Synchronization of mouse L-cells by a velocity sedimentation technique

Overview of Cell Synchronization

The widespread interest in cell synchronization is maintained by the studies of control mechanism involved in cell cycle regulation. During the synchronization distinct subpopulations of cells are obtained representing different stages of the cell cycle. These subpopulations are then used to study regulatory mechanisms of the cycle at the level of macromolecular biosynthesis (DNA synthesis, gene expression, protein synthesis), protein phosphorylation, development of new drugs, etc. Although several synchronization methods have been described, it is of general interest that scientists get a compilation and an updated view of these synchronization techniques. This introductory chapter summarizes: (1) the basic concepts and principal criteria of cell cycle synchronizations, (2) the most frequently used synchronization methods, such as physical fractionation (flow cytometry, dielectrophoresis, cytofluorometric purification), chemical blockade, (3) synchronization of embryonic cells, (4) synchronization at low temperature, (5) comparison of cell synchrony techniques, (6) synchronization of unicellular organisms, and (7) the effect of synchronization on transfection.

Overview of cell synchronization

Widespread interest in cell synchronization is maintained by the studies of control mechanisms involved in cell cycle regulation. During the synchronization distinct subpopulations of cells are obtained representing different stages of the cell cycle. These subpopulations are then used to study regulatory mechanisms of the cycle at the level of macromolecular biosynthesis (DNA synthesis, gene expression, protein synthesis), protein phosphorylation, development of new drugs, etc. Although several synchronization methods have been described, it is of general interest that scientists get a compilation and an updated view of these synchronization techniques. This introductory chapter summarizes: (1) the basic concepts and principal criteria of cell cycle synchronizations, (2) the most frequently used synchronization methods, such as physical fractionation (flow cytometry, dielectrophoresis, cytofluorometric purification), chemical blockade, (3) synchronization of embryonic cells, (4) synchronization at low temperature, (5) comparison of cell synchrony techniques, (6) synchronization of unicellular organisms, and (7) the effect of synchronization on transfection.

Density-independent isolation of type II pneumocytes after partial pneumonectomy

Type II pneumocytes were isolated by immunoglobin G (IgG) panning from the lungs of normal rats and the right lung of rats (180-200 g) subjected to left pneumonectomy. Cells were studied at 7 (pnx-7) and 15 (pnx-15) days postoperative, times during and after, respectively, rapid compensatory growth of the right lung. After 24 h of primary culture, pnx-7 cells contained 32% more protein per DNA, and incorporated thymidine at a rate 224% greater than cells isolated from control rats. Both the protein-to-DNA ratio and thymidine incorporation returned to control values in pnx-15 cells. Uptake of exogenous spermidine also was increased by 50% in pnx-7 cells at 24 h of primary culture and returned to control values in pnx-15 cells. Increased spermidine uptake was due to an increase in the maximal velocity (Vmax) of transport from 30.3 (control) to 45.5 pmol.micrograms DNA-1.h-1 (pnx-7), with no change in the apparent Km of 1.32 microM. No change was observed in the relative rates of phospholipid or neutral lipid biosynthesis. The increases in thymidine incorporation and spermidine uptake were significantly greater than those previously observed [Am. J. Physiol. 254 (Cell Physiol. 23): C684-C690, 1988] in pnx-7 cells isolated by Percoll gradient sedimentation. These results suggest that pnx-7 lungs contain distinct subpopulations of type II pneumocytes, the recovery of which is dependent on the cell isolation protocol employed.

Separation of tumor-infiltrating lymphocytes from tumor cells in human solid tumors. A comparison between velocity sedimentation and discontinuous density gradients

The separation of viable tumor-infiltrating lymphocytes (TIL) from surgical biopsies of human solid tumors was achieved by velocity sedimentation at unit gravity or by discontinuous density gradients. The two methods were adapted to small volumes and cell numbers not exceeding 1 X 10(8). The recovery, purity and composition of the TIL-enriched fractions were comparable in the two methods. Density gradients were more rapid, simpler and more practical for preparation under sterile conditions of TIL from clinical material than velocity sedimentation. Lymphocytes in the TIL-enriched fractions obtained by either of the methods were poorly responsive to mitogens. This poor responsiveness is a characteristic of the human TIL and seems to be related to effects exerted by tumor cells.

