MODULATION OF MONOCYTE/MACROPHAGE ACTIVATION AND MATURATION BY PLANT VIRUS NANOPARTICLES AND FREE FATTY ACIDS: IMPLICATIONS FOR TUMOR IMMUNOTHERAPY

Abstract

Methods that induce immune responses against tumors have emerged as a promising approach for cancer treatment. Recent research has shown that Cow Pea Mosaic Virus (CPMV) nanoparticles can induce anti-tumor immune responses in various animal tumors. The molecular mechanisms underlying these effects, as well as CPMV's potential to activate human cells were investigated in these studies. In vitro stimulation of CD14+ monocytes with CPMV resulted in the induction of HLA-DR, CD86, PD-L1, IL-15R, CXCL10, MIP-1a, and MIP-1b. CPMV also caused activation of dendritic cells and monocyte-derived macrophages. Our findings demonstrated that CPMV activates human monocytes via Syk signaling, endosomal acidification, and recognition by Toll-like Receptor (TLR) 7/8. These findings suggest that CPMV may be a useful immune-based approach in humans. Importantly, immune cell activation is hindered in the tumor microenvironment by immunosuppressive mechanisms. These mechanisms may include an influence on macrophage maturation, ultimately generating macrophages that favor tumor growth. To better understand how tumor microenvironments influence macrophage maturation, we studied the effects of tumor cell supernatants (FaDu and SCC) on monocyte-derived macrophages. Purified monocytes incubated with FaDu or SCC9 supernatants showed improved survival, decreased surface HLA-DR, CD86, IL-15R and 2 IP-10 expression, and increased surface PD-L1, CD14 and CD206 expression. Despite expressing TLR4 and CD14, macrophages matured in tumor supernatants did not respond to the TLR4 agonist LPS. This could be due to overexpression of WIP-1 phosphatase in tumor supernatant-exposed macrophages, resulting in inhibition of p38 phosphorylation that is implicated in many inflammatory signaling pathways. Several phenotypic changes in tumor supernatant exposed macrophages could be reversed by depleting fatty acids from tumor supernatants or by inhibition of fatty acid oxidation. The phenotype of macrophages incubated with fatty acids (palmitic or oleic acid) was also comparable to that of cells incubated with tumor supernatants, suggesting that fatty acids are sufficient to induce such changes. These findings suggest that fatty acids derived from tumor cells can induce macrophages to mature into less pro-inflammatory cells. Blocking the activity of fatty acids in tumor microenvironments may represent a strategy to enhance the effects of immunotherapies.