The microbiome can have profound effects on the innate immune system. Since the innate immune system regulates the adaptive immune response to antigens, we hypothesized that the microbiome may influence anti-tumor responses to immune checkpoint inhibitors. Accordingly, we sought to characterize the anti-tumor effects of PD-L1 blockade therapy between mice with syngeneic tumors in conventional (specific pathogen-free, SPF) and germ-free (GF) environments.
B16-OVA or Lewis Lung Cancer (LLC) cell lines were injected subcutaneously into the flanks of 10-12 week-old C57BL/6 mice in both SPF and germ-free (axenic) environments. Mice with B16-OVA tumors in SPF (n = 6) and GF (n = 12, 6 females and 6 males) environments, and mice with LLC tumors in GF (n = 6) environments were randomized to receive the murine PD-L1 blocking antibody 10B5 or an isotype control. Tumor growth was evaluated every 2-3 days until days 35-40 when all mice were euthanized. Tumor size was compared between treatment groups in each environment at day 24 with the Mann Whitney U test. This project was approved by Mayo Clinic’s Institutional Review Board and Institutional Animal Care and Use Committee. Funding was provided by the NIH (K12 CA90628) and Mayo Clinic’s Center for Individualizing Medicine’s Microbiome Project.
Whereas injection of the anti-PD-L1 antibody (clone 10B5) controlled tumor growth compared to treatment with an isotype control in SPF female mice with B16-OVA (p = 0.05), PD-L1 blockade had no effect on tumor growth in female axenic mice with B16-OVA (p = 0.20) or male axenic mice with B16-OVA (p = 0.34) or axenic mice with LLC (p = 0.56).
PD-L1 blockade therapy loses its anti-tumor efficacy in axenic mice. The microbiome may influence the efficacy of PD-L1 blockade through of its effects on both innate and adaptive immune responses to tumors.
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Legal entity responsible for the study
Aaron Mansfield at Mayo Clinic
National Institutes of Health; Mayo Clinic's Center for Individualizing Medicine Microbiome Project
All authors have declared no conflicts of interest.