Influence of pig age on virus titer and bactericidal activity of porcine reproductive and respiratory syndrome virus (PRRSV)-infected pulmonary intravascular macrophages (PIMs).

Twelve pigs (six 4-week-old and six 4-month-old cross-bred, specific pathogen free pigs) were used as donors for both pulmonary intravascular macrophages (PIMs) and pulmonary alveolar macrophages (PAMs). The PIMs and PAMs were infected in vitro with low (ISU-55) or high (VR-2385) virulence strains of PRRSV at 1 multiplicity of infection (m.o.i.) for comparisons of virus titers at 48 h post infection (PI). PIMs were as permissive as PAMs to infection with both PRRSV isolates yielding similar progeny titers (10(4.81) vs. 10(5.22) TCID50/ml, respectively). Both ISU-55 and VR-2385 were able to infect PIMs and no significant difference in virus replication as measured by virus titers between isolates was found (10(5.33) vs. 10(4.69) TCID50/ml, respectively). PIMs from 4-weak-old pigs yielded a higher virus titer following PRRSV infection than PIMs from 4-month-old pigs (10(5.43) vs. 10(4.59) TCID50/ml, respectively; p < 0.02). VR-2385-infected PIMs had significantly decreased bactericidal (Staphylococcus aureus) activity compared with uninfected PIMS at 48 h PI (p < 0.05). There was no difference in bactericidal activity between ISU-55 (low virulence)-infected PIMs and VR-2385 (high virulence)-infected PIMs. Both ISU-55 and VR-2385 infection significantly decreased the production of superoxide anion (SOA) at 24 and 48 h PI (p < 0.01). In conclusion, (1) PRRSV had a detrimental effect on bactericidal activity and SOA production of PIMs, (2) PIMs from younger pigs were more permissive to PRRSV infection, and (3) the selected PRRSV strains, which differ in their abilities to induce pneumonia in vivo were not different when tested in vitro by measuring virus titer and bactericidal functions.