SERP2, AN INHIBITOR OF THE INTERLEUKIN-1-BETA-CONVERTING ENZYME, IS CRITICAL IN THE PATHOBIOLOGY OF MYXOMA VIRUS

Recently, myxoma virus was shown to encode an additional member of the serpin superfamily. The viral gene, called serp2, was cloned, and the Serp2 protein was shown to specifically bind to interleukin-1 beta (I L-1 beta)-converting enzyme (ICE), thus inhibiting the cleavage of pro -IL lp by the protease (F. Petit, S. Bertagnoli, J. Gelfi, F. Fassy, C . Boucraut-Baralon, and A. Milon, J. Virol. 70:5860-5866, 1996). Here, we address the role of Serp2 in the development of myxomatosis, a let hal infectious disease of the European rabbit. A Serp2 mutant myxoma v irus was constructed by disruption of the single-copy serp2 gene and i nsertion of the Escherichia coli gpt gene serving as the selectable ma rker. A revertant virus was obtained by replacing the E. coli gpt gene by the intact serp2 open reading frame. The Serp2(-) mutant virus rep licated with wild-type kinetics both in rabbit fibroblasts and a rabbi t CD4(+) T-cell line (RL5). Moderate reduction of cell surface levels of major histocompatibility complex I was observed after infection wit h wild-type or Serp2- mutant myxoma virus, and both produced white poc ks on the chorioallantoic membrane of the chick embryo. After the infe ction of European rabbits, the Serp2- mutant virus proved to be highly attenuated compared to wild-type myxoma virus, as demonstrated by the clinical course of myxomatosis and the survival rates of infected ani mals. Pathohistological examinations revealed that infection with wild -type myxoma virus resulted in a blockade of the inflammatory response at the vascular level. In contrast, rapid inflammatory reactions occu rred upon infection with the Serp2(-) mutant virus. Furthermore, lymph ocytes in lymph nodes derived from animals inoculated with Serp2(-) mu tant virus were shown to rapidly undergo apoptosis. We postulate that the virulence of myxoma virus in the European rabbit can be partially attributed to an impairment of host inflammatory processes and to the prevention of apoptosis in lymphocytes. The weakening of host defense is directly linked to serp2 gene function and is likely to involve the inhibition of IL-1 beta-converting-enzyme-dependent pathways.