Frequency of Precursor Cells

There have been several attempts to estimate the frequencies of cells specific to complex antigens or to single epitopes. Many of the experiments were performed before T and B cell cooperation was known, thus making interpretation difficult. Many of the experiments have been, however, repeated under more defined conditions. 

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Table 2. Frequencies of precursor cells estimated by splenic-foci technique...

In Table 4 frequencies of precursor cells from non-immunized mice are compiled (see also Fig. 3). Some authors used spleen cells for limiting dilutions, others bone marrow cells in combination with a constant number of thymus cells. The latter, which guarantees that B cells are titrated, was used by Cudkowicz et al. (1969, I970) and Miller and Cudkowicz (1970). Further-... 

Fig. 3. Estimation of frequencies of precursor cells specific for rat red cells (Brown et al., 1966), poly-ala-BSA (Bosma and Weiler, 1970) and EPS (Moller and Michael, 1971) In all three instances, spleen cells from non-immunized mice were used...

If microculture studies on the frequency of precursor cells are compared with those using the antigen-binding method, splenic-foci technique or in vivo dilution method, the following picture emerges ... 

In the previous chapter it was shown that the frequencies of precursor cells for antibody-producing cells are low. The most straightforward interpretation is, of course, that a small fraction of the cells responds to a given antigen because of the commitment of only a small fraction of the cells to formation of antibody to that antigen. However, this is not the only possible interpretation. The alternative hypothesis is that any lymphocyte is potentially able to respond to any antigen, but the probability of successful triggering is low. If this were the case, any attempt to remove the specific cells would fail, since the essence of this hypothesis is that there are no specific cells. 

The experimental evidence for the single specificity of the receptors carried by a lymphocyte will be reviewed. It will be shown that the frequency of precursors of antibody-forming cells for any given epitope is low, and that these specific precursors can be eliminated or separated from a population of lymphocytes. 

In Table 2 there is presented a summary of estimates of precursor cells obtained using the splenic foci technique. Unfortunately, in most cases it is not possible to determine strictly whether T cells or B cells are limiting. Thus, it is not clear in many cases what precursor cell frequency has been calculated. Nevertheless, in cases where T cells are limiting, the frequency of B cells must be greater than values given in the table. 

Mice exposed to 2,000 ppm of trichloroethylene, 4 hours/day for a 5-day period, had a significant increase in abnormal sperm morphology of 1% 28 days after the exposure (Land et al. 1981). No effect was seen at 200 ppm. A 6% increase in abnormal sperm was observed 4 weeks, but not 4 days or 10 weeks, after mice were exposed to 100 ppm trichloroethylene 7 hours per day for 5 days (Beliles et al. 1980). Based on the time after exposure at which sperm were affected, the study authors indicated that trichloroethylene damages sperm precursor cells but that spermatogonia were either unaffected or were capable of recovery. Reproductive performance was not tested in these studies. Another mouse study tested the effects of a 5-day exposure (6 hours/day) on spermatid micronuclei frequency no effects were observed at exposure levels of up to 500 ppm, the highest concentration tested (Allen et al. 1994). These results were interpreted as evidence that trichloroethylene did not cause meiotic chromosome breakage or loss. No treatment-related reproductive effects were seen in female rats exposed to 1,800 ppm trichloroethylene for 2 weeks (6 hours/day, 7 days/week) before mating (Dorfmueller et al. 1979). 

Abstract. G-CSF Is a major extracellular regulator of hematopoiesis and the most used cytokine in clinical practice. Coherently with and for a long time after the repeated injections of low doses of G-CSF the study of alterations in hematopoietic precursor cells concentration in the bone marrow of mice was undertaken. G-CSF treatment did not affect the number of granulocytes and oligopotent precursor cells (CFU-C). However, frequency of early multipotent stem cells (LTC-IC) decreased one month after the last (7 ) course of G-CSF injections, moreover it halved during the following year. The exhaustion of LTC-IC after G-CSF treatment is discussed. 

