Influence of enzootic bovine leukosis on immune response of naturally infected cattle

Author(s)

Milton Ricardo Azedo

• Document type: Doctoral Thesis
• Press: São Paulo.
• Institution: Universidade de São Paulo (USP). Faculdade de Medicina Veterinária e Zootecnia (FMVZ/SBD)
• Defense date: 2010-04-16

Examining board members

Alice Maria Melville Paiva Della Libera; Fernando José Benesi; Francisco Leydson Formiga Feitosa; Mitika Kuribayashi Hagiwara; Juarez Pinto Fernandes Távora

• Advisor: Alice Maria Melville Paiva Della Libera

Abstract

Enzootic Bovine Leukosis (EBL) is an infectious, multi-symptomatic, chronic, neoplastic disease, which undermines the lymphopoietic organs and is associated with the development of persistent lymphocytosis (PL) and lymphosarcoma. Infected animals present a decrease of production, either by its direct or its indirect harmful effects. However, its effect on the function and quantity of different lymphocyte subpopulations, as well as its role in the establishment of other opportunistic diseases, are unclear. This study evaluated the immune response of Holstein dairy cattle naturally infected with Bovine Leukosis Virus BLV, after antigen challenge provided by vaccination against foot and mouth disease (FMD) virus. To this end, blood samples were collected before challenge and after challenge, weekly, for seven weeks, from ten seropositive cows without PL, from ten seropositive cows expressing PL, and from ten seronegative cows. We evaluated the quantitative changes of different subpopulations of leukocytes; the function of B lymphocytes, through the quantification of different isotypes of immunoglobulins (Ig) serum concentration; the rate of lymphocyte proliferation; the rate of cell death by apoptosis or necrosis; and the serum concentrations of interleukin-10 (IL-10), IL-12, inteferon-&gamma; (IFN-&gamma;), and tumor necrosis factor-&alpha; (TNF-&alpha;). It was verified the normality of distribution of the results using the Anderson-Darling test, and their homoscedasticity, using the F test (for data with normal distribution) or the Levene test (for data without normal distribution). For the evaluation of differences between the average results, according respectively to the presence or absence of homoscedasticity, we used, for data with normal distribution, One-way ANOVA test, followed by the Tukey-Kramer test or the t test, and, for data without normal distribution, the Mann-Whitney test or the Kruskal-Wallis test. Results with p&le;0.05 were considered statistically significant. There were no differences related to serum IgG1, IgM, and IgA concentrations, both among sampling time and, every time, among animals belonging to different groups. IgG2 serum concentrations increased after vaccination in all animals (p<0.05). However, in animals expressing PL, each collection time, 17 days after antigen challenge, IgG2 serum concentration was lower (p<0.01) than those observed in animals belonging to other groups, indicating that animals with PL present less intense and less enduring humoral response. It was observed that there was an increase in the rate of lymphocyte proliferation (p<0.01) 24 days after vaccination against FMD virus, irrespective of the presence of infection by BLV. From this moment, there was an increase in the percentage of &gamma;&sigma;-lymphocytes (p<0.05) and a subsequent decrease in serum IgG2 (p<0.05), indicating regulation of this humoral response by &gamma;&sigma;-lymphocytes. In cattle with PL, the increase in the percentage of &gamma;&sigma;-lymphocytes was higher (p<0.05), leading to more intense and earlier decrease in IgG2 serum concentration. It was found that PL is due to a lower rate of apoptosis, since the percentage of leukocytes undergoing apoptosis was lower (p&le;0.001) among cells obtained from animals expressing PL, when compared to those collected from animals from the other groups. It was found that serum concentrations of Th1 cytokines, specifically IL-12 and IFN-&gamma;, were higher in blood samples from nonlymphocytotic infected animals (p<0.01), whereas serum concentrations of Th2 cytokines, particularly IL-10 and TNF-&alpha;, were higher in blood samples from infected animals expressing LP (p<0.01), indicating that changes in serum cytokines profile may be a cause or a consequence of PL. IL-10 (p<0.01), TNF-&alpha; (p=0.005), and IFN-&gamma; (p<0.01) serum concentrations increased three days after the challenge, and IL-12 serum concentration increased (p<0.001), ten days after the challenge. The increase in IL-10 serum concentration lasts until 31 days after the challenge and may account for the higher rate of &gamma;&sigma;-lymphocyte proliferation found from 31 days after vaccination. It was observed that the majority of circulating B lymphocytes in cattle consists of B1 lymphocytes and that, in BLV-infected animals, PL occurs due to an increase in the percentage of B1a lymphocytes (p<0.05). Moreover, in lymphocytotic BLV-infected animals, the rate between helper and cytotoxic Tlymphocytes are smaller (p<0.01) and the percentage of &gamma;&sigma;-lymphocytes is greater (p<0.01), indicating viral activity in infected cells. Thus, results allow us to conclude that lymphocytotic BLV-infected animals show changes in the immune response after vaccination against FMD virus. (AU)