Susceptibility to lysis by natural killer and natural cytotoxic cells is independent of the mitotic stage of the target cell cycle

Natural cell-mediated cytotoxicity (NCMC) against a number of target cells is mediated by at least two distinct effector populations, with natural killer (NK) and natural cytotoxic (NC) cells being the predominant in the murine system. The studies described in this report examine the role that the phase of the mitotic cycle of the target cell has on its susceptibility to lysis by NC and NK cells. We show that neither the kinetics nor the magnitude of NC cell lysis is altered when assayed using target cells which have been enriched for G1, S, or G2 + M stages of the cell cycle. Similarly, NK cell lysis by fresh or poly-IC augmented effector cells was not effected by target cell cycle.

Early hemopoietic progenitor cells: direct measurement of cell cycle status

We have investigated the cell cycle status of murine hemopoietic progenitors using vital DNA staining and flow sorting. Suspended Balb/c bone marrow cells were stained with Hoechst 33342 dye and separated first on light scattering properties; this procedure allowed a 5-fold enrichment in progenitor cells. A second sorting based on DNA content indicated that 80% of these cells were in G0/G1 and 20% in S-G2 + M. When G0/G1 and S-G2 + M cells were assayed separately in methylcellulose cultures, or with the in vivo colony forming assay, the G0/G1 cells were shown to be markedly enriched in CFU-S, BFU-E, and GM-CFU as compared to S-G2 + M cells with the final recovery increased 20-fold. Comparison of different strains or age groups yielded results identical to those obtained with Balb/c with the exception of C57B1/6. In the latter strain only a 3-fold enrichment could be observed in the G0/G1 fraction. These results demonstrate that the majority of early hemopoietic progenitors are in the G0/G1 phase of the cell cycle.

Use of low temperature for growth arrest and synchronization of human diploid fibroblasts

The growth kinetics of human diploid fibroblasts at two different temperatures were followed. Proliferation of exponentially growing cells is reduced and eventually stops upon incubation at low temperature (i.e. 30 degrees C). The cells which are in S phase at the time of switching to low temperature complete their DNA synthesis and become arrested in the G1 phase of the cell cycle. The arrested cells can be stimulated to proliferate by restoration of the optimal growth temperature (37 degrees C). The kinetics of entry into S phase were investigated by measuring [3H]thymidine incorporation into TCA-precipitable material, by autoradiography and by flow cytofluorimetry. The synchronized cells initiate DNA synthesis at approximately 8 h and DNA synthesis peaks at 20.4 +/- 0.7 h after stimulation. In addition, the rates of UV-induced excision repair at 30 degrees C and 37 degrees C were compared. The results indicate that at 30 degrees C the excision-repair process is operative but at a slightly reduced rate in comparison with repair at 37 degrees C. This method will be useful for the study of S-phase-dependent processes, as well as for repair studies in the absence of cell division.

Synchronization of mammalian cells by selection and additional chemical block studied by DNA distribution analysis and BrdUrd-Hoechst 33258-technique

Ehrlich ascites tumour cells growing in vitro in suspension culture were separated according to volume by the technique of velocity sedimentation in a zonal rotor with a reorienting gradient. Using DNA distribution analysis the sedimentation pattern of the cells could be analysed in detail. With appropriate conditions it was possible to separate pure G1 cells. Samples could also be obtained which were enriched in S or G2 + M cells. The main limitation of the selection in this type of rotor was the reorientation of the gradient which caused disturbances during deceleration of the rotor. The synchronous growth of selected G1 cells has been studied in detail to investigate the reasons for the rather poor synchrony of these cells. The poor synchrony was found to be caused mainly by the small volume of the selected G1 cells compared with the normal volume of G1 cells in an asynchronous population. The synchronization of these cells could be essentially improved by a short treatment with excess thymidine causing a metabolic block at the G1/S border. The duration of this treatment could be minimized using DNA distribution analysis of growing cells after releasing of the block. The durations of the cell cycle phases in synchronized cells agreed with the values calculated in asynchronous cells by DNA distribution analysis and the BrdUrd-Hoechst 33258-technique.

