As recently reported by the world organisation for animal health, 60% of infectious diseases are zoonotic with a significant part associated to ticks. Ticks can transmit various pathogens such as bacteria, viruses and parasites. Among pathogens known to be transmitted by ticks, Ehrlichia ruminantium is an obligate intracellular Gram-negative bacterium responsible for the fatal heartwater disease of domestic and wild ruminants (Allsopp, 2010). E. ruminantium is transmitted by ticks of the genus Amblyomma in the tropical and sub-Saharan areas, as well as in the Caribbean islands. It constitutes a major threat for the American livestock industries since a suitable tick vector is already present in the American mainland and potential introduction of infected A. variegatum through migratory birds or uncontrolled movement of animals from Caribbean could occur (i.e. Deem, 1998 ; Kasari et al 2010). The disease is also a major obstacle to the introduction of animals from heartwater-free to heartwater-infected areas into sub-Saharan Africa and thus restrains breeding programs aiming at upgrading local stocks (Allsopp, 2010).

In this context, it is essential to develop control strategies against heartwater, as developing effective vaccines, for instance. Such an objective requires a better understanding of the early interaction of E. ruminantium and its host cells and of the mechanisms associated with bacterial adhesion to the host-cell. In this study, the authors. studied the role of E. ruminantium membrane protein ERGA_CDS_01230 in the adhesion process to host bovine aortic endothelial cells (BAEC).

After successfully producing the recombinant version of the protein, Pinarello et al (2022) followed the in vitro culture of E. ruminantium in BAEC and observed that the expression of the protein peaked at the extracellular infectious elementary body stages. This result would suggest the likely involvement of the protein in the early interaction of E. ruminantium with its host cells. The authors then showed using flow cytometry, and scanning electron microscopy, that beads coated with the recombinant protein adhered to BAEC. In addition, they also observed that the adhesion protein of E. ruminantium interacted with proteins of the cell's lysate, membrane and organelle fractions. Additionally, enzymatic treatment, degrading dermatan and chondroitin sulfates on the surface of BAEC, was associated with a 50% reduction in the number of bacteria in the host cell after a development cycle, indicating that glycosaminoglycans might play a role in the adhesion of E. ruminantium to the host-cell. Finally, the authors observed that the adhesion protein of E. ruminantium induced a humoral response in vaccinated animals, making this protein a possible vaccine candidate.

As rightly pointed out by both reviewers, the results of this study represent a significant advance (i) in the understanding of the role of the E. ruminantium membrane protein ERGA_CDS_01230 in the adhesion process to the host-cell and (ii) in the development of new control strategies against heartwater as this protein might potentially be used as an immunogen for the development of future vaccines.

References

Allsopp, B.A. (2010). Natural history of Ehrlichia ruminantium. Vet Parasitol 167, 123-135. https://doi.org/10.1016/j.vetpar.2009.09.014

Deem, S.L. (1998). A review of heartwater and the threat of introduction of Cowdria ruminantium and Amblyomma spp. ticks to the American mainland. J Zoo Wildl Med 29, 109-113.

Kasari, T.R. et al (2010). Recognition of the threat of Ehrlichia ruminantium infection in domestic and wild ruminants in the continental United States. J Am Vet Med Assoc. 237:520-30. https://doi.org/10.2460/javma.237.5.520

Pinarello V, Bencurova E, Marcelino I, Gros O, Puech C, Bhide M, Vachiery N, Meyer DF (2022) Ehrlichia ruminantium uses its transmembrane protein Ape to adhere to host bovine aortic endothelial cells. bioRxiv, 2021.06.15.447525, ver. 3 peer-reviewed and recommended by Peer Community in Infections. https://doi.org/10.1101/2021.06.15.447525