While in vitro observations suggest that cross-presentation of antigens is mediated primarily by CD8α + dendritic cells, in vivo analysis has been hampered by the lack of systems that selectively eliminate this cell lineage. Here we show that deletion of the transcription factor Batf3 ablated development of CD8α + dendritic cells, allowing us to examine their role in immunity in vivo. Dendritic cells from Batf3 -/-mice were defective in cross-presentation and Batf3 -/-mice lacked virusspecific CD8 + T cell responses to West Nile virus. Importantly, rejection of highly immunogenic syngeneic tumors was impaired in Batf3 -/-mice. These results suggest an important role for CD8α + dendritic cells and cross-presentation in responses to viruses and in tumor rejection.During antigen 'cross-presentation' (1), antigens generated in one cell are presented by MHC class I molecules of a second cell. It remains unclear whether all antigen presenting cells (APCs) use cross-presentation and whether this pathway plays a role in immune responses in vivo (2). Dendritic cells (DCs) are a heterogeneous group of APCs with two major subsets, plasmacytoid dendritic cells (pDCs) and conventional CD11c + dendritic cells (cDCs) (3). Subsets of cDCs include CD8α + , CD4 + , and CD8α -CD4 -populations that may exert distinct functions in immune responses. Evidence has suggested that CD8α + cDCs are important for cross-presentation during infections, but is based on ex vivo analysis (4-6) or in vitro antigen loading (7). Evidence both for and against a role for cross-presentation in responses against tumors has been reported (8-10).Attempts have been made to study the in vivo role of dendritic cells by selective depletion. Diphtheria toxin treatment can deplete all CD11c hi cells in one transgenic mouse model (11), but affects splenic macrophages and activated CD8 + T cells (12). Gene targeting of transcription factors (e.g., Irf2, Irf4, Irf8, Stat3 and Id2) has caused broad defects in several DC subsets, T cells and macrophages (13). To identify genes regulating DC development, we performed global gene expression analysis across many tissues and immune cells ( fig S1A). Batf3 (p21SNFT) (14) was highly expressed in cDCs, with low to absent expression in other *To whom correspondence should be addressed. E-mail murphy@pathology.wustl.edu. fig. S1B-D).In spleens of Batf3 -/-mice we found a selective loss of CD8α + cDCs, without abnormalities in other hematopoietic cell types or architecture (Fig. 1, fig. S2-S11). CD8α + cDC coexpress DEC205, CD24, and low levels of CD11b (3,15). Batf3 -/-mice lacked splenic CD11c hi CD8α + DEC205 + cells (Fig. 1A), showed a loss of CD11c hi CD11b dull cells and CD11c hi CD8α + CD24 + cells (Fig. 1B), but had normal populations of CD4 + and CD8α -CD4 -cDC subsets (Fig. 1B). Lymph nodes and thymi of Batf3 -/-mice lacked CD8α + DCs but had normal distributions of CD8α -CD11c + cells (Fig. 1C). DEC205 int and DEC205 hi DCs were present in lymph nodes draining the skin of Batf3 -/-mice (Fig. 1C), and show...

Although CD103-expressing dendritic cells (DCs) are widely present in nonlymphoid tissues, the transcription factors controlling their development and their relationship to other DC subsets remain unclear. Mice lacking the transcription factor Batf3 have a defect in the development of CD8α+ conventional DCs (cDCs) within lymphoid tissues. We demonstrate that Batf3−/− mice also lack CD103+CD11b− DCs in the lung, intestine, mesenteric lymph nodes (MLNs), dermis, and skin-draining lymph nodes. Notably, Batf3−/− mice displayed reduced priming of CD8 T cells after pulmonary Sendai virus infection, with increased pulmonary inflammation. In the MLNs and intestine, Batf3 deficiency resulted in the specific lack of CD103+CD11b− DCs, with the population of CD103+CD11b+ DCs remaining intact. Batf3−/− mice showed no evidence of spontaneous gastrointestinal inflammation and had a normal contact hypersensitivity (CHS) response, despite previous suggestions that CD103+ DCs were required for immune homeostasis in the gut and CHS. The relationship between CD8α+ cDCs and nonlymphoid CD103+ DCs implied by their shared dependence on Batf3 was further supported by similar patterns of gene expression and their shared developmental dependence on the transcription factor Irf8. These data provide evidence for a developmental relationship between lymphoid organ–resident CD8α+ cDCs and nonlymphoid CD103+ DCs.

Activator protein 1 (AP-1) transcription factors are dimers of Jun, Fos, MAF and activating transcription factor (ATF) family proteins characterized by basic region and leucine zipper domains1. Many AP-1 proteins contain defined transcriptional activation domains (TADs), but Batf and the closely related Batf3 (refs 2, 3) contain only a basic region and leucine zipper and have been considered inhibitors of AP-1 activity3–8. Here we show that Batf is required for the differentiation of IL-17-producing T helper (TH17) cells9. TH17 cells comprise a CD4+ T cell subset that coordinates inflammatory responses in host defense but is pathogenic in autoimmunity10–13.Batf −/−mice have normal TH1 and TH2 differentiation, but show a defect in TH17 differentiation, and are resistant to experimental autoimmune encephalomyelitis (EAE).Batf −/−T cells fail to induce known factors required for TH17 differentiation, such as RORγt11 and the cytokine IL-21 (refs 14–17). Neither addition of IL-21 nor overexpression of RORγt fully restores IL-17 production in Batf−/− T cells. The IL-17 promoter is Batf-responsive, and upon TH17 differentiation, Batf binds conserved intergenic elements in the IL-17A/F locus and to the IL-17, IL-21 and IL-22 (ref 18) promoters. These results demonstrate that the AP-1 protein Batf plays a critical role in TH17 differentiation.

B and T lymphocyte attenuator (BTLA) provides an inhibitory signal to B and T cells. Previously, indirect observations suggested that B7x was a ligand for BTLA. Here we show that BTLA does not bind B7x; instead, we identify herpesvirus entry mediator (HVEM) as the unique BTLA ligand. BTLA bound the most membrane-distal cysteine-rich domain of HVEM, distinct from regions where the ligands LIGHT and lymphotoxin-alpha bound HVEM. HVEM induced BTLA tyrosine phosphorylation and association of the tyrosine phosphatase SHP-2 and repressed antigen-driven T cell proliferation, providing an example of reverse signaling to a non-tumor necrosis factor family ligand. The conservation of the BTLA-HVEM interaction between mouse and human suggests that this system is an important pathway regulating lymphocyte activation and/or homeostasis in the immune response.