In vivo administration of interferon alpha and interleukin 2 induces proliferation of lymphoid cells in the organs of mice.

We have previously shown that interleukin 2 (IL-2) synergizes with interferon alpha (IFN-alpha) in mediating the regression of established pulmonary and hepatic metastases and the reduction of intradermal tumor in various murine tumor models. To understand the mechanism of synergy, we have examined lymphoid cell proliferation in various organs of mice in response to IL-2 and IFN-alpha administration. We have utilized a technique for labeling newly synthesized DNA in vivo with 5-[125I]iodo-2'-deoxyuridine to examine proliferation of endogenous cells in response to IL-2 and IL-2 plus IFN-alpha. A proliferation index was calculated by dividing cpm in the tissues treated with cytokines by cpm obtained in corresponding tissues of control mice. After 4 days of IL-2 administration, a significant uptake of 5-[125I]iodo-2'-deoxyuridine was observed in the lungs, liver, kidneys, and spleen (proliferation index of 13, 10.3, 3.6, and 3.2, respectively). IFN-alpha alone mediated very little incorporation of radiolabel but when administered in combination with IL-2 a reduction of IL-2-induced proliferation was seen on day 4. For example 19,272 +/- 4,556 cpm (mean +/- SE) were obtained in the liver of IL-2-treated mice, compared to 8,103 +/- 2,111 cpm in livers of IL-2 plus IFN-alpha-treated mice (P less than 0.05). Similar inhibition of IL-2-induced proliferation was observed in the lungs, kidneys, and spleen. In contrast, on days 7 or 8, higher uptake of radiolabel was obtained in IFN-alpha plus IL-2-treated lungs, liver, and kidneys, compared to organs of mice treated with IL-2 alone or IFN-alpha alone. A proliferation index of 30.5, 9.8, and 10 was obtained in the lungs, liver, and kidneys of IL-2- plus IFN-alpha-treated animals, compared to 9.6, 3.6, and 5.5 in the corresponding organs of IL-2-treated mice. The effects of IFN-alpha on IL-2-induced proliferation was dose dependent; very low dosages of IFN-alpha (1,000 units/dose) were able to cause the inhibition of proliferation at 3 days of therapy and increase at 7 days of therapy. Continued proliferation of cells was observed in most organs when IL-2 plus IFN-alpha was injected for 9 consecutive days. Pretreatment irradiation of mice at 500 rad largely eliminated the proliferative response to IL-2 as well as to IFN-alpha plus IL-2 at both 3 and 7 days. Histological studies of lungs receiving cytokine therapy for 3 and 7 days corroborated the results of the 5-[125I]iodo-2'-deoxyuridine incorporation assay. At day 3 a significant infiltration of lymphoid cells was seen in IL-2-treated lungs, whereas little or no lymphocytic infiltration was observed in IL-2- plus IFN-alpha-treated lungs.(ABSTRACT TRUNCATED AT 400 WORDS)