As reported previously, antigenically stimulated guinea pig lymphocytes elaborate a soluble factor which activates macrophages in the sense of promoting increased adherence, spreading, phagocytosis, and glucose oxidation through the hexose monophosphate pathway. Further studies on the characteristics and kinetics of this substance were carried out. The activating factor could not be distinguished from a previously characterized lymphocyte mediator, migration inhibitory factor (MIF), on the basis of Sephadex G-100 gel filtration, CsCl density gradient centrifugation, or sensitivity to neuraminidase. It was, however, shown to be distinct from two other lymphocyte mediators, chemotactic factor for macrophages and lymphotoxin. The kinetics of activation were further studied. The data suggest that the 3 day period required by macrophages to manifest a response to the activating factor consists of two stages. In the first, requiring 1–2 days, the macrophages are refractory to the influence of activating factor, but undergo changes which render them receptive. In the second, they respond to activating factor with increased cell adherence and glucose oxidation. Once macrophages have been activated, the effect persists in the absence of activating factor for 24 h. Finally, it was shown that activation in unfractionated supernatants followed the same time-course as that in more purified fractions. The data suggests that the activating factor is the same as MIF and that, in vitro, macrophages respond to this substance with migration inhibition before they become sensitive to its activating influence.

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