A polypeptide of 8500 molecular weight is described that induces the differentiation of T (thymusderived) cell and B (bone-marrow-derived) cell immunocytes in vitro, apparently via fl-adrenergic receptors and adenylate cyclase activation. This polypeptide shows a high degree of evolutionary conservation, exhibiting close structural, functional, and immunological similarity when isolated from such diverse origins as cells of mammals and higher plants. This polypeptide was detected in animal cells, yeast, bacteria, and higher plants, and so may well be a universal constituent of living cells.The isolation from bovine thymus of the polypeptide hormones thymopoietin I and II has been described previously (1 2). [We are now using the term thymopoietin (3) because valid objections (4) were raised to our previous term "thymin," which is easily confused with the pyrimidine base thymine. ] Pure thymopoietin I and II specifically induce prothymocytes to differentiate into thymocytes (5-7), but these polypeptides were recognized originally not by this evidently physiological faculty of T (thymus-derived) cell differentiation but by a presumably incidental property discovered in the course of studies related to the human disease myasthenia gravis, in which a characteristic neuromuscular disturbance is consistently associated with thymic pathology. When reliable assays of T-cell differentiation in vitro became available, based on the manifestation of T-cell antigens on precursor cells (5,8) it was realized that, although certain pure thymic products (thymopoietins) could induce this step in T-cell differentiation specifically, extracts of tissues other than thymus were also active (10).We describe here the isolation and purification from bovine thymus of a polypeptide subsequently detected in many other tissues, which like thymopoietin induces the differentiation of pro-thymocytes, but which unlike thymopoietin induces differentiation of B (bone-marrow-derived) cells, and is thus distinguished in its mode of action from the physiological inducer thymopoietin. This The isolation of UBIP from bovine thymus is summarized in Fig. 1. Bovine thymus was obtained fresh on wet ice, dissected clean and stored at -20°. Batches were homogenized 25% wet weight/volume in 0.1 M ammonium bicarbonate with a Waring blender. The extract was heated to 700 for 30 min and then centrifuged at 500 X g for 30 min. The supernatant was filtered through gauze and cotton and 0.1% thimerosal was added as a bacteriostatic agent, since the subsequent steps were carried out at room temperature. The extract was processed through a Diaflo XM1OOA membrane (Amicon) in a TCF10 apparatus. The dialysate was then concentrated over a Diaflo UM2 membrane at 55 lb./in.2, using a 402 stirred cell and a reservoir; 3 liters were concentrated to 15 ml. This retentate was fractionated on a 2.5 X 100-cm column of medium Sephadex G-50 (Pharmacia) in 0.1 M ammonium bicarbonate. The fractions shown in Fig. 1 were lyophilized and rerun on the same column. The lyophili...

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Single cells were prepared from mouse tail epidermis by a method which gives high viability counts and so permits their use in cytotoxicity tests. According to tests with standard alloantisera, the antigen phenotype of mouse epidermal cells is H-2+θ+Sk+H-Y+TL-Ly-A-Ly-B,C-PC-. The skin differentiation alloantigen Sk, which is responsible for homograft reactions directed selectively against skin, is expressed also on brain, but not on other cell types; it is present on the transplanted neuroblastoma C1300. Cytotoxicity tests with epidermal cells of H-2 congenic mouse stocks confirm that the Sk locus is not closely linked to H-2. The lymphoid cell differentiation antigen θ also is present on both epidermal cells and brain. Mice frequently retain θ-incompatible or Sk-incompatible skin grafts although they have formed substantial titers of θ or Sk antibody in response to grafting. Male (H-Y) antigen is demonstrable on epidermal cells by cytotoxicity tests with H-Y antibody, as it is also on one other type of cell, spermatozoa.