Detecting Fluorochrome-Labeled Cells

Construction of a versatile fusion protein for targeted therapy and immunotherapy

Antibody (Ab)-based drugs have been widely used in targeted therapies and immunotherapies, leading to significant improvements in tumor therapy. However, the failure of Ab therapy due to the loss of target antigens or Ab modifications that affect its function limits its application. In this study, we expanded the application of antibodies (Abs) by constructing a fusion protein as a versatile tool for Ab-based target cell detection, delivery, and therapy. We first constructed a SpaC Catcher (SpaCC for short) fusion protein that included the C domains of Staphylococcal protein A (SpaC) and the SpyCatcher. SpaCC conjugated with SpyTag-X (S-X) to form the SpaCC-S-X complex, which binds non-covalently to an Ab to form the Ab-SpaCC-S-X protein complex. The "X" can be a variety of small molecules such as fluoresceins, cell-penetrating peptide TAT, Monomethyl auristatin E (MMAE), and DNA. We found that Ab-SpaCC-S-FITC(-TAT) could be used for target cell detection and delivery. Besides, we synthesized the Ab-SpaCC-SN3-MMAE complex by linking Ab with MMAE by SpaCC, which improved the cytotoxicity of small molecule toxins. Moreover, we constructed an Ab-DNA complex by conjugating SpaCC with the aptamer (Ap) and found that Ab-SpaCC-SN3-Ap boosted the tumor-killing function of T-cells by retargeting tumor cells. Thus, we developed a multifunctional tool that could be used for targeted therapies and immunotherapies, providing a cheap and convenient novel drug development strategy.

Cell Staining

When labeled antibodies are used to detect antigens in cells or tissues, several characteristics of an antigen can be readily determined. Most importantly, cell staining will show both the presence and subcellular localization of an antigen. Double-labeling techniques permit the simultaneous detection of two antigens, allowing comparisons of the relative distribution of different antigens. Many cell-staining methods can also be used in conjunction with conventional histological stains and autoradiographic methods to compare the localization of the antigen with other markers. Cell staining can also be used in pathology studies for determining such variables as the type of infectious organism, the progenitor of a neoplastic cell, or the presence of an inflammatory response. With certain modifications, the procedure can be used to purify cells on the basis of the antigenic composition of their cell surface. Cell staining is a versatile technique and, if the antigen is highly localized, can detect as few as a thousand antigen molecules in a cell or tissue. In some circumstances, cell staining may also be used to determine the approximate concentration of an antigen. Improvements in antibody labeling methods, microscopes, cameras, and image analyzers are rapidly extending the sensitivity of cell-staining procedures and are making these techniques more quantitative. Even without these improvements, cell staining can yield important qualitative and semiquantitative data. This introduction describes protocols for cell staining techniques and includes a discussion of major constraints, antibody selection, and troubleshooting.

Fixing and Binding Antibodies to Suspension Cells in Preparation for Staining

Staining of suspension cells after fixation, described here, is normally used only to detect cell-surface antigens. Once cells are fixed and permeabilized, the antibodies are added. The antibodies can be labeled directly or they can be detected by using a labeled secondary reagent that will bind specifically to the primary antibody. Detection reagents for cell staining can be labeled with fluorochromes, enzymes, gold, or iodine. Because the cells are in suspension, the washing steps are tedious, and care should be taken not to centrifuge for long durations or at high speeds.