Bovine Neonatal Pancytopenia (BNP) is a disease of calves, characterised by peripheral blood and bone marrow depletion, which emerged in Europe in 2007. A strong epidemiological association between BNP and the administration of a particular inactivated Bovine Viral Diarrhoea (BVD) vaccine (Pregsure BVD, Pfizer Animal Health) to the dams of affected calves has been reported. Early studies suggested that BNP is mediated by the transfer of alloantibodies in colostrum and that these alloantibodies recognise major histocompatibility complex (MHC) class I molecules. This led to the hypothesis that Pregsure contains bovine MHC I molecules, originating from the MDBK line cell used in vaccine production, and that this is responsible for the generation of alloantibodies in particular cows injected with the vaccine. This project aimed to investigate the mechanisms by which BNP arises and develops. In particular to gain an understanding of the molecular basis of the syndrome and how this influences the number of cows and calves affected and the specificity of the pathology for the haematopoietic system. Haematological analysis of clinically normal calves born on a BNP-affected farm demonstrated that 15% of calves had profoundly abnormal haematology and could be described as affected by subclinical BNP. BNP was reproduced experimentally by feeding pooled colostrum to neonatal calves, confirming the role of colostrum in mediating the condition. Detailed analysis of serial haematology and bone marrow pathology from these calves demonstrated variable alloantibody damages to different haematopoietic lineages. In vitro cellular assays using a panel of MHC I-defined bovine leukocyte cell lines and mouse cells individually transfected with the MDBK-MHC I alleles demonstrated that Pregsure vaccinated cows have significantly higher titres of functionally active MHC I alloantibodies than BVDV unvaccinated cows or cows vaccinated with alternative BVDV vaccines. The alloantibody response was found to be heterogeneous in individual Pregsure vaccinated cows. MHC I expression levels on peripheral blood and bone marrow cells, assessed by flow cytometry, was shown to correlate with levels of in vitro and in vivo alloantibody damage. Overall, the results of this project demonstrate that the pathogenesis of BNP is mediated by the transfer of MHC I-specific alloantibodies via colostrum that cause rapid destruction of peripheral blood and bone marrow cells, and which is dependent on the titre of alloantibody produced by an individual cow, its specificity for the MHC I alleles present, and density of MHC I expression on the specific cells.

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