C58/wm mice that were immunized with formalin-inactivated (1:500) Ib cells were protected when challenged with a lethal dose of viable Ib cells 4 hours before or 4 days after immunization. No early protective effect occurred with formalinized spleen cells obtained from normal C58/wm mice or C58/wm mice bearing transplants of leukemic cells of spontaneous origin. Spleen cells from normal DBA/2 mice, normal BALB/wm mice, and BALB/wm mice with transplants of Rauscher- or Moloney-induced lymphomas had early protective effect. Passive protection tests showed that a factor was not induced by, or released from, inactivated Ib cells that arrested their growth in vivo. Ib cell preparations were fractionated and partially purified by density gradient centrifugation. The cell component responsible for early protective effect had the properties of a lipoprotein and was called protective factor. The growth of Ib cells in C58/wm mice that received no protective factor, and protective factor in combination with Ib cells, was analyzed. The results showed that Ib cells injected alone grew at a linear rate and killed mice in 10 days. Growth of Ib cells in protective factor-treated mice was arrested at 4 days; Ib cells were essentially undetectable by 10 days. Adoptive immunity was demonstrable in mice treated with protective factor and Ib cells. These results are consistent with the conclusion that protective factor accelerated the immune response (had an adjuvant effect) of mice to injected Ib cells, thus making it possible for them to survive immunizing procedures.

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Supported by Grant 06639, National Cancer Institute, NIH, USPHS.

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