Effect of low-dose whole-body irradiation on granulopoietic progenitor cell subpopulations: implications for CFUc release

The reaction of the granulopoietic system to whole body irradiation with 0.80 and 1.60 Gy was studied in dogs by means of colony formation assays in combination with velocity sedimentation and tritiated thymidine cytocidal techniques. Depression of circulating CFUc was associated with a marked shift in the size distribution of granulopoietic progenitors in the bone marrow. This effect lasted much longer in those animals receiving the lower X-ray dose. We conclude that circulating CFUc are not a random proportion of the bone marrow but a subpopulation of cells which are smaller in size. These cells are in an equilibrium with the larger marrow CFUc, which is very sensitive to perturbations.

Cytolytic T lymphocyte function is independent of growth phase and position in the mitotic cycle

We have investigated mitotic cell cycle and growth phase regulation of homogeneous cytolytic T lymphocytes (CTL). Two independently derived CTL clones were stained with the DNA-binding dye Hoechst 33342, sorted in a fluorescence-activated cell sorter according to their position in the cell cycle, and then assayed for specific lytic activity using a short-term (30 min) (51)Cr release assay. Results show that lytic activity remained unchanged throughout the cell cycle. Furthermore, there was no significant difference in the lytic activity of CTL clones growing exponentially or arrested in a plateau phase. These results demonstrate that T cell-mediated cytolysis is independent of growth phase and position in the cell cycle.

Studies on a human melanoma cell line: effect of cell crowding and nutrient depletion on the biophysical and kinetic characteristics of the cells

In an investigation of the changes that occur in cultured neoplastic cells as they outgrow their supply of nutrient, MM96 human melanoma cells were found to diminish in size and to proliferate more slowly. These changes were accompanied by a moderate increase in the proportion of cells with a G1-like DNA content. When replated under favorable conditions, many of these cells gradually resumed active proliferation. Continuing adverse culture conditions led to a continued fall in cell size, loss of reproductive viability, and finally to rapid cell death. Simultaneous buoyant-density and velocity-sedimentation-fractionation experiments showed that cells from exponential cultures were moderately dense and rapidly sedimenting, cells from postexponential cultures were less dense and much more slowly sedimenting, and dye-excluding cells from reproductively nonviable, late postexponential cultures were of widely variable though generally high density, and were moderately rapidly sedimenting. Although neither fractionation method resulted in significant enrichment of clonogenic cells, depletion was seen at both extremes of both types of profile. Cells fractionated by velocity were sorted according to DNA content and hence location in the cell cycle. The relationship between sedimentation rate and cell-cycle location was reflected in the continuous thymidine labeling patterns of the separated cells. Study of these patterns suggested that cycle durations lengthened as crowding increased and nutrient became depleted, and shortened upon reseeding at low density into fresh medium.

