Gene arrays at Pneumocystis carinii telomeres

In the fungus Pneumocystis carinii, at least three gene families (PRT1, MSR, and MSG) have the potential to generate high-frequency antigenic variation, which is likely to be a strategy by which this parasitic fungus is able to prolong its survival in the rat lung. Members of these gene families are clustered at chromosome termini, a location that fosters recombination, which has been implicated in selective expression of MSG genes. To gain insight into the architecture, evolution, and regulation of these gene clusters, six telomeric segments of the genome were sequenced. Each of the segments began with one or more unique genes, after which were members of different gene families, arranged in a head-to-tail array. The three-gene repeat PRT1-MSR-MSG was common, suggesting that duplications of these repeats have contributed to expansion of all three families. However, members of a gene family in an array were no more similar to one another than to members in other arrays, indicating rapid divergence after duplication. The intergenic spacers were more conserved than the genes and contained sequence motifs also present in subtelomeres, which in other species have been implicated in gene expression and recombination. Long mononucleotide tracts were present in some MSR genes. These unstable sequences can be expected to suffer frequent frameshift mutations, providing P. carinii with another mechanism to generate antigen variation.

Mononuclear cell line THP-1 internalizes bactericidal/permeability-increasing protein by a non-receptor-mediated mechanism consistent with pinocytosis

BACKGROUND

Bactericidal/permeability-increasing protein (BPI) binds lipopolysaccharide and neutralizes its toxic effects in vitro and in endotoxemic animals. Our recent work identified physiologically significant interactions between BPI, lipopolysaccharide, and mononuclear cells.

OBJECTIVE

To determine whether the interaction between BPI and mononuclear cells is receptor mediated.

DESIGN

Labeled BPI was incubated with THP-1 cells in the presence of up to 100-fold excess of unlabeled BPI. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis and Western blotting were performed to evaluate competitive binding and total uptake of BPI. Crosslinking was performed to determine whether BPI binds to a single protein entity. Acid washing experiments and flow cytometric analysis were performed to determine whether BPI remains on the cellular surface. Finally, flow cytometry analysis was used to determine whether BPI incubation with THP-1 cells affects the surface expression of the lipopolysaccharide-binding protein-lipopolysaccharide receptor CD14.

RESULTS

Labeled BPI uptake was not inhibited by the presence of 100-fold excess of unlabeled BPI at 37 degrees C or 4 degrees C in the presence of azide. Uptake was not saturable under either condition with incubation concentrations up to 10 microgram/mL. Cross-linking did not show BPI bound to a single entity. Acid washing and flow cytometry experiments disclosed rapid internalization of BPI. Finally, BPI uptake by THP-1 cells had no effect on the surface expression of CD14.

CONCLUSIONS

Bactericidal/permeability-increasing protein is rapidly internalized by mononuclear cells in a nonspecific fashion not saturable at very high doses, which is consistent with pinocytosis. This process may represent a disposal mechanism for lipopolysaccharide in closed-space infections and may be partially responsible for the rapid clearance of BPI from the peripheral circulation.

Regulation of TNF-alpha receptor expression on human neutrophils by various cell activating factors

Since tumor necrosis factor-alpha (TNF-alpha) has been implicated in a wide range of inflammatory states, we sought to examine the regulation of TNF-alpha receptors on human neutrophils (PMNs). We were particularly interested in the relationship of TNF-alpha binding characteristics to various known neutrophil-priming stimuli such as temperature, N-formylmethionyl-leucyl-phenylalanine (fMLP), phorbol myristate acetate (PMA), C5ades Arg, and human recombinant C5a (HrC5a). Competition binding studies indicated that there was no significant change in receptor number or affinity due to the temperature used for neutrophil isolation. PMNs isolated at 4 or 25 degrees C had a single class of receptors with a Kd of 83.1 +/- 3.3 vs 90.0 +/- 8.3 pmole/liter and a similar number of receptors, 1575 +/- 99 vs 1876 +/- 279 receptors/cell. However, when either group of isolated PMNs was further incubated in buffer at 37 degrees C for 15 min, a significant increase in Kd (4-37 degrees C, 147.9 +/- 23.7 pmole/liter; 25-37 degrees C, 165.2 +/- 29.2 pmol/liter) and a decrease in receptor number (4-37 degrees C, 1116 +/- 86 receptors/cell; 25-37 degrees C, 957 +/- 71 receptors/cell) were seen. To determine the effect of other activating substances such as fMLP, PMA, C5ades Arg, HrC5a, 4 degrees C isolated PMNs were used.(ABSTRACT TRUNCATED AT 250 WORDS)

Alterations of neutrophil responses to tumor necrosis factor alpha and interleukin-8 following human endotoxemia

Interleukin-8 (IL-8), a neutrophil chemoattractant and activating cytokine, has been implicated as a proinflammatory mediator in gram-negative sepsis. In vitro data support the notion of IL-8 as an endothelial adherence inhibitor. To evaluate this issue, we infused six volunteers with reference endotoxin and measured plasma levels of IL-8, neutrophil tumor necrosis factor alpha (TNF-alpha) receptors, TNF-alpha-induced adherence to fibronectin, and neutrophil chemotaxis to IL-8 and other attractants. We found that, at 3 h postinfusion, IL-8 but not TNF-alpha plasma levels were elevated. Neutrophils had shed L-selectin (mean channel fluorescence decrease, 79 +/- 9 to 49 +/- 7; P = 0.0625) and TNF-alpha receptors (decrease in number of receptors per cell, 1,596 +/- 340 to 574 +/- 93; P = 0.004). Cells were chemotactically desensitized to IL-8. TNF-alpha-induced adherence to fibronectin was suppressed from 69% +/- 5% of the phorbol myristate acetate response to 38% +/- 7% (P = 0.0154). These findings support the notion that release of IL-8 into the vascular space may be an in vivo mechanism for suppression of neutrophil accumulation at extravascular sites. L-Selectin loss would reduce the ability of neutrophils to adhere to activated endothelial cells. The specific loss of migratory response to IL-8 would impair neutrophil delivery to areas where IL-8 was the predominant chemoattractant. Loss of TNF-alpha-induced adherence to fibronectin would blunt those responses, including production of oxidants, capacitated by adherence.