A lectin model for unresponsiveness. I. Reversible effects of high doses of concanavalin A on spontaneous and induced DNA synthesis.

A mitogen model for unresponsiveness has been studied. In 72-hr cultures the lowest supraoptimal dose of Concanavalin (Con A) showing no stimulation of rat thymocytes above background was 160 mug/ml. This dose was also found to inhibit substantially (50%) the high spontaneous DNA synthesis by thymocytes during the first 24 hr of culture. This effect was most significant 12 to 24 hr after addition of high-dose Con A. It was not necessary for the cells to remain continuously with high-dose Con A, since washing out unbound Con A after 1 hr still led to reduced 3-H-thymidine incorporation at 24 or 72 hr. Furthermore, high-dose Con A treatment suppressed DNA synthesis when added at 48 hr to cultured optimally stimulated with Con A. Interaction with high-dose Con A appears to inhibit the ability of cells to initiate new rounds of DNA synthesis without blocking ongoing DNA replication. Removal of Con A with alpha-methyl-D-mannopyranoside (mannose) led to only partial reversal of these effects after 1 hr although "unresponsiveness" was reversible at 15 min. Suppression was not due to cytotoxic or agglutinating effects of the high-dose Con A. Two times 10-6 thymocytes exposed to 160 mug of Con A for 1 hr bound 1.7 mug, 86% of which was elutable with mannose. There was no reduction in the ability of high-dose treated cells to bind a subsequent optimal dose of Con A. Since Con A binds to essentially all thymocytes, this system is useful in following early events in the induction of lectin "unresponsiveness" as a model for immunologic tolerance.