Precursor frequency The precursor frequency of cells that have divided in response to a stimulus is the proportion of cells present at the time of the addition of the stimulus that were destined to begin division. It is a measure of the potential of a mixed population of cells for response to any particular activator and can be determined, by flow cytometry, with the use of fluorescent tracking dyes that stain cells with stability but dilute by half each time a cell divides. 

In a quadrupole collision cell, the ions undergo multiple collisions. The fragments, as soon as they are formed, are reactivated by collision and can fragment further. In the ion trap, if excitation occurs by irradiation at the secular frequency of the precursor, only this ion is excited, and the product ion may be too cool to fragment further. Figure 2.28 shows an example of this behaviour [17]. 

Carcinoids are the most common tumors arising from the diffuse neuroendocrine system of the GI tract and pancreas. Derived primarily from enterochromaffin cells, these tumors are widely distributed in the body but found with greatest frequency in the GI tract (74%) and respiratory tract (25%). Carcinoids are often classified as APUDomas (flmine precursor uptake and decarboxylation) because of the ability of enterochromaffin cells to take up and decarboxylate amino acid precursors of biogenic amines. In this regard, carcinoid tumors share certain pathological and biological similarities with pheochromocytomas. 

Alteration of Cell Proliferation/Apoptosis Balance by PPARa Activators. PPARa activators produce multiple tumor precursor effects including liver hyperplasia, and altered growth in preneoplastic foci. Increased cell replication induced by PPARa activators may increase the frequency of spontaneous mutations... 

The differences in the structures of the various switch regions raise the possibility of class-specific switch recombination, which would make no sense without a corresponding regulatory mechanism. It is known that the frequency of cells expressing various immunoglobulin classes can be regulated by T cell derived lym-phokines [3-9], although no lymphokine has been clearly demonstrated to induce the switch directly rather than to expand switched cells or switch precursors. 

One method for temporal precnrsor isolation is stored waveform inverse Fourier transform (SWIFT) [40]. In this method, the desired freqnency domain profile (all frequencies except that of the ion of interest) is inversely Fonrier transformed to a time domain waveform. This waveform is then applied to the excite electrodes in the ICR cell and, thns, the precursor ions are isolated in the cell. An alternative techniqne for in-cell isolation is correlated sweep excitation (COSE) [41], also known as correlated harmonic excitation fields (CHEF) [42]. This method involves application of radiofrequency pulses to the excite electrodes. The technique correlates the duration and frequency of the RF-pulses with those appropriate to the ions to be isolated. Both SWIFT and COSE are capable of isolating single isotopomers in peptide and protein ions [43-45]. 

SORI-CID, introduced by Gauthier and co-workers [28], is not beset by these problems. As the name suggests, ions are excited slightly off-resonance (500-2000 Hz). Such excitation results in acceleration and deceleration of the ions with a period equal to the difference between the excitation frequency and the ion cyclotron frequency. The periodic decrease in cyclotron radius means that ions are not ejected from the ICR cell. Prior to off-resonance excitation of the precursor ions, inert gas is leaked into the ICR cell. As the ions are excited, collisions with the gas resnlt in conversion of translational energy to internal energy. Again, as a resnlt of the periodic decrease in cyclotron radius, the product ions are formed close to the center of the cell, eliminating resolution issues. It is possible that the product ions have a cyclotron frequency equal to that of the applied excitation waveform. If this were the case, those product ions would be ejected from the ICR cell (resonant ejection). To avoid this occurrence, off-resonance excitation is performed in both directions, for example, 500 Hz. 

The OSRD cells generated by Mae et al. (2013) could be further differentiated into cells expressing markers typical for various renal structures, gonads and adrenal cortex, consistent with the idea that these cells had properties similar to those of early embryonic urogenital precursor cells. More differentiated renal cell types were obtained at frequencies that were too low for further applications (Mae et al., 2013). 

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