Characterization of the antibody response to Brucella abortus S2308 challenge infection in cattle

Abstract

Attempts to define the antibody response to Brucella abortus vaccination and challenge have been contradictory in assessing the relative importance of specific isotypes and their expression in resistant and susceptible cattle. We approached the resolution of the antibody response to Brucella abortus at both the population level in immunized cattle and in a group of nonimmunized naturally resistant and susceptible cattle. The lack of reagents reactive with newly recognized IgG subisotypes necessitated production of subisotype-specific monoclonal antibodies in addition to allotype-specific reagents. The antibody response to Brucella abortus infection in immunized and nonimmunized cattle was assessed by solid-phase enzyme-linked immunosorbent assay (ELISA). The responsiveness in resistant and susceptible cattle was determined using B. abortus low (2%) protein lipopolysaccharide (LPS) and whole cells of the rough "O" antigen deficient strain, RB51. Irrespective of immunization status, the postchallenge lgG1 RB51-specific and LPS-specific responses were of greatest value in predicting the outcome of infection. Immunized animals received either rough cell envelopes or 2% protein LPS in RIBI adjuvant or were treated with the live vaccine strain of B. abortus (S19). LPS-specific post-immunization levels were highest for the lgG1 and lgG2a subisotypes. The lgG2b total levels postchallenge differentiated culture negative and culture positive cattle. Immunoglobulin allotype expression was evaluated by the LPS or RB51 whole cell ELISA in 24 nonvaccinated cattle challenge exposed with B. abortus 2308. The Brucella antigen-specific ELISA indicated persistent levels of LPS-specific lgG2a in nonimmunized susceptible cattle as long as 36-48 months postchallenge while nonimmunized resistant animals had >10 fold lower lgG2a levels relative to pre-exposure levels. The skewed expression of lgG2a(A1) in immunized culture positive cows and nonimmunized susceptible cows in the Brucella antigen-specific ELISA provides presumptive evidence for an association of the A1 allotype with susceptibility to brucellosis in cattle. Whether this association is due to direct involvement of immunoglobulin heavy chain allotypes in host resistance, through linkage to an as yet undefined immune response gene, or linkage with variable region gene segments is not known but will be resolved in future studies.