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Purpose: Endoglin is a coreceptor for TGFβ ligands that is highly expressed on proliferating endothelial cells and other cells in the tumor microenvironment. Clinical studies have noted increased programmed cell death (PD)-1 expression on cytotoxic T cells in the peripheral blood of patients with cancer treated with TRC105, an endoglin-targeting antibody. In this study, we investigated the combination of endoglin antibodies (TRC105 and M1043) with an anti-PD1 antibody. Experimental Design: The combination anti-endoglin/anti-PD1 antibodies was tested in four preclinical mouse models representing different stages of cancer development. To investigate the underlying mechanism, Fc-receptor–knockout mice were used complemented with depletion of multiple immune subsets in mice. Tumor growth and the composition of immune infiltrate were analyzed by flow cytometry. Finally, human colorectal cancer specimens were analyzed for presence of endoglin-expressing regulatory T cells (Treg). Results: In all models, the combination of endoglin antibody and PD1 inhibition produced durable tumor responses, leading to complete regressions in 30% to 40% of the mice. These effects were dependent on the presence of Fcγ receptors, indicating the involvement of antibody-dependent cytotoxic responses and the presence of CD8+ cytotoxic T cells and CD4+ Th cells. Interestingly, treatment with the endoglin antibody, TRC105, significantly decreased the number of intratumoral Tregs. Endoglin-expressing Tregs were also detected in human colorectal cancer specimens. Conclusions: Taken together, these data provide a rationale for combining TRC105 and anti-PD1 therapy and provide additional evidence of endoglin's immunomodulatory role.

Patients with the mesenchymal subtype colorectal cancer (CRC) have a poor prognosis, in particular patients with stroma-rich tumors and aberrant SMAD4 expression. We hypothesized that interactions between SMAD4-deficient CRC cells and cancer-associated fibroblasts provide a biological explanation. In transwell invasion assays, fibroblasts increased the invasive capacity of SMAD4-deficient HT29 CRC cells, but not isogenic SMAD4-proficient HT29 cells. A TGF-beta/BMP-specific array showed BMP2 upregulation by fibroblasts upon stimulation with conditioned medium from SMAD4-deficient CRC cells, while also stimulating their invasion. In a mouse model for experimental liver metastasis, the co-injection of fibroblasts increased metastasis formation of SMAD4-deficient CRC cells (p = 0.02) but not that of SMAD4-proficient CRC cells. Significantly less metastases were seen in mice co-injected with BMP2 knocked-down fibroblasts. Fibroblast BMP2 expression seemed to be regulated by TRAIL, a factor overexpressed in SMAD4-deficient CRC cells. In a cohort of 146 stage III CRC patients, we showed that patients with a combination of high stromal BMP2 expression and the loss of tumor SMAD4 expression had a significantly poorer overall survival (HR 2.88, p = 0.04). Our results suggest the existence of a reciprocal loop in which TRAIL from SMAD4-deficient CRC cells induces BMP2 in fibroblasts, which enhances CRC invasiveness and metastasis.

BackgroundAdenoviral vectors emerged as important platforms for cancer immunotherapy. Vaccination with adenoviral vectors is promising in this respect, however, their specific mechanisms of action are not fully understood. Here, we assessed the development and maintenance of vaccine-induced tumor-specific CD8+ T cells elicited upon immunization with adenoviral vectors.MethodsAdenoviral vaccine vectors encoding the full-length E7 protein from human papilloma virus (HPV) or the immunodominant epitope from E7 were generated, and mice were immunized intravenously with different quantities (107, 108 or 109 infectious units). The magnitude, kinetics and tumor protection capacity of the induced vaccine-specific T cell responses were evaluated.ResultsThe adenoviral vaccines elicited inflationary E7-specific memory CD8+ T cell responses in a dose-dependent manner. The magnitude of these vaccine-specific CD8+ T cells in the circulation related to the development of E7-specific CD8+ tissue-resident memory T (TRM) cells, which were maintained for months in multiple tissues after vaccination. The vaccine-specific CD8+ T cell responses conferred long-term protection against HPV-induced carcinomas in the skin and liver, and this protection required the induction and accumulation of CD8+ TRM cells. Moreover, the formation of CD8+ TRM cells could be enhanced by temporal targeting CD80/CD86 costimulatory interactions via CTLA-4 blockade early after immunization.ConclusionsTogether, these data show that adenoviral vector-induced CD8+ T cell inflation promotes protective TRM cell populations, and this can be enhanced by targeting CTLA-4.

Purpose: Cancer-associated fibroblasts (CAF) are a major component of the colorectal cancer tumor microenvironment. CAFs play an important role in tumor progression and metastasis, partly through TGF-β signaling pathway. We investigated whether the TGF-β family coreceptor endoglin is involved in CAF-mediated invasion and metastasis. Experimental Design: CAF-specific endoglin expression was studied in colorectal cancer resection specimens using IHC and related to metastases-free survival. Endoglin-mediated invasion was assessed in vitro by transwell invasion, using primary colorectal cancer–derived CAFs. Effects of CAF-specific endoglin expression on tumor cell invasion were investigated in a colorectal cancer zebrafish model, whereas liver metastases were assessed in a mouse model. Results: CAFs specifically at invasive borders of colorectal cancer express endoglin and increased expression intensity correlated with increased disease stage. Endoglin-expressing CAFs were also detected in lymph node and liver metastases, suggesting a role in colorectal cancer metastasis formation. In stage II colorectal cancer, CAF-specific endoglin expression at invasive borders correlated with poor metastasis-free survival. In vitro experiments revealed that endoglin is indispensable for bone morphogenetic protein (BMP)-9–induced signaling and CAF survival. Targeting endoglin using the neutralizing antibody TRC105 inhibited CAF invasion in vitro. In zebrafish, endoglin-expressing fibroblasts enhanced colorectal tumor cell infiltration into the liver and decreased survival. Finally, CAF-specific endoglin targeting with TRC105 decreased metastatic spread of colorectal cancer cells to the mouse liver. Conclusions: Endoglin-expressing CAFs contribute to colorectal cancer progression and metastasis. TRC105 treatment inhibits CAF invasion and tumor metastasis, indicating an additional target beyond the angiogenic endothelium, possibly contributing to beneficial effects reported during clinical evaluations. See related commentary by Becker and LeBleu, p. 6110.

