Interleukin-34

Interleukins (from Latin/Greek inter = between; leukos = white; kinein = to move) are a group of endogenous, short-chain regulatory proteins (cytokines) of the immune system (IL1-IL38). Interleukins are mediators for induction, course and control of T-cell-mediated cytotoxic immune reactions as well as B-cell activation (antibody production). They are mainly formed and secreted by stimulated leukocytes, monocytes and macrophages. So far, about 38 different interleukins have been clearly identified. Each cytokine of the interleukin group is nomenclatically assigned a number for its classification (IL-1 to IL-38; status 2017).

Interleukin 34 (IL-34) is a 38kDa protein composed of 234 amino acids.

Interleukin-34 was discovered as the second ligand for the "colony stimulating factor 1 receptor" (CSF1R). The cytokine thus acts as a growth factor for immune cells such as monocytes.
In humans, IL-34 mRNA is expressed in numerous organs such as thymus, liver, small intestine, colon, prostate, lung, heart, CNS, kidney, testes and ovaries.

In humans, the infiltrating cells of the lamina propria and intestinal epithelia express IL-34 and TNF-alpha in IBDs (inflammatory bowel diseases). This supports the assumption that IL-34 is a modulator of inflammation in Crohn's disease and ulcerative colitis.

IL-34 also plays a pathogenetic role in rheumatoid arthritis by stimulating proliferation and activity of osteoclasts. The cytokine is also important for the differentiation of macrophages and Langerhans cells.

1. Masteller EL et al (2014) Targeting IL-34 in chronic inflammation. Drug Discov Today 19:1212-1216.
2. Nakamichi Y et al. (2013) IL-34 and CSF-1: similarities and differences. J Bone Miner Metab 31:486-495.
3. Wang Y et al (2014) Interkeukin-34, a cytokine crucial for the differentiation and maintenance of tissue resident macrophages and Langerhans cells. Eur J Immunol 44:1575-1581.
4. Zwicker S et al.(2015) Interleukin 34: a new modulator of human and experimental inflammatory bowel disease.Clin Sci (London) 129:281-290.