Inbred strains of rats were used as lymphoid cell donors for the mixed lymphocyte blastogenic reaction (MLR). Assay was by uptake of tritiated thymidine or uridine. Optimal conditions for a maximal response included use of 1) lymph node cell preparations, 2) 10% fresh rat serum as medium supplement, 3) 8 × 106 cells/2 ml of medium, and 4) harvest after 4 to 5 days of incubation. Studies of the early portion of the response indicated at 24-hr lag period, prior to detectable allogeneic stimulation. Unidirectional responses were measured by stimulation with cells rendered unresponsive by graded doses of x-irradiation, or with F1 hybrid cells. The same relative strain responsiveness was obtained by both methods but only x-irradiated cells yielded quantitatively additive responses.

BN × Le combinations had a markedly higher response than BN × Fi mixtures. On the sole basis of their Ag-B locus disparities, these two combinations should react equally. In the presence of Colcemid these reaction differences were shown to be the result of differences in blastogenic, not proliferative, rates. A low-grade but significant response was detected between the Ag-B compatible Fi and Le strains, and this reaction was seen to be solely Fi anti-Le.

Le cells responded to non-responsive BN and DA cells in an additive manner, and the kinetics indicated that this was the result of two independent, simultaneous reactions. Such responses appear to be founded in separate, unipotential cell pools. If the MLR is a true primary immune response, this interpretation is compatible with a clonal selection hypothesis.

This investigation was supported by United States Public Health Service Grant 5-POI-HE-09178 from the National Heart Institute. One of us (D. G. C.) was the recipient of a U. S. Public Health Service Predoctoral Fellowship (5-F1-GM-31, 712-03).

2

A preliminary report was presented at the annual meeting of the American Association of Immunologists, April 1968, Atlantic City, N. J.

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