LeIF, a gene homologue of the eukaryotic initiation factor 4A was first described as a leishmanial antigen that induced a Th1-type T cell response in peripheral blood mononuclear cells (PBMC) from leishmaniasis patients. Moreover, the interferon (IFN)-gamma production by PBMC was found to be interleukin (IL)-12 dependent. Herein, we characterize the effects of LeIF on cytokine production and expression of surface molecules by normal human monocytes as well as by monocyte-derived macrophages and dendritic cells (MoDC). LeIF was a strong inducer of IL-12 and, to a lesser extent, of IL-10 and tumor necrosis factor (TNF)-alpha in macrophages and MoDC. IL-12 production did not require CD40 triggering, confirming that the ability of LeIF to induce IL-12 was not mediated through an effect on T cells. However, addition of soluble CD40 ligand (L) synergistically augmented IL-12 production in macrophages and MoDC. The cytokine-inducing activity of LeIF is located in the N-terminal portion of the molecule and was both proteinase K sensitive and polymyxin B resistant. LeIF, lipopolysaccharide and fixed Staphylococcus aureus all induced comparable amounts of IL-12, validating the potent cytokine-inducing effects of LeIF. Moreover, of these stimuli, LeIF had the highest IL-12/IL-10 and IL-12/TNF-alpha ratio demonstrating the preference of LeIF for IL-12 induction. Studies investigating the expression of surface molecules showed that LeIF up-regulated B7-1 and CD54 (ICAM-1) on macrophages and MoDC. To our knowledge this is the first report describing IL-12 production, up-regulation of co-stimulatory and intercellular adhesion molecules by monocytic antigen-presenting cells in response to a protein from a pathogenic microorganism. These immunomodulatory characteristics of LeIF might be excellent properties for a Th1-type adjuvant.

Previously we showed that His-tagged, recombinant, Leishmania infantum eukaryotic initiation factor (LeIF) was both an RNA-dependent ATPase and an ATP-dependent RNA helicase in vitro, as described for other members of the DEAD-box helicase family. In addition, we showed that LeIF induces the production of IL-12, IL-10, and TNF-α by human monocytes. This study aims to characterize the cytokine-inducing activity in human monocytes of several proteins belonging to the DEAD-box family from mammals and yeast. All tested proteins contained the 11 conserved motifs (Q, I, Ia, GG Ib, II, III, IV, QxxR, V and VI) characteristic of DEAD-box proteins, but they have different biological functions and different percentages of identities with LeIF. We show that these mammalian or yeast recombinant proteins also are able to induce IL-12, IL-10 and TNF-α secretion by monocytes of healthy human subjects. This cytokine-inducing activity is proteinase K sensitive and polymyxin B resistant. Our results show that the induction of cytokines in human monocytes is not unique to the protein LeIF of Leishmania, and it suggests that the activity of certain DEAD-box proteins can be exploited as adjuvant and/or to direct immune responses towards a Th1 profile in vaccination or immunotherapy protocols.