Blood lymphocyte subsets in pigs vaccinated and challenged with Actinobacillus pleuropneumoniae.

Actinobacillus pleuropneumoniae bacterins do not induce protection in pigs while infection with low doses of the CM5 strain of A. pleuropneumoniae given by aerosol induces complete protection. To evaluate possible correlates of protection in blood lymphocyte subset phenotypes, pigs were treated with a commercial bacterin given intramuscularly, low dose (10(5)cfu/ml) aerosol infection with CM5 or control treatments of the bacterin adjuvant or phosphate buffered saline. All pigs were challenged with a high dose (10(7)cfu/ml) of A. pleuropneumoniae. Lymphocytes and sera were collected prior to and following primary and secondary immunizations and challenge, for evaluation of B- and T-cell markers and antibody to four A. pleuropneumoniae antigens. IgM(micro)+ B-cells were increased following primary exposure to antigen in the bacterin-vaccinated group only. An increase in CD4+ cells in the LD aerosol-infected group was apparent following secondary exposure to antigen. These early changes suggest little difference in lymphocyte populations between treatment groups, however, greater differences were observed following high-dose challenge; CD4+ lymphocytes were increased significantly in both bacterin and LD-challenged groups (p<0.05) while CD8+ cells decreased in the LD-group at this time period. Consequently, there were significant differences (p<0.05) in the CD4:CD8 ratio after high-dose challenge compared to earlier time points and control groups. Variation in cellular expression of SLA-DR and DQ was observed but trends correlating to treatment group were not evident. Complete protection or lack of protection associated with LD challenge or immunisation resulted in significant differences in B-cell frequencies and CD4:CD8 ratio phenotypes in pigs, but only changes in CD4:CD8 ratios appeared relevant to protection.

Anti-haemolysin IgG1 to IgG2 ratios correlate with haemolysin neutralization titres and lung lesion scores in Actinobacillus pleuropneumoniae infected pigs.

Currently available porcine Actinobacillus pleuropneumoniae bacterins afford only minimal protection by decreasing mortality but not morbidity. To evaluate a possible role of IgG subclasses in protection, IgG1 and IgG2 responses to A. pleuropneumoniae haemolysin (HLY) were examined in piglets exposed to a low dose (10(5) c.f.u. ml-1) of A. pleuropneumoniae CM5 (LD) given by aerosol (which affords complete protection) or bacterin-vaccinated piglets (no protection). Only the LD group developed HLY neutralizing antibody. These animals produced both IgG1 and IgG2-associated antibody in response to HLY, and there was a positive correlation (r = 0.6) between IgG1 anti-HLY antibody and neutralizing titres. Anti-HLY IgG1 antibody was negatively correlated with pneumonic scores at necropsy (r = -0.67, p < or = 0.005). These results suggest that immunization procedures that bias anti-HLY antibody to IgG1 may be more efficacious than those inducing IgG2.

Antibody- and cell-mediated immune responses of Actinobacillus pleuropneumoniae-infected and bacterin-vaccinated pigs.

Current porcine pleuropneumonia bacterins afford only partial protection by decreasing mortality but not morbidity. In order to better understand the type(s) of immune response associated with protection, antibody- and cell-mediated immune responses (CMIR) were compared for piglets before and after administration of a commercial bacterin, which confers partial protection, or a low-dose (10(5) CFU/ml) aerosol challenge with Actinobacillus pleuropneumoniae CM5 (LD), which induces complete protection. Control groups received phosphate-buffered saline or adjuvant. Serum antibody response, antibody avidity, delayed-type hypersensitivity (DTH), and lymphocyte blastogenic responses were measured and compared among treatment groups to the lipopolysaccharide (LPS), capsular polysaccharide (CPS), hemolysin (HLY), and outer membrane proteins (OMP) of A. pleuropneumoniae. Peripheral blood lymphocytes and sera were collected prior to and following primary and secondary immunization-infection and high-dose A. pleuropneumoniae CM5 (10(7) CFU/ml) aerosol challenge. Serum antibody and DTH, particularly that to HLY, differed significantly between treatment groups, and increases were associated with protection. LD-infected piglets had higher antibody responses (P < or = 0.01) and antibody avidity (P < or = 0.10) than bacterin-vaccinated and control groups. Anti-HLY antibodies were consistently associated with protection, whereas anti-LPS and anti-CPS antibodies were not. LD-infected animals had higher DTH responses, particularly to HLY, than bacterin-vaccinated pigs (P < or = 0.03). The LD-infected group maintained consistent blastogenic responses to HLY, LPS, CPS, and OMP over the course of infection, unlike the bacterin-vaccinated and control animals. These data suggest that the immune responses induced by a commercial bacterin are very different from those induced by LD aerosol infection and that current bacterins may be modified, for instance, by addition of HLY, so as to stimulate responses which better reflect those induced by LD infection.