ASalmonella typhimuriumstrain genetically engineered to secrete effectively a bioactive human interleukin (hIL)-6 via theEscherichia colihemolysin secretion apparatus

Human interleukin-6 (hIL-6) cDNA was genetically fused with the Escherichia coli hemolysin secretorial signal (hlyA[S]) sequence in a plasmid vector. Recombinant E. coli XL-1 Blue and attenuated Salmonella typhimurium secreted a 30 kDa hIL-6-HlyA(S) fusion protein, with an additional form of higher apparent molecular mass produced by S. typhimurium. In S. typhimurium cultures hIL-6-HlyA(S) concentrations entered a plateau at 500 to 600 ng ml(-1) culture supernatant. In contrast to E. coli XL-1 Blue, in S. typhimurium culture supernatants hIL-6-HlyA(S) was accumulated faster reaching three-fold higher maximal concentrations. The cell proliferating activity of hIL-6-HlyA(S) fusion protein(s) was equivalent to that of mature recombinant hIL-6. Furthermore. hIL-6-secreting S. typhimurium were less invasive than the attenuated control strain. Therefore, the bulky hemolysin secretorial peptide at the C-terminus of the fusion protein does not markably affect hIL-6 activity, suggesting that the hemolysin secretion apparatus provides an excellent system to study immunomodulatory effects of in situ synthesized IL-6 in Salmonella vaccine strains.