Synchronization of 9L rat brain tumor cells by centrifugal elutriation

Asynchronous 9L cells were separated into relatively homogeneously-sized populations using centrifugal elutriation with both a conventional collection method and a long collection method. A substantial increase in the homogeneity of the volume distributions and in the degree of synchrony of the separated fractions was obtained using the long collection method. Autoradiographic data indicated that fractions containing greater than or equal to 97% G1 cells, greater than or equal to 80% S cells, and 70-75% G2 cells could be routinely recovered with this procedure. Recovery in these fractions varied from 5 to 8% of the total number of cells elutriated. The colony forming efficiency (CFE) of cells from fractions representing each phase of the cell cycle was a constant 60-70%, which was comparable to the 60-80% usually found for asynchronous 9L cells. The percentage of cells in the G1, S, and G2 phases in the elutriated fractions was more accurately determined from the volume distribution than from computer fits of the DNA histogram obtained from flow cytometry. In genereal, the degree of synchrony was related to the coefficient of variation (CV) of the volume distributions of the elutriated fractions. The CV was about 14% for all elutriated fractions. When the greater than or equal to 97% G1 population was allowed to progress to S and G2, the CVs were about 17 and 20.2%, respectively. Thus, the best nonperturbing method for obtaining synchronous 9L cells in the S or G2 phases was direct elutriation with the long collection method.

Complementation of genetic disease: a velocity sedimentation procedure for the enrichment of heterokaryons

Methodology is described to enrich for heterokaryons after mammalian cell fusion. A heterogeneous cell mixture can be separated on a Sta-Put apparatus into fractions of uniform size cells by sedimentation through a 1% bovine serum albumin-5% Ficoll gradient. Unfused RAG and LM/TK- cells, differing by 10% in diameter, have been sorted by size; following fusion, larger and faster sedimenting cells were shown to be hybrids. This methodology can be utilized in genetic complementation studies of human genetic diseases where selection procedures for proliferating hybrids do not exist. When fibroblasts from individuals with Tay-Sachs disease [deficient in hexosaminidase A (HEX A-)] and Sandhoff-Jatzkewitz disease (HEX A- and HEX B-) are fused, HEX A is generated, demonstrating complementation of two different mutations. After Sta-Put fractionation, the HEX A complementation product was associated with the faster sedimenting multinuclear cells and not with the mononuclear parental cells. This methodology will facilitate detection of genetic differences in fibroblasts from related inherited disorders.

Correlation between cell size and position within the division cycle in suspension cultures of Chang liver cells

Chang liver cells from exponentially growing suspension cultures have been separated by sedimentation at unit gravity. Determinations of the protein content per cell showed that the fractionation procedure resulted in good separation of cells of different size. On the other hand, the DNA content of individual cells from the fractions, as determined cytofluorimetrically, indicated considerable heterogeneity in the size of cells from the same stage of the division cycle. On the basis of earlier results on intermitotic growth and the variation in the length of the cell cycle in homogeneous cell populations, a mathematical model has been constructed and tested using a computer program. The present results on the size distribution of cells from the different stages of the mitotic cycle are consistent with a regeneration of size heterogeneity in each cell generation, as a result of the dispersion of intermitotic times. The variation in cell cycle times may be related to a probabilistic event in the G1 period. In the mathematical model it was necessary to include a mechanism by which the regeneration of abnormally large cells is prevented. The experimental data are compatible with a gradually increasing inhibition of growth in cells larger than a certain size (circa 400 pg protein per cell).

Induction of erythropoietin responsiveness in vitro by a distinct population of bone marrow cells

Bone marrow contains a small population of primitive erythroid progenitor cells which can be detected by their capacity to form large numbers of erythroid progeny in viscous cultures containing erythropoietin (EP). These cells have been termed erythroid 'burst-forming units' (BFUe). The present study demonstrates that expression of the erythroid differentiation potential of BFUe requires the presence of an activity additional to EP. This activity has been designated as BFA (burst feeder activity). It is shown that the number of BFUe detected and their apparent sensitivity to EP are directly related to the BFA concentration of the cultures. BFA was found to be associated with a population of bone marrow cells of high buoyant density and small volume, which are sensitive to irradiation. The radiation dose-effect curve provided strong evidence that bone marrow BFA is independent of cell proliferation; this was supported by showing that BFA is unaffected by in vivo treatment with hydroxyurea. The findings are compatible with a two-step regulation model for erythroid differentiation in which BFA-induced progeny of BFUe acquire sensitivity to EP.

