Malignant mesothelioma is a cancer characterized by a significant stromal component that is dominated by macrophages. Tumor-associated macrophages (TAMs) are supported by macrophage-colony stimulating factor (M-CSF), which not only enhances TAM survival and proliferation, but also their recruitment towards tumors and the expression of a pro-tumor phenotype. TAMs are linked to immune suppression, angiogenesis, and tumor aggressiveness in mesothelioma, which led Dammeijer et al. to the hypothesis that depletion of TAMs could improve the efficacy of a tumor lysate-pulsed dendritic cell (DC) vaccine.

For this study, the researchers used two orthotopic mesothelioma mouse models, BALB/c (AB1) and CBA/J (AC29). The CBA/J (AC29) model was selected for its more prominent presence of macrophages of the pro-tumor phenotype - likely due to an increased expression of M-CSF - and its tendency to form ascites, paralleling the presentation of mesothelioma in cancer patients. In an effort to inhibit the receptor for M-CSF and reduce the number of TAMs in vivo, the researchers used the clinically-tested small molecule tyrosine kinase inhibitor PLX3397 (pexidartinib).

PLX3397 monotherapy effectively reduced the number of TAMs, decreased the volume of ascites, reduced neo-angiogenesis and reduced the number of circulating nonclassical monocytes, which typically express PD-L1. PLX3397 alone, however, did not improve the overall survival of mesothelioma-bearing mice.

Vaccination with tumor lysate-pulsed DCs in isolation, on the other hand, increased survival in ~20% of the mice, increased the number of non-exhausted CD8⁺ T cells in the tumor, and improved the CD8⁺/ T_reg cell ratio in the peripheral blood. Within the circulating CD8⁺ T cell population, DC vaccination raised the percentage of short-lived KLRG1⁺ effector cells from virtually 0% to about 20%.

The combination of the DC vaccine with PLX3397 treatment significantly enhanced survival. The number of circulating T_reg cells decreased even further than it had with DC vaccination alone, and an increased proportion of CD8⁺ T cells was found to co-express CD4 (this double positivity has been associated with improved effector functions). With TAMs and PD-L1⁺ nonclassical monocytes mostly cleared, CD8⁺ T cell populations within the tumor showed reduced surface expression of PD-1, indicating less exhaustion. Overall, this suggests that the depletion of TAMs by PLX3397 helped by improving the phenotypes of tumor-infiltrating lymphocytes recruited following DC vaccination.

Three months after treatment with the combination or the DC vaccine alone, surviving animals were subjected to a tumor rechallenge and proved to be fully protected. Proliferation of CD8⁺ and CD4⁺ T cells was higher in combination-treated mice compared to mice treated with DC monotherapy, indicating a superior memory response.

These results show that the combination of a DC vaccine that efficiently induces an anti-tumor immune response, with an agent that effectively targets inhibitory macrophages in the tumor microenvironment, works in synergy and leads to robust and durable antitumor immunity.

by Lauren Hitchings