Flow cytometric analysis of African swine fever virus-induced plasma membrane proteins and their humoral immune response in infected pigs

Antigenic properties and diagnostic potential of African swine fever virus protein pp62 expressed in insect cells

African swine fever (ASF) is an infectious and economically important disease of domestic pigs. The absence of vaccine renders the diagnostic test the only tool that can be used for the control of new outbreaks of the disease. At present, the enzyme-linked immunosorbent assay (ELISA) test is the most useful method for large-scale ASF serological studies, although false positives have been detected, mainly on poorly preserved sera. In order to improve the current diagnostic test available for ASF, we have studied the antigenic properties of the ASF virus polyprotein pp62 and its suitability for use in a novel ELISA. Two well-known antigenic proteins of ASF virus, p32 and p54, were also included in this study. These proteins were expressed in the baculovirus expression system and used as antigens in ASF serological tests. Our results indicate that the use of these recombinant proteins as antigens in the ELISAs improves the sensitivity and specificity obtained with the conventional diagnosis test used to detect antibodies against ASF virus. Furthermore, the use of polyprotein pp62 in an ELISA for testing poorly preserved sera allows performance of the diagnosis of ASF without the need to confirm the results by the immunoblot test. These features make pp62 one of the most interesting viral proteins to be used for serological ASF diagnosis.

Modulation of immune cell populations and activation markers in the pathogenesis of African swine fever virus infection

African swine fever (ASF) virus induces immune cell alterations that may be detected by changes in peripheral blood cells phenotypic antigens and activation markers which were examined by flow cytometry, analyzing both cell proportion and/or expression intensity of superficial antigens. These studies were conducted in pigs with experimental acute of chronic ASF infection to determine whether changes among important surface activation markers and phenotypic antigens, and their correlative lymph node status, reflected similar or disparate aspects of immune pathology. In acute infection produced by virulent viruses, macrophage and B lymphocyte populations decreased in peripheral blood after a short activation period at the beginning of the infection. A significative decrease of interleukin 2 receptor (IL 2R) expression was also observed in those pigs. These variations correlated with lymph node cell depletion due to an intense lymphoid cell death by apoptosis, affecting mainly the B lymphocyte subpopulation as determined by immunohistochemistry. Nevertheless, pigs infected with an attenuated isolate undergoing chronic persistent infection, presented a distinct pattern of modification, according with a different clinicopathological evolution. Changes consisted in systemic immune activation coincident with the highest viremia titer, with an augmentation in CD8+ T lymphocyte, macrophage, and B cell populations, and MHC (major histocompatibility complex) antigens. Percentage elevation of circulating immune subpopulations was accompanied by cell accumulation with lymphoid hyperplasia but a conserved distribution of B lymphocytes in lymphoid organs of chronically infected pigs.

Application of a monoclonal antibody recognizing protein p30 to detect African swine fever virus-infected cells in peripheral blood

Monoclonal antibody (MAb) 174F11.8 recognizes an epitope of the African swine fever (ASF) virus-induced protein, p30, a protein expressed on the plasma membrane of infected cells. This MAb has been used to analyze infected cell populations in peripheral blood of experimentally inoculated pigs with a virulent or attenuated ASF virus. Flow cytometric analysis of peripheral blood at different days postinfection using this MAb, showed expression of p30 mainly in the monocyte/macrophage cell lineage. Additionally, a small percentage of granulocytes also expressed p30 during infection. This methodology allowed the quantification of fluctuations in the pool of infected monocyte/macrophage cells in the inoculated pigs, maximum percentages ranging between 6 and 31%. Significant differences in the percentages of cell populations expressing p30 were not found between virulent or attenuated virus infection. However, a 2- to 4-day delay in the maximum percentage of cells expressing p30 was observed during infection with the attenuated virus when compared to virulent virus infection. Percentages of infected cells detected by the expression of p30 and viral titres obtained in peripheral blood showed positive correlation. Consequently, MAb 174F11.8 constitutes a marker to follow evolution of ASF virus infection, allowing quantification of percentages of infected cells in peripheral blood.

