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The progressive depletion of quiescent \"bystander\" CD4 T cells, which are nonpermissive to HIV infection, is a principal driver of the acquired immunodeficiency syndrome (AIDS). These cells undergo abortive infection characterized by the cytosolic accumulation of incomplete HIV reverse transcripts. These viral DNAs are sensed by an unidentified host sensor that triggers an innate immune response, leading to caspase-1 activation and pyroptosis. Using unbiased proteomic and targeted biochemical approaches, as well as two independent methods of lentiviral short hairpin RNA–mediated gene knockdown in primary CD4 T cells, we identify interferon-γ–inducible protein 16 (IFI16) as a host DNA sensor required for CD4 T cell death due to abortive HIV infection. These findings provide insights into a key host pathway that plays a central role in CD4 T cell depletion during disease progression to AIDS.

V domain-containing Ig suppressor of T-cell activation (VISTA) is a negative checkpoint regulator that suppresses T cell-mediated immune responses. Previous studies using a VISTA-neutralizing monoclonal antibody show that VISTA blockade enhances T-cell activation. The current study describes a comprehensive characterization of mice in which the gene for VISTA has been deleted. Despite the apparent normal hematopoietic development in young mice, VISTA genetic deficiency leads to a gradual accumulation of spontaneously activated T cells, accompanied by the production of a spectrum of inflammatory cytokines and chemokines. Enhanced T-cell responsiveness was also observed upon immunization with neoantigen. Despite the presence of multi organ chronic inflammation, aged VISTA-deficient mice did not develop systemic or organspecific autoimmune disease. Interbreeding of the VISTA-deficient mice with 2D2 T-cell receptor transgenic mice, which are predisposed to the development of experimental autoimmune encephalomyelitis, drastically enhanced disease incidence and intensity. Disease development is correlated with the increase in the activation of encephalitogenic T cells in the periphery and enhanced infiltration into the CNS. Taken together, our data suggest that VISTA is a negative checkpoint regulator whose loss of function lowers the threshold for T-cell activation, allowing for an enhanced proinflammatory phenotype and an increase in the frequency and intensity of autoimmunity under susceptible conditions.

Mouse spleens contain three populations of conventional ($CD11c^{high}$) dendritic cells (DCs) that play distinct functions. The CD8⁺ DC are unique in that they can present exogenous antigens on their MHC class I molecules, a process known as cross-presentation. It is unclear whether this special ability is because only the CD8⁺ DC can capture the antigens used in cross-presentation assays, or because this is the only DC population that possesses specialized machinery for cross-presentation. To solve this important question we examined the splenic DC subsets for their ability to both present via MHC class II molecules and cross-present via MHC class I using four different forms of the model antigen ovalbumin (OVA). These forms include a cell-associated form, a soluble form, OVA expressed in bacteria, or OVA bound to latex beads. With the exception of bacterial antigen, which was poorly cross-presented by all DC, all antigenic forms were cross-presented much more efficiently by the CD8⁺ DC. This pattern could not be attributed simply to a difference in antigen capture because all DC subsets presented the antigen via MHC class II. Indeed, direct assessments of endocytosis showed that CD8⁺ and CD8⁻ DC captured comparable amounts of soluble and bead-associated antigen, yet only the CD8⁺ DC cross-presented these antigenic forms. Our results indicate that cross-presentation requires specialized machinery that is expressed by CD8⁺ DC but largely absent from CD8⁻ DC. This conclusion has important implications for the design of vaccination strategies based on antigen targeting to DC.

