%0 Journal Article %A Labarque, Geoffrey G. %A Nauwynck, Hans J. %A Van Reeth, Kristien %A Pensaert, Maurice B. %T Effect of cellular changes and onset of humoral immunity on the replication of porcine reproductive and respiratory syndrome virus in the lungs of pigs %D 2000 %J Journal of General Virology, %V 81 %N 5 %P 1327-1334 %@ 1465-2099 %R https://doi.org/10.1099/0022-1317-81-5-1327 %I Microbiology Society, %X Twenty-two 4- to 5-week-old gnotobiotic pigs were intranasally inoculated with 106·0 TCID50 of porcine reproductive and respiratory syndrome virus (PRRSV) (Lelystad) and euthanized at different time intervals post-inoculation (p.i.). Bronchoalveolar lavage (BAL) cell populations were characterized, together with the pattern of virus replication and appearance of antibodies in the lungs. Total BAL cell numbers increased from 140×106 at 5 days p.i. to 948×106 at 25 days p.i. and remained at high levels until the end of the experiment. The number of monocytes/macrophages, as identified by monoclonal antibodies 74-22-15 and 41D3, increased two- to fivefold between 9 and 52 days p.i. with a maximum at 25 days p.i. Flow cytometry showed that the population of differentiated macrophages was reduced between 9 and 20 days p.i. and that between the same time interval, both 74-22-15-positive and 41D3-negative cells, presumably monocytes, and 74-22-15- and 41D3-double negative cells, presumably non-phagocytes, entered the alveolar spaces. Virus replication was highest at 7 to 9 days p.i., decreased slowly thereafter and was detected until 40 days p.i. Anti-PRRSV antibodies were detected starting at 9 days p.i. but neutralizing antibodies were only demonstrated in one pig euthanized at 35 days and another at 52 days p.i. The decrease of virus replication in the lungs from 9 days p.i. can be attributed to (i) shortage of susceptible differentiated macrophages, (ii) lack of susceptibility of the newly infiltrated monocytes and (iii) appearance of anti-PRRSV antibodies in the lungs. Neutralizing antibodies may contribute to the clearance of PRRSV from the lungs. %U https://www.microbiologyresearch.org/content/journal/jgv/10.1099/0022-1317-81-5-1327