Immunological reactivity between chimeric cattle twins. III. Mixed leukocyte reaction.

Leukocytes from chimeric calves invariably failed to mount a proliferative response in excess of control responses when cultured with cotwin cells as stimulators. The levels of cellular proliferation in control cultures of leukocytes from chimeric donors were so low as to exclude the occurrence of any auto-stimulation, and it was clear that the absence of excess proliferation in mixed cultures could not be attributed to elevated background levels. Attempts to restore reactivity in mixed leukocyte culture by removal of cell subpopulations from chimeric calf leukocytes and by modification of cell surfaces were unsuccessful. It was inferred that the unreactivity between chimeric cotwins manifest in the normal lymphocyte transfer and mixed leukocyte reactions was unlikely to be produced by humoral or cellular blocking mechanisms.