DNA synthesis in tonsil lymphocytes. I. Changes in cell population during culture

Nucleoside transport and metabolism in lymphocytes, polymorphonuclear cells and cerebral synaptosomes

The salvage of nucleosides dominates over de novo biosynthesis in lymphocytes, polymorphonuclear cells (PMN) and in central neutral nervous system (CNS) in higher organisms. Earlier works in our laboratory have shown that the salvage of deoxycytidine (dCyd) did not correlate with DNA synthesis. The uptake and metabolism of dCyd was higher in undifferentiated germinal center lymphocytes and in follicles comparing to more differentiated cells. Recently we have compared the transport of thymidine (dThd), dCyd, uridine (Urd) and adenosine (Ado) in the three cell systems in which the salvage of nucleosides is dominating. It was found that dCyd was transported 30 times more effectively into lymphocytes than into PMN and synaptosomes, while Urd was transported about the same rate into the two cells and into synaptosomes. All transport processes could be inhibited by dipyridamole, NBRPR, papaverine and dilazep. The dCyd and dThd was phosphorylated even at 0 degrees C up to TTP and dCTP without incorporation into DNA and into liponucleotides. Our results show that the processes of transport-phosphorylation, as well as the processes of DNA-CDP-phospholipid synthesis are tightly coupled to each other in intact cells and organelles.

Deoxycytidine is salvaged not only into DNA but also into phospholipid precursors. III. dCOP-diacylglycerol formation in tonsillar lymphocytes

In addition to the water-soluble deoxyliponucleotides (Spasokukotskaja et al. (1988), Biochem. Biophys. Res. Commun. 155, 923), a lipid compound was shown to be labeled from external 3H-deoxycytidine (5-3H-CdR) in infant tonsillar lymphocytes. Chlorpromazine enhanced the labeling of this compound, identified by TLC as 3H-dCDP-diacylglycerol (3H-dCOP-DAG). The deoxynucleotide salvage pathway seems to be the main source for dCDP-DAG synthesis, as hydroxyurea increased its labeling from CdR. myo-Inositol induced the disappearance of 5-3H-dCOP-DAG, suggesting its utilization for phosphatidylinositol synthesis. 3H-Arabinosyl-Cytosine (araC) is also incorporated into the lipidic fraction at a rate comparable to its incorporation into DNA, supporting the effect of araC on membrane functions.

Deoxycytidine is salvaged not only into DNA but also into phospholipid precursors. II. Ara-C does not inhibit the later process in lymphoid cells

dCTP formed from exogenous deoxycytidine via the salvage pathways was previously shown to serve deoxyliponucleotide synthesis in lymphocytes (Spasokukotskaja et al, Biochem. Biophys. Res. Commun. (1988) 155, 923-929) and now in lymphoma cells. After treatment with 1-beta-D-arabino-furanosylcytosine (ara-C), much more araCTP as well as araCDP-choline was formed in lymphoma cells than in lymphocytes explaining the high sensitivity of lymphoma cells to this drug. Ara-C did not inhibit labeling of 5-3H-dCDP-choline from exogenous 5-3H-deoxycytidine while inhibiting DNA synthesis. Excess of exogenous ribocytidine diminished labeling of araCDP-choline, without any effect on dCDP-choline. These data suggest that araCDP-choline and dCDP-choline were synthesized from separate pools in these cells.

Follicular cells of tonsils metabolise more deoxycytidine than other cell populations

Nine subpopulations of tonsillar lymphocytes and the unseparated cells were compared in their utilization of exogenous deoxycytidine ([5-3H]CdR) and thymidine ([3H]TdR). Uptake phosphorylation and incorporation of labeled precursors were determined in B and T lymphocytes, in low density (LD; enriched in S phase cells) and in high density (HD; enriched in G0/G1 phase cells) cell fractions as well as in LD and HD subfractions of B and T lymphocytes, and in cells isolated from follicles of tonsils. As expected, LD cells and B lymphocytes were more active in TdR incorporation than HD cells and T lymphocytes. However, the ratio of [5-3H]CdR to [3H]TdR in their total phosphorylation and incorporation into DNA was much lower than the expected value of 1: about 0.5 for total phosphorylation and about 0.3 for incorporation in all subpopulations, except for the follicular cells, where these ratios were 1.0 and 0.7, respectively. These results show that the relative utilization of the two pyrimidine deoxyribonucleoside precursors varies among different lymphocyte subpopulations. However, this variation is not due to the different rate of DNA synthesis; rather, it depends on the differentiation stage of lymphocytes occurring in the germinal center of the follicles.

