Concanavalin A-mediated attachment and ingestion of yeast cells by macrophages

Abstract

The ConA-mediated interaction of yeast cells with macrophages was brought about in two steps. The first step involved the interaction of either macrophages or yeast cells with ConA, MConA or YConA in brief, respectively. The second step consisted of interacting the ConA-coated cells with their non-coated counterpart, yielding MConA-Y or M-YConA. The extent of yeast cell attachment to macrophages depends on the degree of saturation of ConA binding sites on the cell coated with ConA in the first step and on the temperature at which the two cell types interact. The temperature dependence in the range of 10–25 °C implies that cell-cell attachment is sensitive to the physical state of membrane lipids as reflected in increased lateral mobility of ConA receptors in the membrane plane. The extent of ConA-mediated cell association is not influenced significantly by colchicine, cytochalasin B (CB) or hydrocortisone. A mild treatment of macrophages with glutaraldehyde reduces, however, the association of yeast cells, further indicating a need for lateral mobility of ConA receptors. ConA-mediated yeast cell attachment could be totally reversed by α-methyl mannoside in the case of MConA-Y and only partially in the case of M-YConA. Yeast cell ingestion is highly temperature-dependent; in MConA-Y a 50% interiorization of the associated yeast cells is reached at 32 °C and detectable interiorization starts only above 19 °C, while in M-YConA a 50% value of interiorization is reached at 18 °C and about 15% of yeast cells are interiorized already at 5 °C. Interiorization of attached yeast cells is not affected by colchicine. Cytochalasin B (CB) (10 μg/ml) inhibits 82% of yeast interiorization in MConA-Y and only 12% in M-YConA. Hydrocortisone has a similar differential effect of inhibition of ingestion; at 25 °C inhibition in MConA-Y amounts to 78% and in M-YConA to 22%. Sodium azide inhibits 90% of interiorization of yeast cells in both MConA-Y and M-YConA. The following working hypothesis was proposed to explain both the characteristics of attachment and the remarkable difference in ingestion pattern of yeast cells in MConA-Y and M-YConA. ConA-mediated yeast cell attachment to macrophages involves multipoint interaction between the two cells achieved by a certain clustering of ConA receptors in the membrane plane. To achieve interiorization a higher extent of bridge formation between the cells is required, and a higher number of ConA-membrane receptors have to be recruited to the area of apposition of the two membranes. This requires lateral mobility of either ConA receptors conjugates (in the case of MConA) or of mobile non-crosslinked ConA receptors in macrophages interacting with YConA). Mobility of ConA receptor conjugates is more sensitive to membrane fluidity than that of non-crosslinked receptors and hence the differential temperature-dependence of ingestion. The effect of CB suggests an involvement of the cytoskeleton in the reorganization of ConA receptors at the membrane level.