Abstract:
Ehrlichia ruminantium is an obligate intracellular bacterial pathogen which causes heartwater,
a serious tick-borne disease of ruminants throughout sub-Saharan Africa. The
development of promising recombinant vaccines has been reported previously, but none
has been as effective as immunisation with live organisms. In this study we have used
reverse vaccinology to identify proteins that elicit an in vitro cellular immune response
similar to that induced by intact E. ruminantium. The experimental strategy involved four
successive steps: (i) in silico selection of the most likely vaccine candidate genes from the
annotated genome; (ii) cloning and expression of the selected genes; (iii) in vitro screening
of the expressed proteins for their ability to induce interferon-gamma (IFN-ᵧ) production
in E. ruminantium–immune lymphocytes; and (iv) further examination of the cytokine
response profiles of those lymphocytes which tested positive for IFN-ᵧ induction. Based on
their overall cytokine induction profiles the recombinant proteins were divided into four
distinct groups. Eleven recombinant proteins induced a cytokine profile that was similar
to the recall immune response induced by immune peripheral blood mononuclear cells
(PBMC) stimulated with intact E. ruminantium. This response comprised the upregulation
of cytokines associated with adaptive cellular immune responses as well as innate immunity.
A successful vaccine may therefore need to contain a combination of recombinant
proteins which induce both immune pathways to ensure protection against heartwater.
