A major portion of the latent pseudorabies virus genome is transcribed in trigeminal ganglia of pigs

Abstract

Pseudorabies virus (PRV) is a porcine herpesvirus that establishes latent infections in trigeminal ganglia. To determine whether PRV expresses any transcripts that could play a role in latency, the trigeminal ganglia of 14 pigs previously inoculated through the nose and latently infected with PRV(Ka) were assayed by in situ nucleic acid hybridization for the presence of PRV-specific RNA. Hybridizations employing probes encompassing the entire viral genome revealed that an area extending from 0.64 to 0.82 map units was transcriptionally active. The DNA probe that most consistently detected transcripts was BamHI-8, a fragment which contains the gene for the immediate-early protein. With this probe, ganglia from 10 (71%) of 14 pigs scored positive for PRV RNA, although only 1 (8%) of 12 of the ganglia from the opposite side reactivated virus after explantation and culture of latently infected trigeminal ganglia. The RNA was transcribed from the strand opposite to that coding for the immediate-early protein; the signal was neuronally localized, with dense nuclear accumulation accompanied by variable numbers of grains over the cytoplasm. Northern RNA blot analysis showed that a discrete set of poly(A)-PRV transcripts were present in latently infected trigeminal ganglia. Additional in situ nucleic acid hybridization analysis revealed that the 3' limit of the transcriptionally active area was located in a 1.2-kilobase fragment upstream and adjacent to the 5' end of the immediate-early protein RNA, whereas the 5' limit was as much as 4.9 kilobases downstream from the 3' end of this RNA. PRV therefore expresses latent-phase transcripts that, although similar in many respects to latent-phase transcripts reported for other herpesviruses, have some unique properties.

Experimental investigation of herpes simplex virus latency

“…While HSV is the prototype for the alphaherpesviruses, it is important to recognize that there are important areas of divergence between HSV and other members of this group with respect to latency and reactivation. Unlike VZV (discussed above), other members of this group examined (bovine herpesvirus type 1 [BHV-1] equine herpesvirus type 1, and pseudorabies virus) have been shown to express LATs (10,31,159,214,215,227,230,231). While the LATs expressed by these viruses are located in equivalent regions of their respective genomes, the size of the transcripts (and intron species) can vary significantly among them.…”

A deletion at the UL/IR junction reduces pseudorabies virus neurovirulence

“…The LLT␤⌬2 deletion removes the 5Ј end of the second exon of LLT but does not alter the first exon. Other transcripts in the region include two additional poly(A) ϩ and two poly (A) Ϫ transcripts which are expressed during latency (8,22) and four poly(A) ϩ transcripts which are expressed late in the lytic infection cycle (9). The relationship between the lytic versus latent and poly(A) ϩ versus poly(A) Ϫ transcripts has not been fully elucidated.…”

“…The PRV I R sequences encode the immediate-early gene IE180 (7) and the 3Ј portion of the large latency transcript (LLT) (10). LLT, an 8.5-kb spliced poly(A) ϩ transcript, and several smaller RNA spaces are expressed in the TG of latently infected pigs (8,10,22,23). The first and second exons of LLT are antiparallel to and overlap the genes for EP0 (in the U L and a homolog of HSV ICP0) and IE180 (in the I R and a homolog of HSV ICP4).…”

Human Herpesvirus Latency

“…This approach revealed VZV DNA primarily, if not exclusively, in neurons (49). Overall, most data indicate that VZV is latent primarily in neurons (25,31,38,43,49), analogous to other alphaherpesviruses (5,16,48,61,62,68,81).…”