Role of S-layer protein antigenic diversity in the immune responses of sheep experimentally challenged with Campylobacter fetus subsp. fetus.

Surface layer proteins (SLPs) are essential for induction of abortion by Campylobacter fetus subsp. fetus in experimentally challenged ewes. These proteins are encoded by multiple sap genes and vary in size and antigenicity. The role of SLP antigenic variation during experimental ovine infection was investigated. Following subcutaneous challenge, the SLPs were highly antigenic, and antibodies were detected in serum, milk, bile, and urine. Fecal anti-SLP antibodies were detected only in animals challenged orally. Ewes challenged with wild-type strain 23D with variable SLPs developed detectable circulating anti-SLP immunoglobulin G (IgG) antibodies by 2 weeks postchallenge. In contrast, ewes challenged with mutants of 23D that had fixed expression of a single SLP developed antibodies within 1 week postchallenge, suggesting that antigenic variation in SLPs may delay the host antibody response. Although not statistically significant, the data from challenge experiments in which vaccinated ewes were used suggested that SLP-expressing vaccines could protect animals from abortion and that this effect was independent of the SLP expressed, indicating involvement of conserved epitopes in the SLP. The conserved 184-amino-acid N-terminal region of the SLP, identified from previously published sequences, was epitope mapped with rabbit anti-SLP antisera by using overlapping synthetic 20-mer peptides. Two putative epitopes were identified at amino acids 81 to 110 and 141 to 160. Amino acids 81 to 100 also bound serum IgG antibodies from experimentally challenged sheep. Conserved antigenic regions of the SLP that induce protective immune responses may enable development of synthetic vaccine candidates for C. fetus subsp. fetus-associated ovine abortion.