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AFTER INITIAL ACTIVATION MEDIATEDby agents such as antigen, lectin, or antibodies to immunoglobulin, B cells become responsive to putative lineagespecific growth factors (1). One B-cell growth factor (BCGF), which represents the predominant molecular species released by normal lectin-activated T cells, has been defined by its comitogenic effects on activated B lymphocytes derived from either peripheral blood or tonsils (2). The mature secreted form of this BCGF is 12 kD and has a major isoelectric point of 6.3 to 6.6 (3). Multiple human BCGF species of higher molecular weight (60-kD BCGF) have also been detected in the supernatants of hybridomas (4), malignant T cells (5), and normal lectin-activated T cells costimulated with phorbol myristate acetate (PMA) (1). In addition, immortalized human B cells secrete a high molecular weight (60-kD) material with BCGF activity (5). These observations suggest that there are either multiple genes encoding BCGF-like activities [as has been seen in the mouse (6)], or that the mature secreted 12-kD BCGF is a by-product of higher molecular weight BCGF(s). Two murine BCGF genes have recently been identified by recombinant technology and the human homolog to one [B-cell stimulatory factor--1 (BSF-1)] has also been cloned (7). Here we report on the molecular cloning of a human BCGF variant related to the 12-kD secreted product.
I. Morphological and histological study. Twelve types of blood cells were identified in the blood of an ascidian, Styela rustica,by means of phase contrast microscopy, Nomarksy optics and histological analysisof differentially stained sections. They are haemocytoblasts (20%), giantcells (0.3%), hyaline amebocytes (30%), macrophages (2%), cells with eosinophilicgrains (0.2%), cells with eosinophilic granules (2%), cells with colourlessgranules (2%), granulocytes (6%), vacuolated young intermediate and maturemorula cells (about 40% all together). Four later cell types are presumablythe consequent stages of morula cell differentiation pathway, macrophages- the final step of the hyaline amebocytes' maturation. The haemocytoblastsare mainly localised in connective tissue of endocarps, intestinal and subendostylarsinuses. These cells are quickly dividing, their mitotic index being about2%, index of S phase about 10%. They give rise to all other cells, representingat least 6 independent cell lineages. Mature giant cells are located inthe vessels of branchial sac, whereas mature cells with colourless granules,granulocytes and morula cells migrate into the tunic. All blood cells areameboid at least in some degree, but only hyaline amebocytes, macrophagesand morula cells are phagocytic. It seems that macrophages, cells with eosinophilicgranules and cells with colourless granules are specific for members of familyStyelidae, whereas other cell types, i.e. giant, hyaline, granulocytes andmorula cells, are common for ascidians of all three families of the orderStolidobranchia. On the other hand, the signet ring and compartment cellsso characteristic for other ascidian species are absent in the blood of S. rustica and presumably of other members of family Styelidae.
II. Cytochemical study. Blood cells of ascidian Styela rusticawere studied by means of cytochemical methods. It has been shown that eachcell type has its own cytochemical markers, which can be used for exact identificationof the blood cells in other ascidian species. For example, haemocytoblasts,the proliferating stem cells, possess high amount of RNA in their cytoplasm,whereas the most ameboid and phagocytic cells - hyaline amebocytes - showthe highest staining for filamentous actin. Granules of basophilic granulocytesare composed of proteins in complex with acid sulphated polysaccharides. Therefore these cells can be detected easily by PAS-alcian blue reactionor by toluidine blue staining. Specific granules of the cells with eosinophilicinclusions are composed of basic arginine-rich proteins without free aminogroups, and granules of the cells with colourless inclusions (glycoproteincells) contain neutral glycoproteins with high content of cystine and cystein. Four cell types - vacuolated, young, intermediate and mature morula cells- possess the same complex of unique cytochemical markers. They are thebound iron, cationic proteins with high content of free lysine (-amino groups,and enzyme 0-diphenoloxidase in inactive form. The fourth component of thegranules of mature morula cells is represented by the o-diphenolic compounds. It has been shown that accumulation of o-diphenols starts at the late stepof morula cell maturation, i.e. at the stage of intermediate morula cells. Therefore, during their differentiation the morula cells of S. rusticaaccumulate in their granules all the components of phenoloxidase system,which is used for sclerotization of tunic proteins as well as for realisationof different immune reactions.
III. Autoradiographic study on the metabolism of the blood cells.Metabolism of the blood cells of ascidian Styela rusticawas studied in autoradiographic experiments with labeled precursors of polysaccharideand protein synthesis. It has been shown tha t about 50-60% of haemocytoblastsincorporate glucose and different amino acids. These cells may correspondto the fraction of quickly dividing cells. In good agreement with the resultsof previous cytochemical studies, the cells with eosinophilic granules, containingarginine-rich basic proteins, incorporate only amino acids, whereas cellswith colourless (glycoprotein) granules and basophilic granulocytes incorporateboth amino acids and glucose. Morula cells and their precursors show no uptake of labeled glucose or14C-CO2 and do not participate in cellulose synthesis. On the other hand,intermediate and some mature morula cells demonstrate selective uptake oftyrosine, which is more than 50 times higher than incorporation of otheramino acids. In continuous experiments the level of incorporation of aminoacids by haemocytoblasts and glycoprotein cells does not show significantchanges. On the contrary, the labelling of morula cells with tyrosine demonstratesa drastic decrease during the expriment. Thus, total radioactivity of morulacells in experiments with 7, 8-3H-tyrosine drops two times in 24 hours, and10-15 times after 3-7 days. These data suggest that the morula cells usetyrosine not for protein synthesis, but mainly for the synthesis of o-diphenoliccompounds. Therefore, the decrease of their radioactivity may be explainedby the modification of tyrosine side chain during the process of o-diphenolproduction. Both cytochemical and autoradiographic data show clearly thatthe synthesis and accumulation of o-diphenolic compounds during morula celldifferentiation in Styela rustica start at the stage of intermediate morulacells. The main part of the mature morula cell population can thus be consideredas being composed of terminally differentiated and resting defensive cells. 781b155fdc