Summary CD8α+ dendritic cells (DCs) are important in vivo for cross-presentation of antigens derived from intracellular pathogens and tumors. Additionally, secretion of interleukin-12 (IL-12) by CD8α+ DCs suggests a role for these cells in response to Toxoplasma gondii antigens, although it remains unclear whether these cells are required for protection against T. gondii infection. Towards this goal, we examined T. gondii infection of Batf3−/− mice, which selectively lack only lymphoid-resident CD8α+ DCs and related peripheral CD103+ DCs. Batf3−/− mice were extremely susceptible to T. gondii infection, with decreased production of IL-12 and interferon-γ. IL-12 administration restored resistance in Batf3−/− mice, and mice in which IL-12 production was ablated only from CD8α+ DCs failed to control infection. These results reveal that the function of CD8α+ DCs extends beyond a role in cross-presentation and includes a critical role for activation of innate immunity through IL-12 production during T. gondii infection.

Sequential rearrangement of the T cell receptor for antigen (TCR) β and α chains is a hallmark of thymocyte development. This temporal control is lost in TCR transgenics because the α chain is expressed prematurely at the CD4−CD8− double negative (DN) stage. To test the importance of this, we expressed the HYα chain at the physiological CD4+CD8+ double positive (DP) stage. The reduced DP and increased DN cellularity typically seen in TCR transgenics was not observed when the α chain was expressed at the appropriate stage. Surprisingly, antigen-driven selection events were also altered. In male mice, thymocyte deletion now occurred at the single positive or medullary stage. In addition, no expansion of CD8αα intestinal intraepithelial lymphocytes (IELs) was observed, despite the fact that HY transgenics have been used to model IEL development. Collectively, these data establish the importance of proper timing of TCR expression in thymic development and selection and emphasize the need to use models that most accurately reflect the physiologic process.

IL-15 is critical for generation of multiple lymphoid subsets. Recent data have demonstrated a unique aspect of responses to IL-15, in that cells bearing the IL-15Rα chain can bind soluble IL-15 and “transpresent” the cytokine to other cells, allowing the latter to respond to IL-15. However, it is unclear whether IL-15 is normally secreted and then becomes bound to surface IL-15Rα on bystander cells, or whether transpresentation is mediated by the same cells which synthesize IL-15. Using mixed bone marrow chimeric mice, we present evidence for the latter model, showing that development of NK cells and memory phenotype CD8 T cells necessitates that both IL-15 and IL-15Rα be expressed by the same population of cells. These data argue that soluble forms of IL-15 are irrelevant for physiological responses to this cytokine, and the implications of this finding are discussed.

Calcineurin is required for B cell receptor (BCR)-induced proliferation of mature B cells. Paradoxically, loss of NFAT transcription factors, themselves calcineurin targets, induces hyperactivity, which suggests that calcineurin targets other than NFAT are required for BCR-induced proliferation. Here we demonstrate a function for the calcineurin-regulated transcription factor Mef2c in B cells. BCR-induced calcium mobilization was intact after Mef2c deletion, but loss of Mef2c caused defects in B cell proliferation and survival after BCR stimulation in vitro and lower T celldependent antibody responses and germinal center formation in vivo. Mef2c activity was specific to BCR stimulation, as Toll-like receptor and CD40 signaling induced normal responses in Mef2c-deficient B cells. Mef2c-dependent targets included the genes encoding cyclin D2 and the prosurvival factor Bcl-x L . Our results emphasize an unrecognized but critical function for Mef2c in BCR signaling.B cell receptor (BCR) signaling coordinates the development, maintenance, activation and tolerance of B cells. BCR engagement induces the phosphorylation of tyrosine residues in immunoreceptor tyrosine-based activation motifs of immunoglobulin-α and immunoglobulin-β by Src family kinases 1 . Subsequently, other kinases and adaptor molecules orchestrate signaling cascades that eventually activate transcription factors required for the proliferation, survival and differentiation of B cells, including those of the NFAT, AP-1 and NF-κB families 2-5 . BCR engagement can induce opposite outcomes depending on the stage of B cell development 6 . In mature B cells, BCR signaling induces proliferation, affinity maturation and class-switch recombination during T cell-dependent responses and ultimately drives the differentiation of B cells into memory cells and antibody-secreting plasma cells 7 . Alternatively, in immature B cells in the bone marrow and recent bone marrow emigrants in

