Human Pathology (2014) xx, xxx–xxx
www.elsevier.com/locate/humpath
Correspondence
Fibroblast activation protein, a potential diagnostic and therapeutic target for cancer—reply To the Editor, We appreciate the comments from Professor Jun Li and colleagues on our recent publication [1]. We agree with Professor Li that fibroblast activation protein (FAP) could be a versatile therapeutic target in breast cancer, due to its robust expression across all breast cancer subtypes. For that purpose, however, multiple challenges would need to be overcome. So far, studies that have focused on developing FAP as a therapeutic target have been based on 2 conceptually different strategies. The first strategy targets the protein function of FAP, specifically, its proteolytic activity as a dipeptidase. Preclinical studies have shown that inhibiting FAP's enzymatic activity either pharmacologically or by reducing protein expression by knocking-out [2] or knocking-down FAP expression [3] inhibited tumor growth. Instead of inhibiting FAP's enzyme activity, an alternate approach uses the dipeptidase activity to convert a prodrug to its active form with promising antitumor effects in animal studies [4]. A second, perhaps more effective, strategy aims at targeting cells that express FAP by (1) conjugating toxins, cytokines, or radioisotope to FAP-specific antibodies as reviewed in Fischer et al [5]; as such, these toxins or radioisotope are expected to have a “bystander” deleterious effect on both FAP-expressing and non–FAP-expressing cells, that is, adjacent tumor cells; (2) eliminating FAPexpressing cells either using a genetic approach [6] or immunotherapy approach using chimeric antigen receptor T cells (CAR T cell) directed against FAP, which were designed to kill FAP-expressing cells [7]. Although the elimination of FAP-expressing cells by genetic approach reported by Kraman et al [6] has demonstrated impressive antitumor effects along with an improved antitumor immunity when animals were vaccinated, the CAR T cell approach did not result in significant tumor inhibition despite recent success of this immunotherapy approach in patients with refractory chronic lymphocytic leukemia [8]. More importantly, mice treated by CAR T cell directed against FAP developed cachexia and severe toxicity against bone 0046-8177/© 2014 Elsevier Inc. All rights reserved.
marrow, raising safety concerns about its potential application in the clinic [7,9]. Understanding the pattern of expression and biological effects of FAP is, therefore, critical for the design of effective and safe interventions. In this regard, we would like to clarify Professor Li's comment that the expression of FAP could be regulated by human breast tumor–associated macrophages (TAMs). Our data did not show that FAP expression was regulated by TAMs. Rather, we demonstrated that FAP expression in human breast cancer was observed not only in tumor-associated fibroblasts but also in other nonfibroblast stromal cells that expressed CD45 (a panleukocyte marker), CD14, CD11b, and HLA-DR, markers of TAMs. Because TAMs are known to have immunosuppressive functions within the tumor microenvironment, our results offer an alternate (or at least complementary) explanation to the results reported by Kraman et al [6]. As mentioned earlier, depletion of FAP-expressing cells allowed for effective vaccination against an immunogenic tumor, LL2/OVA, in mice and also resulted in stunted tumor growth. Neutralizing tumor necrosis factor α (TNF-α) or interferon γ abrogated the protective effects from the depletion of FAP-expressing cells. In the absence of FAP-expressing cells, it is inferred that there would be an abundance of TNF-α or interferon γ, which was protective against tumor growth. The Kraman study, therefore, demonstrated that FAP-expressing cells have immunosuppressive properties and concluded that these FAP-expressing cells may have directly or indirectly suppressed production of TNF-α or interferon γ. We think that some of these FAP-expressing cells that had contributed to the immunosuppressive effects observed in the Kraman study may be TAMs. In summary, FAP, as a protein and as a surface marker of tumor stromal cells, has the potential of being a cancer therapeutic target. However, much more work is needed to evaluate the safety and efficacy of targeting FAP as a cancer treatment strategy. Julia Tchou MD, PhD Division of Endocrine and Oncologic Surgery Department of Surgery, Rena Rowan Breast Center and the Abramson Cancer Center, Perelman School of Medicine University of Pennsylvania Philadelphia, PA 19104, USA
2
Correspondence Jose Conejo-Garcia MD, PhD The Wistar Institute Philadelphia, PA 19104, USA
http://dx.doi.org/10.1016/j.humpath.2013.11.022
References [1] Tchou J, Zhang PJ, Bi Y, et al. Fibroblast activation protein expression by stromal cells and tumor-associated macrophages in human breast cancer. HUM PATHOL 2013;44:2549-57. [2] Santos AM, Jung J, Aziz N, Kissil JL, Pure E. Targeting fibroblast activation protein inhibits tumor stromagenesis and growth in mice. J Clin Invest 2009;119:3613-25. [3] Cai F, Li Z, Wang C, et al. Short hairpin RNA targeting of fibroblast activation protein inhibits tumor growth and improves the tumor microenvironment in a mouse model. BMB Rep 2013;46: 252-7.
