The inoculation of live Leishmania major to produce a lesion that heals (leishmanization) is to date the only vaccine against cutaneous leishmaniasis that has proven effective in humans, but it still has an unacceptable frequency of large ulcerating lesions that are slow to heal or, in rare cases, non-healing. We have previously shown that C57BL/6 mice vaccinated intradermally with 10 4 L. major/50 lg CpG oligodeoxynucleotides develop little or no dermal lesions and show early containment of parasite growth in the vaccination site, eliminating safety concerns related to the inoculation of live organisms. The addition of CpG to the live vaccine resulted in early activation of dermal dendritic cells and increased IL-6 production, as well as in a reduction in the accumulation of Foxp3 + CD4 + CD25 + regulatory T (T reg ) cells that naturally occurs in the skin following Leishmania infection. Neutralization of IL-6 caused the development of larger lesions and increased local T reg cell numbers. Transfer of vaccine-primed dendritic cells into IL-6-deficient mice mitigated lesion development, indicating that IL-6 reconstitution limited pathology in the vaccination site.
We have previously reported that vaccination with CpG oligodeoxynucleotides delivered concomitantly with live Leishmania major (Lm/CpG) eliminates lesions associated with live vaccination in C57BL/6 mice. The absence of lesions is at least in part a result of the CpG DNA-mediated activation of dermal dendritic cells to produce cytokines such as interleukin (IL)-6. Wild-type C57BL/6 mice and IL-6−/− mice were immunized with the Lm/CpG vaccine and monitored for the development of lesions. IL-6−/− mice developed extensive, nonhealing lesions following live vaccination. The analysis of the inoculation site and draining lymph nodes of the IL-6−/− mice revealed a constitutive reduction in lymphocyte numbers, particularly CD4+ T cells. Live vaccination resulted in the specific expansion of CD4+Foxp3+ regulatory T cells in the knockout mice, and in a decrease of CD4+ IFN-γ -producing cells. These results indicate that IL-6−/− mice may have collateral immune defects that could influence the development of the natural immune response to pathogens, vaccines, or other inflammatory stimuli.
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