Induction of IL-4R alpha-dependent microRNAs identifies PI3K/Akt signaling as essential for IL-4-driven murine macrophage proliferation in vivo

Macrophage (M Phi) activation must be tightly controlled to preclude overzealous responses that cause self-damage. MicroRNAs promote classical M Phi activation by blocking antiinflammatory signals and transcription factors but also can prevent excessive TLR signaling. In contrast, the microRNA profile associated with alternatively activated M Phi and their role in regulating wound healing or anti-helminthic responses has not been described. By using an in vivo model of alternative activation in which adult Brugia malayi nematodes are implanted surgically in the peritoneal cavity of mice, we identified differential expression of miR-125b-5p, miR-146a-5p, miR-199b-5p, and miR-378-3p in helminth-induced M Phi. In vitro experiments demonstrated that miR-378-3p was specifically induced by IL-4 and revealed the IL-4-receptor/PI3K/Akt-signaling pathway as a target. Chemical inhibition of this pathway showed that intact Akt signaling is an important enhancement factor for alternative activation in vitro and in vivo and is essential for IL-4-driven M Phi proliferation in vivo. Thus, identification of miR-378-3p as an IL-4R alpha-induced microRNA led to the discovery that Akt regulates the newly discovered mechanism of IL-4-driven macrophage proliferation. Together, the data suggest that negative regulation of Akt signaling via microRNAs might play a central role in limiting M Phi expansion and alternative activation during type 2 inflammatory settings.