The effect of PEGylation on the efficacy and uptake of an immunostimulatory nanoparticle in the tumor immune microenvironment.

The efficacy of immunotherapies is often limited by the immunosuppressive tumor microenvironment, which is populated with dysfunctional innate immune cells. To reprogram the tumor-resident innate immune cells, we developed immunostimulatory silica mesoporous nanoparticles (immuno-MSN). The cargo of immuno-MSN is a Stimulator of Interferon Gene (STING) agonist, which activates innate immune cells leading to production of interferon (IFN) ß. By proficiently trafficking its cargo into immune cells, the immuno-MSN induced a 9-fold increase of IFN-ß secretion compared to free agonist. While an external PEG shield has historically been used to protect nanoparticles from immune recognition, a PEGylated immunostimulatory nanoparticle needs to strike a balance between immune evasion to avoid off-site accumulation and uptake by target immune cells in tumors. Using the 4T1 mouse model of metastatic breast cancer and flow cytometry, it was determined that the degree of PEGylation significantly influenced the uptake of 'empty' MSNs by tumor-resident innate immune cells. This was not the case for the agonist-loaded immuno-MSN variants. It should be noted the surface charge of the 'empty' MSNs was positive rather than neutral for the agonist-loaded immuno-MSNs. However, even though the cellular uptake was similar at 24 h after injection for the three immuno-MSN variants, we observed a significant beneficial effect on the activation and expansion of APCs especially in lung metastasis using the lightly PEGylated immuno-MSN variant.

Immunostimulatory silica nanoparticle boosts innate immunity in brain tumors.

The high mortality associated with glioblastoma multiforme (GBM) is attributed to its invasive nature, hypoxic core, resistant cell subpopulations and a highly immunosuppressive tumor microenvironment (TME). To support adaptive immune function and establish a more robust antitumor immune response, we boosted the local innate immune compartment of GBM using an immunostimulatory mesoporous silica nanoparticle, termed immuno-MSN. The immuno-MSN was specifically designed for systemic and proficient delivery of a potent innate immune agonist to dysfunctional antigen-presenting cells (APCs) in the brain TME. The cargo of the immuno-MSN was cyclic diguanylate monophosphate (cdGMP), a Stimulator of Interferon Gene (STING) agonist. Studies showed the immuno-MSN promoted the uptake of STING agonist by APCs in vitro and the subsequent release of the pro-inflammatory cytokine interferon ß, 6-fold greater than free agonist. In an orthotopic GBM mouse model, systemically administered immuno-MSN particles were taken up by APCs in the near-perivascular regions of the brain tumor with striking efficiency. The immuno-MSNs facilitated the recruitment of dendritic cells and macrophages to the TME while sparing healthy brain tissue and peripheral organs, resulting in elevated circulating CD8+ T cell activity (2.5-fold) and delayed GBM tumor growth. We show that an engineered immunostimulatory nanoparticle can support pro-inflammatory innate immune function in GBM and subsequently augment current immunotherapeutic interventions and improve their therapeutic outcome.