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Programmed cell death 1 ligand 1 (PD-L1) is an important regulator of T-cell responses and may consequently limit anticancer immunity. We have recently identified PD-L1-specific, cytotoxic CD8+ T cells. In the present study, we develop these findings and report that CD4+ helper T cells spontaneously recognize PD-L1. We examined the locality of a previously identified HLA-A"0201-restricted PD-L1-epitope for the presence of possible CD4+ T-cell epitopes. Thus, we identified naturally occurring PD-L1-specific CD4+ T cells among the peripheral blood lymphocytes of cancer patients and - to lesser extents - healthy donors, by means of ELISP OT assays. PD-L1-specific CD4+ T cells appeared to be TH17 cells exhibiting an effector T-cell cytokine profile. Hence, PD-L1-specific CD4+ T cells released interferon γ (IFNγ), tumor necrosis factor α (TNFα) and interleukin-17 (IL-17) in response to a long PD-L1-derived peptide. Furthermore, we demonstrate that the specific recognition of PD-L1 by CD4+ T cells is MHC Class II-restricted. Natural T-cell responses against PD-L1 are noteworthy as they may play a prominent role in the regulation of the immune system. Thus, cytokine release from PD-L1-specific CD4+ T cells may surmount the overall immunosuppressive actions of this immune checkpoint regulator. Moreover, PD-L1-specific T cells might be useful for anticancer immunotherapy, as they may counteract common mechanisms of immune escape mediated by the PD-L1/PD-1 pathway. [ABSTRACT FROM AUTHOR]

An assessment of antitumor immunity versus autoimmunity as provoked by the specific depletion of FOXP3+ Tregs is now possible with the development of Foxp3-diphtheria toxin receptor-like transgenic mouse models. We have used the poorly immunogenic B16F10 melanoma model to characterize a very heterogeneous antitumor effect of the immune response induced by Treg depletion. Depletion and neutralization studies demonstrated the importance of host T cells and interferon γ (IFNγ) in mediating the antitumor response developing in Treg-depleted mice. Such a response correlated with increased proliferation of granzyme B- and IFNγ-producing T cells in the tumor. Furthermore, enhanced antitumor immunity modulated the expression of MHC Class I molecules by B16F10 melanoma cells in Tregdepleted mice. Since Foxp3+ Treg depletion induced a significantly heterogeneous antitumor response, for the first time we were able to assess antitumor immunity and autoimmunity across different groups of responding mice. Strikingly, the duration of the tumor-immune system interaction provoked in individual Treg-depleted mice positively correlated with their propensity to develop vitiligo. A rapid complete tumor rejection was not associated with the development of autoimmunity, however, a proportion of mice that suppressed, but did not effectively clear, B16F10 melanoma did develop vitiligo. The significant implication is that approaches that combine with Treg depletion to rapidly reject tumors may also diminish autoimmune toxicities. [ABSTRACT FROM AUTHOR]

High endothelial venules (HEVs) are blood vessels especially adapted for lymphocyte trafficking which are normally found in secondary lymphoid organs such as lymph nodes (LN) and Peyer's patches. It has long been known that HEVs develop in non-lymphoid organs during chronic inflammation driven by autoimmunity, infection or allografts. More recently, HEVs have been observed in solid, vascularized tumors and their presence correlated with reduced tumor size and improved patient outcome. It is proposed that newly formed HEV promote antitumor immunity by recruiting naive lymphocytes into the tumor, thus allowing the local generation of cancerous tissue-destroying lymphocytes. Understanding how HEVs develop and function are therefore important to unravel their role in human cancers. In LN, HEVs develop during embryonic and early post-natal life and are actively maintained by the LN microenvironment. Systemic blockade of lymphotoxin-β receptor leads to HEV de-differentiation, but the LN components that induce HEV differentiation have remained elusive. Recent elegant studies using gene-targeted mice have demonstrated clearly that triggering the lymphotoxin-β receptor in endothelial cells (EC) induces the differentiation of HEV and that CD11c+dendritic cells play a crucial role in this process. It will be important to determine whether lymphotoxin-β receptor-dependent signaling in EC drives the development of HEV during tumorigenesis and which cells have HEV-inducer properties. This may reveal therapeutic approaches to promote HEV neogenesis and determine the impact of newly formed HEV on tumor immunity. [ABSTRACT FROM AUTHOR]