An adjuvant formulation containing toll-like receptor 7 agonist stimulates protection against morbidity and mortality due to Anaplasma marginale in a highly endemic region of west Africa

Authors: FUTSE, JAMES - University Of Ghana; ZUMOR-BALIGI, SONGLIEDONG - University Of Ghana; ASHIAGBOR, CHARLES - University Of Ghana; Noh, Susan; FOX, CHRISTOPHER - University Of Washington; PALMER, GUY - Washington State University

Submitted to: PLOS ONE
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 6/11/2024
Publication Date: 8/29/2024
Citation: Futse, J.E., Zumor-Baligi, S., Ashiagbor, C.N., Noh, S.M., Fox, C.B., Palmer, G.H. 2024. An adjuvant formulation containing toll-like receptor 7 agonist stimulates protection against morbidity and mortality due to Anaplasma marginale in a highly endemic region of west Africa. PLOS ONE. https://doi.org/10.1371/journal.pone.0306092.
DOI: https://doi.org/10.1371/journal.pone.0306092

Interpretive Summary: Anaplasma marginale, a tick-borne pathogen that causes bovine anaplasmosis, negatively impacts cattle health and productivity worldwide. In tropical and subtropical regions of Africa the high prevalence of tick borne diseases, including bovine anaplasmosis, greatly limit the use of higher producing breeds of cattle. Efficacious, affordable vaccines area lacking, thus improved methods to control this disease are needed. If animals survive the initial infection, they develop persistent, life long inapparent infection and immunity. In this study, we stimulated the innate immune system of calves, allowed for natural exposure of A. marginale, and compared disease severity in treated and untreated animals. The animals that received the immune stimulant had reduced sickness and death due to bovine anaplasmosis compared to the animals that did not receive the immune stimulant. Importantly, the treated animals had similar weight gain to the untreated animals indicating stimulation of the innate immunity did not have long term negative impacts on growth. Overall, this work demonstrates that significant short term protection against bovine anaplasmosis in highly susceptible cross-bred cattle is possible. This finding is critically important in making the use of higher producing cross bred cattle feasible to increase productivity and ultimately income for smallholder farmers in Africa.

Technical Abstract: Efficient cattle production and provision of animal-sourced foods in much of Africa is constrained by vector-borne bacterial and protozoal diseases. Effective vaccines are not currently available for most of these infections resulting in a continuous disease burden that limits genetic improvement. We tested whether stimulation of innate immunity using the Toll-like Receptor (TLR) 7 agonist imiquimod, formulated with saponin and water-in-oil emulsion, would protect against morbidity and mortality due to Anaplasma marginale, a tick-borne pathogen of cattle highly endemic in west Africa. In Trial 1, haplotype matched Friesian x Sanga (F1) A. marginale negative calves were allocated to either the experimental group (n = 10) and injected with the synthetic TLR 7 agonist/saponin formulation or to an untreated control group (n = 10). TLR7 agonist/saponin injected calves responded with significantly elevated rectal temperature, enlarged regional lymph nodes, and elevated levels of IL-6 post-injection as compared to control group calves. All calves were then allowed to graze in pasture for natural exposure to tick transmission. All calves in both groups acquired A. marginale, consistent with the high transmission rate in the endemic region. The need for antibiotic treatment, using pre-existing criteria, was significantly lower in the experimental group (odds ratio for not requiring treatment was 9.3, p = 0.03) as compared to the control group. Despite treatment, 6/10 calves in the control group died, reflecting treatment failures that are typical of anaplasmosis in the acute phase, while mortality in the experimental group was 1/10 (odds ratio for survival was 13.5, p = 0.03). The trial was then repeated using 45 Friesian x Sanga calves per group. In Trial 2, the odds ratios for preventing the need for treatment and for mortality in the TLR7 agonist/saponin experimental group versus the control group were 5.6 (p = 0.0002) and 7.0 (p = 0.004), respectively, reproducing the findings of the initial trial. Together these findings demonstrate that innate immune stimulation using a TLR7 agonist formulated with saponin and water-in-oil emulsion provides significant protection against disease caused by tick borne A. marginale in highly susceptible cross-bred cattle, critically important for their potential to increase productivity for smallholder farmers in Africa.