IgG and IgM antibodies in normal and leukaemic cattle

MicroRNAs Encoded by Bovine Leukemia Virus (BLV) Are Associated with Reduced Expression of B Cell Transcriptional Regulators in Dairy Cattle Naturally Infected with BLV

Bovine leukemia virus (BLV) is estimated to infect over 83% of dairy herds and over 40% of all dairy cows in the United States. While, BLV only causes leukemia in a small proportion of animals, research indicates that BLV+ cattle exhibit reduced milk production and longevity that is distinct from lymphoma development. It is hypothesized that BLV negatively affects production by interfering with cattle immunity and increasing the risk of secondary infections. In particular, BLV+ cows demonstrate reduced circulating levels of both antigen-specific and total IgM. This study investigated possible mechanisms by which BLV could interfere with the production of IgM in naturally infected cattle. Specifically, total plasma IgM and the expression of genes IGJ, BLIMP1, BCL6, and PAX5 in circulating IgM+ B cells were measured in 15 naturally infected BLV+ and 15 BLV− cows. In addition, BLV proviral load (PVL) (a relative measurement of BLV provirus integrated into host DNA) and the relative expression of BLV TAX and 5 BLV microRNAs (miRNAs) were characterized and correlated to the expression of selected endogenous genes. BLV+ cows exhibited lower total plasma IgM and lower expression of IGJ, BLIMP1, and BCL6. While, BLV TAX and BLV miRNAs failed to correlate with IGJ expression, both BLV TAX and BLV miRNAs exhibited negative associations with BLIMP1 and BCL6 gene expression. The results suggest a possible transcriptional pathway by which BLV interferes with IgM production in naturally infected cattle.

Reduced humoral immunity and atypical cell-mediated immunity in response to vaccination in cows naturally infected with bovine leukemia virus

Bovine leukemia virus (BLV) is a retrovirus that is widely distributed across US dairy herds: over 83% of herds are BLV-infected and within-herd infection rates can approach 50%. BLV infection reduces both animal longevity and milk production and can interfere with normal immune health. With such a high prevalence of BLV infection in dairy herds, it is essential to understand the circumstances by which BLV negatively affects the immune system of infected cattle. To address this question, BLV- and BLV+ adult, lactating Holstein dairy cows were vaccinated with Bovi-Shield GOLD^® FP^® 5 L5 HB and their immune response to vaccination was measured over the course of 28days. On day 0 prior to vaccination and days 7, 14 and 28 post-vaccination, fresh PBMCs were characterized for T and B cell ratios in the periphery. Plasma was collected to measure titers of IgM, IgG1 and IgG2 produced against bovine herpesvirus 1 (BHV1), Leptospira hardjo and L. pomona, as well as to characterize neutralizing antibody titers produced against BHV1 and bovine viral diarrhea virus types 1 and 2. On day 18 post-vaccination, PBMCs were cultured in the presence of BHV1 and flow cytometry was used to determine IFNγ production by CD4+, CD8+ and γδ T cells and to investigate CD25 and MHCII expression on B cells. BLV+ cows produced significantly lower titers of IgM against BHV1, L. hardjo and L. pomona and produced lower titers of IgG2 against BHV1. γδ T cells from BLV+ cows displayed a hyper reactive response to stimulation in vitro, although no differences were observed in CD4+ or CD8+ T cell activation. Finally, B cells from BLV+ cows exhibited higher CD25 expression and reduced MHCII expression in response to stimulation in vitro. All together, data from this study support the hypothesis that BLV+ cows fail to respond to vaccination as strongly as BLV- cows and, consequently, may have reduced protective immunity when compared to healthy BLV- cows.

Bovine leukemia virus: a major silent threat to proper immune responses in cattle

Bovine leukemia virus (BLV) infection is widespread in the US dairy industry and the majority of producers do not actively try to manage or reduce BLV incidence within their herds. However, BLV is estimated to cost the dairy industry hundreds of millions of dollars annually and this is likely a conservative estimate. BLV is not thought to cause animal distress or serious pathology unless infection progresses to leukemia or lymphoma. However, a wealth of research supports the notion that BLV infection causes widespread abnormal immune function. BLV infection can impact cells of both the innate and adaptive immune system and alter proper functioning of uninfected cells. Despite strong evidence of abnormal immune signaling and functioning, little research has investigated the large-scale effects of BLV infection on host immunity and resistance to other infectious diseases. This review focuses on mechanisms of immune suppression associated with BLV infection, specifically aberrant signaling, proliferation and apoptosis, and the implications of switching from BLV latency to activation. In addition, this review will highlight underdeveloped areas of research relating to BLV infection and how it causes immune suppression.

