The present invention is directed to an isolated polynucleotide sequence encoding a chimeric TNFα, comprising a first nucleotide sequence encoding a domain or subdomain of a tumor necrosis factor ligand other than TNFα, wherein the encoded domain or subdomain replaces a cleavage site of native TNFα, and a second nucleotide sequence encoding a domain or subdomain of native TNFα that binds to a TNFα receptor. The encoded chimeric TNFα is significantly less susceptible to cleavage from the cellular surface and, as a result can increase the concentration of a ligand capable of binding to a TNFα receptor on the surface of a cell. The chimeric TNFα is therefore useful in methods for inducing apoptosis of a cell expressing a TNFα receptor, inducing activation of an immune system cell and treating neoplastic cells, by introducing into the cell of interest an isolated polynucleotide sequence encoding a chimeric TNFα that is expressed on the surface of the cell.