Phagocytosis and lymphocyte migration: evidence that lymphocyte trapping in the spleen following carbon injection is not due to direct lymphocyte-macrophage adherence.

The localization of intravenously injected labelled syngeneic lymphoid cells was studied in the spleen of mice and compared with the localization of Indian-ink-containing macrophages. To distinguish between Indian-ink-containing lysosomes of macrophages and silver grains formed in the autoradiographs over the radiolabelled cells, the latter grains were stained blue by a colour-coupling process. Labelled cells were injected 2 h after the Indian ink. Two hours after their injection the bulk of the labelled cells in the spleen was already localized in the white pulp. At this time the Indian ink had been ingested by macrophages in the marginal zone and to a somewhat lesser extent in the red pulp. Twenty-four hours after injection of the cells their concentration in the white pulp appeared constant or had decreased markedly, dependent on the source of the injected cells (spleen, lymph nodes or thymus). At this time carbon-containing macrophages were also found in the white pulp although they contained less carbon than macrophages in the marginal zone and red pulp. A positive correlation between labelled cells and carbon-containing macrophages was never seen in any part of the spleen. It is concluded that, if carbon-containing macrophages induce lymphocyte trapping, as has been supposed by other authors, this trapping must be mediated by the macrophages in an indirect way, e.g. by soluble mediator molecules released in the circulation.