Investigation of the role of CD8+ T cells in bovine tuberculosis in vivo

Abstract

Mycobacterium bovis is the causative agent of bovine tuberculosis (TB), and it has the potential to induce disease in humans. CD8(+) T cells (CD8 cells) have been shown to respond to mycobacterial antigens in humans, cattle, and mice. In mice, CD8 cells have been shown to play a role in protection against mycobacterial infection. To determine the role of CD8 cells in bovine TB in vivo, two groups of calves were infected with the virulent M. bovis strain AF2122/97. After infection, one group was injected with a CD8 cell-depleting monoclonal antibody (MAb), and the other group was injected with an isotype control MAb. Immune responses to mycobacterial antigens were measured weekly in vitro. After 8 weeks, the animals were killed, and postmortem examinations were carried out. In vitro proliferation responses were similar in both calf groups, but in vitro gamma interferon (IFN-gamma) production in 24-h whole-blood cultures was significantly higher in control cattle than in CD8 cell-depleted calves. Postmortem examination showed that calves in both groups had developed comparable TB lesions in the lower respiratory tract and associated lymph nodes. Head lymph node lesion scores, on the other hand, were higher in control calves than in CD8 cell-depleted calves. Furthermore, there was significant correlation between the level of IFN-gamma and the head lymph node lesion score. These experiments indicate that CD8 cells play a role in the immune response to M. bovis in cattle by contributing to the IFN-gamma response. However, CD8 cells may also play a deleterious role by contributing to the immunopathology of bovine TB.

FIG. 1.

(A) Number of PBL per milliliter in peripheral blood. (B to D) Average percentages of PBL stained with MAb in cattle infected with M. bovis. The cells were stained for CD4⁺ (B), CD8⁺ (C), and WC1⁺ (D) cells as described in Materials and Methods. The error bars indicate standard deviations.

FIG. 2.

Proliferative responses of PBL from individual control (A) and CD8⁺-T-cell-depleted (B) calves to PPD-B expressed as SI. The continuous line represents the median SI of the group, and the number next to each bar is its numeric value of the average. Each symbol represents the results for one animal.

FIG. 3.

IFN-γ production in picograms per milliliter by PBL from individual control (A) and CD8-depleted (B) calves against PPD-B. The continuous line represents the median of the group, and the number next to each bar is its numeric value. Each symbol represents the results for one animal.

FIG. 4.

Correlation between cumulative production of IFN-γ by whole blood cells against PPD-B and gross pathology score (P ≤ 0.001; R² = 0.7898). Squares, control calves; crosses, depleted calves.