Apoptotic fraction in lymphoid tissue of FIV-infected SPF cats

In the present study the apoptotic fraction has been investigated in peripheral blood mononuclear cells (PBMCs) and in lymphoid tissue of six clinically asymptomatic serologically positive specific pathogen free (SPF) FIV-infected cats with a decline in peripheral blood CD4+ lymphocytes, compared to five FIV− SPF controls. Apoptosis in PBMCs was scored in relation to cell cycle phases judged by the integrating cytometric measure of DNA content with 3H-thymidine and 3H-leucine incorporation. Apoptosis in lymphoid tissue was revealed with the ApopTag-peroxidase kit, quantified by image analysis and expressed as apoptotic index (number of apoptosis per 100 cells). The high percentage of apoptotic death in lymphocytes from FIV+ cats was chronologically related to the entrance of cells in the S phase of the cell cycle (p<0.0001). No difference in the apoptotic index was revealed comparing the follicular, cortical+paracortical and medullary compartments in lymph nodes of FIV+ and FIV− cats. In each group of cats a similar pattern of apoptosis expression was revealed in lymph nodes: significantly higher in follicular vs. both cortical+paracortical and medullary compartments (p<0.001). In the thymus a significant increase in apoptotic index was revealed in the cortical compartment of the FIV+ cats compared to FIV− (p<0.001), while in the spleen both the red and white pulp expressed a higher value in FIV+ cats compared to FIV− (p<0.05) and the former showed a pattern of expression as follows: in the red pulp significantly higher than in the white pulp (p<0.001). This investigation suggests that the priming signals for apoptosis in FIV infection parallels the S phase of the cell cycle and peripheral blood changes could follow both thymic and splenic modifications in apoptotic expression.