Ayelet Sivan

Commensal Bifidobacterium promotes antitumor immunity and facilitates anti-PD-L1 efficacy.

Gut microbes affect immunotherapy The unleashing of antitumor T cell responses has ushered in a new era of cancer treatment. Although these therapies can cause dramatic tumor regressions in some patients, many patients inexplicably see no benefit. Mice have been used in two studies to investigate what might be happening. Specific members of the gut microbiota influence the efficacy of this type of immunotherapy (see the Perspective by Snyder et al.). Vétizou et al. found that optimal responses to anticytotoxic T lymphocyte antigen blockade required specific Bacteroides spp. Similarly, Sivan et al. discovered that Bifidobacterium spp. enhanced the efficacy of antiprogrammed cell death ligand 1 therapy. Science, this issue, p. 1079 and p. 1084; see also p. 1031 Gut microbes modulate the effectiveness of cancer immunotherapies in mice. [Also see Perspective by Snyder et al.] T cell infiltration of solid tumors is associated with favorable patient outcomes, yet the mechanisms underlying variable immune responses between individuals are not well understood. One possible modulator could be the intestinal microbiota. We compared melanoma growth in mice harboring distinct commensal microbiota and observed differences in spontaneous antitumor immunity, which were eliminated upon cohousing or after fecal transfer. Sequencing of the 16S ribosomal RNA identified Bifidobacterium as associated with the antitumor effects. Oral administration of Bifidobacterium alone improved tumor control to the same degree as programmed cell death protein 1 ligand 1 (PD-L1)–specific antibody therapy (checkpoint blockade), and combination treatment nearly abolished tumor outgrowth. Augmented dendritic cell function leading to enhanced CD8+ T cell priming and accumulation in the tumor microenvironment mediated the effect. Our data suggest that manipulating the microbiota may modulate cancer immunotherapy.

Dormant breast cancer micrometastases reside in specific bone marrow niches that regulate their transit to and from bone

Breast cancer cells traffic to and from the peripheral blood within specific vascular niches, and this migration can be therapeutically targeted. Taking away cancer’s hideouts Breast cancer is notorious for its ability to relapse after many years, long after a patient had completed treatment. Price et al. demonstrate that the culprits responsible for such late metastasis may be dormant cancer cells hiding in perivascular niches. The authors showed that proteins called E-selectin and CXCR4 exert different forces on these cancer cells, with CXCR4 anchoring breast cancer cells to their niches and E-selectin allowing entry of cancer cells into the bone marrow. These findings suggest that combining a CXCR4 inhibitor to force the cells out of their niches and an E-selectin inhibitor to prevent metastasis to the bone marrow could help trap the cells in the vasculature, where they could be killed with chemotherapy. Breast cancer metastatic relapse can occur years after therapy, indicating that disseminated breast cancer cells (BCCs) have a prolonged dormant phase before becoming proliferative. A major site of disease dissemination and relapse is bone, although the critical signals that allow circulating BCCs to identify bone microvasculature, enter tissue, and tether to the microenvironment are poorly understood. Using real-time in vivo microscopy of bone marrow (BM) in a breast cancer xenograft model, we show that dormant and proliferating BCCs occupy distinct areas, with dormant BCCs predominantly found in E-selectin– and stromal cell–derived factor 1 (SDF-1)–rich perisinusoidal vascular regions. We use highly specific inhibitors of E-selectin and C-X-C chemokine receptor type 4 (CXCR4) (SDF-1 receptor) to demonstrate that E-selectin and SDF-1 orchestrate opposing roles in BCC trafficking. Whereas E-selectin interactions are critical for allowing BCC entry into the BM, the SDF-1/CXCR4 interaction anchors BCCs to the microenvironment, and its inhibition induces mobilization of dormant micrometastases into circulation. Homing studies with primary BCCs also demonstrate that E-selectin regulates their entry into bone through the sinusoidal niche, and immunohistochemical staining of patient BMs shows dormant micrometastatic disease adjacent to SDF-1+ vasculature. These findings shed light on how BCCs traffic within the host, and suggest that simultaneous blockade of CXCR4 and E-selectin in patients could molecularly excise dormant micrometastases from the protective BM environment, preventing their emergence as relapsed disease.

Tumor and Host Factors Controlling Antitumor Immunity and Efficacy of Cancer Immunotherapy.

Despite recent clinical advances in immunotherapy, a fraction of cancer patients fails to respond to these interventions. Evidence from preclinical mouse models as well as clinical samples has provided evidence that the extent of activated T cell infiltration within the tumor microenvironment is associated with clinical response to immunotherapies including checkpoint blockade. Therefore, understanding the molecular mechanisms mediating the lack of T cell infiltration into the tumor microenvironment will be instrumental for the development of new therapeutic strategies to render those patients immunotherapy responsive. Recent data have suggested that major sources of intersubject heterogeneity include differences in somatic mutations in specific oncogene pathways between cancers of individual subjects and also environmental factors including commensal microbial composition. Successful identification of such causal factors should lead to new therapeutic approaches that may facilitate T cell entry into noninflamed tumors and expand the fraction of patients capable of responding to novel immunotherapies.

