Abstract IA27: The FAP+ stromal cell and escape from immunosurveillance
Authors: Fearon Douglas T. 
Published: August 2014

An investigator attempting to become informed about the field of tumor immunology encounters pre-clinical and clinical observations that do not easily form the basis of an overarching set of principles that explain and predict the interaction of the tumor with the host immune system. Pre-clinical studies have led to one widely held view termed cancer immunoediting as the basis for the failure of host immunosurveillance. This proposition holds that during the evolution of a cancer, the immune system responds to and eliminates the highly immunogenic cancer cells, but leaves behind non-immunogenic cancer cells that escape immune recognition. Immunoediting, the evidence for which has been obtained almost entirely with murine models of sarcoma, predicts that vaccination with the antigens expressed by these residual cancer cells that are not spontaneously immunogenic will alone lead to control of tumor growth, at least transiently. A second view explains that cancers escape immunosurveillance because they establish microenvironments that are immune suppressive and blunt the capacity of effector T cells to kill cancer cells. In contrast to immunoediting, the occurrence of immune suppressive predicts that spontaneous or vaccine-induced immune responses will be effective only when combined with a therapeutic intervention that interrupts whatever it is in the microenvironment that interferes with the functions of effector T cells. In a pre-clinical spontaneous tumor model of adenocarcinoma of the lung, immunosuppression was the basis of the failure of immunosurveillance, at least inferentially, because antigen-expressing, immunogenic cancer cells continued to grow despite a systemic immune response. Interestingly, careful consideration of these two views reveals that in their strictest interpretation, they are incompatible as explanations for a how a cancer avoids immunosurveillance, since stringent immune suppression would prevent cancer immunoediting, and the occurrence of immunoediting denotes unsuccessful immune suppression.

Clinical studies of immunological interventions have not clearly supported either process as being the better explanation for the tumor-host interaction. Generally, therapeutic cancer vaccines (i.e., vaccines given after cancers have been established) have not been successful, even with virus-induced epithelial cancers, such as that caused by HPV, an observation that does not fulfil the prediction of immunoediting. On the other hand, studies of monoclonal antibodies that promote T cell function, such as anti-CTLA-4 (Ipilimumab), anti-PD-1, and anti-PD-L1, have been effective only in a minority of patients with melanoma, renal cell cancer, and lung cancer, and ineffective in all patients with pancreatic ductal adenocarcinoma (PDA). These results indicate either that spontaneously immunogenic cancer cells had been eliminated in the non-responsive tumors, as in immunoediting, or that these antibodies had not targeted the critical immune suppressive mechanism. That is, these clinical studies do not necessarily support either proposed explanation.

In 2010, we published a study of the effects of depleting a tumoral stromal cell type that can be identified by its expression of the membrane protein, Fibroblast Activation Protein-alpha (FAP), from a transplanted, immunogenic tumor model, the Lewis Lung carcinoma line expressing ovalbumin (LL2/OVA). The FAP+ stromal cell had been identified twenty years ago in essentially all human adenocarcinomas, and had subsequently been found in several examples of chronic inflammatory tissue lesions. These findings suggested to us that the presence of the FAP+ cell in tumors reflected their inflammatory character, and that the function of the cell in this circumstance was related to tissue protection, which included immune suppression. Remarkably, we found that conditionally depleting this cell from established LL2/OVA tumors led to immune control of their growth, without any intervention that directly affected T cells. Perhaps surprisingly, therefore, a stromal cell of mesenchymal origin, had immune suppressive activity that blocked the function of pre-existing, cancer cell-specific T cells. This tumor model, however, was highly artificial, so the relevance of this study to the question of why immunosurveillance fails in human cancers was uncertain.

The conditional depletion of the FAP+ stromal cell in the LL2/OVA tumor was based on its expression of the human diphtheria toxin receptor (DTR), which had been programmed by a BAC transgene in which a cassette encoding the DTR was inserted into the Fap gene. Therefore, it was possible to assess the immunological role of the FAP+ stromal cell in a spontaneous model of PDA by crossing the BAC DTR transgene into the KPC mouse line. This model replicates the immunological phenotype of human PDA in that its growth is unaffected by treatment of mice with anti-CTLA-4. We demonstrated that PDA-bearing mice had spontaneous immune responses to cancer cells, in that tumor cells induced the secretion of IFN-gamma by their CD8+ T cells. Therefore, this model exemplified escape from immune surveillance by tumoral immune suppression rather than immunoediting, since antigen-expressing cancer cells and immune T cells co-existed. Consistent with this surmise was our finding that diminishing the number of tumoral FAP+ stromal cells by giving diphtheria toxin led to T cell-dependent control of PDA growth. This finding then prompted the question of whether the FAP+ stromal cell accounted for the absence of an anti-tumor effect of anti-CTLA-4. This query was answered in the affirmative by observing that depleting the FAP+ cell and anti-CTLA-4 synergistically slowed the growth of the PDA. Thus, there is a hierarchy of tumoral immune suppression, with the FAP+ cell being dominant over at least one T cell-directed intervention. One would predict that immune suppression by the FAP+ cell also would interfere with the efficacy of vaccine-induced immune responses.

We have also conducted studies to determine the relationship of the tumoral FAP+ cell to fibroblast-like stromal cells, such as the carcinoma/cancer-associated fibroblast (CAF) and the alpha-smooth actin+ (aSMA+) “myofibroblast”, and to FAP+ cells that we have found in many normal tissues and organs. The transcriptomes of FAP+ cells from PDA, normal pancreas (likely the “pancreatic stellate cell”), adipose tissue and skeletal muscle were found to be highly similar, and to form a distinct cluster from FAP-negative mouse embryonic fibroblasts when assessed by Principle Component Analysis. All FAP+ cells expressed the inflammatory gene signature reported by Hanahan and colleagues for the CAF, suggesting that this signature identifies this “fibroblast-like” lineage rather than being unique to stromal cells of the tumor microenvironment. FAP+ cells in skeletal muscle are required for the maintenance of this tissue, and FAP+ cells of the bone marrow are necessary for normal hematopoiesis, and are identical to the so-called “CXCL12-associated reticular” cell. Mouse models of cancer-induced cachexia are associated with a loss of FAP-expressing cells from these and many other tissues, a circumstance that may contribute to cachexia and anemia in cancer.

In summary, the FAP+ cell mediates dominant immune suppression in mouse PDA, and is closely related to FAP+ cells of normal tissues that are also affected by cancer. Defining the molecular means of its immune suppression may uncover new immunotherapies of cancer.

Citation Format: Douglas T. Fearon. The FAP+ stromal cell and escape from immunosurveillance. [abstract]. In: Proceedings of the AACR Special Conference on Tumor Invasion and Metastasis; Jan 20-23, 2013; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2013;73(3 Suppl):Abstract nr IA27.