Lymphocyte Apoptosis during Classical Swine Fever: Implication of Activation-Induced Cell Death

Abstract

Infection of pigs with classical swine fever virus (CSFV), a member of the Flaviviridae family, causes a severe leukopenia, particularly notable with the lymphocytes. The goal of this study was to analyze mechanisms behind this CSFV-induced lymphopenia. To this end, the kinetics of leukocyte depletion, the appearance of apoptotic cells, and virus infection of leukocytes after infection of pigs with the virulent CSFV strain Brescia were analyzed. Depletion of B and T lymphocytes was noted as early as 1 day postinfection (p.i.). Circulating viable lymphocytes with reduced mitochondrial transmembrane potential—a particular early marker for apoptosis—were also detectable as early as 1 day p.i. When isolated peripheral blood mononuclear cells were cultured for 6 h, significantly more sub-G1 cells with reduced DNA content were detected among the lymphocytes from CSFV-infected animals, again as early as 1 to 3 days p.i. The first time virus was first found in the plasma, as well as infection of leukocytes, was 3 days p.i. However, throughout the observation time of 1 week, <3% of the circulating leukocytes and no lymphocytes contained virus or viral antigen. Further analysis of the T lymphocytes from infected animals demonstrated an increase in CD49d, major histocompatibility complex class II, and Fas expression. An increased susceptibility to apoptosis in vitro was also observed, particularly after addition of concanavalin A as well as apoptosis-inducing anti-Fas antibody to the cultures. Taken together, these results imply that activation-induced programmed cell death was the mechanism behind lymphopenia during classical swine fever.

Canine distemper virus-induced depletion of uninfected lymphocytes is associated with apoptosis

“…In the acute phase of many other virus infections a lymphopenia with characteristics as those presented here can be observed (Oldstone, 1996). Examples include classical swine fever (Summerfield et al, 1998, bovine viral diarrhea (Ridpath et al, 2000), porcine Circovirus-2 infection (Nielsen et al, 2003), influenza (Tumpey et al, 2000), severe acute respiratory syndrome (SARS) (Peiris et al, 2003), foot and mouth disease (Bautista et al, 2003), Dengue virus infection (Fadilah et al, 1999), cytomegalovirus infection (Einsele et al, 1993), and many others. Interestingly, in many cases a correlation of disease severeness and degree of lymphocyte depletion was observed.…”

Oral immunogenicity of porcine reproductive and respiratory syndrome virus antigen expressed in transgenic banana

“…Lymphocyte proliferation assay was performed using PBMCs from immunized pigs, and swine PBMCs were isolated as described by Summerfield et al (1998). The PBMCs were plated in 96-well round-bottom plates at a concentration of 2 9 10 5 cells/well (100 lL/well) and incubated with or without PRRSV (10 4 TCID 50 / well) in hexaplicate for 72 h at 37°C in 5% CO 2 .…”

Immune evasion during foot‐and‐mouth disease virus infection of swine

“…Some viral infections of swine cause immunopathology by inducing a decline in circulating lymphocytes. Classical swine fever virus, for example, induces a severe lymphopenia of 7-10 days duration (51)(52)(53). PRRSV causes a transient leukopenia (54,55) and pathology in lymphoid tissues of infected swine (56).…”