Universal reporter cell assay for specificity test of novel antigen binding moieties

A method for assessing the specificity of a Fab fragment comprising the steps of:

• a) providing an antigen binding molecule comprising an antigen binding domain and a recognition domain, wherein the antigen binding domain comprises the Fab fragment, wherein the Fab fragment is specific for a target antigen;
• b) contacting the antigen binding molecule with a target cell comprising the target antigen on the surface;
• c) contacting the antigen binding molecule with a chimeric antigen receptor (CAR) expressing reporter T (CAR-T) cell wherein the reporter CAR-T cell comprises:
• i. a CAR capable of specific binding to the recognition domain wherein the CAR is operationally coupled to a response element;
• ii. a reporter gene under the control of the response element;
• and
• d) determining T cell activation by determining the expression of the reporter gene to establish the specificity of the Fab fragment;
• wherein the recognition domain is an Fc domain.