Compositions and methods for identification of vhh antibodies that bind a target antigen

A method for identifying a VHH antibody that binds a target antigen, the method comprising:

• (a) preparing a library of expression vectors encoding chimeric antigen receptors (CARs), each comprising a VHH domain generated in response to an antigen of interest;
• (b) expressing each member of the library of expression vectors in an immortalized immune cell, wherein the immortalized immune cell comprises a selection vector comprising an activation induced promoter operably linked to a resistance gene and a sensitizing gene, wherein the resistance gene provides for positive selection with a positive selection agent and the sensitizing gene provides for negative selection with a negative selection agent;
• (c) contacting the immune cell with an antigen that is not the antigen of interest and the negative selection agent;
• and
• (d) contacting the immune cell with the antigen of interest and the positive selection agent, thereby identifying chimeric antigen receptors comprising VHH domains that selectively bind the target antigen.