Immunologic properties of bacterial lipopolysaccharide (LPS). IV. Cellular basis of the unresponsiveness of C3H/HeJ mouse spleen cells to LPS-induced mitogenesis.

Lymphoid cells obtained from the C3H/HeJ mouse strain respond abnormally to LPS in vitro, as shown by the fact that they are unable to make a mitogenic response to some LPS preparations and make only a low mitogenic response to other LPS preparations. In contrast, cells from a closely related C3H substrain, the C3H/St, are highly responsive to both types of LPS preparations. Experiments were carried out to determine the cellular basis of these genetically determined LPS response differences. This question was approached by studying the mitogenic response to LPS in cultures containing mixtures of various combinations of B cells, T cells, and macrophages from C3H/HeJ and C3H/St mice. Experiments utilizing an LPS preparation to which the C3H/HeJ is totally unresponsive (negative LPS) revealed, first, that either spleen cells, or partially purified T cells and/or macrophages obtained from C3H/St, could not restore the ability of C3H/HeJ spleen cells to respond to LPS, indicating that the C3H/HeJ is not deficient in an LPS-specific helper cell population which may be required for mitogenesis. Secondly, the addition of either spleen cells or partially purified T cells or macrophages from the C3H/HeJ to spleen cells from the C3H/St did not inhibit the mitogenic response to LPS, suggesting that the presence of suppressor cell activity is also not involved. Experiments analogous to those described, except utilizing another LPS preparation to which the C3H/HeJ is partially responsive (positive LPS), also failed to demonstrate reconstitutive or suppressive effects when C3H/HeJ and C3H/St spleen cells were admixed. The results obtained indicate that the defect in the C3H/HeJ mouse strain that limits its responsiveness to positive LPS and which renders it totally unresponsive to negative LPS appears to be an intrinsic defect in the capacity of B cells to react to the mitogenic stimulus of LPS.     

Changes in testicular lactate dehydrogenases of the rat (Rattus norvegicus) during growth and development

1. We have quantified lactate dehydrogenase (LDH) A, B and C enzyme levels during the first twelve weeks of life in rat testes. 2. Proteins were electrophoresed under non-denaturing conditions, isoenzymes identified by activity staining and quantified by densitometry scanning. 3. LDH A declined 2-3-fold during testes growth, changing from the most to the least abundant isoenzyme form. LDH B remained fairly constant, and LDH C increased from undetectable levels at birth to become the most abundant of the three polypeptides from week 3 onwards.     

The effect of gamma radiation and neocarzinostatin on NAD and ATP levels in mouse leukaemia cells.

When mouse leukemia cells are treated with gamma-radiation or neocarzinostatin the intracellular NAD and ATP levels fall rapidly. We have shown that the ATP response is a consequence of the decreased NAD level. We suggest that this low NAD level results in decreased glycolytic activity and that there is a subsequent accumulation of phosphorylated sugars associated with the fall in ATP. Under these extreme conditions, therefore, the NAD level probably regulates the rate of glycolysis in cells which are utilising a rapidly metabolisable sugar as their energy source.     

The effect of gamma radiation and neocarzinostatin of NAD and ATP levels in mouse leukaemia cells

When mouse leukaemia cells are treated with γ-radiation or neocarzinostatin the intracellular NAD and ATP levels fall rapidly. We have shown that the ATP response is a consequence of the decreased NAD level. We suggest that this low NAD level results in decreased glycolytic activity and that there is a subsequent accumulation of phosphorylated sugars associated with the fall in ATP. Under these extreme conditions, therefore, the NAD level probably regulates the rate of glycolysis in cells which are utilising a rapidly metabolisable sugar as their energy source.     

Frequent burning promotes invasions of alien plants into a mesic African savanna

Fire is both inevitable and necessary for maintaining the structure and functioning of mesic savannas. Without disturbances such as fire and herbivory, tree cover can increase at the expense of grass cover and over time dominate mesic savannas. Consequently, repeated burning is widely used to suppress tree recruitment and control bush encroachment. However, the effect of regular burning on invasion by alien plant species is little understood. Here, vegetation data from a long-term fire experiment, which began in 1953 in a mesic Zimbabwean savanna, were used to test whether the frequency of burning promoted alien plant invasion. The fire treatments consisted of late season fires, lit at 1-, 2-, 3-, and 4-year intervals, and these regularly burnt plots were compared with unburnt plots. Results show that over half a century of frequent burning promoted the invasion by alien plants relative to areas where fire was excluded. More alien plant species became established in plots that had a higher frequency of burning. The proportion of alien species in the species assemblage was highest in the annually burnt plots followed by plots burnt biennially. Alien plant invasion was lowest in plots protected from fire but did not differ significantly between plots burnt triennially and quadrennially. Further, the abundance of five alien forbs increased significantly as the interval (in years) between fires became shorter. On average, the density of these alien forbs in annually burnt plots was at least ten times as high as the density of unburnt plots. Plant diversity was also altered by long-term burning. Total plant species richness was significantly lower in the unburnt plots compared to regularly burnt plots. These findings suggest that frequent burning of mesic savannas enhances invasion by alien plants, with short intervals between fires favouring alien forbs. Therefore, reducing the frequency of burning may be a key to minimising the risk of alien plant spread into mesic savannas, which is important because invasive plants pose a threat to native biodiversity and may alter savanna functioning.