: The protein secretory apparatus of host cells is inhibited early in poliovirus (PV) infection. Preliminary evidence supports the hypothesis that poliovirus 3A protein specifically inhibits protein traffic between the endoplasmic reticulum (ER) and Golgi by preventing anterograde traffic from the ER. When cells are infected with mutant PV that expresses a 3A protein which does not inhibit protein secretion, increased levels of IFN-b, IL-6 and IL-8 are secreted and MHC class I-mediated presentation of antigens to CD8+ CTL is increased. Thus, an important function of 3A is to limit cellular antiviral responses, thereby affecting viral pathogenesis. In this proposal experiments are described which will investigate the molecular mechanism by which 3A disrupts ER-to-Golgi traffic. A survey of the ability of 3A and 2B alleles from other picornaviral families and serotypes will be performed to test the relationship between the inhibition of host protein secretion and the pathogenic potential of these viruses. PV variants with various degrees of ability to inhibit host protein secretion will be selected by a novel genetic screen. These variants will be characterized and reconstructed into viral replicons as candidate picornaviral vaccines. Transgenic mice that express the PV receptor will be infected with packaged replicons that do or do not inhibit host protein secretion and the CD8+ CTL response will be quantified using specific MHC tetramers. It is anticipated that this method will allow the regulation of the antigen presentation to the cellular immune response by picornaviral-based vectors, which could be crucial in vaccine design.