A cytokine fusion protein selected from the group consisting of: A.) a cytokine fusion protein comprising the extracellular domain of a first ligand of the tumor necrosis factor superfamily and the extracellular domain of a second ligand of the TNF superfamily, wherein the first ligand and the second ligand are different, and wherein the extracellular domains are covalently linked; and B.) a cytokine fusion protein comprising (i) three extracellular domains of a first ligand of the tumor necrosis factor superfamily forming a first homotrimer capable of binding to a receptor of the first ligand and (ii) three extracellular domains of a second ligand of the TNF superfamily forming a second homotrimer capable of binding to a receptor of the second ligand, wherein the first ligand and the second ligand are different, and wherein the first homotrimer and the second homotrimer are covalently linked, preferably via one or more peptide linkers, and wherein in the cytokine fusion protein of A.) and B.) the first ligand is CD40L, and the second ligand is 4-1BBL, wherein the extracellular domain of CD40L comprises or consists of amino acid residues 51 to 261 or 116 to 261 of SEQ ID NO: 1 and the extracellular domain of 4-1BBL comprises or consists of amino acid residues 71 to 254 of SEQ ID NO: 3, wherein the cytokine fusion protein of A.) comprises three subunits with the general formula: N'- A-L-B- C' (Formula II), wherein A comprises the extracellular domain of the first ligand, and B comprises the extracellular domain of the second ligand, wherein L comprises a peptide linker, and wherein the cytokine fusion protein of A.) is present as a trimeric complex, wherein three extracellular domains of the first ligand form a first homotrimer capable of binding to a receptor of the first ligand, and three extracellular domains of the second ligand form a second homotrimer capable of binding to a receptor of the second ligand; and wherein the cytokine fusion protein of B.) comprises a molecule/structure having the general formula N' -A-L A -A-L A -A-L-B-L B -B-L B -B- C' (Formula I), wherein A comprises the extracellular domain of the first ligand, and B comprises the extracellular domain of the second ligand, and wherein L comprises a peptide linker, and L A and L B are, at each occurrence, independently selected from a covalent bond and a peptide linker. A.) a cytokine fusion protein comprising the extracellular domain of a first ligand of the tumor necrosis factor superfamily and the extracellular domain of a second ligand of the TNF superfamily, wherein the first ligand and the second ligand are different, and wherein the extracellular domains are covalently linked; and B.) a cytokine fusion protein comprising (i) three extracellular domains of a first ligand of the tumor necrosis factor superfamily forming a first homotrimer capable of binding to a receptor of the first ligand and (ii) three extracellular domains of a second ligand of the TNF superfamily forming a second homotrimer capable of binding to a receptor of the second ligand, wherein the first ligand and the second ligand are different, and wherein the first homotrimer and the second homotrimer are covalently linked, preferably via one or more peptide linkers, and wherein in the cytokine fusion protein of A.) and B.) the first ligand is CD40L, and the second ligand is 4-1BBL, wherein the extracellular domain of CD40L comprises or consists of amino acid residues 51 to 261 or 116 to 261 of SEQ ID NO: 1 and the extracellular domain of 4-1BBL comprises or consists of amino acid residues 71 to 254 of SEQ ID NO: 3, wherein the cytokine fusion protein of A.) comprises three subunits with the general formula: N'- A-L-B- C' (Formula II), wherein A comprises the extracellular domain of the first ligand, and B comprises the extracellular domain of the second ligand, wherein L comprises a peptide linker, and wherein the cytokine fusion protein of A.) is present as a trimeric complex, wherein three extracellular domains of the first ligand form a first homotrimer capable of binding to a receptor of the first ligand, and three extracellular domains of the second ligand form a second homotrimer capable of binding to a receptor of the second ligand; and wherein the cytokine fusion protein of B.) comprises a molecule/structure having the general formula N' -A-L A -A-L A -A-L-B-L B -B-L B -B- C' (Formula I), wherein A comprises the extracellular domain of the first ligand, and B comprises the extracellular domain of the second ligand, and wherein L comprises a peptide linker, and L A and L B are, at each occurrence, independently selected from a covalent bond and a peptide linker. wherein A comprises the extracellular domain of the first ligand, and B comprises the extracellular domain of the second ligand, wherein L comprises a peptide linker, and wherein the cytokine fusion protein of A.) is present as a trimeric complex, wherein three extracellular domains of the first ligand form a first homotrimer capable of binding to a receptor of the first ligand, and three extracellular domains of the second ligand form a second homotrimer capable of binding to a receptor of the second ligand; and wherein the cytokine fusion protein of B.) comprises a molecule/structure having the general formula N' -A-L A -A-L A -A-L-B-L B -B-L B -B- C' (Formula I), wherein A comprises the extracellular domain of the first ligand, and B comprises the extracellular domain of the second ligand, and wherein L comprises a peptide linker, and L A and L B are, at each occurrence, independently selected from a covalent bond and a peptide linker.