DNA synthesis and nucleoside metabolism in human tonsillar lymphocyte subpopulations

High (HD) and low (LD) density cells were separated on 25% BSA gradient from tonsils of 3-6 years old children. Early B lymphocyte markers and sIg-s were found on the surface of 59-82% of the LD cells. This cell population was 5-6 times more active in DNA synthesis (3H-thymidine incorporation, DNA polymerase activity) than the HD cells. The total uptake of 3H-deoxycytidine was about the same as that of 3H-thymidine. As long as practically all thymidine taken up by the cells was immediately incorporated into DNA (90-95%), only 10-15% of deoxycytidine was incorporated into DNA under the same conditions, indicating different pool sizes for the DNA precursors. The majority of deoxycytidine (70%) was converted and incorporated as dTMP. A considerable part of labeled deoxycytidine could be detected in the soluble pool in form of nucleotides (3-8%), and in an unknown form, called substance X (8-14%). Substance X was purified by TL chromatography and identified by HPLC as deoxycytidine containing liponucleotides, probable precursors for plasmamembranes. The preferential utilisation of deoxycytidine for DNA and membrane synthesis in immature B lymphocytes draws the attention to its function in early events of lymphocyte maturation.

Preferential utilisation of deoxycytidine by undifferentiated (peanut positive) tonsillar lymphocytes

Peanut agglutinin (PNA), a D-galactose specific lectin, agglutinated 10-18% of lymphocytes isolated from tonsils of 3- to 6-yr-old children. PNA+ cells were found to be mainly B lymphocytes showing a 9.6 times higher specific activity of DNA polymerase alpha compared to the PNA- cells. The specific activity of deoxycytidine kinase as well as the incorporation of [5-3H]deoxycytidine were also much higher in PNA+ cell fraction than in PNA- fraction (7.7-fold and 6-fold, respectively). On the other hand, thymidine kinase activity and [5-3H]deoxythymidine incorporation were only 3.6 and 3.9 times higher, respectively. The data presented here show a high degree of DNA synthesis and preferential utilisation of [5-3H]deoxycytidine for DNA synthesis in this undifferentiated B lymphocyte fraction.

Reversible permeabilization of lymphocytes destroys the incorporation of deoxythymidine but not of deoxycytidine

The total uptake, phosphorylation and incorporation of thymidine (dThd) and deoxycytidine (dCyd) were compared in intact and reversibly permeabilized human tonsillar lymphocytes. The total uptake of [3H]dThd was lower than that of [5-3H]dCyd, but almost all of [3H]dThd was incorporated into DNA. However, the main part of [5-3H]dCyd taken up by the lymphocytes was found in the pool as phosphorylated nucleoside (55%), and only a smaller part (13%) was incorporated into DNA. Phosphorylated nucleosides were determined by DEAE-cellulose sheets in the ethanol-soluble fraction of the cells. The reversible permeabilization of lymphocytes by Dextran T-150 destroys totally the [3H]dThd incorporation, while [5-3H]dCyd incorporation decreased only to 60% of intact cells. During permeabilization the phosphorylation of both nucleosides increased severalfold. After permeabilization all [3H]dThd was in dTMP form, while [5-3H]dCyd was also found in dCDP (3%) and dCTP (38%) form. In the meanwhile, 22% of thymidine kinase, 63% of deoxycytidine kinase and 98% of DNA polymerase activity were measured in permeabilized cells as compared to intact cells. The results suggest different relationships between the lymphocyte plasma membrane and the salvage pathways of the two pyrimidine nucleosides.

Cultured tonsillar lymphocytes excrete [3H]thymidine labeled DNA and revert to resting state

Large tonsillar lymphocytes labeled with [3H]thymidine reverted to small lymphocytes with concomitant loss of [3H]DNA upon culturing. The decrease of labeled DNA content and size of large lymphocytes was demonstrated by flow cytometry and cell sorting. These observations suggest that stimulated lymphocytes may revert from their proliferative phase to resting phase by shedding 'extra'DNA under cell culture conditions. This released DNA is not due to cell damage and can be hybridized to chromosomal DNA.

