To combat TAMs, a key strategy involves inhibiting their polarization toward the immunosuppressive M2 phenotype, which is often accomplished through specific cytokines or immunomodulators [
149,
150]. Kubara
et al. developed an LNP-based mRNA vaccine (L17-F05) encapsulating mRNA encoding the tumor antigens Gp100 and Trp2 [
151]. The L17-F05 NPs employ ionizable lipids with a pH-responsive design to enhance LNs targeting, enabling efficient delivery to LN-resident macrophages and DCs. In TDLNs, these LNPs activate macrophages
via the stimulator of interferon genes (STING) type-I interferon (IFN1) pathway, inducing the production of IFN-I and the upregulation of co-stimulatory molecules (CD40, CD80, CD86) in DCs. This immune activation enhances the maturation and antigen-presentation capacity of DCs, culminating in a robust CD8
+ T cell-mediated anti-tumor response that significantly inhibits tumor growth in melanoma models. The Toll like receptors (TLR)7/8 agonist R848 is among the most effective molecules for inducing macrophage polarization toward the M1 phenotype
in vitro [
152]. Beta-cyclodextrin NPs (CDNP-R848) loaded with R848 effectively accumulate within macrophages in tumors and TDLNs due to their physicochemical properties [
152].
In vivo experiments demonstrated that CDNP-R848 specifically targets TAMs within tumors, promoting their phenotypic transformation and inducing the expression of IL-12. This transformation subsequently activates the immune response of Th1 cells. Solely applied CDNP-R848 demonstrates efficacy in regulating tumor growth and improving the survival rate of tumor-bearing mice. However, when combined with anti-PD-1, CDNP-R848 exhibits a pronounced synergistic effect, achieving superior control over tumor growth and further enhancing the survival probability of tumor-bearing mice. Tong
et al. developed pH-sensitive, size-switchable nanoclusters (SPN-R848) by conjugating the TLR7/8 agonist R848 with polyamidoamine (PAMAM) derivatives [
153]. These nanoclusters target TAMs and TDLNs, and their size reduces under acidic conditions, facilitating their penetration and the repolarization of M2-TAMs to an antitumor M1 phenotype. Silva
et al. employed biodegradable NPs (pR848M-NPs) to co-deliver poly(I:C), R848, and MIP3
α, reprogramming the immune TME within tumors and TDLNs [
154]. R848 significantly facilitated the transition of TAMs from an M2-like immunosuppressive phenotype (Ly6Cmed) to an acute inflammatory phenotype associated with M1-TAMs (Ly6Chi). poly(I:C) and R848 activate TLR3 and TLR7/8, markedly augmenting the expression of CD40, CD80, and CD86 on the surface of DCs and stimulating the secretion of IL-12. This subsequently modulates Th1 cell differentiation and the cytotoxic activity of cytotoxic T lymphocytes (CTLs). MIP3
α recruits immune cells expressing CCR6/CD196, including immature DCs, T cells, and natural killer (NK) cells, to the tumor site, significantly enhancing immune cell diversity and quantity within the TME. Xu
et al. engineered an oral NP system (MTG/siSIRP
α/pMUC1) with surface modifications of glycocholic acid and mannose, encapsulating the immune checkpoInt. inhibitor signal regulatory protein
α (SIRP
α) small interfering RNA (siRNA) and the mucin-1 (MUC1) pDNA vaccine [
155]. Glycocholic acid facilitates the transepithelial passage of NPs through the intestinal epithelium into the lymphatic system, enhancing their accumulation in TDLNs. Mannose modification further enhances NPs binding to macrophages. Xu
et al. engineered mannose and glycocholic acid-modified trimethyl chitosan (MTG) nanoparticles. The SIRP
α siRNA payload in MTG/siSIRP
α/pMUC1 NPs downregulates SIRP
α expression in macrophages, promoting their phenotypic shift from the tumor-promoting M2 to the anti-tumor M1 phenotype. Simultaneously, the MUC1 pDNA vaccine stimulates T-cell immunity against the MUC1 antigen, amplifying the systemic anti-tumor response. Nano-PI is an albumin-based NP encapsulating the phosphatidylinositol 3-kinase (PI3K
γ) inhibitor (IPI-549) and the chemotherapy drug paclitaxel (PTX) [
156]. Leveraging the natural circulation properties of albumin, Nano-PI specifically targets and accumulates in macrophages within TDLNs and tumors. Nano-PI delivers IPI-549, inhibiting the PI3K
γ signaling pathway and promoting the polarization of immunosuppressive M2-TAMs into tumor-fighting M1-TAMs. Additionally, Nano-PI activates M1-TAMs and DCs, boosting antigen presentation in TDLNs. This increases the number of active CD4
+ and CD8
+ T cells, reduces exhausted T cells and Tregs, and strengthens the local anti-tumor immune response.