Enhancement of human MLR by very low concentrations of lipopolysaccharide and blocking of this enhancement by polymyxin B.

Addition of 50 micrograms LPS/ml to as little as 5 pg of LPS/ml to the allogeneic human mixed leukocyte reaction (MLR) enhanced [3H]thymidine incorporation 2-4-fold. The same concentrations of LPS had no effect on thymidine incorporation in cultures without the allogeneic cells. The Limulus amebocyte lysate (LAL) test we used was sensitive only to 15 pg LPS/ml indicating that the MLR is more sensitive to LPS than the LAL test. A commercially available RPMI 1640 culture medium used by many investigators was found to contain concentrations of LPS that would significantly enhance the MLR. Polymyxin B blocked the MLR-enhancing effect of LPS while having no inhibitory or stimulating effect itself. We concluded that low concentrations of LPS are helpful in revealing MLR incompatibilities by enhancing the lymphocyte responses and that the addition of polymyxin B is essential in the study of factors affecting the MLR by eliminating any LPS contribution.