Differential apoptotic effects of BVDV and BHV-1 on PBMCs comparing animals from BVDV seronegative and immunized dairy herds

Apoptosis can prevent the replication and spread of viral infections, therefore many viruses have developed strategies to prevent this phenomenon. However, in a variety of acute viral infections, there is an increasing evidence that immunodepression is directly associated with the induction of apoptosis in immune cells in order to facilitate viral dissemination. Bovine viral diarrhea virus (BVDV) and bovine herpesvirus 1 (BHV-1) infections give rise to acute and persistent or latent infections, being considered key predisposing factors in the development of the Bovine Respiratory Disease Complex as a result of their immunosuppressive features. Since synergic interactions have been observed in vivo between BVDV and BHV1, we have established an in vitro model for viral coinfection in which bovine peripheral blood mononuclear cells (PBMCs) are infected with both viruses to characterize the susceptibility of animals in contact with an immunotolerant calf to BVDV for apoptotic processes following a secondary infection. Thus, PBMCs were collected from eight animals, four collected from a seronegative dairy herd and from other four suffering an ongoing infection with a persistently infected (PI) animal to BVDV in Lecco and Como provinces. PBMCs were separated into 4 groups of infection: Uninfected control, infected with noncytopathic BVDV-1a, infected with BHV-1 subtype 1 and infected with both viruses, being incubated for 18, 24, 48, and 72 hours. Dual staining with Annexin V-FITC and Propidium Iodide (PI) was used to discriminate by flow cytometry early apoptotic, necrotic/late apoptotic and live cells (MBL MEBCYTO Apoptosis kit; MBL). Additionally, caspases 3/7 are part of a common complex where the two apoptotic routes converge, and once activated, irreversibly execute cell death. Expression levels of Caspases 3/7 were determined by an Apo-ONE© Homogeneous Caspase-3/7 kit. Total leukocyte and platelet counts from the animals used in this study had values included within the normal range for the bovine species, except for the PI animal that suffered a marked lymphocytosis. Our results showed that in vitro infection with BVDV and/or BHV-1 induces an apoptotic effect in PBMCs primary cultures. Moreover, when we compare PBMCs from BVDV pre-infected and healthy animals, we observed that exposure to the PI animal played an important role in the development of an increased susceptibility of PBMCs against secondary infections, observing less cell survival at the in vitro conditions tested and, in particular, after viral infections. Cell death was associated with a marked activation of Caspases-3/7, which appears to be directly related with massive PBMCs apoptosis, being the progression from early to late apoptosis/necrosis more intense and of sooner appearance in the animals in contact with the PI calf. These evidences could suggest a possible association between BVDV pre-infection and the apoptotic mechanisms by which this virus would establish immunosuppression in susceptible cattle. BVDV and BHV-1 ability to induce apoptosis in vitro on T lymphocytes, B lymphocytes and monocytes might also have important implications in vivo, by affecting cell cytotoxicity, cytokine and antibody production, as well as having an inhibitory effect on the lymphocyte proliferative response.