Regulation of in vitro lymphocyte responses: I. Adjuvant effect of lipopolysaccharide (LPS) on low-zone unresponsiveness to concanavalin A

Simultaneous addition of concanavalin A (Con A) and lipopolysaccharide (LPS) to cultures of rat spleen lymphocytes resulted in a synergistic effect on DNA synthesis as measured by increased ³H]thymidine uptake after 3 days. This effect was maximal when 10 μg of LPS was added to understimulating doses of Con A (synergistic index = 14) and diminished with increasing doses of the mitogen. In contrast to increasing concentrations of serum factors, LPS was not able to unblock the nonresponse of lymphocytes stimulated with supraoptimal doses of Con A. LPS did not exert its adjuvant effect by stimulating lymphocytes with the help of soluble factors released by Con A-activated cells. Both Con A and LPS seem rather to act together on a distinct population of T-cells which can be separated on nylon columns and respond twice as much as nonseparated cells to their synergistic combination. Rat B-cells were unresponsive to stimulation with Con A and LPS added alone or simultaneously. These results help to better understand some of the mechanisms involved in the immunological enhancement observed with LPS.