Targeting PD-L2/RGMb overcomes microbiome-related immunotherapy resistance

The gut microbiota has been shown to be a critical regulator of anti-tumor immunity during immune checkpoint inhibitor therapies. Several bacteria that promote an anti-tumor response to immune checkpoint inhibitors have been identified in mice, and transplantation of fecal specimens from responders has been shown to improve the efficacy of anti-PD-1 therapy in melanoma patients. However, the increased efficacy from fecal transplants is variable and how gut bacteria promote anti-tumor immunity remain unclear. Here, we show that the gut microbiome downregulates expression of PD-L2 and its binding partner Repulsive Guidance Molecule b (RGMb) to promote anti-tumor immunity, and identify bacterial species that mediates this effect. PD-L1 and PD-L2 share PD-1 as a binding partner, but PD-L2 can also bind RGMb. We demonstrate that blockade of PD-L2 or RGMb interactions can overcome microbiome-dependent resistance to PD-1 pathway inhibitors: antibody-mediated blockade of the PD-L2/RGMb pathway or conditional deletion of RGMb in T cells combined with anti-PD-1 or anti-PD-L1 promotes anti-tumor responses in multiple mouse tumor models that do not respond to anti-PD-1 or anti-PD-L1 alone (Germ Free mice, antibiotic treated mice, and even mice colonized with non-responder patient stool). These studies identify downregulation of the PD-L2/RGMB pathway as a specific mechanism by which the gut microbiota can promote responses to PD-1 checkpoint blockade, and define a potentially effective immunological strategy for treating patients who do not respond to PD-1 cancer immunotherapy.