Proinflammatory cytokines are widely explored in different adoptive cell therapy protocols for enhancing survival and function of the transferred T cells, but their systemic administration is often associated with severe toxicity which limits their clinical use. To confine cytokine availability to the therapeutic T cells, we expressed 3 key cytokines, IL-2, IL-12, and IL-15, as integral T-cell membrane proteins. To prevent permanent activation of growth signaling pathways, we delivered these genes to T cells through mRNA electroporation. The engineered cytokines could be detected on the surface of mRNA-transfected cells and binding to their cell-surface receptors mainly occurred in cis. The 3 human cytokines supported the ex vivo growth of activated human CD8 and CD4 T cells for at least 6 days posttransfection, comparably to high-dose soluble IL-2. Similarly, membrane IL-2, membrane IL-12, and, to a lesser extent, membrane IL-15, were comparable with their soluble counterparts in supporting proliferation of splenic mouse CD8 T cells. Following electroporation of human CD8 T cells and antimelanoma tumor-infiltrating lymphocytes, membrane cytokines synergized with constitutively active toll-like receptor 4 in inducing interferon-γ secretion. Efficient cooperation with TLR4 was also evident in the upregulation of the activation molecules CD25, CD69, CD137 (4-1BB), and CD134 (OX40). Taken together, membrane cytokines expressed through mRNA transfection emerge as effective tools for enhancing T-cell proliferation and function and may have potential use in adoptive T-cell therapy.