Absence of Tumor Necrosis Factor Supports Alternative Activation of Macrophages in the Liver after Infection with Leishmania major

TLDR

Lack of TNF renders normally resistant mice susceptible to lethal Leishmania major infection, yet the underlying mechanism remained unclear. TNF‑deficient mice develop severe hepatic infection with an expanding M2‑polarized macrophage population driven by IL‑6, and TNF counteracts IL‑6‑mediated STAT3/STAT6 signaling, underscoring TNF’s role in macrophage inflammatory activation.

Abstract

The absence of tumor necrosis factor (TNF) causes lethal infection by Leishmania major (L. major) in normally resistant C57BL/6J (B6.WT) mice. The underlying pathogenic mechanism of this fatal disease has so far remained elusive. We found that B6.WT mice deficient for the tnf gene (B6.TNF-/-) displayed not only a non-healing cutaneous lesion but also a serious infection of the liver upon L. major inoculation. Infected B6.TNF-/- mice developed an enlarged liver that showed increased inflammation. Furthermore, we detected an accumulating monocyte-derived macrophage population (CD45+F4/80+CD11bhiLy6Clow) that displayed a M2 macrophage phenotype with high expression of CD206, Arginase-1, and IL-6. supporting the notion that IL-6 could be involved in M2 differentiation. In in vitro experiments we demonstrated that IL-6 upregulated M-CSF receptor expression and skewed monocyte differentiation from dendritic cells to macrophages. This was countered by the addition of TNF. Furthermore, TNF interfered with the activation of IL-6-induced gp130-STAT3 and IL-4-STAT6 signaling, thereby abrogating IL-6-facilitated M2 macrophage polarization. Therefore, our results support the notion of a general role of TNF in the inflammatory activation of macrophages and define a new role of IL-6 signaling in macrophage polarization downstream of TNF.