A single-chain antibody construct was prepared containing the VH and VL regions of anti-(integrin α4) antibody HP1/2, an interchain linker and a KDEL endoplasmic reticulum retention sequence. Intracellular expression of this single-chain antibody caused cell-surface expression of α4β1 integrin to be decreased by 80% on selected RD cells and by 65–100% on selected Jurkat cells, relative to mock transfectants. Immunoprecipitation from single-chain-antibody-transfected cells showed that the single-chain antibody was complexed with the integrin α4 and β1 subunits, and the diminished sizes of α4 and β1 were consistent with impaired maturation. Furthermore, cell adhesion to α4β1 ligands [VCAM-1 (vascular cell adhesion molecule-1), FN40 (40 kDa chymotryptic fragment of fibronectin) and CS1] was greatly impaired in both RD and Jurkat cells, and cell spreading on immobilized FN40 protein was almost completely eliminated. Thus we conclude that intracellular single-chain antibodies may be used to reduce or eliminate cell-surface expression of a specific integrin, with specific functional consequences. This approach should be generally applicable to other integrin subunits.

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