The composition of the microbiota modulates allograft rejection

Transplantation is the only cure for end-stage organ failure, but without immunosuppression, T cells rapidly reject allografts. While genetic disparities between donor and recipient are major determinants of the kinetics of transplant rejection, little is known about the contribution of environmental factors. Because colonized organs have worse transplant outcome than sterile organs, we tested the influence of host and donor microbiota on skin transplant rejection. Compared with untreated conventional mice, pretreatment of donors and recipients with broad-spectrum antibiotics (Abx) or use of germ-free (GF) donors and recipients resulted in prolonged survival of minor antigen-mismatched skin grafts. Increased graft survival correlated with reduced type I IFN signaling in antigen-presenting cells (APCs) and decreased priming of alloreactive T cells. Colonization of GF mice with fecal material from untreated conventional mice, but not from Abx-pretreated mice, enhanced the ability of APCs to prime alloreactive T cells and accelerated graft rejection, suggesting that alloimmunity is modulated by the composition of microbiota rather than the quantity of bacteria. Abx pretreatment of conventional mice also delayed rejection of major antigen-mismatched skin and MHC class II-mismatched cardiac allografts. This study demonstrates that Abx pretreatment prolongs graft survival, suggesting that targeting microbial constituents is a potential therapeutic strategy for enhancing graft acceptance.

Cancer Immunotherapy Targets Based on Understanding the T Cell-Inflamed Versus Non-T Cell-Inflamed Tumor Microenvironment

Most cancers express tumor antigens that can be recognized by T cells of the host. The fact that cancers become clinically evident nonetheless implies that immune escape must occur. Two major subsets of human melanoma metastases have been identified based on gene expression profiling. One subgroup has a T cell-inflamed phenotype that includes expression of chemokines, T cell markers, and a type I IFN signature. In contrast, the other major subset lacks this phenotype and has been designated as non-T cell-inflamed. The mechanisms of immune escape are likely distinct in these two phenotypes, and therefore the optimal immunotherapeutic interventions necessary to promote clinical responses may be different. The T cell-inflamed tumor microenvironment subset shows the highest expression of negative regulatory factors, including PD-L1, IDO, FoxP3+ Tregs, and evidence for T cell-intrinsic anergy. Therapeutic strategies to overcome these inhibitory mechanisms are being pursued, and anti-PD-1 mAbs have been FDA approved. The presence of multiple inhibitory mechanisms in the same tumor microenvironment argues that combination therapies may be advantageous, several of which are in clinical testing. A new paradigm may be needed to promote de novo inflammation in cases of the non-T cell-infiltrated tumor microenvironment. Natural innate immune sensing of tumors appears to occur via the host STING pathway, type I IFN production, and cross-priming of T cells via CD8α+ DCs. New strategies are being developed to engage this pathway therapeutically, such as through STING agonists. The molecular mechanisms that mediate the presence or absence of the T cell-inflamed tumor microenvironment are being elucidated using parallel genomics platforms. The first oncogene pathway identified that mediates immune exclusion is the Wnt/β-catenin pathway, suggesting that new pharmacologic strategies to target this pathway should be developed to restore immune access to the tumor microenvironment.

Evidence implicating the commensal microbiota in shaping anti-tumor immunity in melanoma

T cell infiltration of solid tumors is found in a subset of cancer patients and is associated with favorable patient outcomes. This demonstrates the capacity of the immune system to mount protective anti-tumor responses in some cancer patients, yet the underlying mechanisms mediating the presence or absence of a T cell infiltrate are not well understood. Individuals exhibit remarkable diversity in their commensal microbiota, which in turn influences systemic immune responses, yet the role of microbiota in shaping spontaneous immunity against tumors is not known. Here we sought to examine the effect of microbial composition on the immune response to melanoma by comparing subcutaneous B16 melanoma growth in genetically identical C57BL/6 mice derived from two different facilities, Jackson laboratories and Taconic farms, which have been shown to differ in their commensal microbes. We found that B16.SIY tumors exhibited an increased growth rate in mice derived from Taconic (p < 0.02), which was associated with reduced induction of tumor-specific IFN-γ-producing CD8+ T cells (p < 0.009). A markedly increased number of SIY antigen-specific CD8+ T cells was found to accumulate within the tumor microenvironment in Jackson versus Taconic mice (p < 0.03). Strikingly, the differences in tumor growth, endogenous T cell priming, and SIY antigen-specific T cell infiltration were ablated in animals cohoused for at least two weeks prior to injection of tumor cells, consistent with a microbiota-derived effect. We thus asked whether transfer of Jackson fecal material into Taconic mice by oral gavage prior to tumor inoculation was sufficient to confer protective anti-tumor responses. Indeed, Taconic mice that received Jackson feces exhibit dramatically slower tumor growth (p < 0.015) and increased numbers of SIY antigen-specific CD8+ T cells at the tumor site, compared to Taconic mice that received Taconic feces. To pursue mechanisms that might explain these differences, we transferred CFSE-labeled SIY-specific 2C TCR Tg T cells into tumor-bearing Jackson and Taconic mice and tested their proliferation and acquisition of IFN-γ production ex vivo. 2C T cells responding to tumors in Taconic mice produced significantly less IFN-γ than 2C T cells responding to tumors in Jackson mice, likely pointing to a difference at the level of antigen-presenting cells. No difference in Th17 induction was observed. Our data provide evidence implicating the microbiota in shaping anti-tumor immunity in a pre-clinical model of melanoma. Gaining further understanding of the mechanisms underlying this effect may provide the foundation for rational manipulation of commensal microbes as a cancer therapeutic.