Improvement of African swine fever virus neutralization assay using recombinant viruses expressing chromogenic marker genes

Antibody neutralization of African swine fever (ASF) virus measured by a plaque reduction assay presents frequent difficulties because of the absence or delay in plaque formation by many strains, especially low-passage viruses. To overcome this problem, a new ASF virus neutralization test has been developed. The new test consists of a conventional plaque reduction assay in which the viral plaques are detected by expression of marker genes. For the development of this neutralization assay 4 mutant viruses were generated by homologous recombination, containing beta-galactosidase or beta-glucuronidase reporter genes inserted into the thymidine kinase locus of the viral genome. These recombinant viruses have the following advantages with respect to parental viruses: (1) the neutralization assay takes less than a third of the time needed using non-recombinant viruses; (2) the small plaques can be detected more accurately by color contrast; and (3) the neutralization-resistant virus clones can be recovered easily post-plaque counting. Additionally, these recombinant viruses permit differentiation by chromogenic staining of individual infected pig macrophages, the natural host cell for ASF virus, facilitating neutralization assays in these primary cultures as described in cell lines.

Characterization and molecular basis of heterogeneity of the African swine fever virus envelope protein p54

It has been reported that the propagation of African swine fever virus (ASFV) in cell culture generates viral subpopulations differing in protein p54 (C. Alcaraz, A. Brun, F. Ruiz-Gonzalvo, and J. M. Escribano, Virus Res. 23:173-182, 1992). A recombinant bacteriophage expressing a 328-bp fragment of the p54 gene was selected in a lambda phage expression library of ASFV genomic fragments by immunoscreening with antibodies against p54 protein. The sequence of this recombinant phage allowed the location of the p54 gene in the EcoRI E fragment of the ASFV genome. Nucleotide sequence obtained from this fragment revealed an open reading frame encoding a protein of 183 amino acids with a calculated molecular weight of 19,861. This protein contains a transmembrane domain and a Gly-Gly-X motif, a recognition sequence for protein processing of several ASFV structural proteins. In addition, two direct tandem repetitions were also found within this open reading frame. Further characterization of the transcription and gene product revealed that the p54 gene is translated from a late mRNA and the protein is incorporated to the external membrane of the virus particle. A comparison of the nucleotide sequence of the p54 gene carried by two virulent ASFV strains (E70 and E75) with that obtained from virus Ba71V showed 100% similarity. However, when p54 genes from viral clones generated by cell culture passage and coding for p54 proteins with different electrophoretic mobility were sequenced, they showed changes in the number of copies of a 12-nucleotide sequence repeat. These changes produce alterations in the number of copies of the amino acid sequence Pro-Ala-Ala-Ala present in p54, resulting in stepwise modifications in the molecular weight of the protein. These duplications and deletions of a tandem repeat sequence array within a protein coding region constitute a novel mechanism of genetic diversification in ASFV.

Uses of flow cytometry in virology

This article reviews some of the published applications of flow cytometry for in vitro and in vivo detection and enumeration of virus-infected cells. Sample preparation, fixation, and permeabilization techniques for a number of virus-cell systems are evaluated. The use of flow cytometry for multiparameter analysis of virus-cell interactions for simian virus 40, herpes simplex viruses, human cytomegalovirus, and human immunodeficiency virus and its use for determining the effect of antiviral compounds on these virus-infected cells are reviewed. This is followed by a brief description of the use of flow cytometry for the analysis of several virus-infected cell systems, including blue tongue virus, hepatitis C virus, avian reticuloendotheliosis virus, African swine fever virus, woodchuck hepatitis virus, bovine viral diarrhea virus, feline leukemia virus, Epstein-Barr virus, Autographa californica nuclear polyhedrosis virus, and Friend murine leukemia virus. Finally, the use of flow cytometry for the rapid diagnosis of human cytomegalovirus and human immunodeficiency virus in peripheral blood cells of acutely infected patients and the use of this technology to monitor patients on antiviral therapy are reviewed. Future prospects for the rapid diagnosis of in vivo viral and bacterial infections by flow cytometry are discussed.

African swine fever virus genome content and variability

A 55 kilobase pair (kb) region from the right end of the virulent African swine fever virus isolate, Malawi LIL20/1, has been sequenced. The 68 major open reading frames (ORFs) encoded are generally closely spaced and read from both DNA strands across the complete sequence. Comparison of the amino acid sequences of predicted ORFs with sequence databases identified 15 ORFs which encode proteins that are similar to proteins of known function. Two ORFs are homologous to copies of multigene family 360 (MGF360) and one ORF is homologous to copies of multigene family 110 (MGF110). Both of these multigene families have been described previously.