Effect of lipopolysaccharide on the morphology and integrin immunoreactivity of ramified microglia in the mouse brain and in cell culture.
Microglial cells form the first line of defense in brain infection. They are related to monocytes and macrophages and can be readily activated by cell wall components of bacteria such as lipopolysaccharides (LPS). In the present study, we explored the effect of this endotoxin in mouse on the morphology of microglia and their immunoreactivity for the integrin family of cell adhesion molecules in vitro and in vivo. Subcutaneous injection of LPS led to a dose-dependent activation of alpha M beta 2-positive microglia, with a saturating effect at 1 microg LPS in the blood-brain barrier deficient area postrema, at 10 microg in the directly adjacent tissue, and at 100 microg throughout the brainstem and cerebellum. Morphologically, this activation was characterized by the swelling of the microglial cell body, a thickening of the proximal processes, and a reduction in distal ramification. Microglial immunoreactivity for the integrins alpha 4 beta 1, alpha 5 beta 1, alpha 6 beta 1, and alpha M beta 2 was strongly increased. In vitro, ramified microglia were obtained using a coculture on top of a confluent astrocyte monolayer. Two days exposure to LPS resulted in a morphological activation of the cultured cells with an increase of the integrin immunoreactivity for alpha 5 (5.7-fold), alpha 4 (3.1-fold), beta 1 (2.3-fold), and alpha M (1.5-fold), and a decrease in the alpha 6-staining intensity by 39%. Even a sublethal dose of LPS (3 mg in vivo and 500 microg/ml in vitro, respectively) did not induce the phagocyte-associated integrin alpha X beta 2 (CD11c/CD18, p150,95) and did not lead to a morphological transformation of the ramified microglia into phagocytes.