Immune responses in vitro. ii. mixed leukocyte interaction in a protein‐free medium

Addition of mercaptoethanol to the culture medium has permitted a mixed leukocyte interaction (MLI) to be achieved with mouse spleen cells in a chemically defined protein‐free medium. Results which confirm those obtained in culture medium supplemented with heterologous sera are: 1) the maximum incorporation of isotope occurred between 60 and 72 h of culture; 2) the index of stimulation was 3–15; 3) the number of responder and stimulator cells required for maximum incorporation was 3 x 10 6 – 4 x 10 6 and 4) immunization resulted in a higher level of incorporation that occurred earlier than the maximum response of normal cells. Other results obtained confirm previous reports which are in apparent conflict: i.e. , 1) stimulation of parental strains (P 1 ) by F 1 hybrids (P 1 x P 2 ) may be either equal to or one‐half that induced by the other parental strains (P 2 ); and 2) F 1 hybrids may or may not be stimulated by the parental strains. These latter two results were found to be dependent on the strains used. Other advantages of the system are discussed in terms of the elimination of artifacts that occur in medium containing serum and the usefulness in examination of molecular events occurring early in immune reactions.