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Regulatory CD4+ T cells (Treg) prevent tumor clearance by conventional T cells (Tconv) comprising a major obstacle of cancer immune-surveillance. Hitherto, the mechanisms of Treg repertoire formation in human cancers remain largely unclear. Here, we analyze Treg clonal origin in breast cancer patients using T-Cell Receptor and single-cell transcriptome sequencing. While Treg in peripheral blood and breast tumors are clonally distinct, Tconv clones, including tumor-antigen reactive effectors (Teff), are detected in both compartments. Tumor-infiltrating CD4+ cells accumulate into distinct transcriptome clusters, including early activated Tconv, uncommitted Teff, Th1 Teff, suppressive Treg and pro-tumorigenic Treg. Trajectory analysis suggests early activated Tconv differentiation either into Th1 Teff or into suppressive and pro-tumorigenic Treg. Importantly, Tconv, activated Tconv and Treg share highly-expanded clones contributing up to 65% of intratumoral Treg. Here we show that Treg in human breast cancer may considerably stem from antigen-experienced Tconv converting into secondary induced Treg through intratumoral activation., The mechanisms that shape the regulatory T cell repertoire in patients with cancer are not completely understood. Here, the authors observe that, in breast cancer patients, tumor-resident regulatory T cells do not show clonal relationship with their circulating counterpart, but share a common origin with intratumoral antigen-experienced conventional T cells.

Endogenous antitumor effector T-cell responses and immune-suppressive regulatory T cells (Treg) critically influence the prognosis of patients with cancer, yet many of the mechanisms of how this occurs remain unresolved. On the basis of an analysis of the function, antigen specificity, and distribution of tumor antigen–reactive T cells and Tregs in patients with breast cancer and transgenic mouse tumor models, we showed that tumor-specific Tregs were selectively activated in the bone marrow (BM) and egressed into the peripheral blood. The BM was constantly depleted of tumor-specific Tregs and was instead a site of increased induction and activity of tumor-reactive effector/memory T cells. Treg egress from the BM was associated with activation-induced expression of peripheral homing receptors such as CCR2. Because breast cancer tissues express the CCR2 ligand CCL2, the activation and egress of tumor antigen–specific Tregs in the BM resulted in the accumulation of Tregs in breast tumor tissue. Such immune compartmentalization and redistribution of T-cell subpopulations between the BM and peripheral tissues were achieved by vaccination with adenoviral vector–encoded TRP-2 tumor antigen in a RET transgenic mouse model of spontaneous malignant melanoma. Thus, the BM simultaneously represented a source of tumor-infiltrating Tregs and a site for the induction of endogenous tumor-specific effector T-cell responses, suggesting that both antitumor immunity and local immune suppression are orchestrated in the BM.

Effector memory T cells (T(EM)) have an important role in immunity against infection. However, little is known about the factors regulating T(EM) maintenance and proliferation. In this study, we investigated the role of direct interactions between CD4(+) and CD8(+) T cells (TC) for human T(EM) expansion. Proliferation of separated or mixed CD4(+) and CD8(+)T(EM) populations was analyzed after polyclonal stimulation in vitro. Compared to each isolated subset mixed T(EM) populations showed increased proliferation and expansion of both CD4(+) and CD8(+)T(EM) subpopulations. Combined activation of CD4(+) and CD8(+) memory T cells (Tmem) induced an increased expression of CD40L and CD40 on both populations. Subsequently, CD40/CD40L caused a bi-directional stimulation of CD40(+)CD4(+)T(EM) by CD40L(+)CD8(+)T(EM) and of CD40(+)CD8(+)T(EM) by CD40L(+)CD4(+)T(EM). Blocking of CD40L on activated CD8(+)T(EM) selectively inhibited proliferation of CD4(+)T(EM), while blocking of CD40L on CD4(+)T(EM) abrogated proliferation of CD8(+)T(EM). Taken together, we demonstrate for the first time that the expression of CD40L is exploited on the one hand by CD8(+)T(EM) to increase the proliferation of activated CD4(+)T(EM) and on the other hand by CD4(+)T(EM) to support the expansion of activated CD8(+)T(EM). Thus, efficient T(EM) expansion requires bi-directional interactions between CD4(+) and CD8(+)T(EM) cells.