A heterologous prime/boost immunisation strategy protects against virulent E. ruminantium Welgevonden needle challenge but not against tick challenge

Heterologous prime/boost immunisation strategies using the Ehrlichia ruminantium 1H12 pCMViUBs_ORFs [Pretorius A, Collins NE, Steyn HC, Van Strijp F, Van Kleef M, Allsopp BA. Protection against heartwater by DNA immunisation with four Ehrlichia ruminantium open reading frames. Vaccine 2007;25(12):2316–24] were investigated in this study. All the animals immunised twice with a recombinant (r) DNA cocktail of four 1H12 pCMViUBs_ORFs followed by a r1H12 protein and those immunised 3× with 1H12 plasmid rDNA showed 100% protection against a virulent E. ruminantium Welgevonden needle challenge. In addition, 90% of the sheep immunised twice with rDNA and boosted with r1H12 lumpy skin disease virus (LSDV) survived. Only the lymphocytes isolated from the r1H12 protein boost group showed specific proliferation and increased interferon (IFN)-γ expression. In contrast, only 20% protection was obtained in animals immunised with the rDNA prime/r1H12 protein boost when subjected to natural tick challenge in the field. Thus this heterologous prime/boost immunisation strategy had not conferred any significant protection against a field challenge.