In a mixed leukocyte culture (MLC) reaction of allogenic mouse spleen cells differing for H-2K or H-2D, only a weak cytotoxic response is generated. This cytotoxic response is augmented significantly if bacterial lipopolysaccharide (LPS), 5 microgram/ml, or polyadenylic acid (poly A):polyuridylic acid (poly U), 20 microgram/ml, is present in the culture. The cytotoxic cells generated in the presence of these two agents are specific for sensitizing H-2K or H-2D antigen. Two lines of evidence suggest that these two agents exert their effect at different steps in the development of cytotoxic lymphocytes: (a) the effect of poly A:U depends on the presence of adherent cells, whereas the effect of LPS is independent of the presence of adherent cells and (b) LPS promotes the development of cytotoxic cells when ultraviolet light-treated stimulating cells are used in the MLC whereas poly A:U does not.

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