Humanization of T cell-mediated immunity in mice.

Despite the enormous promise of T cell therapies, the isolation and study of human T cell receptors (TCRs) of dedicated specificity remains a major challenge. To overcome this limitation, we generated mice with a genetically humanized system of T cell immunity. We used VelociGene technology to replace the murine TCRαß variable regions, along with regions encoding the extracellular domains of co-receptors CD4 and CD8, and major histocompatibility complex (MHC) class I and II, with corresponding human sequences. The resulting "VelociT" mice have normal myeloid and lymphoid immune cell populations, including thymic and peripheral αß T cell subsets comparable with wild-type mice. VelociT mice expressed a diverse TCR repertoire, mounted functional T cell responses to lymphocytic choriomeningitis virus infection, and could develop experimental autoimmune encephalomyelitis. Immunization of VelociT mice with human tumor-associated peptide antigens generated robust, antigen-specific responses and led to identification of a TCR against tumor antigen New York esophageal squamous cell carcinoma-1 with potent antitumor activity. These studies demonstrate that VelociT mice mount clinically relevant T cell responses to both MHC-I­ and MHC-II­restricted antigens, providing a powerful new model for analyzing T cell function in human disease. Moreover, VelociT mice are a new platform for de novo discovery of therapeutic human TCRs.

Cis-acting pathways selectively enforce the non-immunogenicity of shed placental antigen for maternal CD8 T cells.

Maternal immune tolerance towards the fetus and placenta is thought to be established in part by pathways that attenuate T cell priming to antigens released from the placenta into maternal blood. These pathways remain largely undefined and their existence, at face value, seems incompatible with a mother's need to maintain a functional immune system during pregnancy. A particular conundrum is evident if we consider that maternal antigen presenting cells, activated in order to prime T cells to pathogen-derived antigens, would also have the capacity to prime T cells to co-ingested placental antigens. Here, we address this paradox using a transgenic system in which placental membranes are tagged with a strong surrogate antigen (ovalbumin). We find that although a remarkably large quantity of acellular ovalbumin-containing placental material is released into maternal blood, splenic CD8 T cells in pregnant mice bearing unmanipulated T cell repertoires are not primed to ovalbumin even if the mice are intravenously injected with adjuvants. This failure was largely independent of regulatory T cells, and instead was linked to the intrinsic characteristics of the released material that rendered it selectively non-immunogenic, potentially by sequestering it from CD8α(+) dendritic cells. The release of ovalbumin-containing placental material into maternal blood thus had no discernable impact on CD8 T cell priming to soluble ovalbumin injected intravenously during pregnancy, nor did it induce long-term tolerance to ovalbumin. Together, these results outline a major pathway governing the maternal immune response to the placenta, and suggest how tolerance to placental antigens can be maintained systemically without being detrimental to host defense.

Chemokine gene silencing in decidual stromal cells limits T cell access to the maternal-fetal interface.

The chemokine-mediated recruitment of effector T cells to sites of inflammation is a central feature of the immune response. The extent to which chemokine expression levels are limited by the intrinsic developmental characteristics of a tissue has remained unexplored. We show in mice that effector T cells cannot accumulate within the decidua, the specialized stromal tissue encapsulating the fetus and placenta. Impaired accumulation was in part attributable to the epigenetic silencing of key T cell-attracting inflammatory chemokine genes in decidual stromal cells, as evidenced by promoter accrual of repressive histone marks. These findings give insight into mechanisms of fetomaternal immune tolerance, as well as reveal the epigenetic modification of tissue stromal cells as a modality for limiting effector T cell trafficking.

Coordinate regulation of tissue macrophage and dendritic cell population dynamics by CSF-1.

Tissue macrophages (Mφs) and dendritic cells (DCs) play essential roles in tissue homeostasis and immunity. How these cells are maintained at their characteristic densities in different tissues has remained unclear. Aided by a novel flow cytometric technique for assessing relative rates of blood-borne precursor recruitment, we examined Mφ and DC population dynamics in the pregnant mouse uterus, where rapid tissue growth facilitated a dissection of underlying regulatory mechanisms. We demonstrate how Mφ dynamics, and thus Mφ tissue densities, are locally controlled by CSF-1, a pleiotropic growth factor whose in situ level of activity varied widely between uterine tissue layers. CSF-1 acted in part by inducing Mφ proliferation and in part by stimulating the extravasation of Ly6C(hi) monocytes (Mos) that served as Mφ precursors. Mo recruitment was dependent on the production of CCR2 chemokine receptor ligands by uterine Mφs in response to CSF-1. Unexpectedly, a parallel CSF-1-regulated, but CCR2-independent pathway influenced uterine DC tissue densities by controlling local pre-DC extravasation rates. Together, these data provide cellular and molecular insight into the regulation of Mφ tissue densities under noninflammatory conditions and reveal a central role for CSF-1 in the coordination of Mφ and DC homeostasis.

Dendritic cell entrapment within the pregnant uterus inhibits immune surveillance of the maternal/fetal interface in mice.

Embryo implantation induces formation of the decidua, a stromal cell-derived structure that encases the fetus and placenta. Using the mouse as a model organism, we have found that this tissue reaction prevents DCs stationed at the maternal/fetal interface from migrating to the lymphatic vessels of the uterus and thus reaching the draining lymph nodes. Strikingly, decidual DCs remained immobile even after being stimulated with LPS and exhibiting responsiveness to CCL21, the chemokine that drives DC entry into lymphatic vessels. An analysis of maternal T cell reactivity toward a surrogate fetal/placental antigen furthermore revealed that regional T cell responses toward the fetus and placenta were driven by passive antigen transport and thus the tolerogenic mode of antigen presentation that predominates when there is negligible input from tissue-resident DCs. Indeed, the lack of involvement of tissue-resident DCs in the T cell response to the fetal allograft starkly contrasts with their prominent role in organ transplant rejection. Our results suggest that DC entrapment within the decidua minimizes immunogenic T cell exposure to fetal/placental antigens and raise the possibility that impaired development or function of the human decidua, which unlike that of the mouse contains lymphatic vessels, might lead to pathological T cell activation during pregnancy.