The present disclosure relates to cells capable of co-expressing T cell receptors (“TCR”) together with membrane-bound IL-15 polypeptides and/or CD8 polypeptides and the use thereof in adoptive cellular therapy. The present disclosure further provides for modified IL-15, IL-15Rα, IL-15/IL-15Rα fusion polypeptide, and IL-15Rα/IL-15 fusion polypeptide sequences, vectors, and associated methods of making and using the same. The present disclosure further provides for modified CD8 sequences, vectors, and associated methods of making and using the same.