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Redirecting circulating antibodies via ligand-hapten conjugates eliminates target cells in vivo
A R Lussow1, R Buelow, L Fanget
1SangStat Medical Corporation, Menlo Park, CA 94025, USA.
The elimination of cell populations in vivo often relies on reagents that are self-limiting, are difficult to design and produce or contain highly toxic components. Here we describe a novel immunotherapy using molecules that combine a cell-specific ligand and a hapten binding to preexisting antibodies in serum. The F(ab')2 fragment of a polyclonal anti-thymocyte globulin (ATG) preparation was used as a T-cell-specific ligand, and fluorescein isothiocyanate (FITC), as the hapten. Clearance of ligand-hapten conjugates from the circulation through formation of immune complexes was prevented through controlled synthesis of conjugates so that they contained one F(ab')2 fragment and one FITC molecule. Administration of a single dose of F(ab')2 or F(ab')2ATG-FITC into naive mice had no effect on the number of circulating T cells. In contrast, injection of F(ab')2ATG-FITC into mice with circulating anti-FITC antibodies resulted in the elimination of peripheral T cells. The reduction in cell numbers was equivalent to that obtained with a corresponding dose of intact ATG. Experiments in thymectomized mice demonstrated that the reduction of circulating T cells was due to target-cell elimination and not to immunomodulation or cellular sequestration. The adaptability of the model to other sources of effector antibodies and more useful ligands is discussed.
The elimination of cell populations in vivo often relies on reagents that are self-limiting, are difficult to design and produce or contain highly toxic components. Here we describe a novel immunotherapy using molecules that combine a cell-specific ligand and a hapten binding to preexisting antibodies in serum. The F(ab')2 fragment of a polyclonal anti-thymocyte globulin (ATG) preparation was used as a T-cell-specific ligand, and fluorescein isothiocyanate (FITC), as the hapten. Clearance of ligand-hapten conjugates from the circulation through formation of immune complexes was prevented through controlled synthesis of conjugates so that they contained one F(ab')2 fragment and one FITC molecule. Administration of a single dose of F(ab')2 or F(ab')2ATG-FITC into naive mice had no effect on the number of circulating T cells. In contrast, injection of F(ab')2ATG-FITC into mice with circulating anti-FITC antibodies resulted in the elimination of peripheral T cells. The reduction in cell numbers was equivalent to that obtained with a corresponding dose of intact ATG. Experiments in thymectomized mice demonstrated that the reduction of circulating T cells was due to target-cell elimination and not to immunomodulation or cellular sequestration. The adaptability of the model to other sources of effector antibodies and more useful ligands is discussed.