Induction of murine erythroleukemia differentiation by actinomycin D

Murine erythroleukemia cells are induced to differentiate by 0.5-5 ng of actinomycin D per ml. Murine erythroleukemia cells cultured with actinomycin D prolong cell doubling time but achieve the same density after 5 days as cells without inducer. Actinomycin D causes over 95% of the cells to become benzidine-reactive. [(3)H]Actinomycin D uptake into DNA can be detected within 2 hr and reaches a maximum (approximately 0.1 pmol/10(6) cells) by 10-12 hr. It is estimated that about one out of 10(5) dG.dC pairs is bound to actinomycin D. Commitment to differentiation, assayed by transfer of cells to culture without inducer, was detected as early as 5 hr. Unlike Me(2)SO, which causes a transient prolongation in G(1) at about 15-20 hr, cells cultured with actinomycin D show a more sustained increase in the proportion of the cells in G(1). Globin mRNA accumulation was detectable by 19 hr in culture. Alteration in DNA stability in alkaline sucrose gradients was detected by 19 hr. Actinomycin D induces synthesis of Hb(maj) and Hb(min) in approximately equal amounts. A decrease in rates of synthesis of RNA, DNA, and total protein occurs in cells cultured with actinomycin D, as well as in cells cultured with Me(2)SO. No evidence for an early action of actinomycin D at the plasma membrane was obtained by measurement of changes in cell volume or (86)RbCl uptake. Taken together, the present results indicate that actinomycin D is a potent inducer of differentiation of murine erythroleukemia cells and suggest that the target of its effect may be at the level of DNA.

Mouse spleen lymphoblasts generated in vitro. Recovery in high yield and purity after floatation in dense bovine plasma albumin solutions

Mouse spleen lymphoblasts, stimulated to divide in vitro, acquired a low cell density and could be separated by isopycnic techniques. Cultured cells were suspended in BPA columns, rho = 1.080, and spun to equilibrium. The method was simple, fast, accomodated large numbers of cells, and was reproducible. It provided lymphoblasts in high yield and purity (at least 80% of the low density cells were blasts). It allowed for the recovery of proliferating cells in their first cell cycle, and did not alter the subsequent ability of cells to proliferate when recultured in vitro. Certain properties of mouse spleen lymphoblasts were analyzed in detail. Lymphoblasts induced by LPS, FCS, con A (tetravalent and succinylated), and MLC were very similar except in the absolute numbers that were induced. The blasts exhibited the classic cytologic features of enlarged nucleoli and abundant cytoplasmic polyribosomes (basophilia). As a population, they were enlarged in size relative to nondividing cells, but this seemed to apply primarily to cells in the S and G2+ M phase of the cell cycle rather than G1. The cell cycle distribution of lymphoblasts was analyzed by flow microfluorometry. By analyzing low density cells obtained at varying intervals after mitogen stimulation, FMF indicated that lymphoblasts enter the S phase of their first cell cycle beginning at 20-24 h after stimulation.

Characterization of functionally distinct lymphoid and myeloid cells from human blood and bone marrow. II. Separation by velocity sedimentation

Velocity sedimentation in a zonal rotor using gradients of uniform osmolarity was used to separate leukocyte subpopulations from human blood and bone marrow. The separations were performed at high sedimentation rates having the advantage of rapidity over conventional unit gravity separations. Myeloid stem cells (CFU-C) and cells reactive with phytohemagglutinin (PHA), Concanavalin A (Con A), and in mixed leukocyte culture (MLC) were separated and their sedimentation profiles obtained. CFU-C sedimented ahead of lymphoid cells and behind mature myeloid elements. Two distinct marrow subpopulations separated by velocity sedimentation were consistently stimulated by Con A, and variably stimulated by PHA and MLC reactions. Both large cells (predominantly myeloid) and small cells (predominantly lymphoid) from bone marrow were stimulated by Con A in [3H]thymidine incorporation assays. When separated subpopulations showing stimulation by Con A were mixed, inhibition of [3H]thymidine incorporation resulted.

