William C. Sha

Inhibition of Th1 development mediated by GATA-3 through an IL-4-independent mechanism.

Recently, the transcription factor GATA-3 was shown to be selectively expressed in Th2 but not Th1 cells and to augment Th2-specific cytokines. Here, we show that loss of GATA-3 expression by developing Th1 cells requires IL-12 signaling through Stat4 and does not simply result from an absence of IL-4. Moreover, we demonstrate a novel role for GATA-3 in directly repressing Th1 development distinct from its positive actions on Th2-specific cytokines. GATA-3 inhibits Th1 cytokines by a cell-intrinsic mechanism that is not dependent on IL-4 and that may involve repression of IL-12 signaling. Thus, GATA-3 expression and IL-12 signaling are mutually antagonistic, which facilitates rapid dominance of one pathway during early Th development, producing a stable divergence in cytokine profiles.

B7h, a Novel Costimulatory Homolog of B7.1 and B7.2, Is Induced by TNFα

In a screen to identify genes induced by NF-kappaB/Rel transcription factors, we cloned a novel gene, b7h, that is a close homolog of B7 costimulatory ligands expressed on antigen-presenting cells. B7h can costimulate proliferation of purified T cells through a receptor on T cells distinct from CD28 or CTLA-4. Surprisingly, although B7h is expressed in unstimulated B cells, its expression is induced in both 3T3 cells and embryonic fibroblasts treated with TNFalpha, and it is upregulated in nonlymphoid tissues of mice treated with LPS, a potent activator of TNFalpha. These data define a novel costimulatory ligand for T cells and suggest that induction of B7h by TNFalpha may function as a mechanism to directly augment recognition of self during inflammation.

Enhancement of CD8+ T Cell Responses by ICOS/B7h Costimulation

Although the recently identified ICOS/B7h costimulatory counterreceptors are critical regulators of CD4+ T cell responses, their ability to regulate CD8+ responses is unclear. Here we report using a tumor-rejection model that ectopic B7h expression can costimulate rejection by CD8+ T cells in the absence of CD4+ T cells. Although responses of naive T cells were significantly augmented by priming with B7h, B7h was surprisingly effective in mobilizing recall responses of adoptively transferred T cells. To explore why secondary responses of CD8+ T cells were particularly enhanced by B7h, kinetics of ICOS up-regulation, proliferative responses, and cytokine production were compared from both naive and rechallenged 2C-transgenic T cells costimulated in vitro. Although B7h costimulated proliferative responses from both CD8+ populations, rechallenged cells were preferentially costimulated for IL-2 and IFN-γ production. These results indicate that ICOS/B7h counterreceptors likely function in vivo to enhance secondary responses by CD8+ T cells.

The right place at the right time: novel B7 family members regulate effector T cell responses.

Recent identification of novel members of the B7-family of costimulatory ligands has illustrated their importance for costimulation, not only for initiation of adaptive immune responses, but also for regulation of activated effector lymphocytes. Two key features that distinguish these novel molecules from classical B7.1 and B7.2 costimulatory ligands are their broader expression in non-lymphoid tissues and their binding to receptors induced on activated T cells. Whereas B7.1/B7.2-CD28 interactions are important for priming naïve T cells, novel costimulatory interactions appear critical in regulating effector lymphocytes at sites of infection in the periphery.

The combined absence of NF-κB1 and c-Rel reveals that overlapping roles for these transcription factors in the B cell lineage are restricted to the activation and function of mature cells

Transcription factors NF-κB1 and c-Rel, individually dispensable during embryogenesis, serve similar, yet distinct, roles in the function of mature hemopoietic cells. Redundancy among Rel/NF-κB family members prompted an examination of the combined roles of c-Rel and NF-κB1 by using mice that lack both proteins. Embryonic development and the maturation of hemopoietic progenitors were unaffected in nfkb1−/−c-rel−/− mice. Peripheral T cell populations developed normally, but follicular, marginal zone, and CD5+ peritoneal B cell populations all were reduced. In culture, a failure of mitogen-stimulated nfkb1−/−c-rel−/− B cells to proliferate was caused by a cell cycle defect in early G1 that prevented growth. In vivo, defects in humoral immunity and splenic architecture seen in nfkb1−/− and c-rel−/− mice were exacerbated in the double mutant mice. These findings demonstrate that in the B lineage overlapping roles for NF-κB1 and c-Rel appear to be restricted to regulating the activation and function of mature cells.

