Date Full Report Received10/13/2008
Date Abstract Report Received10/13/2008
InvestigationInstitution: University of Iowa
Primary Investigator: John Butler
Co-Investigators: Kelly Lager
Funded ByNational Pork Board
Viral infection exposes the host immune system to epitopes that are normally recognized by T cells and which leads to both cytotoxic responses against virus-infected cells and to B cells that make viral neutralizing (VN) antibodies. The anti-viral response to PRRSV is reported to be delayed, the response to porcine circovirus (PCV-2) is poorly understood but the response to swine influenza (SIV) is robust and classical and resolves the infection in 2-3 weeks. Therefore we wondered if the T cell receptor Vb gene usage (important in specificity and an indicator of T cell superantigen activity) was unusual in PRRSV-infected piglets versus those infected with SIV. In addressing our first objective we compared four major TCRVb families; Vb 4,-5,-7-, 12 in PRRSV, PVC-2 and SIV infected isolator piglets. Isolator piglets were used in an effort to remove other sources of immune pressure, e.g. other pathogens, gut flora, etc. We proposed and initially addressed TCRVb usage by cloning and hybridization using Vb gene family-specific probes but later developed a Cyber Green-based real-time PCR assay for quantitation of TCRVb. The data remain incomplete but suggest: (a) there is little evidence for preferential TCRVb usage in PRRSV, PCV-2 and SIV infections and (b) there is no T cell superantigen effect in any piglet we have examined. In pursuing our second objective, we studied the affect of PRRSV, PCV-2 and SIV on B cell activity by quantifying IgG, IgM and IgA in serum and bronchial alveolar lavage and through adaptation of real-time PCR to measure the proportion of T- and B cells in lymphoid tissue of infected animals and sham controls. The latter method failed to show that PRRSV preferentially causes B cell proliferation; a conclusion not supported by data on Ig levels and earlier histological studies. This raised our concern about the value and validity of the real-time PCR. Thus we are collaborating with Dr. Harry Dawson to compare two forms of real-time PCR with cloning and hybridization; a full report on these comparative studies will be available at year’s end. In summary the delayed appearance of VN in PRRS is unlikely to be due to some type of unusual T cell repertoire usage. Rather it is more likely to be a B cell immune dysregulation phenomenon (see NPB 05-174). This phenomenon may be the cause of the delayed protective response to PRRSV and the incomplete protection provided by current vaccines.