Recent studies by Dorshkind, Yoder, and colleagues show that embryonic (E9) B-cell progenitors located in the yolk sac and intraembryonic hemogenic endothelium before the initiation of circulation give rise to B-1 and marginal zone B cells but do not give rise to B-2 cells. In studies here, we confirm and extend these findings by showing that distinct progenitors for B-1 and B-2 cells are present in the adult spleen. Furthermore, we show that the splenic B-cell progenitor population (lin⁻CD19⁺/B220lo/⁻/CD43⁻) that gives rise to B-1 cells is likely to be heterogeneous because, in some recipients, it also gives rise to B cells expressing the marginal zone phenotype (B220hiIgMhiIgDloCD21hi) and to some (CD19⁻CD5hi) T cells. In addition to the well-known function differences between B-1 and B-2, our studies demonstrate that substantial developmental differences separate these B-cell lineages. Thus, consistent with the known dependence of B-2 development on IL-7, all B-2 progenitors express IL-7R. However, >30% of the B-1 progenitors do not express this marker, enabling the known IL-7 independent development of B-1 cells in IL-7⁻/⁻ mice. In addition, marker expression on cells in the early stages of the B-2 development pathway (CD19⁻/c-Kitlo/⁻/Sca-1lo/⁻) in adult bone marrow distinguish it from the early stages of B-1 development (CD19hi/c-Kit⁺/Sca-1⁺), which occur constitutively in neonates. In adults, in vivo inflammatory stimulation (LPS) triggers B-1 progenitors in spleen to expand and initiate development along this B-1 developmental pathway.

B-cell chronic lymphocytic leukemia (B-CLL), the most common leukemia in the Western world, occurs in two forms, aggressive (showing for the most part high ZAP-70 expression and unmutated IgH V H ) and indolent (showing low ZAP-70 expression and mutated IgH V H ). We found that miR-29a is up-regulated in indolent human B-CLL as compared with aggressive B-CLL and normal CD19⁺ B cells. To study the role of miR-29 in B-CLL, we generated Eμ-miR-29 transgenic mice overexpressing miR-29 in mouse B cells. Flow cytometric analysis revealed a markedly expanded CD5⁺ population in the spleen of these mice starting at 2 mo of age, with 85% (34/40) of miR-29 transgenic mice exhibiting expanded CD5⁺ B-cell populations, a characteristic of B-CLL. On average, 50% of B cells in these transgenic mice were CD5 positive. At 2 y of age the mice showed significantly enlarged spleens and an increase in the CD5⁺ B-cell population to ∼100%. Of 20 Eμ-miR-29 transgenic mice followed to 24—26 mo of age, 4 (20%) developed frank leukemia and died of the disease. These results suggest that dysregulation of miR-29 can contribute to the pathogenesis of indolent B-CLL.

The cancer stem cell hypothesis postulates that tumor growth is driven by a rare subpopulation of tumor cells. Much of the supporting evidence for this intriguing idea is derived from xenotransplantation experiments in which human leukemia cells are grown in immunocompromised mice. We show that, when lymphomas and leukemias of mouse origin are transplanted into histocompatible mice, a very high frequency (at least 1 in 10) of the tumor cells can seed tumor growth. We suggest that the low frequency of tumor-sustaining cells observed in xenotransplantation studies may reflect the limited ability of human tumor cells to adapt to growth in a foreign (mouse) milieu.

Type I IFN production in response to the DNA virus herpes simplex virus type-1 (HSV-1) is essential in controlling viral replication. We investigated whether plasmacytoid dendritic cells (pDC) were the major tissue source of IFN-α, and whether the production of IFN-α in response to HSV-1 depended on Toll-like receptor 9 (TLR9). Total spleen cells or bone marrow (BM) cells, or fractions thereof, including highly purified pDC, from WT, TLR9, and MyD88 knockout mice were stimulated with known ligands for TLR9 or active HSV-1. pDC freshly isolated from both spleen and BM were the major source of IFN-α in response to oligodeoxynucleotides containing CpG motifs, but in response to HSV-1 the majority of IFN-α was produced by other cell types. Moreover, IFN-α production by non-pDC was independent of TLR9. The tissue source determined whether pDC responded to HSV-1 in a strictly TLR9-dependent fashion. Freshly isolated BM pDC or pDC derived from culture of BM precursors with FMS-like tyrosine kinase-3 ligand, produced IFN-α in the absence of functional TLR9, whereas spleen pDC did not. Heat treatment of HSV-1 abolished maturation and IFN-α production from all TLR9-deficient DC but not WT DC. Thus pDC and non-pDC produce IFN-α in response to HSV-1 via both TLR9-independent and -dependent pathways.

