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Where are we with a vaccine for ASFv?


Around the world, various institutes and companies are working towards a vaccine to control African Swine Fever. Some are promising, for example a recent report showed promising preliminary tests by the Plum Island Animal Disease Center in the USA. So, what is being done worldwide?


BY C.V. HERST, PHD, AND ERIC HARRELSON, FLOW PHARMA, CALIFORNIA, USA AND VINCENT TER BEEK, EDITOR, PIG PROGRESS


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SFv is a large and complex DNA virus (~170-190 kbp) encoding for about 150 proteins, with 23 genotypes identified thus far. The Georgia 2007 strain, a member of ASFv genotype II, is highly vir-


ulent and has been the focus of laboratories around the world actively investigating the molecular pathology of ASFv with the intent of developing a vaccine. The work of Dr Linda Dixon’s laboratory at the Pirbright In- stitute in the United Kingdom has focused on determining which of the ASFv proteins are essential for immunological protection by measuring the T-cell response to the viral pro- teins. T-cells, specifically cytotoxic T lymphocytes (CTLs), are key players in cellular immunity and are critical in defending against a viral challenge by recognising, attacking, and killing virally-infected cells. Recent work has examined small amino acid sequences (referred to as peptides) from each of 133 ASFv proteins to identify the most antigenic proteins and peptide sequences within. These results have demonstrated that AS- Fv-specific immune responses, both cellular (CTLs) and hu- moral (B-cells or antibody-mediated), are increased to a select


▶PIG PROGRESS | Volume 36, No. 1, 2020


group of these peptides. When the genes encoding these pep- tides were expressed by viral vectors and injected into pigs, vi- raemia (i.e., the amount of virus in the blood) was reduced sig- nificantly in pigs following a challenge with ASFv. Possible future directions may include delivering these peptides in combination with other molecules that can boost the immune response (adjuvants) to protect animals from disease. An im- portant consideration for any peptide-based vaccine to work effectively is to administer only those peptides that will be cor- rectly processed by the immune system based on the animal’s major histocompatibility loci (MHC; akin to tissue typing). On a positive note, the pig MHC loci, referred to as Swine Leukocyte Antigens (SLA), have been thoroughly documented and are available through various free-access Internet databases.


Killed or inactivated ASFv In the past, various research groups have used killed/inacti- vated ASFv as the immunogen, but this format did not pro- tect animals from ASFv challenges. More recently, trials with live attenuated virus have been performed. Although anti- bodies were induced, they did not protect the animals from infection and disease. In addition, there are always concerns using attenuated whole virus, as genetic rearrangements may restore their virulence. Dr Borca and Dr Gladue at the Plum Island Animal Disease Center in New York are developing genetically modified ASFv that lacks certain genes so that reversion to a virulent form is not possible. In early experiments, vaccination with these modified viruses proved effective against a ASFv challenge. For more information, see box. Dr Yolanda Revilla and others at the Severo Ochoa


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