Natural cell-mediated cytotoxicity to cells infected with infectious bovine rhinotracheitis virus

Bovine natural killer cells

Natural killer (NK) cells have received much attention due to their cytotoxic abilities, often with a focus on their implications for cancer and transplantation. But despite their name, NK cells are also potent producers of cytokines like interferon-gamma. Recent discoveries of their interplay with dendritic cells and T-cells have shown that NK cells participate significantly in the onset and shaping of adaptive cellular immune responses, and increasingly these cells have become associated with protection from viral, bacterial and parasitic infections. Furthermore, they are substantially present in the placenta, apparently participating in the establishment of normal pregnancy. Consequently, NK cells have entered arenas of particular relevance in veterinary immunology. Limited data still exist on these cells in domestic animal species, much due to the lack of specific markers. However, bovine NK cells can be identified as NKp46 (CD335) expressing, CD3(-) lymphocytes. Recent studies have indicated a role for NK cells in important infectious diseases of cattle, and identified important bovine NK receptor families, including multiple KIRs and a single Ly49. In this review we will briefly summarize the current understanding of general NK cell biology, and then present the knowledge obtained thus far in the bovine species.

Role of a distinct population of bovine gamma delta T cells in the immune response to viral agents

A distinct population of bovine gamma delta T cells was isolated from peripheral blood mononuclear cells of foot-and-mouth disease (FMD)-vaccinated cattle; these lymphocytes were shown to exert a natural killer-like activity against cells infected by different viruses. The antiviral activity was dependent upon cognate recognition of target cells and could operate by both cytostatic and cytotoxic mechanisms. Among these, secretion of a serine esterase was shown after binding to target cells. This population of bovine gamma delta T cells is recognized by murine monoclonal antibodies 1E7, 5D4, and 6F9, raised in our laboratory. To define an in vivo antiviral role, four heifers were infected with a strain of bovid herpesvirus 1 by the intranasal/intravaginal routes and contact exposure. The prevalence of 1E7+/5D4+ cells among peripheral blood lymphocytes increased dramatically in the first days after infection; the same held true for in-contact cattle, albeit with a different time kinetics. In another experiment, colonization of mucosae was demonstrated by immunoperoxidase staining on tongue and palate sections of healthy cattle. The infiltration of gamma delta T cells altogether in the palate mucosa was much more accentuated in foot-and-mouth disease-vaccinated, as compared to nonvaccinated, control calves.

Target recognition by bovine mononuclear, MHC-unrestricted cytotoxic cells

A population of bovine non B/non T, cytotoxic lymphocytes with natural killer activity against virus-infected and non-infected embryonic kidney cells was functionally characterized. The data obtained in experiments of flow cytometry and immuno-peroxidase staining show that a CD2-, CD4-, CD8-, TcR gamma delta-, CD3+, CD45+, FcR+ lymphoid killer cell does exist within bovine peripheral blood leucocytes. This population can detect the down-regulation of class I MHC antigens or the expression of embryonic forms thereof, as shown by experiments of 17-hour 51Cr release and binding to target cells. This model was tested in vitro in experiments on virus-infected bovine kidney cells. The emerging picture was substantially in agreement with the "missing self" theory as a possible option for target cell recognition. In this respect, the profound alteration of MHC Class I expression could represent a major early event, recognized on virus-infected cells by the immune system.

Characterization and activation requirements of bovine lymphocytes acquiring cytotoxic activity after interleukin-2 treatment

Interleukin-2 (IL-2) treatment of cells and generation of non-major histocompatibility complex (MHC)-restricted cytotoxic cells from peripheral blood mononuclear leukocytes (PBML) was studied. Effector-target conjugate assays demonstrated that bovine PBML bound but did not lyse K562, HL60S and HL60R cells unless activated with IL-2. The magnitude of IL-2-activated killing of tumor cells as well as the magnitude of antibody-dependent cellular cytotoxicity depended on the IL-2 concentration. A short treatment (12-18 h) of effector cells with IL-2 was sufficient for development of cytotoxic activity. Withdrawal of IL-2 from the culture resulted in a reduction of cytotoxic activity that could be restored by further addition of IL-2. Cytotoxic activity of IL-2-activated populations obtained after nylon wool or Sephadex G-10 passage, and Percoll gradient centrifugation of PBML suggests that lymphokine-activated killer (LAK) cell activity in PBML is mainly mediated by a non-adherent lymphocyte lacking markers for B-cells. Positive and negative selection experiments using cell sorting confirmed these findings and demonstrated that the cell responsible for LAK cell activity in cattle belongs to a non-monocyte, non-B, CD2+ lymphocyte population. Furthermore, cytotoxic activity could not be generated in CD2+ populations enriched for cells expressing molecules equivalent to human and murine CD4 and CD8. These findings suggest that effector cells mediating non MHC-restricted cytotoxicity in cattle prevail in a population bearing a CD2+, CD4-, CD8- phenotype and that this population depends on the continuous presence of IL-2 for optimal cytotoxic function.

Demonstration of the in vitro antiviral properties of bovine lymphokine-activated killer (LAK) cells

In cattle, cells with functional characteristics similar to those of natural killer (NK) cells are difficult to detect. However, lymphokine-activated killer (LAK) cells can be detected readily after in vitro activation of peripheral blood mononuclear leukocytes (PBML) with interleukin-2 (IL-2). In the present study, we demonstrated that IL-2-activated PBML preferentially lyse bovine herpesvirus type 1 (BHV-1)-infected cells and that the cell responsible for the lysis copurified with the cell responsible for lysis of K562. The IL-2-activated effector cells were capable of significant reducing virus production. The reduction in virus yield seemed to be by an interferon (IFN)-independent mechanism, as the amount of IFN induced in effector cells by BHV-1 was not altered by the addition of IL-2. Furthermore, enrichment of cytotoxic cells by passage through nylon wool columns removed the capability of PBML to produce IFN in response to the virus. These results suggest that activation of LAK mechanisms in cattle plays a role in controlling virus spread.

Bovine natural cell mediated cytotoxicity (NCMC): activation by cytokines

Incubation of bovine peripheral blood mononuclear leukocytes (PBML) with the cytokines (CK) IL-2, alpha-IFN, gamma-IFN or IL-4 resulted in significant increases in natural cell mediated cytotoxicity (NCMC) over endogenous levels, as determined in an 18 h 51Cr-release assay using the human K562 or mouse Yac-1 target cell lines. Endogenous cytotoxic activity of bovine natural effector cells (NEC) using K562 or Yac-1 target cells was minimal (killing less than 8%). After 18 h of incubation with the CK hurIL-2, alpha-bovrIFN, gamma-bovrIFN or hurIL-4, NEC had significant increases in cytotoxic activity for both K562 and Yac-1 target cells. Significant increases in cytotoxic activity were not found after incubation of NEC with IL-1 or beta-IFN. Specific killing varied with CK concentration in a dose dependent manner and was proportional to effector:target cell ratio. Activation of the bovine NEC by CK was rapid, occurring within 6-12 h of incubation with alpha-IFN or gamma-IFN and within 12-18 h of incubation with IL-2. Incubation of bovine PBML with IL-2 and alpha- or gamma-IFN or with alpha-IFN and gamma-IFN showed that these CK do not act in a synergistic manner to increase NCMC in the bovine NEC.