Microenvironment and Immunology T-Cell Trafficking Facilitated by High Endothelial Venules Is Required for TumorControl after Regulatory T-Cell Depletion

The evolution of immune blockades in tumors limits successful antitumor immunity, but the mechanisms underlying this process are not fully understood. Depletion of regulatory T cells (Treg), a T-cell subset that dampens excessive inflammatory and autoreactive responses, can allow activation of tumor-specific T cells. However, cancer immunotherapy studies have shown that a persistent failure of activated lymphocytes to infiltrate tumors remains a fundamental problem. In evaluating this issue, we found that despite an increase in Tcell activation and proliferation following Treg depletion, there was no significant association with tumor growth rate. In contrast, therewas a highly significant association between low tumor growth rate and the extent of T-cell infiltration. Further analyses revealed a total concordance between low tumor growth rate, high T-cell infiltration, and the presence of high endothelial venules (HEV). HEV are blood vessels normally found in secondary lymphoid tissue where they are specialized for lymphocyte recruitment. Thus, our findings suggest that Treg depletionmay promote HEV neogenesis, facilitating increased lymphocyte infiltration and destruction of the tumor tissue. These findings are important as they point to a hitherto unidentified role of Tregs, themanipulation of whichmay refine strategies for more effective cancer immunotherapy. Cancer Res; 72(21); 1–10. 2012 AACR. Introduction Several laboratories have shown that CD4þ CD25þ Foxp3þ regulatory T cells (Treg), which normally serve to control autoimmunity and inflammation, also inhibit immune responses to tumor antigens (1). Thus, strategies aimed at boosting antitumor responses, throughmanipulation of Tregs, are being intensely investigated. In mice, prophylactic depletion of Tregs using CD25-specific antibodies can limit and even prevent tumor development and/or progression (2–4). The development of transgenic mice expressing the primate diphtheria toxin receptor (DTR) on Foxp3þ cells has allowed specific and complete depletion of Tregs by administration of diphtheria toxin (DT; 5). Using the melanoma cell line B16, it has been shown that selective Treg depletion using anti-CD25 antibodies (6) or DT (7) results in activation of a tumor-specific CD8þ T-cell response and slower tumor growth. Use of the DTR-Foxp3 transgenic mice has also shown that Treg depletion canprevent development and even limit the progression of tumors induced by the carcinogen methylcholanthrene (MCA) in a manner that is dependent on CD8þ T cells and IFNg (8). The success of these interventions however remains suboptimal. Treg depletion, even when combined with vaccination, does not readily result in the elimination of established tumors as tumor rejection is often observed in only a proportion of treated mice. Several factors, including the extent of T-cell activation and/or tumor infiltration, may account for this failure (6). In this study, we set out to identify factors limiting the success of antitumor immune responses. For this purpose, we used the MCA chemical carcinogenesis model in combination with Foxp3 mice (5), to identify the key features that distinguish progressing tumors from those that are controlled after Treg depletion. Our findings clearly indicate that T-cell infiltration and not the extent of activation, is a critical bottleneck to tumor destruction in Tregdepleted animals. Moreover, our extensive immunohistochemical analyses of MCA-induced tumors from Tregreplete and Treg-depleted mice revealed that control of tumor growth, observed in Treg-depleted mice, was determined by development of high endothelial venules (HEV), specialized blood vessels that alter both the composition and size of the T-cell infiltrate (9, 10). Overall, these data provide a new perspective on the impact of Treg depletion, demonstrating that Tregs control immune responses, not only through limiting immune activation, but also through influencing blood vessel differentiation. Authors' Affiliations: Infection and Immunity, School of Medicine, Henry Wellcome Building, Cardiff University, Cardiff, United Kingdom; and MRC Centre for Immune Regulation, Institute for Biomedical Research, College of Medical and Dental Sciences, University of Birmingham, Birmingham, United Kingdom Note: Supplementary data for this article are available at Cancer Research Online (http://cancerres.aacrjournals.org/). J.P. Hindley and E. Jones have contributed equally to this work. Corresponding Author: Awen M. Gallimore, Infection and Immunity, School of Medicine, Henry Wellcome Building, Cardiff University, Cardiff, CF144XN, United Kingdom. Phone: 44-2920-687012; Fax: 44-2920687079; E-mail: [email protected] doi: 10.1158/0008-5472.CAN-12-1912 2012 American Association for Cancer Research. Cancer Research www.aacrjournals.org OF1 Research. on April 13, 2017. © 2012 American Association for Cancer cancerres.aacrjournals.org Downloaded from Published OnlineFirst September 7, 2012; DOI: 10.1158/0008-5472.CAN-12-1912