PD-1/PD-L1 checkpoint therapy for cancer is commonly considered to act by reactivating T cells in the tumor microenvironment. Here, we present data from 2 mouse tumor models demonstrating an essential involvement of tumor-draining lymph nodes in PD-1 and PD-L1 therapeutic efficacy. Immune activation induced by checkpoint treatment was predominantly observed in the tumor-draining, but not nondraining, lymph nodes and was reflected in local accumulation of CD8+ T cells. Surgical resection of these lymph nodes, but not contralateral lymph nodes, abolished therapy-induced tumor regressions and was associated with decreased immune infiltrate in the tumor microenvironment. Moreover, inhibitor FTY720, which locks lymphocytes in lymph organs, also abrogated checkpoint therapy, suggesting that the tumor-draining lymph nodes function as sites of T cell invigoration required for checkpoint blockade therapy. Now that PD-1/PD-L1 checkpoint treatment is applied in earlier clinical stages of cancer, our preclinical data advocate for enrolling patients with their tumor-draining lymph nodes still in place, to optimally engage the antitumor immune response and thereby enhance clinical benefit.

Chronic liver damage leads to the onset of fibrogenesis. Rodent models for liver fibrosis have been widely used, but are less suitable for screening purposes. Therefore the aim of our study was to design a novel model for liver fibrosis in zebrafish embryos, suitable for high throughput screening. Furthermore, we evaluated the efficacy of mesenchymal stromal cells (MSCs) to inhibit the fibrotic process and thereby the applicability of this model to evaluate therapeutic responses. Zebrafish embryos were exposed to TAA or CCL4 and mRNA levels of fibrosis-related genes (Collagen-1α1, Hand-2, and Acta-2) and tissue damage-related genes (TGF-β and SDF-1a, SDF-1b) were determined, while Sirius-red staining was used to estimate collagen deposition. Three days after start of TAA exposure, MSCs were injected after which the fibrotic response was determined. In contrast to CCL4, TAA resulted in an upregulation of the fibrosis-related genes, increased extracellular matrix deposition and decreased liver sizes suggesting the onset of fibrosis. The applicability of this model to evaluate therapeutic responses was shown by local treatment with MSCs which resulted in decreased expression of the fibrosis-related RNA markers. In conclusion, TAA induces liver fibrosis in zebrafish embryos, thereby providing a promising model for future mechanistic and therapeutic studies.

Background Mesenchymal stromal cells (MSCs) are a potential therapeutic modality in inflammatory bowel diseases (IBDs) because of their immunomodulatory and regenerative properties. However, when injected systemically, only a small portion of the cells, if any, reach the inflamed colon. In this study, we assessed whether endoscopic injections of MSCs into the intestinal wall of the inflamed colon affect the course of experimental colitis. Furthermore, we investigated if injection of aggregated MSCs in spheroids could enhance their therapeutic ability. Methods Expression levels of in vivo MSC aggregates and in vitro MSC spheroids were compared with monolayer cultured MSCs for both anti-inflammatory and pro-regenerative factors. Subsequently, MSCs and MSC spheroids were injected endoscopically in mice with established dextran sulfate sodium (DSS)-induced colitis. Results Endoscopically injected MSCs and MSC spheroids both alleviated DSS-induced colitis. Furthermore, both in vivo and in vitro MSC spheroids showed increased expression of factors important for immunomodulation and tissue repair, compared with monolayer cultured MSCs. Despite differential expression of these factors, MSC spheroids showed similar clinical efficacy in vivo as single-cell suspension MSCs. Analysis of serum samples and colon homogenates showed that local MSC therapy resulted in increased levels of interferon-γ, indoleamine 2,3-dixoygenase, and interleukin-10. Conclusions Endoscopic injections of MSCs and MSC spheroids in the inflamed colon attenuate DSS-induced colitis. Our data show that endoscopic injection can be a feasible and effective novel application route for MSC therapy in patients with luminal IBD.

Immunotherapy with PD-1/PD-L1-blocking antibodies is clinically effective for several tumor types, but the mechanism is not fully understood. PD-L1 expression on tumor biopsies is generally regarded as an inclusion criterion for this cancer therapy. Here, we describe the PD-L1-blocking therapeutic responses of preclinical tumors in which PD-L1 expression was removed from cancer cells, but not from immune infiltrate. Lack of PD-L1 expression on malignant cells delayed tumor outgrowth in a CD8+ T cell-mediated fashion, showing the importance of this molecule in immune suppression. PD-L1 expression was evident on myeloid-infiltrating cells in the microenvironment of these tumors and targeting stromal PD-L1 with blocking antibody therapy had additional antitumor effect, demonstrating that PD-L1 on both malignant cells and immune cells is involved in the mechanism of immunotherapeutic antibodies. Importantly, comparable results were obtained with PD-1-blocking therapy. These findings have implications for inclusion of cancer patients in PD-1/PD-L1 blockade immunotherapies.