Effects of aging on antibody avidity to Mycoplasma pulmonis

Antibody avidity of serum and lung lavage responses was examined in rats to determine aging effects on functional differences in antibody to Mycoplasma pulmonis. Three age groups of animals (weanling, adult and senescent) were immunized with either of two doses of formalinized M. pulmonis as the antigen, or a placebo control. Total immunoglobulin levels and specific antibody responses were examined in serum and lung lavage fluids and subsequently avidity measurements of the same samples were made for the specific antibody to M. pulmonis. The concentration of NH4SCN that dissociated 50% of the antibody was used to determine the avidity index of the serum and lung lavage samples. Total serum IgG and IgA were decreased in the weanling animals when compared to the other two age groups of animals. In contrast, serum IgA levels were substantially increased in senescent animals. Significant increases in serum IgA levels were noted following immunization that was not observed for IgG levels. Substantial increases in both serum antibody and lung lavage antibody were observed in response to immunization with either dose of antigen, but only the lung lavage samples showed both IgG and IgA isotypes differences that were attributable to age. Serum IgG avidity indices gradually increased over time following immunization with higher indices being observed in the weanling animals immunized with the higher M. pulmonis dose. Serum IgA avidity indices also increased over time with no significant differences noted among the age groups. Lung lavage IgG avidity demonstrated slightly higher indices in the weanling animals, while lung lavage IgA avidity showed higher avidity indices in the senescent animals at the higher antigen dose. These data suggest that senescent animals are capable of producing an apparently functional antibody response and that differences noted in increased disease susceptibility in older animals may be attributed to mechanisms other than a dysfunctional humoral immune response at mucosal surfaces.