Detection of mitogen-activated T and non-T lymphocytes by virus plaque assay. Virus plaque assay on the cells fractionated by unit gravity sedimentation

Virus plaque assay (VPA) was utilized for the quantitative evaluation of activated lymphocytes. We examined what types of cells, especially which of activated T and non-T lymphocytes, were detected as infective centres after infection with vesicular stomatitis virus. Marked increases in DNA synthesis and in virus-plaque forming cells (V-PFC) were observed not only during the activation of T lymphocytes with Con A, but also, though to a lesser extent, during the activation with lipopolysaccharide (LPS) of non-T lymphocyte preparations of nude spleen from which theta-positive lymphocytes and macrophages were completely depleted. The latter observation was further confirmed by the VPA on the populations enriched in LPS-activated non-T lymphocytes fractionated by the unit gravity sedimentation method. Fast sedimenting cells were found to be more active in DNA synthesis and contained more infective centres after infection than those sedimenting slowly and original unfractionated cells. Both the capacity for DNA synthesis and virus-replication were considered to be general properties accompanying lymphocyte activation.

Early and late volume changes during erythroid differentiation of cultured Friend leukemic cells

Friend erythroleukemic cells (FLC) can be induced to differentiate in vitro by addition of dimethylsulfoxide (DMSO). We have studied the kinetics of induction by measuring cell volume, volume coefficient of variation and cell doubling time. Two distinct volume changes (early and late) are observed after the addition of the inducing agent. The early change occurs after ten hours and consist of a 10-20% decrease in volume compared to an untreated control population. This shift persists for two days and its magnitude is proportional to both the concentration of DMSO and the number of differentiated cells seen on day 5. FLC lines which induce weakly or not all with DMSO exhibit a reduced or absent early volume shift. Inclusion of a local anaesthetic in the culture prevents the appearance of differentiated cells and also counteracts the early volume shift. The exact time of the early volume change is a function of cell growth rate and appears to be cell cycle related. Synchronized cell populations exposed to DMSO during G2 and S phase undergo one round of mitosis before expression of the volume change whereas cells in G2-M express the change only after a second mitosis. A later, more gradual decrease in volume is observed in those cultures which begin to produce hemoglobin. It occurs after approximately five doubling times and coincides with the first appearance of hemoglobin-containing cells. Volume distribution parameters indicate that only a proportion of the population becomes smaller in size.

Rapid separation of cells at unit gravity

An apparatus is described for the rapid separation of cells at unit gravity. The apparatus is filled with a discontinuous density gradient, followed by the sample and then an overlay. The sedimentation chamber is turned from a vertical to a horizontal position before sedimentation takes place in order, (a) to form a continuous gradient out of the discontinuous gradient and, (b) to reduce the time required for the separation. After the sedimentation is completed the chamber is returned to a vertical position and the gradient fractionated by hydrostatic pressure using a newly developed floating device. The usefulness of the method is illustrated by the separation of leukemia cells in different phases of the life cycle.

The mechanism of T cell mediated cytotoxicity. III. Changes in target cell susceptibility during the cell cycle

Sedimentation velocity techniques have been used to fractionate cells according to cell cycle phase. Mastocytoma P815 shows higher susceptibility to specific anti-H2d cytotoxic T cells during the early phases of the cycle. The changes in H2d antigen density as measured by susceptibility to limiting dilutions of anti-H2d and complement vary independently from the changes in susceptibility to T cells. Further there is no change in P815 susceptibility to antibody-dependent cytotoxicity by K cells during the cell cycle. A methylcholanthrene induced tumour (Meth A) is less susceptible to cytotoxic T cells than is P815. This is shown not to be due to differences in H2d antigen density. Fractionation of this cell line indicated that suscep­tibility to cytotoxic T cells did not change significantly during the cell cycle. It is suggested that differences in susceptibility between different cell lines are found mainly in the early phases of the cell cycle. Although the causes of the changes in susceptibility remain unexplained, these obser­vations point to an important contribution by the target cell, apart from the possession of the relevant antigen, to the events leading to lysis by T cells.

