Polyclonal activation of B cells occurs in lymphoid organs from porcine reproductive and respiratory syndrome virus (PRRSV)-infected pigs

Porcine reproductive and respiratory syndrome virus (PRRSV) induces a persi stent viral infection associated with an inefficient humoral immune respons e. A study of lymphoid B cells and specific humoral immune response was per formed in blood and several lymphoid organs collected from PRRSV experiment ally-infected pigs. Groups of specific pathogen-free (SPF) pigs were infect ed with the LHVA-93-3 isolate of PRRSV, and blood, tonsils, spleen and medi astinal lymph nodes (MLN) were collected at various times postinfection (p. i.) (3-60 days). Lymphoid cells were isolated, immunolabeled for cytofluoro metric determination of B cell percentages, used for counting specific anti -PRRSV antibody secreting B cells by an ELISPOT assay, or cultured for meta bolic activity. The presence of anti-PRRSV antibodies in the serum of infec ted pigs was determined using a commercial ELISA assay. Virus detection was performed in all tissues, including lungs, by virus isolation and RT-PCR. The results show that percentages of B cells increased in tonsils as soon a s 3 days until 17 days p.i. in PRRSV-infected pigs while they increased in spleen at 3 days p.i. only, due to an increase of larger Ig(high)-producing B cells. Metabolic activity of lymphoid cells from blood and spleen increa sed at 3 days p.i. only while lymphoid cells from tonsils and MLN transient ly decreased at that time and increased thereafter up to 60 days p.i. Anti- PRRSV antibody-secreting B cells occurred in tonsils after 10 days p.i. and strongly increased up to 60 days p.i. However, specific anti-PRRSV-secreti ng B cells were detected in blood and spleen after 17 days p.i and in MLN o nly after 45 days p.i. Specific antibodies were detectable in serum at 10 d ays p.i., reached the maximum level at 45 days and remained high up to 60 d ays p.i. Infectious virus was detected in lungs and MLN as soon as 3 days p .i., and remained detectable up to 45 days p.i. in tonsils of one pig while viral RNA was detected in most organs up to 60 days p.i. In vitro experime nts revealed that inactivated virus induced a stimulation of lymphoid cells isolated from PRRSV-infected pigs while it was cytotoxic for lymphoid cell s from control pigs. Taken together, these results indicate that viral infe ction induced simultaneously a polyclonal activation of B cells, mainly in tonsils. and an exaggerated and prolonged specific humoral immune response due to persistent viral infection in lymphoid organs. (C) 2001 Elsevier Sci ence B.V. All rights reserved.