The innate immune response to bacteria is essential for survival but the systemic release of inflammatory mediators results in the life-threatening septic-shock reaction. Now, reporting in Nature, Thierry Roger and co-workers show that a cytokine associated with endotoxic shock, macrophage migration inhibitory factor (MIF), regulates the innate response to Gram-negative bacteria.
MIF-deficient mice are resistant to endotoxic shock but the molecular mechanism behind this resistance has been unknown. Macrophages are the main source of MIF and produce high basal levels of it. To invesigate the effects of MIF on innate responses, mouse macrophages were stably transfected with a plasmid encoding an antisense Mif mRNA. These macrophages had impaired production of the inflammatory cytokines tumour necrosis factor α and interleukin 6 in response to lipopolysaccharide (LPS) and Gram-negative bacteria, but their responses to Gram-positive bacteria were unaffected.
Why does inhibition of MIF selectively affect responses to Gram-negative bacteria? LPS and Gram-negative bacteria are recognized by a receptor complex consisting of a signalling subunit (Toll-like receptor 4, TLR4) and two accessory proteins (MD2 and CD14). The levels of CD14 and MD2 in Mif-antisense macrophages are normal and the LPS-binding capacity of the macrophages is unaffected, but TLR4 production is markedly reduced. By contrast, levels of TLR2, a component of the receptors for fungi and Gram-positive bacteria, are normal. In agreement with a key role for MIF in responses to Gram-negative bacteria, macrophages from MIF-deficient mice had defective responses to Gram-negative but not Gram-positive bacteria.
How, then, does MIF modulate TLR4 production? Transfection of MIF-deficient macrophages with a Tlr4-promoter–luciferase reporter construct showed that the Tlr4 promoter is only 40% as active as when MIF is present. This indicates that MIF regulates TLR4 at the level of transcription. The transcription factor PU.1 is important for TLR4 expression in myeloid cells, and basal PU.1-DNA-binding activity is impaired in MIF-deficient macrophages.
Together, these results indicate that, through PU.1, MIF causes optimal TRL4 expression and hence optimal LPS responses in macrophages. MIF might, therefore, be a useful, specific target for preventing endotoxic shock in cases of Gram-negative sepsis.
ORIGINAL RESEARCH PAPER
Roger, T. et al. MIF regulates innate immune responses through modulation of Toll-like receptor 4. Nature 414, 920–924 (2001)
Medzhitov, R. Toll-like receptors in innate and adaptive immunity. Nature Rev. Immunol. 1, 135–146 (2001)
Calandra, T. et al. Protection from septic shock by neutralization of macrophage migration inhibitory factor. Nature Med. 6, 164–170 (2000)