In vitro maintenance of differentiation marker synthesis by subpopulations of mouse thymocytes

A specific biosynthetic marker for immature thymic lymphoblasts. Active synthesis of thymus-leukemia antigen restricted to proliferating cells

Large cortical thymocytes from C57BL/6-Tla(a) mice have been prepared rapidly and in high yield by a combination of centrifugal elutriation and differential binding to peanut agglutinin (PNA)-coated plates. The cells in these lymphoblast-rich fractions were clearly distinct from the majority of thymocytes, with up to 70 percent in the S or G(2) + M phases of the cell cycle and an average rate of [(35)S]methionine incorporation per cell up to 20 times higher than that of the majority population. The populations of cells resolved in this fractionation were characterized by monitoring their rates of synthesis of specific glycoproteins, thymus- leukemia antigen (TL) and the Lyt-2, Lyt-3 complex (Lyt-2/3), relative to their total protein synthesis. Cells that bound to PNA synthesized high levels of Lyt-2/3, consistent with their identification as cortical thymocytes. Those that failed to bind made little or no Lyt-2/3, as expected for medullary cells, The fraction of dividing lymphoblasts that bound to PNA was enriched in cortical thymocyte precursors, including all the large cells detectably active in synthesizing Lyt-2. It differed sharply from the small cortical cells, however, in the synthesis of TL. Although both populations displayed abundant surface TL, the TL glycoprotein was produced actively in fractions containing dividing cells but made at a drastically reduced rate by the nondividing majority of cortical thymocytes. Thus, TL seems to be made at a narrowly circumscribed stage of early thymocyte development that is correlated with rapid proliferation. In most of the descendants of such blast cells, the TL glycoprotein is presumably retained on the cell surface as long as no substantial membrane turnover takes place. Ongoing TL synthesis may therefore serve as a marker for a unique developmental state which terminates rapidly in normal differentiation but may be extended by agents that give rise to TL(+) thymic lymphomas.

Structure and expression of glycoproteins controlled by the Qa-1a allele

The alloantigen controlled by the Qa-1a allele is a glycoprotein that exists in two forms. The first, an intracellular molecule of apparent Mr 44 000 daltons, appears to be a kinetic precursor of the second, a cell-surface molecule with an apparent size of 47 000 daltons. The intracellular form of Qa-1 is distinct from that of the TL glycoprotein in two ways: (1) its polypeptide backbone is approximately 5000 daltons shorter, and (2) it possesses three sites of high-mannose carbohydrate attachment, while TL has only one. In the cell-surface form of Qa-1, all three carbohydrate chains are processed to structures that resist endoglycosidase H digestion, presumably complex-type oligosaccharides. Concomitant with these late carbohydrate-processing steps is the formation of stable complexes between Qa-1 and beta 2-microglobulin. The timing of this association provides a further contrast between Qa-1 and TL, which is associated with beta 2-microglobulin shortly after its synthesis. The Qa-1 glycoproteins have been identified genetically by their synthesis in B6-TL+ (Qa-1a/Tlaa) splenocytes but not in splenocytes of congenic B6.K1 and B6.K2 (Qa-1b/Tlab) mice, and by their absence from the products of BALB/c (Qa-1b/Tlac) splenocytes. The cells synthesizing Qa-1 are at least as prevalent in Ig+ spleen cell populations as in T-cell-enriched splenic Ig- populations. Thus, active Qa-1 synthesis appears to take place at a high rate in normal splenic B cells without mitogenic stimulation.