Localization in the rat spleen of carbon-laden macrophages introduced into the splenic artery: a subpopulation of macrophages entering the white pulp

Heavily carbon-laden (HC) macrophages, largely derived from the red pulp of the donor spleen, were injected into the splenic artery of recipient rats. Immediately after injection, HC macrophages were found only in the marginal sinus and in the splenic cords. With time after injection, they appeared successively at the periphery of the white pulp, in the deeper white pulp, and finally in and near the germinal centers, suggesting migration of HC macrophages from the marginal sinus towards the germinal centers. The number of HC macrophages in and near the germinal centers reached a peak at 12 h. Most of the HC macrophages in the white pulp were spherical or ovoid in shape with a diameter of 7-11 microns in sections, having an eccentric round or oval nucleus often with a distinct nucleolus and a cap-like or horseshoe-like cytoplasm filled with carbon. When immunostained with monoclonal antibodies against rat macrophage subpopulations, more than 90% of HC macrophages in the white pulp were found to be ED1+2-3-. A population of the same type of macrophages, both in morphology and phenotype, were found in the red pulp of the donor spleen. They were different from the major residents, red pulp scavenger macrophages, which were ED1+2+3- and larger in size and irregular in shape. These results suggest the presence of a distinct subpopulation of macrophages which actively migrate into the splenic white pulp including the germinal centers. A discharge of transferred macrophages from the red pulp to the general circulation is also suggested.