[4] Brennen WN, Rosen DM, Wang H, Isaacs JT, Denmeade SR. Targeting carcinoma-associated fibroblasts within the tumor stroma with a fibroblast activation protein-activated prodrug. J Natl Cancer Inst 2012;104:1320-34. [5] Fischer E, Chaitanya K, Wuest T, et al. Radioimmunotherapy of fibroblast activation protein positive tumors by rapidly internalizing antibodies. Clin Cancer Res 2012;18:6208-18. [6] Kraman M, Bambrough PJ, Arnold JN, et al. Suppression of antitumor immunity by stromal cells expressing fibroblast activation proteinalpha. Science 2010;330:827-30. [7] Tran E, Chinnasamy D, Yu Z, et al. Immune targeting of fibroblast activation protein triggers recognition of multipotent bone marrow stromal cells and cachexia. J Exp Med 2013;210:1125-35. [8] Porter DL, Levine BL, Kalos M, Bagg A, June CH. Chimeric antigen receptor-modified T cells in chronic lymphoid leukemia. N Engl J Med 2011;365:725-33. [9] Roberts EW, Deonarine A, Jones JO, et al. Depletion of stromal cells expressing fibroblast activation protein-alpha from skeletal muscle and bone marrow results in cachexia and anemia. J Exp Med 2013;210:1137-51.
www.elsevier.com/locate/humpath
Correspondence
Fibroblast activation protein, a potential diagnostic and therapeutic target for cancer—reply To the Editor, We appreciate the comments from Professor Jun Li and colleagues on our recent publication [1]. We agree with Professor Li that fibroblast activation protein (FAP) could be a versatile therapeutic target in breast cancer, due to its robust expression across all breast cancer subtypes. For that purpose, however, multiple challenges would need to be overcome. So far, studies that have focused on developing FAP as a therapeutic target have been based on 2 conceptually different strategies. The first strategy targets the protein function of FAP, specifically, its proteolytic activity as a dipeptidase. Preclinical studies have shown that inhibiting FAP's enzymatic activity either pharmacologically or by reducing protein expression by knocking-out [2] or knocking-down FAP expression [3] inhibited tumor growth. Instead of inhibiting FAP's enzyme activity, an alternate approach uses the dipeptidase activity to convert a prodrug to its active form with promising antitumor effects in animal studies [4]. A second, perhaps more effective, strategy aims at targeting cells that express FAP by (1) conjugating toxins, cytokines, or radioisotope to FAP-specific antibodies as reviewed in Fischer et al [5]; as such, these toxins or radioisotope are expected to have a “bystander” deleterious effect on both FAP-expressing and non–FAP-expressing cells, that is, adjacent tumor cells; (2) eliminating FAPexpressing cells either using a genetic approach [6] or immunotherapy approach using chimeric antigen receptor T cells (CAR T cell) directed against FAP, which were designed to kill FAP-expressing cells [7]. Although the elimination of FAP-expressing cells by genetic approach reported by Kraman et al [6] has demonstrated impressive antitumor effects along with an improved antitumor immunity when animals were vaccinated, the CAR T cell approach did not result in significant tumor inhibition despite recent success of this immunotherapy approach in patients with refractory chronic lymphocytic leukemia [8]. More importantly, mice treated by CAR T cell directed against FAP developed cachexia and severe toxicity against bone 0046-8177/© 2014 Elsevier Inc. All rights reserved.
