Provided herein are CasX:gNA systems, and compositions and methods relating thereto, the systems comprising CasX proteins, guide nucleic acids (gNAs), and optionally donor template nucleic acids useful for the modification cell genes encoding proteins involved in antigen processing, antigen presentation, antigen recognition, and/or antigen response, as well as methods of producing and using populations of cells comprising these modified genes. In some embodiments, the modified cells further express chimeric antigen receptors (CAR) or engineered T cell receptors (TCR). Such systems are useful for preparing cells for immunotherapy.