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straightforward, since it regulates the LPS-induced expression of pro-inflammatory cytokines differently,
[60]
stimulating the production of IFN-γ, IL-1, IL-6, and IL-12, but suppressing that of TNF-α . Experiments
on knock-out mice suggested that IRF-2 can play a role in macrophage survival and inhibition of apoptosis,
[61]
regulating the expression of caspase-1 . Moreover, IRF-2 was shown to be important for the immune
[62]
response to bacterial infection . The overexpression of IRF-2 had a protective effect in a liver ischemia-
[63]
reperfusion injury model via suppression of inflammation and hepatic damage mediated by IRF-1 .
Taken together, these observations suggest that the role of IRF-2 in macrophage activation can be context-
dependent, promoting inflammation during infection, while dampening inflammation in sterile conditions.

Role of IRF/STAT signaling in M2 differentiation
Polarization of anti-inflammatory M2 macrophages is mediated by STAT-3 and STAT-6 signaling in
response to IL-4 and IL-13 with the involvement of IRF-3 and IRF-4. In monocytes, IRF-3 prevents the
pro-inflammatory polarization to M1 macrophages while IRF-4 promotes the anti-inflammatory M2a
phenotype. During M2 polarization, IRF-4 strongly cooperates with the epigenetic regulator histone
demethylase Jumonji D3 (JMJD3), which activates the expression by demethylating repressive histone H3
lysine 27 H3K27 trimethylation (H3K27me3) epigenetic marks in the promoters of many immune-related
[64]
[65]
genes . JMJD3 can activate TGF-β signaling through the SMAD3 pathway and induce M2 polarization
[66]
through STAT-6 signaling .
STAT-6
STAT-6 is involved in IL-4-induced expression of JMJD3 by binding to consensus sites of the Jmjd3
[66]
promoter . Up-regulated expression of JMJD3 reduces demethylation and trimethylation of H3K27 and
stimulates the expression of specific M2 marker genes. STAT6 is important for transcriptional regulation of
IL-4/IL-13-induced M2 activation. The receptors of both cytokines share the common α-chain (IL4Rα) that
[67]
mediates signal transduction . Interaction of either IL-4 or IL-13 with the ligand-binding receptor subunit
results in Jak1/Jak3- or Jak1/Tyk2-mediated activation of STAT-6 respectively. Phosphorylated STAT-6 forms
[68]
a transcriptionally active dimer, which in cooperation with IRF-4 conducts transcription of target genes .

Like most other STATs, STAT-6 binds to IFN-γ-activated sites, which represent a palindromic sequence
TTCNNNGAA separated by a 3-bp spacer. STAT-6 binds with a greater affinity to the sites with 4-bp spacers
[69]
like TTCNNNNGAA . In murine macrophages, STAT-6 mediates the transcription of M2-specific marker
genes such as Fizz1 (resistin-like-α; Retnla), mannose receptor 1 (CD206), Ym1 (chitinase 3-like 3; Chi3l3),
[70]
and arginase-1 and contributes to IL-4-dependent down-regulation of anti-inflammatory genes . STAT-6-
deficient mice lack IL-4-dependent Th2 responses, M2 polarization, and increased IFN-γ-STAT-1 signaling
that suggests a key role of this immunoregulator in providing IL-4-dependent stimuli and commitment of
[71]
the M2 phenotype .
STAT-3
[72]
STAT-3 mediates the effects of anti-inflammatory cytokine IL-10 on gene transcription . In macrophages,
IL-10 binding to the receptor stimulates STAT-3 recruitment via phosphorylation by Jak1. Up-regulation
of STAT-3 is followed by down-regulation of pro-inflammatory cytokines including IL-1, IL-12, TNF-α,
and IFN-γ through IL-10-dependent signaling indicating the role of STAT-3 in M1-M2 switch. Deficiency
of STAT-3 in mice is characterized by impaired antimicrobial immune responses and increased release
[73]
of pro-inflammatory cytokines and IL-10 in response to stimulation by LPS suggesting an important
role of STAT-3 in IL-10-mediated inhibition of inflammation. However, STAT-3 can provide either
anti-inflammatory or pro-inflammatory responses in a signal-dependent manner. In severe combined
immunodeficient mice, IL-10 produced by transplanted regulatory T cells induces STAT-3-mediated M2
[72]
polarization in macrophages, while activation with IL-6 and IFN-β is pro-inflammatory .
STAT-3 is negatively regulated by SOCS3, which can be induced by pro-inflammatory mediators in order