Memory T cells are critical for immunity to various intracellular pathogens. Recent studies have indicated that CD8 secondary memory cells, induced by prime-boost approaches, show enhanced protective function compared with primary memory cells and exhibit phenotypic and functional characteristics that distinguish them from primary memory cells. However, little is known about the cytokine requirements for generation and maintenance of boosted memory CD8 T cells. We studied the role of IL-15 in determining the size and composition of the secondary (2°) memory CD8 T cell pool induced by Listeria monocytogenes infection in mice. Following boosting, IL-15–deficient animals failed to generate a subset of CD8 effector memory cells, including a population of IL-7Rαlow cells, which were prominent among secondary memory cells in normal mice. IL-15 deficiency also resulted in changes within the IL-7RαhighCD62Llow subset of 2° memory CD8 T cells, which expressed high levels of CD27 but minimal granzyme B. In addition to these qualitative changes, IL-15 deficiency resulted in reduced cell cycle and impaired Bcl-2 expression by 2° memory CD8 T cells, suggesting a role for IL-15 in supporting both basal proliferation and survival of the pool. Analogous qualitative differences in memory CD8 T cell populations were observed following a primary response to Sendai virus in IL-15−/− animals. Collectively, these findings demonstrate that IL-15 plays an important role in dictating the composition rather than simply the maintenance of the CD8 memory pool.

Production of IFN-γ by CD4 T cells is generally thought to be mediated by TCR triggering, however, Ag-nonspecific activation of effector CD8 T cells has been reported in infection models. In this study, we demonstrate that Ag-experienced CD4 T cells in the spleen of Salmonella-infected mice acquire the capacity to rapidly secrete IFN-γ in response to stimulation with bacterial lysate or LPS. This innate responsiveness of T cells was transient and most apparent during, and immediately following, active Salmonella infection. Furthermore, innate T cell production of IFN-γ in response to bacterial lysate or LPS was Ag independent and could be induced in Listeria-infected mice and in the absence of MHC class II expression. IL-18 was required for maximal innate responsiveness of CD4 T cells in Salmonella-infected mice and for optimal bacterial clearance in vivo. These data demonstrate that CD4 T cells acquire the capacity to respond to innate stimuli during active bacterial infection, a process that may contribute significantly to amplifying effector responses in vivo.

One-time treatment with an antibody against BTLA provides long-term protection against graft-versus-host disease without affecting effector T cell responses to tumors or pathogens.

Naive T cells undergo slow homeostatic proliferation in response to T cell lymphopenia, which is also called lymphopenia-induced proliferation (LIP). IL-7 is critically required for this process, but previous studies suggested IL-15 was expendable for LIP of naive CD8 T cells. In contrast, we show that IL-15 is important for sustained CD8 T cell proliferation and accumulation in a lymphopenic setting, as revealed by truncated LIP in IL-15−/− hosts. At the same time, we find that IL-12 enhances LIP by acting directly on the CD8 T cells and independently of IL-15, suggesting distinct pathways by which cytokines can regulate homeostatic proliferation. Interestingly, the memory-phenotype CD8 T cell generated by LIP in IL-15−/− hosts are phenotypically distinct from the rare endogenous memory-phenotype cells found in IL-15−/− animals, suggesting these cells are generated by different means. These findings demonstrate that cytokine requirements for LIP change during the process itself, illustrating the need to identify factors that regulate successive stages of lymphopenia-driven proliferation.