A cellular analysis of immunosuppression in cattle with mucosal disease.

Eleven cattle with mucosal disease (MD) were investigated to determine the constitution and function of the cells participating in the immune response. The total numbers of blood mononuclear cells varied within the normal range but a poor response of the lymphocytes to mitogen stimulation indicated a state of immunosuppression in the cattle with MD. Identification of subpopulations of the mononuclear cells revealed that eight of the cattle with MD had an increased proportion of B cells. However, the proportions of monocytes and T cells were in the normal ranges, resulting in a low proportion of cells not identified as being either B or T cells (null cells). Three of the cattle with MD had an increased proportion of cells with receptor for the Fc part of IgG (Fc gamma + cells). Bovine Fc gamma + cells have previously been shown to exert suppressor activity, as was also demonstrated in the present study by increased mitogen induced lymphocyte stimulation following removal of the Fc gamma + cells. The effect of depletion of Fc gamma + cells was at least five-fold greater in the two cattle with MD tested than the two controls. It appears that a diminished number of null cells and, or, increased suppressor cell activity might contribute to the observed immunosuppression in cattle affected with MD.