Interleukin 34 (IL-34) is a four helical homodimeric protein that does not belong to any already identified cytokine family. It is expressed by several different tissues upon inflammatory stimuli. Notably, IL-34 shares structural homology with M-CSF and signals via the colony stimulating factor (CSF) receptor 1, and induces myeloid cell expansion and differentiation, similarly to other CSF (G-CSF, M-CSF, and GM-CSF). IL-34 promotes survival of monocytes and proliferation of macrophages, and plays a hematopoietic differentiation role in the bone marrow, inducing colony forming units of macrophages and promoting osteoclast differentiation, adhesion, and proliferation. A role of IL-34 in the development of myeloid compartments, especially dendritic cells and monocyte, but also Langerhans cells and microglia, is under investigation. Mouse IL-34 shares 71% protein sequence identity with human IL-34.
