Reappearance of terminal deoxynucleotidyl transferase containing cells in rat bone marrow following corticosteroid administration.

Administration of dexamethasone to rats lyses greater than 75% of bone marrow and thymic lymphocytes. including those containing terminal deoxynucleotidyl transferase (TdT). Reappearance of TdT containing lymphocytes in bone marrow appears to be a phase of synchronized regeneration in which a cohort of cells differentiates together over a period of 2 wk. Days 1 -5 of marrow regeneration are characterized by the rapid proliferation of large. TdT-negative lymphoid cells. On days 6-1 1 , these cells divide less rapidly, contain TdT, and morphologically are transitional lymphocytes. The TdTcontaining cells do not appear to originate in thymus because the TdT antigen is nuclear rather than cytoplasmic and the cells in marrow do not react with antiserum to thymocytes. By days 1 2-1 5. the lymphocytes are small. very few are dividing. and TdT is absent from most. Whether the cells were dividing was estimated by pulse labeling with 3H-thymidine. The relative percentage of TdT-containing cells and activity of TdT per cell are maximal on day 9 after dexamethasone. Cells from bone marrow were separated on Hypaque-Ficoll discontinuous density gradients on day 9 and a fraction containing high levels of TdT was isolated. TdT antigen was present in 8% and 34% of cells in fraction 3 from gradient separations of normal and day 9 bone marrow. respectively. TdT activity in cells containing TdT antigen was estimated to be 125 U/108 cells from normal marrow and 56 U/b8 cells from day 9 marrow. These values are greater than twice the value usually observed in rat thymocytes and 60-1 00 times the activity per nucleated cell in unfractionated normal rat marrow. Cells containing TdT do not have surface immunoglobulin and do not adhere to nylon-wool columns. The fraction rich in TdT-containing cells was also characterized by a high rate of incorporation of 3H-thymidine into DNA. high level of DNA polymerase-a activity, and low level of adenosine deaminase activity. Bone marrow regenerating after dexamethasone may be an excellent source of lymphocytes at various stages of differentiation. The cells may be particularly useful for biochemical study because they are available in large numbers.