Leishmaniasis affects 12 million people, but there are no vaccines. Immunological correlates of vaccine efficacy are unclear. Polarized Th1 vs Th2 responses in Leishmania major-infected mice suggested that a shift in balance from IL-4 to IFN-γ was the key to vaccine success. Recently, a role for IL-10 and regulatory T cells in parasite persistence was demonstrated, prompting re-evaluation of vaccine-induced immunity. We compared DNA/modified vaccinia virus Ankara heterologous prime-boost with Leishmania homolog of the receptor for activated C kinase (LACK) or tryparedoxin peroxidase (TRYP). Both induced low IL-4 and high IFN-γ prechallenge. Strikingly, high prechallenge CD4 T cell-derived IL-10 predicted vaccine failure using LACK, whereas low IL-10 predicted protection with TRYP. The ratio of IFN-γ:IL-10 was thus a clear prechallenge indicator of vaccine success. Challenge infection caused further polarization to high IL-10/low IFN-γ with LACK and low IL-10/high IFN-γ with TRYP. Ex vivo quantitative RT-PCR and in vitro depletion and suppression experiments demonstrated that Ag-driven CD4+CD25+ T regulatory 1-like cells were the primary source of IL-10 in LACK-vaccinated mice. Anti-IL-10R treatment in vivo demonstrated that IL-10 was functional in determining vaccine failure, rendering LACK protective in the presence of high IFN-γ/low IL-5 responses.

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Successful resolution of infections by intracellular pathogens requires gamma interferon (IFN-␥

Leishmaniasis affects 12 million people, but there are no vaccines in routine clinical use. Th1 polarizing vaccines that elicit long-term protection are required to prevent disease in susceptible populations. We recently showed that heterologous priming-boosting with tryparedoxin peroxidase (TRYP) DNA followed by TRYPmodified vaccinia virus Ankara (TRYP MVA) protected susceptible BALB/c mice from Leishmania major. Here we compared treatment with TRYP DNA with treatment with TRYP DNA/TRYP MVA. We found that equivalent levels of protection during the postvaccination effector phase correlated with equivalent levels of serum immunoglobulin G2a and gamma interferon (IFN-␥) in draining lymph nodes. In contrast, challenge infection during the memory phase revealed that there was enhanced clinical efficacy with TRYP DNA/TRYP MVA. This correlated with higher levels of effector phase splenic IFN-␥, sustained prechallenge levels of memory phase IFN-␥, and a more polarized post-L. major challenge Th1 response compared to the Th2/T reg response. Thus, TRYP DNA/TRYP MVA, but not TRYP DNA alone, provides long-term protection against murine leishmaniasis.Leishmaniasis is caused by intracellular protozoan parasites transmitted by the bite of a sand fly, and the diverse clinical manifestations range from localized cutaneous lesions to fatal visceral infection. The disease prevalence is estimated to be 12 million people, and there are 1.5 million new cases annually. There are no vaccines in routine use. Experimental infections of inbred mice with Leishmania major defined the Th1/Th2 paradigm (for reviews, see references 16 and 26) and demonstrated that primary immunity to L. major in resistant mice requires the development of a polarized Th1 response (11,31,32). In contrast, susceptibility in BALB/c mice was associated with an aberrant Th2 response resulting from the early production of interleukin-4 (IL-4) by a restricted population of V␤4V␣8 CD4 ϩ T cells (12, 13). These studies supported the hypothesis that immunotherapy shifting the balance from IL-4 to gamma interferon (IFN-␥) would provide the key to vaccine success. The challenge for developing a vaccine against Leishmania spp., like the challenge for developing vaccines against other intracellular pathogens, such as Mycobacterium tuberculosis, is thought to be induction and maintenance of a cellmediated immune response that produces IFN-␥ to activate macrophages to kill the pathogen.The vaccination strategies employed to induce protective immunity in experimental models of leishmaniasis have included vaccination with recombinant Leishmania antigens, such as a Leishmania homologue of the receptor for activated C kinase (LACK) plus IL-12 as an adjuvant, vaccination with live attenuated parasites, vaccination with plasmid DNA encoding single or multiple parasite antigens, and vaccination with live recombinant vectors, such as Salmonella spp., Mycobacterium bovis BCG, or vaccinia virus (for a review, see reference 19). While all of these studies have resulted in some degree of...