Orthobunyavirus

Vectors

The primary vectors of Orthobunyaviruses are hematophagous insects of the Culicidae family, including members from a number of mosquito genera (including Aedes, Coquillettidia, Culex, Culiseta, and Anopheles) and biting midges (such as Culicoides paraensis).^[11][8] Although transmission by ticks and bed bugs may also occur. Viral vector preference is generally strict, with only a one or very small number of vectors transmitting a specific virus in the region, even where multiple viruses and vectors overlap.^[12] Organisms related to the preferential vector may be able to carry a virus but not competently transmit it.^[8]

The vector arthropod acquires the virus while taking a blood meal from an infected host. In mosquitoes, replication of orthobunyaviruses is enhanced by immune modulation that occurs as a result of blood protein digestion producing GABA and the activation of GABAergic signalling.^[13] Infection is transmitted to a new host via viral particles in vector saliva.^[13] Orthobunyavirus infection in arthropod cells is not fully understood, but is generally non-cytopathological and deleterious effects are minimal.^[14][12] Infected mosquitoes may experience an increase in fitness.^[12] Transorvarial transmission has been observed among mosquitoes infected with orthobunyaviruses of the California serogroup^[8] Like mosquitoes, only female culicoid midges feed on blood; they prefer indoor feeding particularly during rain.^[8]

Sylvatic Cycle Hosts

In the slyvatic cycle, viruses are transmitted between mammalian hosts by the arthropod vector. A diverse range of mammals have been identified or implicated as hosts or reservoirs of orthobunyaviruses including: non-human primates, sloths, wild and domestic birds, marmosets, rodents, and large mammals such as deer, moose, and elk.^[11][8]

Infection

Infection begins with the bite of an infected competent vector organism. Viral entry proceeds by receptor-mediated (clathrin-dependent) endocytosis, but which receptors unknown.^[14] Although, Heparan sulfate and DC-SIGN (CD209 or Dendritic cell-specific intracellular adhesion molecule-3-grabbing non-integrin) have been identified as viral entry components in some orthobunyaviruses.^[12][14] Gn/Gc heterodimers on the viral surface are responsible for target cell recognition,^[15] with Gc is considered the primary attachment protein, although Gn has been suggested as the attachment protein for LACV in arthropod cells.^[12] Acidification of the endosome triggers a conformational change in the Gc fusion peptide, uncoating the ribonuclearprotein (RNP) as it is released into the cytoplasm.^[15]

Upon release into the cytoplasm, primary transcription begins with an endonuclease domain on L protein engaging in a process known as "cap-snatching."^[12][15] During cap-snatching, 10-18 nucleotides of 5' 7-methylguanylate primers are cleaved from host mRNAs and attached to prime the 5' end of the viral RNAs.^[8] Like all negative-sense RNA viruses, orthobunyaviruses require ongoing, concurrent translation by the host cell to produce full-length viral mRNAs, consequently the 3' end of orthobunyavirus mRNAs lack polyadenylation.^[8] Notably they are also missing the signal for polyadenylation; instead the 3' ends are thought to form a stem-loop structure.^[8][12] Antigenomes (full length positive-sense RNAs) used as templates for replication of the viral genome are produced by L protein RdRp without the need for primers.^[8] Both negative-sense genomes and positive-sense antigenomes are associated with N proteins (forming RNPs) at all times during the replication cycle.^[16] Thus, N and L are the minimum proteins required for transcription and replication^[15][12]

The M genome segment codes for the Gn-NSm-Gc polyprotein on a single open-reading frame (ORF) which is cotranslationally cleaved by internal signal peptides and host signal peptidase.^[15][8] The free glycoproteins Gc and Gn insert into the membrane of the endoplasmic reticulum and form heterodimers. A Golgi retention signal on Gn, permits transport of the heterodimers to the Golgi apparatus, where glycosylation occurs. The presence of the viral glycoproteins modifies the Golgi membrane to enable budding of RNPs into a Golgi derived tubular viral factory (viroplasm).^[14][8] As segmented viruses, orthobuynaviruses require precise packaging of one of each of the three genomic segments into the final virion to produce a mature, infectious particle. Packaging appears to be directed by signals contained entirely within UTR sequences.^[12] The packaged genomes acquire a lipid membrane as they bud into the viral factories, are then transported to the host cell plasma membrane and released via exocytosis. A final gylcoprotein modification upon release produces a mature, infectious particle.^[12]