br Modulating the Tumor Microenvironment
3.3 Modulating the Tumor Microenvironment
Spontaneous tumors are comprised of many cell populations including tumor cells, immune SCR7 (both immune-stimulating and immunosuppressive populations), fibroblasts, and endothelial cells in addition to acellular components. These are collectively referred to as the tumor microenvironment (TME). It is now appreciated that non-tumor cell components of the TME are critical for tumoral proliferation, migration, and therapeutic responsiveness. The TME represents both a physical and biochemical barrier to infiltrating immune cells. Immunosuppressive cell populations within the TME pose a major barrier to immune-mediated tumor clearance and cancer immunotherapies. These include Th2 CD4+ regulatory T-cells (T-regs), M2 TAMs, and tumor-associated fibroblasts (TAFs). Strategies which deplete inhibitory cell populations or convert them into immunostimulatory phenotypes have the potential to improve the efficacy of conventional cancer therapies and cancer immunotherapy. Unfortunately, many immune-modulating drugs and cytokines are poorly tolerated systemically and selective ablation of immunosuppressive but not proimmunogenic cell populations is difficult with conventional drug delivery. Extensive preclinical investigations have identified a handful of cell surface markers that are selectively overexpressed in tumoral populations of inhibitory cell types that can potentially be targeted by stealth NPs. Nanomedicines are well-poised to enhance the feasibility of these approaches by decreasing the systemic toxicity of novel compounds and improving their delivery to relevant cellular populations.
There are two principle classes of TAMs: immunostimulatory M1 macrophages, which facilitate tumor clearance, and immunosuppressive M2 macrophages that inhibit cytotoxic CD8+ T-cells and promote tumor cell proliferation/survival98. The basis of immune modulation by TAMs is secretion of high-levels of cytokines in the TME. M1 TAMs secrete high levels of pro-inflammatory cytokines including INF-γ, TNF-α, and IL-12 which promote maturation of APCs, Th1 Tregs, and adaptive immune responses. M2 macrophages, in contrast, secrete high levels of immunosuppressive cytokines including IL-10 and TGF-β which inhibit MHC-mediated antigen presentation and stimulate apoptosis of various lymphocyte populations. As such, the relative ratio of M2 to M1 TAMs in the TME is critical to determining sensitivity of tumors to conventional chemotherapies and immunotherapies including T-cell checkpoint inhibitors. Accordingly, high levels of M2 macrophages are associated with resistance to T-cell checkpoint inhibitors and poor long-term survival99,100. Fortunately, TAMs are highly plastic and several strategies to promote polarization towards the pro-immunogenic M1 phenotype have been identified. One strategy involves activation of specific toll-like receptors (TLR) on the macrophage surface. A number of TLR agonists have been identified including R848, a selective TLR7/8 agonist that potently drives macrophages towards the M1 phenotype. One group demonstrated that cyclodextran NPs efficiently deliver R848 to TAMs in vivo and stimulate M1 polarization101. This in turn sensitizes non-immunogenic tumors to T-cell checkpoint inhibitors leading to sustained tumor rejection and resistance to subsequent tumor challenge. M1 macrophages also express high levels of the mannose receptor
MRc1 (CD206) that can be used for NP targeting. Mannosylated cationic albumin carriers have been used to enhance in vivo delivery of the TLR agonist CpG-ODN to facilitate M1 polarization. Ai et al. showed that the inclusion of mannose moieties enhanced M1 polarization (as evidenced by increased secretion of pro-inflammatory cytokines such as IL-12 and IL-6) compared to standard cationic albumin carriers102.
Other well-characterized cytokines can also alter polarization of TAMs. Pro-inflammatory cytokines including GM-CSF, INFβ, and INFγ enhance M1 polarization primarily through activation of STAT1 and STAT5 pathways103. Systemically-administered GM-CSF exerts minimal effect on cell populations in the TME. NP-delivered GM-CSF, in contrast, appears to effectively polarize TAMs to the M1 phenotype and improve immune-mediated cytotoxicity. Researchers in one study generated micellar formulations of polymer-CSF conjugates linked by an acid-labile calcium carbonate linker. Very little GM-CSF was released from the micelles at physiologic pH. However, acid-mediated cleavage in the TME stimulated rapid release of active GM-CSF that enhanced M1 differentiation and improved tumor growth delay104.
Polarization of TAMs towards the M1 phenotype can also be accomplished with NPs by targeting inflammatory signaling pathways. miR155 is an important microRNA that regulates inflammatory signaling pathways in myeloid and lymphoid pathways105,106. Yang et al. delivered miR155 to immune cells in the TME using layered double hydroxide NPs107. These particles were efficiently taken up by TAMs and demonstrated efficient endosomal escape and cytosolic delivery. miR155 markedly reduced the expression and activation of STAT-3, ERK1/2 and NFκB and polarized TAMs towards the M1 phenotype. These changes in inflammatory signaling were also associated with decreased recruitment and formation of activated myeloid derived suppressor cells (MDSCs) in the TME. miR155 treatment increased tumoral infiltration of CD8+ T-cells and enhanced responsiveness to checkpoint inhibitors.