Towards the development of an effective vaccine against malignant catarrhal fever

Alcelaphine herpesvirus 1 (AlHV-1) persists in wildebeest asymptomatically but induces malignant catarrhal fever (MCF), a fatal lymphoproliferative disease upon transmission to several ruminants, including cattle. The significant socio-economic impact of MCF in East-Africa urges for the development of new vaccination strategies. We have previously shown using the experimental rabbit model that the latency-associated gene ORF73 was essential for MCF induction while a ORF73-deficient (ORF73−) strain C500 could induce sterile immunity against a wild-type (WT) challenge. In the present study, we first infected 4-month-old calves with WT or ORF73− virus. Intranasal infection with the WT virus induced typical MCF clinical signs and lesions. However, ORF73− virus did not cause any clinical sign but induced a complete protection against an intranasal challenge with the WT virus. These results were encouraging for future prospects of vaccination against MCF. However, AlHV-1 is highly cell-associated and viral titres remain low, potentially hampering effective vaccine production. Interestingly, attenuated strain WC11 of AlHV-1 displays increased viral growth and cell-free infectious particles. Whole-genome sequencing of strain WC11 revealed few genomic changes including full deletion of the gene A7. A7 is a positional homolog of Epstein-Barr virus (EBV) BZLF2 encoding the C-type lectin-like glycoprotein gp42. Gp42 is expressed in the envelope of EBV virions and mediates entry into B cells. Hence, we used the C500 BAC clone to generate an A7STOP recombinant strain. We observed that a lack of A7 expression resulted in significant increased viral growth in fibroblasts in vitro. Also, the plaque size over time and the morphology of the plaques were modified in absence of A7. Finally, infection of rabbits demonstrated that A7 is essential for the development of MCF. In conclusion, the lack of A7 significantly alters the replication of the virus in vitro and the development of MCF in rabbits. Thus, joined impairments of A7 and ORF73 could lead to an optimized vaccine.