Interleukin 21 (IL-21) is a pleiotropic cytokine produced by CD4 T cells that affects the differentiation and function of T, B, and NK cells by binding to a receptor consisting of the common cytokine receptor γ chain and the IL-21 receptor (IL-21R). IL-21, a product associated with IL-17-producing CD4 T cells (TH17) and follicular CD4 T helper cells (TFH), has been implicated in autoimmune disorders including the severe systemic lupus erythematosus (SLE)-like disease characteristic of BXSB-Yaa mice. To determine whether IL-21 plays a significant role in this disease, we compared IL-21R-deficient and -competent BXSB-Yaa mice for multiple parameters of SLE. The deficient mice showed none of the abnormalities characteristic of SLE in IL-21R-competent Yaa mice, including hypergammaglobulinemia, autoantibody production, reduced frequencies of marginal zone B cells and monocytosis, renal disease, and premature morbidity. IL-21 production associated with this autoimmune disease was not a product of TH17 cells and was not limited to conventional CXCR5⁺ TFH but instead was produced broadly by ICOS⁺ CD4⁺ splenic T cells. IL-21 arising from an abnormal population of CD4 T cells is thus central to the development of this lethal disease, and, more generally, could play an important role in human SLE and related autoimmune disorders.

Natural regulatory T cells (${\\rm{T}}_{{\\rm{reg}}} $) represent a distinct lineage of T lymphocytes committed to suppressive functions, and expression of the transcription factor Foxp3 is thought to identify this lineage specifically. Here we report that whereas the majority of natural ${\\rm{CD4}}^ + {\\rm{Foxp3}}^ + $ T cells maintain stable Foxp3 expression after adoptive transfer to lymphopenic or lymphoreplete recipients, a minor fraction enriched within the CD25- subset actually lose it. Some of those Foxp3- T cells adopt effector helper T cell (${\\rm{T}}_{\\rm{h}} $) functions, whereas some retain \"memory\" of previous Foxp3 expression, reacquiring Foxp3 upon activation. This minority \"unstable\" population exhibits flexible responses to cytokine signals, relying on transforming growth factor-β to maintain Foxp3 expression and responding to other cytokines by differentiating into effector ${\\rm{T}}_{\\rm{h}} $ in vitro. In contrast, ${\\rm{CD4}}^ + {\\rm{Foxp3}}^ + {\\rm{CD25}}^{{\\rm{high}}} $ T cells are resistant to such conversion to effector ${\\rm{T}}_{\\rm{h}} $ even after many rounds of cell division. These results demonstrate that natural ${\\rm{Foxp3}}^ + $ T cells are a heterogeneous population consisting of a committed ${\\rm{T}}_{{\\rm{reg}}} $ lineage and an uncommitted subpopulation with developmental plasticity.

Glycogen synthase kinase 3β (GSK3β) is involved in metabolism, neurodegeneration, and cancer. Inhibition of GSK3β activity is the primary mechanism that regulates this widely expressed active kinase. Although the protein kinase Akt inhibits GSK3β by phosphorylation at the N terminus, preventing Akt-mediated phosphorylation does not affect the cell-survival pathway activated through the GSK3β substrate β-catenin. Here, we show that p38 mitogen-activated protein kinase (MAPK) also inactivates GSK3β by direct phosphorylation at its C terminus, and this inactivation can lead to an accumulation of β-catenin. p38 MAPK-mediated phosphorylation of GSK3β occurs primarily in the brain and thymocytes. Activation of β-catenin-mediated signaling through GSK3β inhibition provides a potential mechanism for p38 MAPK-mediated survival in specific tissues.