Macrophages and the kidney.

Macrophage infiltration is a hallmark of all forms of inflammatory and non-inflammatory renal injury. However, the classical view of macrophages as cells that cause injury has been superseded with evidence of their heterogeneous role, i.e. with involvement in all stages of the inflammatory process including tissue repair and healing. This review summarizes the major advances in macrophage biology achieved in the last year, highlighting the different activation states, how these are regulated, and their relevance in renal disease. New concepts have emerged concerning the factors controlling monocyte recruitment into inflamed tissue and their subsequent differentiation into activated macrophages. There is now compelling evidence for the heterogeneity of macrophages in clinical disease, i.e. they appear to be able to both promote and downregulate inflammation. An increased understanding of the factors regulating the expression of pro-inflammatory or reparative characteristics by macrophages is establishing how their function can be manipulated to attenuate renal inflammation in experimental models. An understanding of the role of macrophages at different time-points in renal inflammation, and the development of techniques for modulating macrophage activation in vivo, will provide a powerful method for exploiting the reparative attributes of these cells in clinical settings, restoring regulation to the inflammatory process and promoting healing.