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The ability of meta-nitrobenzenediazonium fluoborate (m-NBDF)-labeled thymus and spleen (S) cells to transfer immunity to 2,4-dinitrophenyl (DNP) into irradiated syngeneic recipients was investigated. There was a significant increase in the number of anti-DNP plaque-forming cells (PFC) when m-NBDF-labeled thymus cells and normal spleen cells, or normal thymus cells and m-NBDF-labeled spleen cells were transferred, but not when both thymus- and S-cell populations were labeled and injected together into irradiated recipients. The ability of these cell populations to cooperate and enhance the in vivo immune response to DNP is discussed. The T cells seem to be actively involved in the development of this response; they participate beyond the mere role of carrying and presenting antigen to the B cells. It is suggested that cell to cell contact between T and B cells may be an important factor in the elicitation of an immune response. In addition, the cellular interaction is affected by irradiating the thymus cell preparation and the initiating interaction required for antibody synthesis probably occurs within 48 h after injecting the cell populations into the syngeneic irradiated recipients.

Assay of the first enzyme unique to methionine biosynthesis, homoserine-O-transsuccinylase, in metJ and metK regulatory mutants of Salmonella typhimurium showed that synthesis of the enzyme was derepressed seven- and fourfold, respectively. The possibility of noncoordinate regulation of the methionine enzymes is discussed. In metA feedback-resistant mutants, the enzyme activity can be inhibited in vitro by 10 mmS-adenosylmethionine but not by 10 mm l-methionine; hence, the synergistic inhibition found for the wild-type enzyme is not effective in these latter mutants.

A higher percentage of specific antigen-binding cells can be detected not only in normal CBA/J mouse spleen cell preparations (0.99%), but also in the normal thymus cell preparations (0.15%) with the use of [(125)I]2,4-dinitrophenyl-human IgG (DNP-HGG) as compared with most other antigens employed under similar conditions. The receptors on these cells are mainly specific for the DNP group as shown by the inhibition studies with DNP-lysine and the other DNP conjugates. In addition, it was shown by the inhibition studies with DNP-lysine that the thymus cells seem to have a lower avidity for DNP than the spleen cells. Preincubation of cell suspensions with antisera to immunoglobulins showed that the DNP-HGG antigen-binding cells in the thymus are inhibited predominantly with anti-micro-chain serum and the spleen cells with both anti-micro-chain and anti-gamma-chain sera; both cell populations were also significantly inhibited with the antisera to kappa-chains and Fab fragments. These data indicate that the nature of the receptor on the T cell differs from that on the majority of spleen cells.