Cell-size distribution of human tonsillar lymphocytes

Cell-size distributions of human tonsillar lymphocytes (TL) and adenoid lymphocytes (AL) were determined using a computerized cell-size analyser. In comparison with peripheral blood lymphocytes, TL, and AL comprised a greater number of large lymphocytes. Child TL were characterized by an enrichment of not only B-lymphocytes but also of large lymphocytes. A decrease in the number of large B-lymphocytes with advancing age was considered to be indicative of the morphological profile of tonsillar involution.

Amino acid transport in human tonsillar lymphocytes with regard to patient's age and tonsillar diseases

Amino acid transport in human tonsillar lymphocytes was investigated in 32 patients of various ages and with different tonsillar diseases. Tonsillar lymphocytes appeared to possess at least four different transport systems for neutral amino acids including the activated Na+-dependent A system transport. The transport activity of neutral amino acid was significantly higher in child cases than in adult cases. In adult cases with focal tonsillitis, tonsillar lymphocytes were taken from patients with rheumatoid arthritis (RA) and skin diseases as secondary lesions who showed good improvement of the skin lesions after tonsillectomy. These lymphocytes were revealed to have more activated Na+-dependent transport than those from patients with skin diseases who showed poor improvement of the skin lesions after the tonsillectomy, and those from patients with recurrent tonsillitis. The characteristics of amino acid transport in human tonsillar lymphocytes and changes influenced by age and tonsillar diseases are discussed.

Spontaneous DNA synthesis in tonsillar lymphocytes and its clinical implications

Spontaneous DNA synthesis (SDS) in tonsillar lymphocytes was investigated in 58 cases by [3H]-thymidine incorporation under a certain culture condition without any mitogens. Tonsillar lymphocytes showed an extremely higher uptake of [3H]thymidine than peripheral blood lymphocytes. Cells exhibiting SDS were mainly B cells, but, however, they needed the helper functions of T cells and glass-adherent cells to synthesize DNA. SDS in tonsillar lymphocytes decreased with age and was lowest in the 21-30 year age group. SDS in cases with complications such as chronic sinusitis, nasal allergy or secretory otitis media was significantly higher than that in cases with no complications.

Discrepancies between flow cytometric analysis and [3H]thymidine incorporation in stimulated lymphocytes

The DNA synthesis system of freshly isolated tonsillar lymphocytes and those stimulated by phytohaemagglutinin were compared by different methods. Both cell populations had high DNA polymerase alpha and thymidine kinase activities, as well as a high rate of incorporation of [3H]thymidine into DNA. However, the two cell populations differed when their DNA distributions were compared by flow cytometry. Freshly isolated cells contained many less (6%) cells in S phase than were found in phytohaemagglutinin-stimulated lymphocytes (18%) as detected by flow cytometry. The labelling of different subpopulations of lymphocytes was studied by sorting them electrically with a fluorescence-activated cell sorter. Analysis of the radioactivity of [3H]thymidine pulse-labelled cells, sorted according to their DNA content, showed that cells in the G1 peak of DNA distribution had a significant amount of incorporated [3H]thymidine. Sorting of cells according to their size (i.e., by light scattering) revealed that only large cells were labelled with [3H]thymidine.

Biochemical characterization of the proliferation pool of lymphatic cells in the human tonsil and demonstration of the age-dependent pool size reduction

The DNA and RNA content and also the thymidine and uridine incorporation kinetics were determined in peripheral lymphocytes and lymphatic cells of the tonsil. The existence of a proliferation pool of lymphoid cells in the tonsil could be established by the comparison of all data. An age-conditioned reduction of the pool size was assumed from the uridine incorporation rate which takes place faster than the organo-specific cell decrease likewise conditioned by age in the tonsil. The abscess tonsil could be explained as a pathological state with considerable reduction of proliferation.

Excretion of newly synthesized DNA by tonsil lymphocytes

The main part of newly synthesized DNA is preferentially released in vitro both by non-stimulated or phytohemagglutinin-stimulated tonsil lymphocytes. Freshly isolated cells excrete DNA faster than phytohemagglutinin-stimulated ones. The acid-precipitable 3H-thymidine lost can be accounted for in the culture medium as labeled, double stranded DNA isolated on hydroxyapatite column.