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Two fusion proteins, composed of interleukin 2 (IL-2) or tumor necrosis factor (TNF) coupled to an antibody [fusion protein
(FuP); IL-2-FuP or TNF-FuP], were capable of retarding growth of a human malignant melanoma in the severe combined immunodeficient
mouse depending on the concomitant application of human peripheral blood leukocytes. Here we have analyzed the mechanisms
that determine the therapeutic effect. Tumor-bearing severe combined immunodeficient mice received once per week an i.v. injection
of HLA-matched peripheral blood leukocytes and twice per week i.v. or intratumoral injections of FuPs. Leukocyte recovery
and their activation state were monitored. The number of draining lymph node cells (LNCs) and tumor-infiltrating leukocytes
increased continuously, and the yield of draining LNCs was improved significantly when the FuPs were applied locally. In IL-2-FuP-treated
mice, the majority of draining LNCs and tumor-infiltrating lymphocytes expressed T-cell activation markers and IL-2, thus
being classified as T helper type 1 cells. These cells displayed strong proliferative activity and initiated activation of
lymphokine-activated killer cells and CTLs. TNF-FuP supported activation of CTLs and of monocytes as revealed by TNF expression
and cytostatic activity. Neither the antibody, nor IL-2, nor TNF, nor the mixture of antibody and cytokines exhibited the
full-fledged activational potency of the FuPs. Notably, activation of immune effector mechanisms was much stronger when the
FuPs were applied intratumorally. This is the first report to show that FuPs are efficient immunostimulants in vivo for native leukocytes. Although IL-2-FuP induced a T helper type 1 response with recruitment of LAK and CTL, TNF-FuP efficiently
recruited and activated monocytes and, in a less pronounced manner, CTLs.