DLA-D-specific suppressor cells characterized by monoclonal antibodies

Canine lymphocytes alloantigen-primed in mixed leukocyte culture (MLC) were tested for their ability to suppress MLC reactivity. These cells were added to fresh responder cells in MLC at ratios of 1:50 and 1:20 and suppressed reactivity 90 +/- 7% (mean +/- 1 SD) and 91 +/- 3% respectively. At lower ratios no suppression was observed. Monoclonal antibody E11 (IgG3) recognized a subset of canine Thy1+ T cells, and with complement treatment was capable of depleting suppressor cells from bulk MLC (11 +/- 6% of control suppression), but DT-2 (IgG2a), another canine T-cell antibody, had no such effect (84 +/- 11%) (P = 0.028). Antibody A5 (IgG2b), reactive with all T and most non-T cells, also eliminated suppression. Positively selected E11+ bulk-MLC cells suppressed (91%) and E11- bulk-MLC cells did not (4%). At the ratios described, suppression was specific for the DLA-D phenotype of the stimulator cells. In tests with the original stimulators, suppression was 91 +/- 7%; in tests with DLA-D identical stimulators it was 92 +/- 5%; and for DLA-D nonidentical stimulators it was 38 +/- 25% (P = 0.005). Nonspecific suppression increased with increasing numbers of suppressor cells. At ratios of 1:10 and 1:5 nonspecific suppression was 72 +/- 13% and 92 +/- 8%, respectively. These studies show that under appropriate conditions, Thy1+, E11+, A5+, DT-2- alloantigen-activated canine T lymphocytes function as DLA-D-specific suppressor cells and DT-2+ cells do not.