Low CD40 Expression Levels in Leishmania Infantum-Infected Bone Marrow Dendritic Cells Evoke Regulatory Responses by Down-Regulating Interleukin-12 Production: Role of ERK1/2

Abstract

Dendritic cells (DCs) play a pivotal role in promoting resistance to leishmaniasis, both by activating CD4+ T cells and endorsing their differentiation into Th1 cells by producing interleukin (IL)-12. High level of IL-12 production, a decisive component of the DC maturation, requires not only microbial stimuli but also strong CD40-CD40L interactions. Until now, the mechanisms by which Leishmania (L.) infantum parasites affect DC functional maturation and consequently T cell polarization are not fully understood. In the present study, we investigated the response that is elicited when L. infantum promastigote-infected bone marrow-derived DCs (BM-DCs) to CD40 engagement and this way mimicking DC-T cells interactions at the early stages of infection. We found that L. infantum promastigotes-infected BM-DCs following CD40 engagement were capable of inducing significant amounts of TNF-α and IL-10, whereas IL-12 production remained unaffected compared to infected untreated cells. Interestingly, infected BM-DCs did not up-regulate CD40 surface expression. On the other hand, BM-DC stimulation with soluble Leishmania antigen (SLA) resulted not only in significant increase of co-stimulatory molecule expression but also IL-12 and IL-10 production. CD40 engagement on L. infantum-infected BM-DCs sustained ERK1/2 activation induced by the parasite alone. Inhibition of ERK1/2 activation with the use of PD98059 inhibitor prior to CD40 engagement on L. infantum-infected BM-DCs resulted in significant up-regulation of p38 MAPK phosphorylation and IL-12 production, whereas it did not affect TNF-α and IL-10 production. These findings suggest that L. infantum has evolved specific strategies to avoid efficient DC-T cell interactions by suppressing CD40 expression and consequently leading CD40 signaling pathways to ERK1/2 activation.