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Type I interferons (IFN)1 play an essential role in the innate immune response against virus infection (1). In uninfected cells, the expression of IFN genes is tightly regulated. Virus infection activates transcription of type I IFN genes, and the cis-acting virus-responsive elements (VRE), located within the 110 nucleotides 5Ј of the transcription initiation site, are sufficient for virus-mediated activation (2, 3). The VRE of IFNA genes contain purine-rich GAAANN motifs that constitute the specific binding sites (IRF-E and PRDI/III) for the proteins of the interferon regulatory factor (IRF) family, whereas the VRE in the promoter of the IFNB gene contain not only PRDI/ PRDIII elements but also an NF-B (PRDII)-binding site (4).Nine cellular IRF and three viral homologues (vIRF) have been identified (4 -9). All of the cellular IRF share a region of homology in the amino terminus encompassing a highly conserved DNA binding domain (DBD) that is characterized by five tryptophan repeats (4). Three of the repeats contact DNA recognizing the GAAA or AANNGAAA sequences (10 -12). KSHV-encoded IRF that contain an imperfect DNA binding domain are not able to bind DNA with the same specificity as cellular IRF.Several of the cellular IRF were implicated in the regulation of type I IFN gene expression in virus-infected cells. IRF-1 was first identified as an activator of the IFNB gene, whereas IRF-2 antagonized the IRF-1-mediated activation and acted as a suppressor (13-15). Whereas the infected embryonic fibroblasts from IRF-1 Ϫ/Ϫ mice expressed normal levels of IFNA and -B (16, 17), it was observed that IRF-1 associates with the VRE of both human IFNA and IFNB promoters in vivo, thus suggesting that IRF-1 may contribute to the transcriptional regulation of the human IFNA genes (18,19). Three members of the IRF family (IRF-3, IRF-5, and IRF-7) were shown to be direct transducers of virus-mediated signaling and to play a crucial role in the expression of type I IFN genes (19 -26). Whereas IRF-3 is constitutively expressed in all cell types (27), constitutive expression of IRF-7 was observed only in lymphoid cells but could be induced by type I IFN in other cell types (20,28). Expression of IRF-5 was detected primarily in B cells and dendritic cells and was further enhanced by type I IFN (5, 9, 21).In the murine system, induction of type I IFN genes in virus-infected primary embryonic fibroblasts was proposed to proceed by two sequential phases. During the initial phase, which does not require protein synthesis, transcription of IFNB and IFNA4 genes was activated. The second phase, during which the rest of the IFNA subtypes were induced, depended on the IFN-mediated induction of IRF-7 expression (23, 29). However, in recently generated mice with a homozygous deletion of the IRF-7 gene, production of IFN␣ in the isolated mouse embryo fibroblasts was completely abolished, whereas the expression of IFN␤ was significantly reduced (30). These results are consistent with the observations in human cells, where expression of IRF-3 co...

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Leishmanization is the inoculation of live Leishmania into the host to vaccinate against subsequent infections. This approach has been largely discontinued due to safety concerns. We have previously shown that combining CD40 ligand (CD40L) with Leishmania antigen preferentially induces a type 1 immune response and provides some protection to vaccinated mice (G. Chen, P. A. Darrah, and D. M. Mosser, Infect. Immun. 69:3255-3263, 2001). In the present study, we developed transgenic L. major organisms which express and secrete the extracellular portion of CD40L (L. major CD40LE). We hypothesized that these organisms would be less virulent but more immunogenic than wild-type organisms and therefore be more effective at leishmanization. Transgenic parasites expressing CD40L mRNA and protein were developed. BALB/c mice infected with these parasites developed significantly smaller lesions containing fewer parasites than animals infected with wild-type organisms. Infection of resistant C57BL/6 mice with low doses of transgenic parasites induced a significant amount of protection against subsequent high-dose infection with wild-type organisms. These results demonstrate that transgenic organisms expressing CD40L are less virulent than wild-type organisms while retaining full immunogenicity.