Transcriptional Profiling of Antigen-Dependent Murine B Cell Differentiation and Memory Formation

Humoral immunity is characterized by the generation of Ab-secreting plasma cells and memory B cells that can more rapidly generate specific Abs upon Ag exposure than their naive counterparts. To determine the intrinsic differences that distinguish naive and memory B cells and to identify pathways that allow germinal center B cells to differentiate into memory B cells, we compared the transcriptional profiles of highly purified populations of these three cell types along with plasma cells isolated from mice immunized with a T-dependent Ag. The transcriptional profile of memory B cells is similar to that of naive B cells, yet displays several important differences, including increased expression of activation-induced deaminase and several antiapoptotic genes, chemotactic receptors, and costimulatory molecules. Retroviral expression of either Klf2 or Ski, two transcriptional regulators specifically enriched in memory B cells relative to their germinal center precursors, imparted a competitive advantage to Ag receptor and CD40-engaged B cells in vitro. These data suggest that humoral recall responses are more rapid than primary responses due to the expression of a unique transcriptional program by memory B cells that allows them to both be maintained at high frequencies and to detect and rapidly respond to antigenic re-exposure.

Constitutive Expression of the B7h Ligand for Inducible Costimulator on Naive B Cells Is Extinguished after Activation by Distinct B Cell Receptor and Interleukin 4 Receptor–mediated Pathways and Can Be Rescued by CD40 Signaling

The recently described ligand–receptor pair, B7h–inducible costimulator (ICOS), is critical for germinal center formation and antibody responses. In contrast to the induced expression of the related costimulatory ligands B7.1 and B7.2, B7h is constitutively expressed on naive B cells and is surprisingly extinguished after antigen engagement and interleukin (IL)-4 cytokine signaling. Although signaling through both B cell receptor (BCR) and IL-4 receptor (R) converge on the extinction of B7h mRNA levels, BCR down-regulation occurs through Ca2+ mobilization, whereas IL-4R down-regulation occurs through a distinct Stat6-dependent pathway. During antigen-specific B cell activation, costimulation through CD40 signaling can reverse both BCR- and IL-4R–mediated B7h down-regulation. These data suggest that the CD40–CD40 ligand signaling pathway regulates B7h expression on activated B cells and may control whether antigen-activated B cells can express B7h and costimulate cognate antigen–activated T cells through ICOS.

ICOS-induced B7h shedding on B cells is inhibited by TLR7/8 and TLR9.

We report in this study that B7h, the ligand for the ICOS costimulatory receptor, is rapidly shed from mouse B cells following either ICOS binding or BCR engagement. Shedding occurs through proteolytic cleavage that releases the extracellular ICOS-binding region of B7h. Prior exposure of B7h-expressing APCs to ICOS-expressing cells inhibits their subsequent ability to costimulate IFN-γ and IL-4 production from CD4+ T cells. Shedding is regulated as TLR7/8 and TLR9 ligands inhibit B7h shedding. A shedding-resistant B7h mutant elicits greater costimulation of IFN-γ production from CD4+ T cells than does wild-type B7h. These data define shedding of B7h as a novel mechanism for controlling costimulatory signaling by B7-CD28 family members that is regulated on B cells by TLR signaling.

CD28-B7 bidirectional signaling: a two-way street to activation.

Engagement of CD28 by B7 molecules results in the activation of T cells. In addition to this T cell stimulation through CD28, the engagement of B7 on dendritic cells by CD28 can lead to the activation of dendritic cells.

Host metabolism regulates growth and differentiation of Toxoplasma gondii

A critical step in the pathogenesis of Toxoplasma gondii is conversion from the fast-replicating tachyzoite form experienced during acute infection to the slow-replicating bradyzoite form that establishes long-lived tissue cysts during chronic infection. Bradyzoite cyst development exhibits a clear tissue tropism in vivo, yet conditions of the host cell environment that influence this tropism remain unclear. Using an in vitro assay of bradyzoite conversion, we have found that cell types differ dramatically in the ability to facilitate differentiation of tachyzoites into bradyzoites. Characterization of cell types that were either resistant or permissive for conversion revealed that resistant cell lines release low molecular weight metabolites that could support tachyzoite growth under metabolic stress conditions and thereby inhibit bradyzoite formation in permissive cells. Biochemical analysis revealed that the glycolytic metabolite lactate is an inhibitory component of supernatants from resistant cells. Furthermore, upregulation of glycolysis in permissive cells through the addition of glucose or by overexpression of the host kinase, Akt, was sufficient to convert cells from a permissive to a resistant phenotype. These results suggest that the metabolic state of the host cell may play a role in determining the predilection of the parasite to switch from the tachyzoite to bradyzoite form.