Fig. 1.

Homeostatic control of leucocyte accumulation at sites of inflammation. The left image shows the normal steady state whereby cell recruitment, emigration, cell death and division are stable and controlled to prevent the excessive build-up of leucocytes with a tissue. The right image shows how fibroblasts at an inflammatory site orchestrate the maintenance of a leucocytic infiltrate by producing survival factors such as type 1 interferon (IFN-1), interleukin (IL)-15 and B cell activating factor (BAFF) to prevent cell death. Chemokines such as CXCL12 and CCL21 prevent cell emigration and CXCL1/5 and IL-8 act to recruit leucocytes to the lesion. Unless there is active removal of survival factors and reordering of chemokine gradients, the net result is chronic accumulation, survival and retention of leucocytes at sites of disease.