The neuronal-like behaviour of macrophages.

Tissue-resident macrophages (TRMs) contribute to both organ physiology and pathology by communicating with the nervous system and tissue-resident cells. They have been increasingly reported to be extremely plastic and adaptable, as shown by their broad range of molecular signatures and functional roles well beyond their classical duties as phagocytes. It has recently been found that macrophages can reside in proximity to muscle spindles, which are sensory neurons sensing muscle length. Surprisingly muscle spindle macrophages express neuron-like gene expression signatures and support muscle contraction by releasing glutamate. This behaviour is more in keeping with neurons rather than immune cells. Here, the potential additional roles for muscle spindle macrophages in muscle contraction and locomotion at homeostasis and in disease states will be discussed. It will also be debated whether the use of neurotransmitters by myeloid cells as shared currency to optimise cell to cell communication, to favour neural circuit development, homeostasis and repair is an exception or could rather be the rule.