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Cell-specific role of Hfe in Salmonella infection
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Figure 6. Elevated iron levels correlate with reduced IFN-g production and increased bacterial numbers in the serum. Serum complement factor C3 (A), IL-6 (B) and IFN-γ (C) concentrations were analyzed in AlfpCre+ Hfefl/fl mice infected for 72 hours with Salmonella. n=19-20 for AlfpCre– Hfefl/fl, n=8-14 for AlfpCre+ Hfefl/fl. sIndependently, wild-type (WT) mice were fed an iron-adequate (IA) or iron-enriched diet (IO) 3 weeks prior to and during S. enterica Typhimurium (S. Tm.) infection. Serum bacterial load (D), IL-6 (E) and IFN-g (F) concentrations were determined. Statistically significant differences as calculated by Mann-Whitney test are indicated. n= 8-9 for IA, n=8 for IO.
IFN-g detected in the spleen and serum of infected AlfpCre+ Hfefl/fl mice and of WT mice maintained on a high iron diet may offer a partial explanation. Unlike AlfpCre+ Hfefl/fl mice in steady state,25 we herein exclusively report on the setting of Salmonella infection and observed that Salmonella-infect- ed AlfpCre+ Hfefl/fl mice show normal iron content in the spleen and bone marrow macrophages, suggesting that high serum iron levels impair IFN-g production and its anti- microbial activity as shown in vitro and in vivo.32,33
To a large extent, host defense against intracellular microbes relies on direct antimicrobial effector functions of macrophages.28,34 Reactive nitrogen (RNS) and oxygen species, generated by Nos2 and phagocyte oxidase (phox), interfere with bacterial metabolism and exert toxic effects to limit Salmonella replication within macrophages and counteract systemic spread in infected mice.35-38 TNF and IFN-g have partly overlapping functions in the sense that both of them stimulate the expression of Nos2 and the assembly of phox subunits in Salmonella-infected macrophages.36 IL-6 in contrast, is the major cytokine inducer of the acute-phase reaction and centrally involved in the adaptation of iron homeostasis upon inflammatory stress.39 In the setting of Salmonella infection, IL-6 fine- tunes myeloid cell functions as it promotes bacterial killing but is also associated with alternative macrophage activation.40
Given unaltered bacterial loads in the mononuclear phagocyte system, the high numbers of bacteria found in the serum of AlfpCre+ Hfefl/fl mice may result from enhanced extracellular proliferation rather than differen- tial phagocytosis, which is supported by the enhanced bacterial growth we observed in AlfpCre+ Hfefl/fl serum- spiked medium. Hfe-/- mice, in contrast, have reduced numbers of Salmonella in the serum, which may in part be attributable to increased serum Lcn2 concentrations (Figure 3), which are already present in the absence of infection.13,41 Our findings also support the concept that after invasion of myeloid cells, Salmonella has limited access to serum and hepatocellular iron pools. Rather, Salmonella may use intramacrophage iron sources such as ferritin.26
Iron metabolism and immune function have multiple interconnections including effects of iron availability on immune cell differentiation as well as direct effects of iron on cytokine formation and innate immune responses.42,43 In addition, iron genes and their products modulate the body’s response to inflammation.44 Here we extend these observations by demonstrating that the expression of the antimicrobial enzyme Nos2 is partially affected by Hfe: Nos2 expression in the spleen was highest in Hfe-/- mice, moderately increased in the setting of myeloid-specific Hfe deficiency and markedly reduced in mice with hepa-
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