The incorporation of 3H-thymidine in newly formed DNA was studied in guinea pig spleen cells stimulated by phytohemagglutinin (PHA) and/or concanavalin A (Con A). In spleen cells stimulated by PHA, two peaks of thymidine uptake were observed for two different doses of lectin whatever the number of cultured cells. Furthermore, thymidine uptake is proportional to the number of the cultured cells. During Con A-induced mitogenesis, two peaks were also observed but only when using a great concentration of cells. Maximal thymidine incorporation depended on both the cell number and the concentration of Con A. When cells were stimulated by both PHA and Con A, thymidine uptake was strongly depressed as compared to the one observed using PHA or Con A alone. On the other hand, supernatants from unstimulated spleen cells had opposite effects on blastogenesis induced by Con A or by PHA; they depressed PHA-induced blastogenesis while enhancing the one induced by Con A. These results suggest that, in guinea pigs, both PHA and Con A induce thymidine incorporation in at least two lymphocyte populations through different mechanisms. This heterogeneity of lectin-induced T cell mitogenesis has to be taken in account when studying the mechanisms by which the immunomodulators are active at the T cell level.
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