Increased MHC class II and CD25 expression on lymphocytes in the absence of persistent lymphocytosis in cattle experimentally infected with bovine leukemia virus

We recently observed that a group of cattle experimentally infected with bovine leukemia virus (BLV) had enhanced antibody responses to recall antigens. None of the cattle in this group were classified as persistently lymphocytotic, but they did have significantly increased numbers of circulating T and B cells. In order to investigate the potential mechanisms of BLV-induced immune activation, dual-color flow cytometry was used to compare the expression of MHC class II (MHC II) molecules and the inducible IL-2 receptor alpha chain, CD25, on lymphocyte subsets in freshly isolated and cultured PBMC from these same BLV-infected cattle (n=5) with that of age-matched, uninfected controls (n=3). Freshly isolated peripheral blood mononuclear cells (PBMC) from BLV-infected cattle were found to contain a significantly higher percentage of B cells that expressed MHC II molecules (p<0.01). In addition, an increased proportion of CD4+ T cells from BLV-infected cattle expressed MHC II molecules after 20 h of Concanavalin A (Con A) stimulation (p<0.05), and MHC II expression was increased on both CD4+ (p<0.01) and CD8+ (p<0.05) T cells from BLV-infected cattle after 68 h in vitro, even in the absence of exogenous mitogen. Although CD25 expression was not increased on freshly isolated lymphocytes from BLV-infected cattle, an increased percentage of B cells from BLV-infected cattle expressed CD25 after 20 h of culture, either in the presence (p<0.05) or absence (p<0.01) of Con A. Thus, in addition to causing alterations in absolute numbers of circulating lymphocytes, BLV infection appears to cause a functional activation of both B and T cells, even in cattle that are non-lymphocytotic. It is likely that these BLV-induced alterations in lymphocyte activation status contributed to the previously observed enhancement of antibody responses in vivo.

Detrimental effect of bovine leukemia virus (BLV) on the immunological state of cattle

Bovine leukemia virus (BLV) is a retrovirus which seems to affect both the humoral and the cellular immune response. Cows affected by enzootic bovine leukemia (EBL) showed a reduction of IgM-producing cells in the spleen and lymph nodes. Experimentally infected calves had lower levels of secretory IgM and a decrease in T lymphocytes in the peripheral blood. The reduction in the amount of T cells was noticed mainly in cells bearing the CD4 markers. BLV-infected animals showed diminished responsiveness to newly encountered antigens. Cows naturally infected by BLV produced Igs with impaired structural or biological reactivity. The primary immune response was shown to be deficient in BLV-infected cows following vaccination with synthetic antigen. A marked shift in the proportion of PBL, especially of the CD5+ subset, was noticed. Peripheral blood mononuclear cells from BLV-infected cows secrete elevated levels of certain cytokines and contain increased levels of cytokine mRNA. High levels of cytokines are also found in the sera of BLV-infected cows compared to non-infected animals. A correlation was found between BLV infection and lack of spontaneous recovery from Trichophyton verrucosum infection. Moreover, some studies ascertained a significant association between the herd BLV infection status and disease incidence. The culling rate was higher and milk production lower in BLV-infected vs. BLV-free herds. It seems that BLV infection affects the immune system of a cow to such an extent that it ceases to be productive enough to be kept and, in most cases, the animal is culled before any symptoms of illness associated with persistent immunodeficiency become apparent.

Lymphoid leukosis viruses, their recognition as 'persistent' viruses and comparisons with certain other retroviruses of veterinary importance

Diseases caused by lymphoid leukosis virus (LLV), a retrovirus, take a long time after infection to develop and have a wide variety of pathological manifestations. This long latent period is characteristic of 'persistent virus infections'. Disease produced by LLV infection and its underlying mechanisms is compared with 'persistent' infections caused by other retroviruses in birds and mammals of veterinary importance. The diseases considered for comparison are those caused by reticuloendotheliosis, feline leukaemia, bovine leukosis and equine infectious anaemia viruses. There are significant changes in the immunological status in all diseases caused by these viruses. LLV infections follow this trend with, in manifestations of neoplastic disease, a perturbation of the normal switch that occurs from IgM to IgG synthesis. There are also indications of other immunological disturbances. Factors other than immunological disturbances may contribute to the length of time after infection required for the many forms of LLV infection to appear. Such additional factors may include the operation of 'biological clocks', such as the arrival of sexual maturity, and also the very nature of retroviruses. These factors, like the immunological changes, play major roles in the maintenance and progression of persistent retrovirus infections.