Biophysics of cell separations

To the Chemist, it has long been second nature to work with separate, purified substances. Biochemists and molecular biologists have now largely achieved the same working position, with their powerful separation techniques of ultracentrifugation, electrophoresis, chromatography and the like. For the cell biologists, it is a much more recent phenomenon that widespread attention is being directed towards quantitative measurement of properties in isolated, homogeneous cell types. The richness and diversity of cell systems beg for cleverness on the part of the experimenter.

Generation of cytotoxic T lymphocytes in vitro. III. Velocity sedimentation studies of the differentiation and fate of effector cells in long-term mixed leukocyte cultures

Separation of cells by velocity sedimentation at unit gravity was utilized to investigate the physical properties of cytotoxic thymus-derived lymphocytes (CTL) generated in long-term mixed leukocyte cultures (MLC). In kinetic studies, CTL were found almost exclusively in the large cell fractions at the peak of the response on day 4, whereas the majority of CTL in day 14 MLC had the sedimentation properties of small lymphocytes. Reculture until day 14 of cells fractionated on the basis of size on day 4 indicated that the small CTL were derived exclusively from cells which had been large on day 4. Re-exposure of day 14 MLC cells to the original stimulating alloantigens resulted in significant cell proliferation and rapid regeneration of CTL activity. Cell fractionation experiments demonstrated that the cells in the day 14 MLC population which responded to the secondary allogeneic stimulus were small T lymphocytes, and that these cells rapidly developed into large, highly cytotoxic CTL following stimulation. Moreover, by restimulating on day 14 fractions which were selected on the basis of size on day 4, it was found that the responding small lymphocytes were themselves the progeny of cells which were large at the peak of the response. Since CTL and CTL progenitors showed concomitant changes in physical properties with time, the possibility exists that they belong to the same cell lineage, and hence that CTL can differentiate into cells which are no longer cytotoxic, but capable of mounting an anamnestic response.

Sedimentation changes of L cells in a density gradient early after infection with vaccinia virus

By 2 h postinfection, LM cells infected with vaccinia virus show a shift in their distribution when separated on a Ficoll density gradient. This shift is dependent on both time and the multiplicity of infection and is due, at least in part, to an increase in cell size. Those cells which do shift in position in the gradient represent infected members of the population. Physical changes induced in virus-infected cells can be utilized for studying early events in virus replication.

Restriction of specificity in the precursors of bone marrow-associated lymphocytes

Previous work has shown that the immediate precursor of B lymphocytes (PB cell) has many properties that distinguish it from both B lymphoctes and hemopoietic stem cells. Size, density, tissue distribution, and sensitivity to cytotoxic antisera differ for each type of cell. The work described here was designed to study three aspects of the differentiation of PB cells. First, since PB cells probably have immunoglobulin surface receptors, fluorescein-labeled anti-immunoglobulin antiserum was used in an attempt to investigate directly the physical properties of PB cells. The use of this labeled antiserum revealed a population of cells with properties similar to the PB cells defined by the functional assays. Second, the differentiative potential of PB cells was studied by comparing the size of the total population of PB cells, as determined with fluorescein-labeled anti-immunoglobulin antiserum, to the size of the population of PB cells responding in a functional assay with a specific antigen. The cells responding in the functional assay represent only 0.1% of the total population of PB cells. This observation suggests that PB cells are not pluripotent stem cells of the immune system. Finally, the kinetics of the differentiation of PB cells to B lymphocytes was studied. The differentiation to mature lymphocytes involves at least one intermediate stage in which cells larger than mature B cells are active in a functional assay for B cells. These large B cells are present in irradiated mice soon after transplantation of PB cells, but by 20 days the majority of the B cells are typical small lymphocytes.