMs into M1-like macrophages, enhancing antitumor immunity while minimizing immune side effects . To improve targeting specificity, micellar nanodrugs with pH-sheddable PEG coronas have been designed, allowing the encapsulated siRNA and inhibitors to selectively act on M2 TAMs in the acidic tumor microenvironment. This strategy effectively suppressed tumor growth and metastasis, while avoiding off-target macrophage repolarization in non-tumor tissues, enhancing both safety and efficacy .

Messenger RNA (mRNA) therapeutics: mRNA-based therapies enable the expression of therapeutic proteins within macrophages, providing a versatile approach to modulate their polarization. mRNAs encoding anti-inflammatory cytokines, such as IL-10, have been used to promote M2 polarization, facilitating tissue repair and inflammation resolution in autoimmune diseases.

For instance, LNPs containing IL-10 mRNA, formulated with the ionizable amino lipid Dlin-MC3-DMA (MC3) and other lipids like DSPC, cholesterol, and DMG-PEG2000, have demonstrated significant efficacy in inducing the M2 phenotype, reducing inflammation and tissue damage in RA models . Beyond cytokines, mRNA therapies have targeted macrophage-specific markers to induce phenotype switching. Polyethylenimine NPs grafted with mannose ligands have been used to deliver genes encoding CD163, a hallmark of M2 macrophages. This system successfully converted pro-inflammatory M1 macrophages into M2 macrophages in vitro, enhancing the release of anti-inflammatory cytokines and potentially mitigating inflammatory responses in vivo .

Transcription factors are another promising target for mRNA-based therapies. Delivering mRNAs encoding transcription factors such as PPARγ has shown the potential to promote M2 polarization, thereby supporting tissue regeneration and reducing chronic inflammation. These systems demonstrate the versatility of mRNA-based therapies in addressing various pathological conditions.

However, translating RNA-based therapeutics into clinical practice requires overcoming key challenges, such as ensuring molecular stability, achieving targeted delivery, and minimizing immune activation. Ongoing research is addressing these barriers by advancing RNA delivery systems . For example, mesoporous silica NPs have been utilized to co-deliver miRNAs and small-molecule drugs to macrophages, successfully reprogramming TAMs in cancer models to favor antitumor immunity .

Recent breakthroughs in mRNA delivery technologies, exemplified by the success of the Moderna and Pfizer-BioNTech COVID-19 vaccines, provide a clear pathway for improving mRNA-based therapies. These vaccines use LNPs designed to facilitate endosomal escape, preventing RNA degradation within lysosomes and improving intracellular delivery efficacy .

Table 2: Targeting strategies and approaches to modulate M1/M2 macrophages.