Regulatory T (T(reg)) cells mediate homeostatic peripheral tolerance by suppressing autoreactive T cells. Failure of host antitumor immunity may be caused by exaggerated suppression of tumor-associated antigen-reactive lymphocytes mediated by T(reg) cells; however, definitive evidence that T(reg) cells have an immunopathological role in human cancer is lacking. Here we show, in detailed studies of CD4(+)CD25(+)FOXP3(+) T(reg) cells in 104 individuals affected with ovarian carcinoma, that human tumor T(reg) cells suppress tumor-specific T cell immunity and contribute to growth of human tumors in vivo. We also show that tumor T(reg) cells are associated with a high death hazard and reduced survival. Human T(reg) cells preferentially move to and accumulate in tumors and ascites, but rarely enter draining lymph nodes in later cancer stages. Tumor cells and microenvironmental macrophages produce the chemokine CCL22, which mediates trafficking of T(reg) cells to the tumor. This specific recruitment of T(reg) cells represents a mechanism by which tumors may foster immune privilege. Thus, blocking T(reg) cell migration or function may help to defeat human cancer.

Suppression of dendritic cell function in cancer patients is thought to contribute to the inhibition of immune responses and disease progression. Molecular mechanisms of this suppression remain elusive, however. Here, we show that a fraction of blood monocyte-derived myeloid dendritic cells (MDCs) express B7-H1, a member of the B7 family, on the cell surface. B7-H1 could be further upregulated by tumor environmental factors. Consistent with this finding, virtually all MDCs isolated from the tissues or draining lymph nodes of ovarian carcinomas express B7-H1. Blockade of B7-H1 enhanced MDC-mediated T-cell activation and was accompanied by downregulation of T-cell interleukin (IL)-10 and upregulation of IL-2 and interferon (IFN)-gamma. T cells conditioned with the B7-H1-blocked MDCs had a more potent ability to inhibit autologous human ovarian carcinoma growth in non-obese diabetic-severe combined immunodeficient (NOD-SCID) mice. Therefore, upregulation of B7-H1 on MDCs in the tumor microenvironment downregulates T-cell immunity. Blockade of B7-H1 represents one approach for cancer immunotherapy.

Tumor-associated macrophages are a prominent component of ovarian cancer stroma and contribute to tumor progression. B7-H4 is a recently identified B7 family molecule. We show that primary ovarian tumor cells express intracellular B7-H4, whereas a fraction of tumor macrophages expresses surface B7-H4. B7-H4+ tumor macrophages, but not primary ovarian tumor cells, suppress tumor-associated antigen-specific T cell immunity. Blocking B7-H4-, but not arginase-, inducible nitric oxide synthase or B7-H1 restored the T cell stimulating capacity of the macrophages and contributes to tumor regression in vivo. Interleukin (IL)-6 and IL-10 are found in high concentrations in the tumor microenvironment. These cytokines stimulate macrophage B7-H4 expression. In contrast, granulocyte/macrophage colony-stimulating factor and IL-4, which are limited in the tumor microenvironment, inhibit B7-H4 expression. Ectopic expression of B7-H4 makes normal macrophages suppressive. Thus, B7-H4+ tumor macrophages constitute a novel suppressor cell population in ovarian cancer. B7-H4 expression represents a critical checkpoint in determining host responses to dysfunctional cytokines in ovarian cancer. Blocking B7-H4 or depleting B7-H4+ tumor macrophages may represent novel strategies to enhance T cell tumor immunity in cancer.

Although 13 years have passed since identification of human immunodeficiency virus-1 (HIV-1) as the cause of AIDS, we do not yet know how HIV kills its primary target, the T cell that carries the CD4 antigen. We and others have shown an increase in the percentage of apoptotic cells among circulating CD4+ (and CD8+) T cells of HIV-seropositive individuals and an increase in frequency of apoptosis with disease progression. However, it is not known if this apoptosis occurs in infected or uninfected T cells. We show here, using in situ labelling of lymph nodes from HIV-infected children and SIV-infected macaques, that apoptosis occurs predominantly in bystander cells and not in the productively infected cells themselves. These data have implications for pathogenesis and therapy, namely, arguing that rational drug therapy may involve combination agents targeting viral replication in infected cells and apoptosis of uninfected cells.

Dendritic-cell (DC) trafficking and function in tumors is poorly characterized, with studies confined to myeloid DCs (DC1s). Tumors inhibit DC1 migration and function, likely hindering specific immunity. The role of plasmacytoid DCs (DC2s) in tumor immunity is unknown. We show here that malignant human ovarian epithelial tumor cells express very high levels of stromal-derived factor-1, which induces DC2 precursor (preDC2) chemotaxis and adhesion/transmigration, upregulates preDC2 very late antigen (VLA)-5, and protects preDC2s from tumor macrophage interleukin-10-induced apoptosis, all through CXC chemokine receptor-4. The VLA-5 ligand vascular-cell adhesion molecule-1 mediated preDC2 adhesion/transmigration. Tumor preDC2s induced significant T-cell interleukin-10 unrelated to preDC2 differentiation or activation state, and this contributed to poor T-cell activation. Myeloid precursor DCs (preDC1s) were not detected. Tumors may weaken immunity by attracting preDC2s and protecting them from the harsh microenvironment, and by altering preDC1 distribution.