Tumour immunology and the FAP+ stromal cell
Even though spontaneous or vaccine-induced systemic immune responses to cancers occur, the stromal microenvironment of tumours protects cancer cells from immune attack. We have recently found that a stromal cell identified by its expression of fibroblast activation protein-α (FAP) mediates immune suppression in murine tumours. We seek ways to block its immune suppressive functions to improve clinical tumour immunotherapy.
The FAP+ stromal cell in tumours
The proposal of the immune surveillance of cancer, as put forward by Macfarlane Burnet and Lewis Thomas, hypothesizes that cancers may sufficiently differ from normal cells so that they would be recognized by the immune system and eliminated. Today we know that cancers, either because they are virally induced and express foreign viral antigens, or are genetically unstable and express mutated self antigens, do induce systemic immune responses, but we also recognize that cancers usually escape immune control.
Two general mechanisms have been proposed for the ability of cancers to circumvent an immune response: establishing an immune suppressive microenvironment within the tumour, and the generation and immune selection of cancer cell variants that are not immunologically recognized. Evidence for both exists, but we decided to concentrate on immune suppression because it would dominate over immune selection, and it offered the possibility of therapeutic approaches.
Early evidence for an immune suppressive microenvironment within tumours was the observation that an established tumour, containing not only cancer cells but also non‑cancer “stromal cells”, was resistant to killing by tumour antigen-specific T cells. However, cancer cells alone, without an accompanying stroma, were eliminated. This finding was made more than a quarter of a century ago, but the realization that attention must be directed to the tumour stroma has been slow to develop, perhaps for three reasons: a continued emphasis on the possibility that the cancer cell itself was responsible for tumoural immune suppression, gaps in our understanding of how the immune system worked, and the complexity of the tumour stroma.
The tumour stroma is comprised of three general cell types, those involved with forming the tumour vasculature, cells of the innate and adaptive immune systems, and mesenchymal cells, or fibroblasts. Most work has been directed to understanding the roles of the cells of the immune systems, with the reasonable rationale that the processes intrinsic to this system, which control auto-immunity, also would be involved in immune suppression in the tumour microenvironment. This approach has been productive and has led to the development of a clinically approved treatment for metastatic melanoma, ipilimumab, an antibody that blocks the function of CTLA-4, a lymphocyte receptor. This treatment, however, causes systemic autoimmunity because it does not selectively target immune suppression in the tumour microenvironment.
To determine the cellular basis for immune suppression within the tumour microenvironment, we focused on stromal cells of mesenchymal origin, which have usually been referred to as myofibroblasts or carcinoma‑associated fibroblasts (CAFs). These cells have been examined for their ability to promote tumour growth, but not by an immunological mechanism. Over the last 20 years, however, an interesting correlation was found between the occurrence of chronic inflammatory lesions of various types, such as atherosclerosis, rheumatoid arthritis, cirrhosis, and dermal scars, and the presence of a mesenchymal cell that was first observed in most human adenocarcinomas by its expression of a membrane protein, FAP. The recognition that tumours contain the same inflammatory cells that characterize these chronic lesions, the likelihood that these lesions represent attempts at tissue repair, and the possibility that immune suppression is a normal component of tissue repair led to the consideration that the FAP+ stromal cell might have a role in tumoural immune suppression.
We tested this possibility by developing a mouse line in which the primate diphtheria toxin receptor is expressed in FAP+ stromal cells to enable their conditional depletion by the administration of diphtheria toxin. The experiment was informative in that depleting FAP+ cells from the stroma of established tumours caused immune control of tumour growth. This finding was initially made with immunogenic, ectopic tumours caused by injecting cultured cancer cells, but has been extended now to the Tuveson laboratory model of spontaneous pancreatic ductal adenocarcinoma in which cancer cells express mutant Trp53R172H and KrasG12D alleles (Figure 1). The FAP+ stromal cell is a non‑redundant element of tumoural immune suppression, and the presence of these cells in human adenocarcinomas suggests that these findings in the mouse may be relevant to human cancer.
Figure 1. A mouse pancreatic ductal adenocarcinoma showing the FAP+ stromal cells (red) surrounding the ductal cancer cells (green) with Trp53R172H+ nuclei (white). Photograph courtesy of James Jones.
The FAP+ stromal cell and normal tissues
The possibility that FAP+ stromal cells might have functions in normal tissues was raised by their presence in the somites of developing mouse embryos, and in the uterus and placenta. To examine this possibility, we developed a mouse in which luciferase was expressed in FAP+ cells, which has revealed that FAP+ cells are present in almost all tissues of the adult mouse. Thus, FAP expression may denote a mesenchymal lineage with both shared and tissue-specific homeostatic functions, as well as its immune suppressive function in tumours, which may be an elicited activity that is potentially available to “injured” tissues throughout the body.
We have begun to define the functions of FAP+ cells in several normal tissues, including skeletal muscle in which we have shown that they are required for the maintenance of normal muscle mass. Remarkably, cancer may also affect some tissue homeostatic functions of FAP+ cells, in that in two mouse models of cancer-induced cachexia, which is the loss of skeletal muscle mass that may occur independently of food intake, FAP+ cell numbers are decreased in skeletal muscle, perhaps accounting for the cachexia. Cachexia is a serious clinical problem, and these findings may lead to an improved understanding of this process.
The depletion of FAP+ cells is not a reasonable option for enhancing the ability of the immune system to control tumour growth because they are necessary for the functions of normal tissues. Therefore, we must determine the molecular basis of the immune suppressive function of the tumoural FAP+ cell, and develop therapies that will interrupt this function. Our strategy is to identify among the genes that are selectively expressed in the tumoural FAP+ cells candidates for immune suppression. We are also determining how FAP+ cells accumulate in the tumour. Conceivably, they may be generated by replication from FAP+ cells in the local tissue, or they may come from another site, such as the bone marrow, where we have shown them to proliferate. Either of these two research directions may lead to therapeutic opportunities for enhancing immune control of tumour growth.