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Leishmanial Amastigote Antigen P‐2 Induces Major Histocompatibility Complex Class II‐Dependent Natural Killer‐Cell Reactivity in Cells from Healthy Donors

Innate mechanisms involving natural killer cells have been implied to play an important role in immunity against Leishmania infection. Previous studies have evaluated responses to three purified amastigote antigens, P‐2, P‐4 and P‐8, of Leishmania pifanoi. The P‐4 and P‐8 antigens have been demonstrated to induce protection in mouse models, as well as to induce cellular responses in American cutaneous leishmaniasis patients. Cells from Leishmania aethiopica‐infected leishmaniasis patients preferentially responded to P‐8 and, to a lesser extent, to the cysteine proteinase, P‐2. In this study, it is shown that cells from healthy donors, including cells from truly naïve donors (cord blood), could be stimulated to proliferation and cytokine production by P‐2. The main proliferating cell types in healthy adult donors were CD16/56+ and the CD8+ cells. Blocking of major histocompatibility complex (MHC) class II with α‐MHC class II antibodies markedly inhibited proliferation and interferon‐γ (IFN‐γ) production, whereas interleukin‐10 production was not affected. Experimental evidence indicates that CD4+ cells were not necessary for the proliferative and IFN‐γ responses; however, an adherent cell population was required. Furthermore, CD16/56+ cells expressing MHC class II were expanded following P‐2 stimulation. The responses to P‐2 show a striking similarity to responses induced by the vaccine candidate Leishmania homologue of receptors for activated C‐kinase (LACK) in healthy donors. The responses described here may not be desirable when aiming at inducing protective immune responses with a vaccine, and the implications of these results for the development of vaccines against leishmaniasis are discussed.