Preparing Feeder Cell Cultures to Support Hybridoma Growth

Cross-Linking Mass Spectrometry on P-Glycoprotein

The ABC transporter P-glycoprotein (Pgp) has been found to be involved in multidrug resistance in tumor cells. Lipids and cholesterol have a pivotal role in Pgp's conformations; however, it is often difficult to investigate it with conventional structural biology techniques. Here, we applied robust approaches coupled with cross-linking mass spectrometry (XL-MS), where the natural lipid environment remains quasi-intact. Two experimental approaches were carried out using different cross-linkers (i) on living cells, followed by membrane preparation and immunoprecipitation enrichment of Pgp, and (ii) on-bead, subsequent to membrane preparation and immunoprecipitation. Pgp-containing complexes were enriched employing extracellular monoclonal anti-Pgp antibodies on magnetic beads, followed by on-bead enzymatic digestion. The LC-MS/MS results revealed mono-links on Pgp's solvent-accessible residues, while intraprotein cross-links confirmed a complex interplay between extracellular, transmembrane, and intracellular segments of the protein, of which several have been reported to be connected to cholesterol. Harnessing the MS results and those of molecular docking, we suggest an epitope for the 15D3 cholesterol-dependent mouse monoclonal antibody. Additionally, enriched neighbors of Pgp prove the strong connection of Pgp to the cytoskeleton and other cholesterol-regulated proteins. These findings suggest that XL-MS may be utilized for protein structure and network analyses in such convoluted systems as membrane proteins.

Generating Monoclonal Antibodies

Antibodies that are produced by hybridomas are known as monoclonal antibodies. Here we introduce methods for generating and screening monoclonal antibodies, including developing the screening procedure and producing hybridomas.

Single B cell technologies for monoclonal antibody discovery

Monoclonal antibodies (mAbs) are often selected from antigen-specific single B cells derived from different hosts, which are notably short-lived in ex vivo culture conditions and hence, arduous to interrogate. The development of several new techniques and protocols has facilitated the isolation and retrieval of antibody-coding sequences of antigen-specific B cells by also leveraging miniaturization of reaction volumes. Alternatively, mAbs can be generated independently of antigen-specific B cells, comprising display technologies and, more recently, artificial intelligence-driven algorithms. Consequently, a considerable variety of techniques are used, raising the demand for better consolidation. In this review, we present and discuss the major techniques available to interrogate antigen-specific single B cells to isolate antigen-specific mAbs, including their main advantages and disadvantages.

Growing Hybridomas

This introduction discusses the techniques used to grow and maintain myeloma and hybridoma cell lines, the production and collection of monoclonal antibodies, and methods for drug selection used in hybridoma work.

Selecting for and Checking Cells with HGPRT Deficiency for Hybridoma Production

For drug-selective media to work for hybridoma selection, myeloma cells expressing a mutation abrogating the function of their HGPRT gene (and subsequently unable to produce purines for DNA biosynthesis) are used. HGPRT will recognize 8-AG as a substrate and convert it to the monophosphate nucleotide. The 8-AG-containing nucleotide is then processed further and incorporated into DNA and RNA, where it is toxic. Therefore, cells with a functional HGPRT enzyme grown in the presence of 8-AG will die. Cells that are deficient in HGPRTase cannot incorporate 8-AG in vivo and thus continue to grow. Cells that have been selected for resistance to 8-AG should be checked periodically to ensure that they maintain sensitivity to drugs that block the de novo synthesis of DNA. In addition, all myeloma cell lines should be checked periodically for reversion of their drug selection markers. Any line that is not killed completely by drug selection should either be reselected or replaced with a new line.

Liquid Nitrogen Storage of Hybridoma Cells

Hybridoma and myeloma cell lines can be stored by slowly freezing cells in an appropriate solution of nutrients and a cryoprotectant such as glycerol or dimethyl sulfoxide (DMSO). In this protocol, cells are centrifuged at 4°C, resuspended in cold freezing solution (10% DMSO in FBS), and then transferred to an appropriate freezing vial. The vials are slowly frozen to -70°C in Styrofoam racks and then stored in liquid nitrogen (LN₂). Cells stored in LN₂ will remain viable for years. Once a frozen vial has been removed from LN₂ storage, it should be thawed as described, grown out into log phase, and refrozen.

Single-Cell Cloning of Hybridoma Cells by Limiting Dilution

Isolating a stable clone of hybridoma cells that all secrete the correct antibody is the most time-consuming step in the production of hybridomas. Single-cell cloning ensures that cells that produce the antibody of interest are truly monoclonal and that the secretion of this antibody can be stably maintained. The original positive well will often contain more than one clone of hybridoma cells, and many hybrid cells have an unstable assortment of chromosomes. Both of these problems may lead to the desired cells being outgrown by cells that are not producing the antibody of interest. Cloning hybridoma cells by limiting dilution is the easiest of the single-cell-cloning techniques. Two approaches are presented here, one rapid technique for generating cultures that are close to being single-cell-cloned and one for single-cell cloning directly. Even though every attempt is made to ensure that the cells are in a single-cell suspension before plating, there is no way to guarantee that the colonies do not arise from two cells that were stuck together. Therefore, limiting dilution cloning should be performed at least twice to generate a clonal population.

Single-Cell Cloning of Hybridoma Cells by Growth in Soft Agar

Single-cell cloning during hybridoma production ensures that cells that produce the antibody of interest are truly monoclonal and that the secretion of this antibody can be stably maintained. Cloning of hybridoma cells in semisolid medium is one of the most commonly used methods for producing single-cell clones. The technique is easy, but, because it is performed in two stages, it does take longer than other methods. Not all cells will grow in soft agar, and there may be a bias on the type of colony that appears. However, most of the commonly used myeloma fusion partners have relatively good cloning efficiencies in soft agar, and, consequently, so do most hybridomas. Even though every attempt is made to ensure that the cells are in a single-cell suspension before plating, there is no way to guarantee that the colonies do not arise from two cells that were stuck together. Therefore, single-cell cloning in soft agar should be repeated at least twice before the cells are considered clonal.