Activation of early enzyme production in small lymphocytes in response to high, nonmitogenic concentrations of concanavalin A

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Abstract
Lymphocyte [bovine] mitogenesis is generally assessed by measuring the incorporation of [3H]thymidine into DNA. By this criterion, small lymphocytes, which are activated by relatively low doses of concanavalin [Con] A, are unresponsive to or inhibited by higher concentrations. Because lymphocytes begin to synthesize DNA about 24 h after addition of mitogen, the response is far removed temporally from the initial stimulus. The induction of S-adenosylmethionine decarboxylase (S-adenosyl-L-methionine carboxy-lyase, EC 4.1.1.50) was used to assess early activation events in bovine lymphocytes. Adenosylmethionine decarboxylase induction is bimodal, with an initial phase beginning 3 h after addition of con A and a 2nd wave coinciding with the onset of DNA synthesis. The initial accumulation of the decarboxylase (0-9 h) in cultures treated with nonmitogenic levels of con A (108 .mu.g/ml) was similar to that observed in cultures stimulated with optimally mitogenic doses (18 .mu.g/ml). The early induction of ornithine decarboxylase (L-ornithine carboxy-lyase, EC 4.1.1.17) was similar under these 2 culture conditions. The 2nd phase of adenosylmethionine decarboxylase accumulation, the induction of thymidine kinase (ATP: thymidine 5''-phosphotransferase, EC 2.7.1.21), and DNA replication were blocked at the higher concentrations of con A. The inhibition of late events by high doses of con A was reversible. Cells treated with .alpha.-methyl-D-mannopyranoside 25 h after addition of a high dose of lectin responded with a 2nd period of adenosylmethionine decarboxylase accumulation, induction of thymidine kinase and progression through S phase. Initial lymphocyte activation probably occurs normally at high doses of con A, but the cells are reversibly blocked prior to induction of late enzymes and progression through S phase.