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The functional role of subpopulations of bovine cells in the regulation of pokeweed mitogen (PWM)-induced proliferative and antibody responses of peripheral blood mononuclear cells (PBM) was analysed. Subpopulations of bovine PBM identified by specific monoclonal antibodies (mAbs) were isolated by sorting them through the fluorescence activated cells sorter (FACS). The depletion from PBM of T cells bearing the CD4 marker, recognised by mAb IL-A12, resulted in a reduction of PWM-induced responses, which could be partly reversed by the addition of CD4 positive T cells. The depletion of cells belonging to the macrophage/monocyte lineage also resulted in a reduction of PWM-induced proliferative responses. PBM depleted of CD8 positive T cells, recognised by mAb IL-A51, showed increased PWM-induced responses, which in turn were reduced by the addition of mAb IL-A51 positive cells. Proliferative and antibody responses were also obtained by PWM stimulation of FACS-purified B cells, in the presence of bovine T cell growth factor.

Epitopes specific for IgM on peripheral blood lymphocytes from 47 cattle were examined with three class-specific monoclonal antibodies, IL-A30, IL-A50 and B5/4. In all 47 animals tested, mAb IL-A30 detected a similar percentage of peripheral blood lymphocytes as a mAb that recognizes all immunoglobulin classes. However, in some animals mAbs IL-A50 and B5/4 detected a lower percentage of B cells compared with IL-A30 or the mAb against total Ig. They both reacted with a proportion of the serum IgM from these animals, while IL-A30 reacted with all serum IgM. Therefore, it is probable that mAb-A30 recognizes an IgM isotypic determinant and mAbs IL-A50 and B5/4 recognize different IgM allotypic determinants. Using mAb IL-A30 it was found that the percentage of peripheral blood IgM+ lymphocytes varied widely between healthy cattle (4-31%) but remained constant, with only minor variations, within individual animals.