Hepatitis A virus (HAV) is a picornavirus that causes acute hepatitis in humans. Our lab identified HAVcr-1, a gene coding for a surface glycoprotein on the GL37 clone of African green monkey kidney (AGMK) cells, as a receptor for HAV (Kaplan et al., EMBO J. 15:4282-4269, 1996). Molecular cloning and sequence analysis of the HAVcr-1 cDNA revealed that it codes for a class I, integral membrane glycoprotein termed havcr-1. The extracellular domain of havcr-1 contains an N-terminal cys-rich region followed by a mucin-like region. We recently showed that i) the cys-rich region of havcr-1 is required for binding of HAV and MAb 190/4 (Thompson et al., J. Virol. 72:3751-3761, 1998), ii) havcr-1 expressed at the cell surface of BS-C-1 and CV-1 cells, two widely used AGMK cell lines, contain a K108Q substitution responsible for the lack of reaction with MAb 190/4 which did not affect their HAV-receptor function (Feigelstock et al., J. Virol. 72:6218-6222, 1998), and iii) the human homolog of havcr-1 (huHAVcr-1), which is 79% identical to the simian havcr-1, binds HAV suggesting that it is a functional receptor for HAV (Thompson et al., J. Virol. 72:6621-6628, 1998). Mouse cell transfectants expressing havcr-1 gain limited susceptibility to HAV infection due to intracellular block(s) to HAV replication. Since we are interested in developing a transgenic mouse model for pathogenesis of HAV, we adapted HAV to grow in mouse Ltk- cells. The resulting mouse adapted HAV, termed HAV-L, was able to grow in Ltk- cells expressing havcr-1 but not in Ltk- cells expressing havcr-1 containing a deletion of the cys-rich region and Ltk- cells transfected with vector alone. These results clearly show that havcr-1 is a functional receptor for HAV and that HAV-L contains mutations that alows it to replicate efficiently in mouse cells (J. Lu and G. Kaplan, manuscript in preparation). We have recently overexpressed in CHO cells the cys-rich region of havcr-1 fused to the Fc of human IgG1. Several milligrams of the reulting fusion protein, termed Fc/havcrD1, was purified from the supernatant of CHO cells using protein A-agarose affinity chromatography. HAV bound to the purified Fc/havcrD1 at a very low level but this binding was not sufficient to show a significant level of soluble receptor-mediated neutralization of HAV. Further constructs containing larger portions of the extracellular domain of havcr-1 fused to the Fc of human IgG1 are currently being expressed in CHO cells to determine whether the TSP-rich region of havcr-1 is required for efficient binding of HAV to soluble receptors. The cell entry mechanism of picornaviruses is poorly understood. To further study the interaction of picornaviruses, including HAV, with their cellular receptors, we first made a comparative analysis of soluble receptor binding to poliovirus and rhinovirus (Xing et al., submitted for publication) which are two model picornaviruses. We are currently extending this analysis to the interaction of HAV with soluble havcr-1.
