Leukocyte-Derived High-Mobility Group Box 1 Governs Hepatic Immune Responses to Listeria monocytogenes.

High-mobility group box 1 (HMGB1) is a nucleoprotein with proinflammatory functions following cellular release during tissue damage. Moreover, antibody-mediated HMGB1 neutralization alleviates lipopolysaccharide (LPS)-induced shock, suggesting a role for HMGB1 as a superordinate therapeutic target for inflammatory and infectious diseases. Recent genetic studies have indicated cell-intrinsic functions of HMGB1 in phagocytes as critical elements of immune responses to infections, yet the role of extracellular HMGB1 signaling in this context remains elusive. We performed antibody-mediated and genetic HMGB1 deletion studies accompanied by in vitro experiments to discern context-dependent cellular sources and functions of extracellular HMGB1 during murine bloodstream infection with Listeria monocytogenes. Antibody-mediated neutralization of extracellular HMGB1 favors bacterial dissemination and hepatic inflammation in mice. Hepatocyte HMGB1, a key driver of postnecrotic inflammation in the liver, does not affect Listeria-induced inflammation or mortality. While we confirm that leukocyte HMGB1 deficiency effectuates disseminated listeriosis, we observed no evidence of dysfunctional autophagy, xenophagy, intracellular bacterial degradation, or inflammatory gene induction in primary HMGB1-deficient phagocytes or altered immune responses to LPS administration. Instead, we demonstrate that mice devoid of leukocyte HMGB1 exhibit impaired hepatic recruitment of inflammatory monocytes early during listeriosis, resulting in alterations of the transcriptional hepatic immune response and insufficient control of bacterial dissemination. Bone marrow chimera indicate that HMGB1 from both liver-resident and circulating immune cells contributes to effective pathogen control. Conclusion: Leukocyte-derived extracellular HMGB1 is a critical cofactor in the immunologic control of bloodstream listeriosis. HMGB1 neutralization strategies preclude an efficient host immune response against Listeria.

IRF4 deficiency abrogates lupus nephritis despite enhancing systemic cytokine production.

The IFN-regulatory factors IRF1, IRF3, IRF5, and IRF7 modulate processes involved in the pathogenesis of systemic lupus and lupus nephritis, but the contribution of IRF4, which has multiple roles in innate and adaptive immunity, is unknown. To determine a putative pathogenic role of IRF4 in lupus, we crossed Irf4-deficient mice with autoimmune C57BL/6-(Fas)lpr mice. IRF4 deficiency associated with increased activation of antigen-presenting cells in C57BL/6-(Fas)lpr mice, resulting in a massive increase in plasma levels of TNF and IL-12p40, suggesting that IRF4 suppresses cytokine release in these mice. Nevertheless, IRF4 deficiency completely protected these mice from glomerulonephritis and lung disease. The mice were hypogammaglobulinemic and lacked antinuclear and anti-dsDNA autoantibodies, revealing the requirement of IRF4 for the maturation of plasma cells. As a consequence, Irf4-deficient C57BL/6-(Fas)lpr mice neither developed immune complex disease nor glomerular activation of complement. In addition, lack of IRF4 impaired the maturation of Th17 effector T cells and reduced plasma levels of IL-17 and IL-21, which are cytokines known to contribute to autoimmune tissue injury. In summary, IRF4 deficiency enhances systemic inflammation and the activation of antigen-presenting cells but also prevents the maturation of plasma cells and effector T cells. Because these adaptive immune effectors are essential for the evolution of lupus nephritis, we conclude that IRF4 promotes the development of lupus nephritis despite suppressing antigen-presenting cells.

Ischemia reperfusion induces IFN regulatory factor 4 in renal dendritic cells, which suppresses postischemic inflammation and prevents acute renal failure.

Ischemia reperfusion (IR) activates TLRs causing subsequent sterile inflammation, for example in postischemic acute renal failure. Unexpectedly, TLR signaling predominates in intrinsic renal cells and not in intrarenal APCs in the postischemic kidney. We hypothesized that certain factors suppress APC activation and thereby limit sterile renal inflammation, for example, IFN regulatory factor 4 (IRF-4), an inducible inhibitor of LPS signaling. Oxidative stress was a trigger for IRF4 induction in myeloid cells in vitro as well as in CD45(+)/CD11c+ cells in the postischemic kidney. Lack of IRF4 aggravated acute renal failure 24 h after renal artery clamping together with increased intrarenal expression of TNF-alpha, IL-6, CXCL2, and CCL2 as well as excessive tubular necrosis and peritubular neutrophil influx as compared with wild-type IR kidneys. This effect almost entirely depended on the role of IRF4 to suppress TNF-alpha release by intrarenal APCs because either clodronate liposome depletion of these cells or TNF-alpha blockade with etanercept entirely abrogated the aggravation of cytokine expression and acute renal failure in Irf4-deficient mice. Thus, loss-of-function mutations in the IRF4 gene predispose to IR injury because the postischemic induction of IRF4 in resident APCs like CD11c(+) dendritic cells, suppresses them to secrete TNF-alpha, and thereby limits inappropriate immunopathology.