Immune response of the host

Circulating antibodies in infected pigs can be detected by radioimmunoassay or ELISA 3-4 days after infection and they persist for life. Neutralizing antibodies have never been demonstrated. After recovery from infection with less virulent isolates, pigs are persistently infected for at least 6 months and are resistant to challenge with homologous virus, although the challenge virus may undergo limited replication without producing clinical disease; they are usually fully susceptible to infection with different virus genotypes.

Pigs given passive antibody or piglets given colos-tral antibody have reduced levels of virus in the blood and tissues when challenged with virulent virus and show little or no clinical signs of disease. Studies in vitro have also shown that antibodies can reduce virus yield from infected pig macrophages in the absence of complement but the transfer of immune lymphocytes has no effect on the course of disease.

Antibodies which mediate lysis of ASF virus infected pig kidney cells (IBRS2) in vitro, by either antibody-dependent cell-mediated cytotoxicity (ADCC) or complement-mediated lysis, are present in serum 12-14 days after infection but are not able to lyse infected pig macrophages.

Because the humoral immune response does not appear to mediate the immunity of pigs which recover from infection, the cell-mediated immune responses have been investigated to determine what immune mechanisms may be important, and the following observations have been made.

Natural cytotoxic cells are also unable to lyse infected macrophages but may be important as producers of interferon y (IFNy). Some isolates of ASF virus can induce IFNy production, and recombinant bovine IFNa and porcine IFNy reduce infectious virus production by pig alveolar macrophages and monocytes in vitro.

Stimulation of peripheral blood lymphocytes from uninfected pigs with recombinant porcine interleukin 2 (IL-2) will induce a lymphokine-activated killer (LAK) cell which will lyse infected pig macrophages. Lymphocytes from recovered animals stimulated with ASF virus in vitro will produce IL-2, which indicates that a lymphokine-activated cytotoxic lymphocyte may be involved. ASF virus-specific cytotoxic T lymphocytes obtained from infected inbred pigs with defined SLA haplotypes are major histocompatibility complex (MHC) class I restricted.

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