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

Abstract

Canine distemper virus (CDV), a negative stranded RNA morbillivirus, causes a multisystemic disease in dogs, which is associated with a severe immune suppression. The aim of the study was to examine the influence of early CDV infection on leukocyte depletion, lymphopenia and virus-induced cell death in dogs infected with a virulent CDV strain. From 10 infected dogs, peripheral blood leukocytes were harvested periodically, phenotyped and analyzed for CDV antigen content and apoptosis using Annexin V-FITC and propidium iodide labeling. CDV infection induced a severe CD3+ T cell and CD21+ B cell depletion in all animals at 3 days post-infection (d.p.i.). For dogs with severe distemper, developing virus persistence in the lymphoid tissue and central nervous system, this lymphopenia lasted until the end of the experiment. Increased levels of lymphocyte apoptosis were found at 3 d.p.i., and monocyte apoptosis at 6 d.p.i. This was more prominent in the group of animals with severe distemper. At 3 d.p.i. no leukocyte infection was detectable indicating that the early lymphocyte depletion and apoptosis was not a direct consequence of virus infection. Taken together, our results demonstrate that CDV-induced lymphopenia is an early event and that the degree of lymphocyte depletion correlates with the severity of disease and virus persistence in the lymphoid tissue and central nervous system.

Fig. 1

Leukocyte counts after CDV infection. The number of leukocytes/μl peripheral blood from dogs experimentally infected with CDV was determined before infection (day −23) and at different time points p.i. The upper graph shows the data for the CDV-Ag⁻ group and the lower graph for the CDV-Ag⁺ group, as defined in Table 1.

Fig. 2

Absolute counts of CD3 (A), CD21 (B) and CD14 (C) positive leukocytes before (day −23) and at different time points after experimental CDV infection. The upper graphs show the data for the CDV-Ag⁻ group and the lower graphs for the CDV-Ag⁺ group, as defined in Table 1.

Fig. 3

Absolute counts of CD4⁺ (A) and CD8⁺ (B) positive lymphocytes before (day −23) and at different time points after experimental CDV infection. The upper graphs show the data for the CDV-Ag⁻ group and the lower graphs for the CDV-Ag⁺ group, as defined in Table 1.

Fig. 4

Influence of experimental CDV infection on the percentage of Annexin V⁺ leukocytes. The values before (day −23) and at different time points p.i. are shown. The upper graphs show the data for the CDV-Ag⁻ group and the lower graph for the CDV-Ag⁺ group, as defined in Table 1.

Fig. 5

Influence of experimental CDV infection on the percentage of Annexin V⁺ CD3⁺, CD4⁺, CD8⁺, CD21⁺ and CD14⁺ leukocytes. The values before (day −23) and at different time points p.i. are shown. The gray boxes represent the data for the CDV-Ag⁻ group, and the empty boxes for the CDV-Ag⁺ group, as defined in Table 1.

Fig. 6

Detection of CDV N protein in leukocytes of dogs infected with CDV. The percentages at different time points p.i. are shown. The upper graphs show the data for the CDV-Ag⁻ group and the lower graph for the CDV-Ag⁺ group, as defined in Table 1.