Leishmania promastigotes evade interleukin 12 (IL-12) induction by macrophages and stimulate a broad range of cytokines from CD4+ T cells during initiation of infection.

Abstract

Leishmania major are intramacrophage parasites whose eradication requires the induction of T helper 1 (Th1) effector cells capable of activating macrophages to a microbicidal state. Interleukin 12 (IL-12) has been recently identified as a macrophage-derived cytokine capable of mediating Th1 effector cell development, and of markedly enhancing interferon gamma (IFN-gamma) production by T cells and natural killer cells. Infection of macrophages in vitro by promastigotes of L. major caused no induction of IL-12 p40 transcripts, whereas stimulation using heat-killed Listeria or bacterial lipopolysaccharide induced readily detectable IL-12 mRNA. Using a competitor construct to quantitate a number of transcripts, a kinetic analysis of cytokine induction during the first few days of infection by L. major was performed. All strains of mice examined, including susceptible BALB/c and resistant C57BL/6, B10.D2, and C3H/HeN, had the appearance of a CD4+ population in the draining lymph nodes that contained transcripts for IL-2, IL-4, and IFN-gamma (and in some cases, IL-10) that peaked 4 d after infection. In resistant mice, the transcripts for IL-2, IL-4, and IL-10 were subsequently downregulated, whereas in susceptible BALB/c mice, these transcripts were only slightly decreased, and IL-4 continued to be reexpressed at high levels. IL-12 transcripts were first detected in vivo by 7 d after infection, consistent with induction by intracellular amastigotes. Challenge of macrophages in vitro confirmed that amastigotes, in contrast to promastigotes, induced IL-12 p40 mRNA. Reexamination of the cytokine mRNA at 4 d revealed expression of IL-13 in all strains analyzed, suggesting that IL-2 and IL-13 may mediate the IL-12-independent production of IFN-gamma during the first days after infection. Leishmania have evolved to avoid inducing IL-12 from host macrophages during transmission from the insect vector, and cause a striking induction of mRNAs for IL-2, IL-4, IL-10, and IL-13 in CD4+ T cells. Each of these activities may favor survival of the organism.

Transcriptional Inhibition of Interleukin-12 Promoter Activity in Leishmania Spp.–Infected Macrophages

“…2) and IFN-γ + CD40L (data not shown). It previously has been shown that L. major amastigotes induce IL-12 production in murine MP (Reiner et al, 1994;Piedrafita et al, 1999) and L. major log-phase (procyclic) promastigotes induce IL-12p40 in human PBMC (Sartori et al, 1996). In our studies, L. major metacyclic and log-phase promastigote, as well as amastigote infection, inhibited luciferase production (Fig.…”

“…Interestingly, it previously has been reported that L. major lesion-derived amastigotes (Reiner et al, 1994;Piedrafita et al, 1999) and log-phase promastigotes (Sartori et al, 1997) stimulate IL-12p40 production in murine MP and human PBMC, respectively. We were unable to confirm this observation using the RAW 264.7 MP cell line, as procyclic promastigotes or amastigotes did not induce any luciferase expression in the presence or absence of LPS stimulation.…”

“…life-cycle stages in the inhibitory process. Reiner et al (1994) were the first to report that Leishmania major amastigotes stimulate, rather than inhibit, IL-12 production in murine bone marrow-derived MP (BMDM), an observation that was confirmed in the murine MP cell line J774 (Piedrafita et al, 1999 (Carrera et al, 1996).…”

“…life-cycle stages in the inhibitory process. Reiner et al (1994) were the first to report that Leishmania major amastigotes stimulate, rather than inhibit, IL-12 production in murine bone marrow-derived MP (BMDM), an observation that was confirmed in the murine MP cell line J774 (Piedrafita et al, 1999 (Carrera et al, 1996).The fact that Leishmania spp.-conditioned medium is able to inhibit SAC-induced IL-12 production in human PBMC (Sartori et al, 1997) suggests that soluble parasite components mediate IL-12 inhibition. Specifically, both purified and synthetic phosphoglycans (PG) have been reported to inhibit IL-12 production in murine MP (Piedrafita et al, 1999).…”

Mechanisms of Immune Evasion in Leishmaniasis

“…Pro-inflammatory cytokines (such as IL-12), which are essential for the generation of a successful immune response against Leishmania , are generally suppressed, facilitating a ‘silent entry’ of the parasite into host macrophages (Belkaid et al, 2000; McDowell & Sacks, 1999; Reiner, Zheng, Wang, Stowring, & Locksley, 1994; Weinheber, Wolfram, Harbecke, & Aebischer, 1998). Conversely, IL-10 which is essential for parasite survival and disease progression is induced by Leishmania in infected monocytes and macrophages (Chandra & Naik, 2008; Meddeb-Garnaoui, Zrelli, & Dellagi, 2009).…”