The effect of lymphokine treatment of mouse and human fibroblast cell lines on the growth of Rickettsia prowazekii within the fibroblasts was studied. Treatment of mouse L929 cells with concanavalin A- or antigen-induced mouse lymphokines both before and after infection with R. prowazekii led to clearance of the rickettsiae from a substantial proportion of the cells and suppression of rickettsial growth in those cells which remained infected. Similar but less dramatic anti-rickettsial effects were observed in L929 cells treated with mouse lymphokines either only before or after infection with rickettsiae. Mouse lymphokine treatment of L929 cells had similar anti-rickettsial effects on the avirulent E strain and the virulent Breinl strain of R. prowazekii. Addition of cycloheximide or emetine to L929 cells at the same time as the lymphokines markedly suppressed the inhibition of rickettsial growth by the lymphokines. Mouse lymphokine treatment inhibited rickettsial survival and growth in mouse 3T3-A31 cells as well as in mouse L929 cells, but had no effect on rickettsial survival and growth in human foreskin fibroblasts. Conversely, concanavalin A-induced human lymphokines inhibited rickettsial survival and growth in human foreskin fibroblasts but had no effect on rickettsial survival and growth in mouse L929 cells. The rickettsia inhibitory activity in concanavalin A-induced mouse lymphokines was destroyed by heating the lymphokines at 80 degrees C for 10 min or by holding the lymphokines at pH 2 for 24 h but was retained after heating at 56 degrees C for 30 min.

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