Tumor eradication after cyclophosphamide depends on concurrent depletion of regulatory T cells: a role for cycling TNFR2-expressing effector-suppressor T cells in limiting effective chemotherapy

Abstract

Tumor cell death potentially engages with the immune system. However, the efficacy of anti-tumor chemotherapy may be limited by tumor-driven immunosuppression, e.g., through CD25+ regulatory T cells. We addressed this question in a mouse model of mesothelioma by depleting or reconstituting CD25+ regulatory T cells in combination with two different chemotherapeutic drugs. We found that the efficacy of cyclophosphamide to eradicate established tumors, which has been linked to regulatory T cell depletion, was negated by adoptive transfer of CD25+ regulatory T cells. Analysis of post-chemotherapy regulatory T cell populations revealed that cyclophosphamide depleted cycling (Ki-67(hi)) T cells, including foxp3+ regulatory CD4+ T cells. Ki-67(hi) CD4+ T cells expressed increased levels of two markers, TNFR2 and ICOS, that have been associated with a maximally suppressive phenotype according to recently published studies. This suggest that cyclophosphamide depletes a population of maximally suppressive regulatory T cells, which may explain its superior anti-tumor efficacy in our model. Our data suggest that regulatory T cell depletion could be used to improve the efficacy of anti-cancer chemotherapy regimens. Indeed, we observed that the drug gemcitabine, which does not deplete cycling regulatory T cells, eradicates established tumors in mice only when CD25+ CD4+ T cells are concurrently depleted. Cyclophosphamide could be used to achieve regulatory T cell depletion in combination with chemotherapy.

The Role of Regulatory T Cells in Mesothelioma

“…It potentially inhibits effector T cell accumulation, retention, survival, proliferation or function within tumours [42] but also dendritic cell (DC) activation and maturation [38]. Tumour necrosis factor receptor-2 (TNFR2) and inducible T cell costimulator (ICOS) have both been associated, against the accepted norm of Treg cell anergy, with proliferation of Treg cells (Ki-67 expression) in the AB1 murine mesothelioma model [43]. Within this murine mesothelioma model Treg cells were active suppressors that increased suppressor function in response to tumour antigen.…”

Harnessing the immune response to treat cancer

“…However, a different Treg depletion strategy using the cytotoxic agent cyclophosphamide may be more readily translatable to the clinic. At high doses, cyclophosphamide is cytotoxic and causes immunosuppression, but at low doses it preferentially depletes numbers of Tregs (Ghiringhelli et al, 2004van der Most et al, 2009b) and impairs Treg function (Lutsiak et al, 2005). A single dose of cyclophosphamide-depleted Tregs and when followed by an immunotherapy cured mice with established tumours (Ghiringhelli et al, 2004).…”

“…This phenomenon potentially offers a window to skew the regenerating T-cell response back towards active antitumour activity. Depletion of regulatory T cells by cyclophosphamide (Ghiringhelli et al, 2004Lutsiak et al, 2005;van der Most et al, 2009b) or depletion of myeloid-derived suppressor cells with gemcitabine (Suzuki et al, 2005) may enhance antitumour activity through removal of negative regulation. Lymphodepletion in combination with tumour vaccines has shown efficacy in mice (Dummer et al, 2002;Hu et al, 2002) and humans (Dudley et al, 2002).…”

“…Treatment with gemcitabine results in increased antigen crosspresentation and priming of tumour-specific CD8 cells (Nowak et al, 2003a). Chemotherapy can also sensitize those cells not directly lysed by treatment to subsequent killing by immune cells, through upregulation of death receptors Fas (CD95) or TRAIL receptors (DR5) (Mattarollo et al, 2006;van der Most et al, 2009a).…”

“…Inducing tumour cell death through cytotoxic chemotherapy, radiotherapy or locally administered therapies may alter the amount or the way in which tumour antigens engage with the immune system, through exposure of hidden antigens, increased exposure of antigens present at low concentration or modification of low-affinity antigens. In this way, we can use tumour antigens to prime a more effective immune response without ever knowing their identities; in effect the tumour functions as its own vaccine (van der Most et al, 2006;Jackaman et al, 2008). Studies using chemotherapy to load tumour antigens have confirmed the feasibility of this notion (Nowak et al, 2003a).…”