Serum immunoglobulin concentrations in cattle naturally infected with bovine leukemia virus

In order to elucidate whether natural infection of BLV in cattle might induce humoral immunological responses, changes in IgG1, IgG2, and IgM concentrations in the sera of infected cattle were determined. Twelve BLV-infected cattle were used. Cattle of different breeds were classified serologically and haematologically into BLV + PL+, BLV + PL- and BLV-free groups. Ig concentrations in the sera of the three groups were quantitated using a commercial single radial immunodiffusion assay. The findings were compared to those of BLV-free cattle. The serum IgM concentrations were significantly lower in cattle with PL (P less than 0.001) than in BVL + PL- and BLV-free cattle. The IgM concentrations tended to be lower in BLV+ PL- than those of BVL-free cattle. There were no significant differences in IgG1 and IgG2 serum concentrations between the three experimental cattle groups. IgG1 was the predominant subtype in the sera of all cattle groups.

Neutrophil function in sheep experimentally infected with bovine leukemia virus

In vitro neutrophil adherence, random migration, chemotaxis, resting and phagocytosis-associated oxygen consumption and bactericidal responses were assessed in sheep experimentally infected with bovine leukemia virus (BLV). Neutrophil function was examined in two groups of 9 control and 9 BLV-infected sheep at 0, 1, 2, 3, 5, 7, 11 and 15 weeks post-infection. Enhanced neutrophil adherence, chemotaxis and resting oxygen consumption responses were found in the infected group at 2, 11 and 15 weeks respectively. Significant alterations between groups were not demonstrated during the other time intervals.

Immunocompetence of sheep experimentally infected with bovine leukemia virus

The humoral and cellular immunological reactivity of sheep were studied throughout the first 32 weeks following experimental infection with bovine leukemia virus (BLV). Seroconversion of BLV-inoculated sheep occurred within 4 weeks, but infection was not transmitted to contact control sheep. Despite the persistence of the viral infection, no differences were demonstrated in leukograms, serum IgG concentrations, humoral response to immunization with an irrelevant antigen (rabbit red blood cells), phytomitogen (Concanavalin A and Pokeweek mitogen)-induced lymphocyte blastogenesis, or chemical (1-chloro, 2-4 dinitrobenzene) skin contact hypersensitivity, between BLV-infected and uninfected contact control sheep. These results demonstrate the absence of a nonspecific immunosuppressive effect of BLV and further negate the influence of a generalized immunological deficit on the development of clinical disease in BLV-infected animals.

Possible association of antibody responses to human serum albumin and (T,G)-A--L with the bovine major histocompatibility complex (BoLA)

Antibody responses to human serum albumin (HSA) and (T,G)-A--L were determined in 130 young bulls in Norway and the BoLA types of the bulls were defined. Significant associations of some BoLA antigens with immune responsiveness were shown, indicating the likely existence of an immune response (Ir) region linked to the BoLA class I antigens. High response to HSA seems to be a dominant trait. BoLA w2 showed an association with low response to HSA. This may reflect the effect of a specific MHC-associated immune suppressor gene.

Humoral and cellular responses in calves experimentally infected with bovine leukemia virus (BLV)

In order to elucidate whether infection by Bovine Leukemia Virus (BLV) might induce an immunodeficient state, we inoculated sixteen calves with BLV. The calves were followed up for two years and were tested for humoral and cellular responses using various parameters, namely the appearance of antibodies to the BLV antigens, the changes in the numbers of lymphocytes involved, and the ratio between the two main populations of lymphocytes. Antibodies to the BLV antigens were of both the IgG and the IgM classes of immunoglobulins. The levels of antibodies of the IgM class were higher than those of IgG. There was a temporary decrease of reactive antibodies to the BLV antigens, to below detectable levels, during the 14-24 weeks post infection. A significant decrease in the level of plasma IgM was found in all BLV infected calves exhibiting lymphocytosis, while the level of IgG in the plasma of all experimental calves did not diverge significantly from the initial values, throughout the experiment. BLV infection was followed by lymphocytosis of B-cells in most infected calves, which persisted for the whole course of the experiment, while a decrease in the population of T-cells in peripheral blood was observed for a period of several months in all infected calves.

Use of a synthetic antigen to predetermine the responsiveness of cattle to vaccine-induced immunity

Groups of healthy, virus-infected and leukaemic cattle were injected with synthetic multichain poly(Tyr, Glu)-poly(Ala)--poly(Lys) in order to predetermine responsiveness to vaccine-induced immunity. Immunospecific purification of antibodies, precipitin reactions and enzyme linked immunosorbent assay (ELISA) tests were carried out on sera collected at various intervals after immunization. Of the methods employed, the ELISA had the advantage of specificity, sensitivity and speed for screening the humoral immune response of cattle to antigenic stimulation.