Date Full Report Received06/11/2018
Date Abstract Report Received06/11/2018
InvestigationInstitution: ARS, Beltsville Area, USDA
Primary Investigator: Dr. Joan K. Lunney Ph.D.
Co-Investigators: Jack C.M. Dekkers, James Reecy
Funded ByNational Pork Board
Despite extensive efforts to eliminate PRRS from US production facilities, it remains a key disease issue and poses a continued economic threat to the industry, particularly in pig dense areas. A major factor that complicates PRRS control is viral persistence. This project proposed to identify pigs which have persistent PRRSV infections. Pigs with persistent PRRSV infections, or carrier pigs, are a continuing threat to each production unit. Viral survival is maintained because a proportion of the herd have virus which persists in lymphoid tissues (tonsil, lymph nodes) and can be shed occasionally (due likely to stress, diseases or other factors). This shed virus then infects the remaining herd pigs many of which are naïve and thus susceptible. Transmission studies have verified that pigs can harbor the virus for >160 days, and likely longer. Closing a herd for 200 days was thought to be effective before new “clean pigs” could be reintroduced; some would argue for longer times before the PRRS virus becomes extinct.
Currently there is no good technology to accurately identify PRRSV carrier pigs, nor are there procedures to treat pigs to eliminate persistent virus from their tissues. This proposal determined the frequency of pigs with persistent PRRSV by quantitating viral RNA levels in tonsil as a surrogate measure of viral persistence. To perform this, we took advantage of the repository of samples that were collected through the NPB funded PRRS Host Genetics Consortium (PHGC). Each PHGC pig, provided at weaning from current commercial breeding stocks, was infected with PRRSV and followed for 42 days post infection (dpi). Every pig that survived to 42 dpi had tonsil tissue archived. All data is preserved in the PHGC database (www.animalgenome.org/lunney/index.php) including the pig’s pedigree, response to PRRSV infection (serum viral and antibody levels and weight gain data), along with extensive genotypic information.
Our work with tonsils from pigs infected with type 2 PRRSV (virulent NVSL or moderately virulent KS-06) showed limited relationship between serum viral load (VL; 0-21 dpi) with tonsil viral RNA levels (TV). This limited correlation is not unexpected since PRRSV is known to persist in tissues, particularly the tonsil, well past the time when serum viremia has cleared. When comparing the two viruses our data indicated that there is much lower cumulative virus with KS-06 as compared to NVSL infected pigs in serum and in tonsil. This grant affirmed that pigs infected with the moderately virulent PRRSV (KS-06) exhibited similar tonsil persistence characteristics to the virulent NVSL infection.
Our statistical analysis (Hess et al. 2018) verified that pigs identified as having persistent serum viremia levels for NVSL had significantly higher TV than pigs that had already cleared serum viremia (P<0.001). This trend was similar for KS-06 infection but not as significant. Tonsil virus level was estimated to be lowly heritable for both type 2 PRRSV isolates. Data indicated that an earlier and faster serum virus clearance was associated with lower TV for both NVSL and KS-06 infected pigs. Analysis of weekly weight gain did not reveal any associations with TV (Hess et al. 2018).
Mapping studies revealed several genomic regions that explained a proportion of genetic variance for tonsil viral levels. Several strong candidate genes were identified and may be involved with the host’s ability to control viral infiltration/replication and the ability to clear infected cells from tissue and are useful targets for gene expression analyses on tonsil tissue that are underway. These data may provide insight into alternative genes involved in host genetic control of tonsil virus levels and viral persistence (Hess et al., 2018).
Finally, we probed for host genes that might be involved in persistence by analyzing pig gene expression in tonsil RNA. Overall, 12,597 genes were determined to be expressed in the tonsil with 1646 and 336 differentially expressed genes (DE genes, q≤0.2) identified between PRRSV isolate and TVclass, respectively (Dong et al., 2018, in preparation). Pathway analysis results showed that both KS-06 and high TV were associated with DE genes predicted to increase the quantity, proliferation, differentiation, cell movement and adhesion of immune cells, especially T cells, compared to NVSL and low TV. Preliminary results indicate that there were 4 DE genes that were significantly up-regulated in tonsils from pigs from high versus low tonsil viral level: CXCL10, TBX21, CCL5 and CCL19. Overall, nursery pigs infected with a less pathogenic PRRSV isolate, or that have higher tonsil viral level, have a stronger tonsil immune response. Collectively, these DE gene results suggest that KS-06 infection may result in less tonsil tissue damage by regulating genes related to cell and tissue morphology. High tonsil virus levels may activate the expression of genes that trigger cellular immune responses to clear virus that persists in tonsils and inhibits virus replication.
These findings contribute to our understanding of the mechanisms involved in tonsil pathology induced by PRRSV infection in pigs. Based on this, efforts can be planned to selectively breed for pigs with lower tissue persistence or, alternately, to identify means of stimulating anti-viral responses in pigs with persistent PRRSV infections.