Abstract
In this paper we describe a unique method for selection of agonist antibodies that regulate stem cell fate. Unbiased combinatorial libraries (not preselected by phage panning) were used and, thus, no constraints were placed on the discovery process. Remarkably, we found that a single agonist antibody against the alpha chains of integrins can induce human stem cells to become dendritic cells. These studies are important because they help deconvolute an otherwise complex developmental process. This approach is analogous to forward genetics and should be widely used to identify previously undescribed proteins involved in important cellular pathways.
When combinatorial antibody libraries are rendered infectious for eukaryotic cells, the integrated antibody genotype and cellular phenotype become permanently linked and each cell becomes a selection system unto itself. These systems should be ideal for the identification of proteins and pathways that regulate differentiation so long as selection systems can be devised. Here we use a selection system based on the ability of secreted antibodies to alter the morphology of colonies expressing them when grown in soft agar. Importantly, this approach is different from all previous studies in that it used a pure discovery format where unbiased libraries that were not preselected against any known protein were used as probes. As such, the strategy is analogous to classical forward genetic approaches except that it operates directly at the protein level. This approach led to the identification of integrin-binding agonist antibodies that efficiently converted human stem cells to dendritic cells.
