Sorting with counter selection using sequence similar peptides

A method for selecting a eukaryotic cell expressing on its surface an antigen-binding protein specifically and/or selectively binding to a protein antigen of interest (PAI) comprising the following steps:

• (i) providing a eukaryotic cell population;
• (ii) contacting the eukaryotic cell population of step (i) with a first antigen complex (1st AC) comprising the PAI and a detectable label A or with the PAI comprising a detectable label A;
• (iii) contacting the eukaryotic cell population of step (i) with at least a second antigen
• complex (2nd AC) comprising a similar protein antigen (SPA),
• wherein the amino acid sequence of the SPA differs by 1-20 amino acids from the amino acid sequence of the PAI, optionally by 2-10 amino acids from the amino acid sequence of the PAI, and
• wherein the 2nd AC comprises a detectable label B;
• or with the SPA and a detectable label B;
• and
• (iv) selecting at least one eukaryotic cell that specifically and/or selectively binds to the 1st AC,
• wherein the detectable label A and the detectable label B are detectably different from each other, and
• wherein the selected eukaryotic cell is a T-cell, a B-cell, or a mammalian cell expressing a heterologous antigen binding protein selected from the group consisting of a T-cell receptor (TCR) or antigen binding fragments thereof, a B-cell receptor (BCR) or antigen binding fragments thereof, and a chimeric antigen receptor (CAR) or antigen binding fragments thereof.