Designing potent silencing triggers is key to the successful application of RNA interference (RNAi) in mammals. Recent studies suggest that the assembly of RNAi effector complexes is coupled to Dicer cleavage. Here we examine whether transfection of optimized Dicer substrates results in an improved RNAi response. Dicer cleavage of chemically synthesized short hairpin RNAs (shRNAs) with 29-base-pair stems and 2-nucleotide 3' overhangs produced predictable homogeneous small RNAs comprising the 22 bases at the 3' end of the stem. Consequently, direct comparisons of synthetic small interfering RNAs and shRNAs that yield the same small RNA became possible. We found synthetic 29-mer shRNAs to be more potent inducers of RNAi than small interfering RNAs. Maximal inhibition of target genes was achieved at lower concentrations and silencing at 24 h was often greater. These studies provide the basis for an improved approach to triggering experimental silencing via the RNAi pathway.

We have identified and characterized a novel src homology 2 (SH2) and pleckstrin homology (PH) domain–containing adaptor protein, designated Bam32 (for B cell adaptor molecule of 32 kD). cDNAs encoding the human and mouse Bam32 coding sequences were isolated and the human bam32 gene was mapped to chromosome 4q25–q27. Bam32 is expressed by B lymphocytes, but not T lymphocytes or nonhematopoietic cells. Human germinal center B cells show increased Bam32 expression, and resting B cells rapidly upregulate expression of Bam32 after ligation of CD40, but not immunoglobulin M. Bam32 is tyrosine-phosphorylated upon B cell antigen receptor (BCR) ligation or pervanadate stimulation and associates with phospholipase Cγ2. After BCR ligation, Bam32 is recruited to the plasma membrane through its PH domain. Membrane recruitment requires phosphatidylinositol 3-kinase (PI3K) activity and an intact PI(3,4,5)P3-binding motif, suggesting that membrane association occurs through binding to 3-phosphoinositides. Expression of Bam32 in B cells leads to a dose-dependent inhibition of BCR-induced activation of nuclear factor of activated T cells (NF-AT), which is blocked by deletion of the PH domain or mutation of the PI(3,4,5)P3-binding motif. Thus, Bam32 represents a novel B cell–associated adaptor that regulates BCR signaling downstream of PI3K.

CD8+ T cell tolerance to self-proteins prevents autoimmunity but represents an obstacle to generating T cell responses to tumor-associated antigens. We have made a T cell receptor (TCR) transgenic mouse specific for a tumor antigen and crossed TCR-TG mice to transgenic mice expressing the tumor antigen in hepatocytes (gag-TG). TCRxgag mice showed no signs of autoimmunity despite persistence of high avidity transgenic CD8+ T cells in the periphery. Peripheral CD8+ T cells expressed phenotypic markers consistent with antigen encounter in vivo and had upregulated the antiapoptotic molecule Bcl-2. TCRxgag cells failed to proliferate in response to antigen but demonstrated cytolytic activity and the ability to produce interferon γ. This split tolerance was accompanied by inhibition of Ca2+ flux, ERK1/2, and Jun kinasephosphorylation, and a block in both interleukin 2 production and response to exogenous interleukin 2. The data suggest that proliferation and expression of specific effector functions characteristic of reactive cells are not necessarily linked in CD8+ T cell tolerance.

Ag presented by activated APCs promote immunogenic responses whereas Ag presented by resting APCs leads to tolerance. In such a model, the regulation of cytokine release by the presence or absence of costimulation might potentially play a critical role in dictating the ultimate outcome of Ag recognition. C-C chemokines are a structurally defined family of chemoattractants that have diverse effects on inflammation. We were interested in determining the activation requirements for chemokine production by CD4+ T cells. Our data demonstrate for T cell clones and previously activated T cells from TCR-transgenic mice that stimulation with anti-TCR alone results in the production of copious amounts of macrophage-inflammatory protein-1α (MIP-1α) and other C-C chemokines, and that addition of anti-CD28 gives very little augmentation. Furthermore, MIP-1α production is nearly equivalent from both anergic and nonanergic cells. For naive T cells, anti-CD3 stimulation alone led to as much MIP-1α production as Ag + APC stimulation. The addition of costimulation gave a 3–10-fold enhancement, but this was 70-fold less than the effect of costimulation on IL-2 production. Thus, although C-C chemokines play a broad role in influencing inflammation, their production by signal 1 alone makes them unlikely to play a critical role in the decision between a tolerogenic and an immunogenic response. Furthermore, the production of MIP-1α by anergic T cells, as well as following signal 1 alone, raises the possibility that in vivo this chemokine serves to recruit activated T cells to become tolerant.

Costimulation (signal 2) has been proposed to inhibit the induction of T cell clonal anergy by either directly antagonizing negative signals arising from TCR engagement (signal 1) or by synergizing with signal 1 to produce IL-2, which in turn leads to proliferation and dilution of negative regulatory factors. To better define the cellular events that lead to the induction of anergy, we used the immunosuppressive agent rapamycin, which blocks T cell proliferation in late G1 phase but does not affect costimulation-dependent IL-2 production. Our data demonstrate that full T cell activation (signal 1 plus 2) in the presence of rapamycin results in profound T cell anergy, despite the fact that these cells produce copious amounts of IL-2. Similar to conventional anergy (induction by signal 1 alone), the rapamycin-induced anergic cells show a decrease in mitogen-activated protein kinase activation, and these cells can be rescued by culture in IL-2. Interestingly, the rapamycin-induced anergic cells display a more profound block in IL-3 and IFN-γ production upon rechallenge. Finally, in contrast to rapamycin, full T cell activation in the presence of hydroxyurea (which inhibits the cell cycle in early S phase) did not result in anergy. These data suggest that it is neither the direct effect of costimulation nor the subsequent T cell proliferation that prevents anergy induction, but rather the biochemical events that occur upon progression through the cell cycle from G1 into S phase.