Analysis of thymus-independent type 2 antigen uptake by marginal zone macrophages in thin slices of viable lymphoid tissue in vitro.

There is evidence to suggest that the B cell population in the marginal zone (MZ) of the spleen is responsible for the antibody response to thymus-independent type 2 antigens (TI-2). The macrophage (M phi) population in the MZ has been shown to take up TI-2 antigens selectively, and this uptake is potentially important in understanding antigen handling in TI-2 responses. Uptake has not been studied in vitro because isolation of MZ M phi completely abrogates uptake. We have adapted a technique for cutting thin slice of viable spleen which retained this M phi function under in vitro conditions and allowed manipulation of the system. This technique may have widespread application in the study of tissue M phi which are difficult to isolate. Using this method, we show that MZ M phi in splenic slices in vitro selectively take up a TI-2 antigen, in a way apparently identical to that seen in vivo. This is not due to high pinocytic activity by these cells nor to their anatomical location. We provide evidence for a receptor-mediated uptake system, whose function is sensitive to collagenase. The ligand specificity of the uptake system showed unexpected cross-reactivity with the mannosyl-fucosyl receptor with high affinity for mannan, but was not identical to it, and may be indicative of the natural ligands for the TI-2 antigen uptake system.