Coxsackievirus A21 (CAV21) is classified within the species Human enterovirus C (HEV-C) of the Enterovirus genus of picornaviruses. HEV-C share striking homology with the polioviruses (PV), their closest kin among the enteroviruses. Despite a high level of sequence identity, CAV21 and PV cause distinct clinical disease typically attributed to their differential use of host receptors. PV cause poliomyelitis, whereas CAV21 shares a receptor and a propensity to cause upper respiratory tract infections with the major group rhinoviruses. As a model for CAV21 infection, we have developed transgenic mice that express human intercellular adhesion molecule 1, the cell-surface receptor for CAV21. Surprisingly, CAV21 administered to these mice via the intramuscular route causes a paralytic condition consistent with poliomyelitis. The virus appears to invade the CNS by retrograde axonal transport, as has been demonstrated to occur in analogous PV infections. We detected human intercellular adhesion molecule 1 expression on both transgenic mouse and human spinal cord anterior horn motor neurons, indicating that members of HEV-C may share PV's potential to elicit poliomyelitis in humans.

Like cellular FLIPs, the v-FLIP contains death effector domains (DEDs) (9), but unlike its host homologs, it does not appear to directly modulate caspase activation from the plasma membrane. Rather, it interacts with cytosolic IKK-␥ (10), leading to phosphorylation and proteasomal degradation of IB, the inhibitor of the transcription factor NF-B. As a result, NF-B undergoes constitutive activation, leading to the expression of a proinflammatory (11) and antiapoptotic (12) program. The antiapoptotic aspect of this program is essential for B-cell survival in primary effusion lymphoma (PEL) (13), and the proinflammatory signaling is speculated to play a role in the inflammatory phenotype of Kaposi's sarcoma (KS). NF-B activation in human primary endothelial cells is also responsible for the spindle-like elongation characteristic of the infected endothelial cells of KS (14). Finally, v-FLIP-mediated NF-B activation helps to stabilize the latent state by impairing lytic reactivation (15,16), and inhibition of NF-B triggers enhanced lytic induction (14). Therefore, v-FLIP expression is considered to be an important feature of KSHV latency in most lineages examined to date.Despite this, how v-FLIP is expressed in infection has been difficult to rigorously define. Expression of the protein appears to be extremely inefficient; it is difficult to detect by immunoblotting in latently infected cells, and attempts to express it from potent recombinant vectors generally result in unexpectedly low levels of protein accumulation (see reference 17 and Fig. 3 below). Northern blots of latently infected cells do not show the presence of a monocistronic mRNA for v-FLIP; rather, v-FLIP sequences detected by this method are found as the 3=-most open reading frame (ORF) in bi-or tricistronic transcripts directed from the LANA promoter (a map of the known transcripts from the MLL is shown in Fig. 1) (18-22). This seeming paradox led to the identification of sequences in the v-cyclin gene that can function as an internal ribosome entry site (IRES) element in recombinant vectors (17,23), and since that time, it has been presumed that v-FLIP is expressed from polycistronic mRNAs via IRES-mediated internal translational initiation. Direct evidence for this mechanism in infected cells has been lacking, however. In fact, reverse transcription (RT)-PCR-based studies have detected low-level expression of a spliced monocistronic v-FLIP RNA (Fig. 1) in some latently infected cells (17). Moreover, the reasons for the difficulty in

In many respects, picornaviruses are well suited for their proposed use as immunization vectors. However, their inherent genetic instability hinders application for prophylactic purposes. We demonstrate the improved expression and stability of a heterologous insert through a novel vector design strategy that partially replaces noncoding regulatory sequences with coding sequences for foreign gene products.