PRRSV-macrophage interaction and putative receptors (2000)

\nPorcine reproductive and respiratory syndrome virus (PRRSV), \na member of the Arteriviridae family, has a restricted cell tropism \nin its host, the pig. Mainly cells of the monocyte/macrophage \n(mø) \nlineage are susceptible, which are situated in different locations \nsuch as the tonsils, lungs, lymph nodes and spleen. The observation\n that several subpopulations of this cell lineage do not permit a \ncomplete replication is important. Blood monocytes, peritoneal \nmacrophages and progenitor cells in the bone marrow are examples\n of refractory mø. The basis for the restricted replication of \nPRRSV in certain cell types can be found in the different subsequent\n steps which have to be gone through successfully before a new virus \nis produced: attachment, entry, release of the genome, transcription, \ntranslation and assembly. The inhibition of one of these steps will \nresult in a non-permissive status of the cell. The early events of the \nreplication cycle have already been studied extensively and will be discussed.\n The entry of PRRSV starts with attachment. Binding of PRRSV to alveolar \nmacrophages happens in a dose dependent way and is completed within one \nhour at $4\\,^\\circ{\\rm C}$. \nThe number of binding sites on the surface of a single \nmacrophage is quite variable but exceeds \n$10^4$ per cell. Recently, two \nmonoclonal antibodies were produced which are able to partially block\n virus attachment to the plasma membrane and to inhibit infection. \nThey recognise a 210 kDa protein. Since a clear colocalisation exists\n between attached virus and aggregates of this protein on the plasma\n membrane, it was concluded that these monoclonal antibodies either \nspecifically recognise the PRRSV receptor or a protein which is in close \ncontact with it. The fact that all PRRSV-positive cells in lungs and lymphoid \ntissues carry this 210 kDa protein gives further evidence that monoclonal antibodies\n are directed against a putative receptor. Further research is currently underway \nto provide definitive proof that the 210 kDa protein corresponds to the PRRSV \nreceptor. The next step in the entry-process is the endocytosis of PRRSV. This \nhas been visualised by confocal microscopy using labelled virus. Within one hour,\n bound virus is taken up by the cell. Since a clear colocalisation exists between \nthe virus and clathrin during engulfment, it can be concluded that clathrin is \ninvolved. The fact that colocalisation is no longer present once the virus is \ninternalised provides evidence that clathrin is released from the membrane of\n the endosomes during the movement of the virus-containing endosome inside the\n cell. This finding is in agreement with the knowledge that clathrin is recycled\n during clathrin-dependent, receptor-mediated endocytosis processes. Although\n attachment and endocytosis of PRRSV is observed in almost all alveolar \nmacrophages, viral antigens are only produced in 30% of these cells. This \ndiscrepancy suggests that one of the next steps in the replication cycle, \nwhich is the release of the viral genome in the cytoplasm or translation, \nis blocked in one way or another. How the viral genome gets free from its\n envelope and arrives in the cytoplasm for transcription and translation \nhas not yet been clarified. It has, however, already been shown by the use \nof the acidotropic agents, NH$_4$Cl and chloroquine, that a low pH phase is\n essential during this period. It is now believed, in relation to the \nknowledge of other viruses, which need a low pH-phase during early replication, \nthat acidification causes conformational changes in envelope glycoproteins and/or\n capsid structures and triggers membrane fusion. In conclusion, it can be stated \nthat during the early stages of PRRSV-replication, a complex cascade of events takes\n place and that the non-permissive status of a cell may be caused by a block at the \nlevel of one of these events.\n