marrow, raising safety concerns about its potential application in the clinic [7,9]. Understanding the pattern of expression and biological effects of FAP is, therefore, critical for the design of effective and safe interventions. In this regard, we would like to clarify Professor Li's comment that the expression of FAP could be regulated by human breast tumor–associated macrophages (TAMs). Our data did not show that FAP expression was regulated by TAMs. Rather, we demonstrated that FAP expression in human breast cancer was observed not only in tumor-associated fibroblasts but also in other nonfibroblast stromal cells that expressed CD45 (a panleukocyte marker), CD14, CD11b, and HLA-DR, markers of TAMs. Because TAMs are known to have immunosuppressive functions within the tumor microenvironment, our results offer an alternate (or at least complementary) explanation to the results reported by Kraman et al [6]. As mentioned earlier, depletion of FAP-expressing cells allowed for effective vaccination against an immunogenic tumor, LL2/OVA, in mice and also resulted in stunted tumor growth. Neutralizing tumor necrosis factor α (TNF-α) or interferon γ abrogated the protective effects from the depletion of FAP-expressing cells. In the absence of FAP-expressing cells, it is inferred that there would be an abundance of TNF-α or interferon γ, which was protective against tumor growth. The Kraman study, therefore, demonstrated that FAP-expressing cells have immunosuppressive properties and concluded that these FAP-expressing cells may have directly or indirectly suppressed production of TNF-α or interferon γ. We think that some of these FAP-expressing cells that had contributed to the immunosuppressive effects observed in the Kraman study may be TAMs. In summary, FAP, as a protein and as a surface marker of tumor stromal cells, has the potential of being a cancer therapeutic target. However, much more work is needed to evaluate the safety and efficacy of targeting FAP as a cancer treatment strategy. Julia Tchou MD, PhD Division of Endocrine and Oncologic Surgery Department of Surgery, Rena Rowan Breast Center and the Abramson Cancer Center, Perelman School of Medicine University of Pennsylvania Philadelphia, PA 19104, USA
2
Correspondence Jose Conejo-Garcia MD, PhD The Wistar Institute Philadelphia, PA 19104, USA
http://dx.doi.org/10.1016/j.humpath.2013.11.022
References [1] Tchou J, Zhang PJ, Bi Y, et al. Fibroblast activation protein expression by stromal cells and tumor-associated macrophages in human breast cancer. HUM PATHOL 2013;44:2549-57. [2] Santos AM, Jung J, Aziz N, Kissil JL, Pure E. Targeting fibroblast activation protein inhibits tumor stromagenesis and growth in mice. J Clin Invest 2009;119:3613-25. [3] Cai F, Li Z, Wang C, et al. Short hairpin RNA targeting of fibroblast activation protein inhibits tumor growth and improves the tumor microenvironment in a mouse model. BMB Rep 2013;46: 252-7.
[4] Brennen WN, Rosen DM, Wang H, Isaacs JT, Denmeade SR. Targeting carcinoma-associated fibroblasts within the tumor stroma with a fibroblast activation protein-activated prodrug. J Natl Cancer Inst 2012;104:1320-34. [5] Fischer E, Chaitanya K, Wuest T, et al. Radioimmunotherapy of fibroblast activation protein positive tumors by rapidly internalizing antibodies. Clin Cancer Res 2012;18:6208-18. [6] Kraman M, Bambrough PJ, Arnold JN, et al. Suppression of antitumor immunity by stromal cells expressing fibroblast activation proteinalpha. Science 2010;330:827-30. [7] Tran E, Chinnasamy D, Yu Z, et al. Immune targeting of fibroblast activation protein triggers recognition of multipotent bone marrow stromal cells and cachexia. J Exp Med 2013;210:1125-35. [8] Porter DL, Levine BL, Kalos M, Bagg A, June CH. Chimeric antigen receptor-modified T cells in chronic lymphoid leukemia. N Engl J Med 2011;365:725-33. [9] Roberts EW, Deonarine A, Jones JO, et al. Depletion of stromal cells expressing fibroblast activation protein-alpha from skeletal muscle and bone marrow results in cachexia and anemia. J Exp Med 2013;210:1137-51.
