Faculty Bibliography

The membrane glycoprotein CD59 protects host cells from homologous complement attack by inhibiting the assembly of the membrane attack complex. CD59 binds to C8 and C9 in the nascent membrane attack complex and interferes with C9 membrane insertion and polymerization. We show here that a synthetic peptide from the putative C9 hinge region, postulated to be involved in the rearrangement of C9 globular domains during membrane insertion, binds specifically to CD59 and enhances lysis of human erythrocytes by the terminal complement C5b-9 complex. The peptide, C9H, caused a dose-dependent increase in the sensitivity of human erythrocytes to C5b-9-mediated lysis by interfering with the final C9 binding and/or membrane insertion step. C9H exhibited species-specificity, since it had no activity against guinea pig C8 and C9 or on the putative functional homologues of CD59 in guinea pig erythrocytes. A direct association between CD59 and C9H was suggested by two different binding experiments: C9H inhibited the binding of 125I-labeled CD59 to immobilized C9, and C9H immobilized to microtiter plates bound purified CD59 and selectively recognized CD59 from extracts of detergent-solubilized human erythrocyte membranes. These data indicate that the peptide C9H corresponds to a region of the CD59 binding site of C9

Erythrocytes shed membrane vesicles in response to many stimuli. It has been previously demonstrated that glycan phosphatidylinositol-linked (GPI-linked) proteins such as decay accelerating factor and acetylcholinesterase are concentrated in these vesicles relative to the erythrocyte membrane. We have examined the requirement for GPI-linked proteins for the process of vesiculation. Erythrocytes that do not express GPI-linked proteins, obtained from patients with paroxysmal nocturnal hemoglobinuria (PNH), release between 10% and 50% of the quantity of vesicles as normal cells in response to the Ca2+ ionophore A23187. Platelets from the same patients produced 10% to 20% of the amount of vesicles as normal platelets. In addition, a mutant B-lymphoblastoid cell line that lacks GPI-linked molecules produces about half of the number of vesicles as compared with the wild-type cell line in response to the Ca2+ ionophore. Prior findings indicate that vesiculation is one of the mechanisms that the cell uses to remodel the plasma membrane, as well as protect itself from membrane-damaging agents such as the terminal complement components C5b-9. On the basis of the present results, we conclude that GPI-linked proteins play an important role in membrane vesiculation

Nucleated cells can resist attack by C by exocytosis or endocytosis of the terminal C components C5b-9 (membrane attack complex) (MAC), but it is generally accepted that formation of a single MAC channel on E leads to lysis (one-hit theory). We find that human and guinea pig E, but not SRBC, can eliminate the MAC from the membrane in the form of microvesicles and escape destruction. When guinea pig or human E are incubated with C5b-9, vesiculation proceeds without a lag and is detected at nonlytic doses of C9. Continuous Ca2+ influx is required for vesiculation. The amount of released vesicles is in direct relation to Ca2+ concentration, and the increase in vesiculation is associated with a parallel decrease in lysis. SRBC, which do not vesiculate when Ca2+ loaded, are lysed by C5b-9 with the same efficiency in the presence or absence of Ca2+. Vesicles released from guinea pig RBC under C5b-9 attack are enriched in C9 by a factor of 10, compared with the unlysed cells, and by a factor of 3 to 4, compared with ghosts. We conclude that E are protected from lysis not only by CD59 and C8bp/HRF, which prevent MAC assembly, but also by selective elimination of the MAC

The isoform of Fc gamma RIII (CD16) expressed on PMN has a GPI membrane anchor, and in paroxysmal nocturnal hemoglobinuria (PNH) there is a deficiency in Fc gamma RIII expression on PMN. Contrary to expectation, however, CD16 expression is preserved (albeit at reduced levels) in all affected PNH PMN that completely lack the GPI-anchored proteins DAF (CD55) and CD59. Fc gamma RIII negative PMN are not observed in any of the six PNH patients examined in this study. Analysis of the molecular weight of both glycosylated and deglycosylated Fc gamma RIII from PMN with reduced Fc gamma RIII expression indicates no variations in size relative to normal donor Fc gamma RIIIPMN. Indeed, the Fc gamma RIII expressed at intermediate levels is phosphatidylinositol-specific phospholipase C (PI-PLC)-sensitive. Thus, there is no evidence suggestive of expression of a transmembrane isoform and all data indicate that Fc gamma RIIIPMN on affected cells in PNH is a GPI-linked isoform. With Fc gamma RIIIPMN expression preserved at reduced levels on affected cells in PNH, PMN from PNH patients retain the capacity to internalize the Fc gamma RIIIPMN-specific probe E-ConA (at reduced levels) as well as IgG-opsonized erythrocytes. Reduced expression of GPI-anchored molecules on PNH PMN is not restricted to Fc gamma RIIIPMN since intermediate levels of CD59 were observed in the PNH PMN that were decay-accelerating factor (DAF)-negative and Fc gamma RIIIPMN intermediate. In addition, discordant expression of GPI-linked molecules in individual cells is not restricted to PMN since DAF+/CD14- monocytes were observed in one PNH patient. These data suggest that, when analyzed on an individual cell level, the GPI anchor defect in PNH is not absolute and must involve either a hierarchy of access of different protein molecules to available GPI anchors, distinct anchor biochemistries for the different proteins, or differential regulation of protein-anchor assembly

Here we compare the properties of leukocyte antigens H19 and CD59 with those of the PI-linked 18,000-20,000 Mr molecules which inhibit lysis of human cells by the autologous terminal complement components C5b-9. H19, a 19,000 Mr protein found on human erythrocytes, monocytes, neutrophils, T-lymphocytes and other cells, is one of the ligands involved in the spontaneous rosette formation between human T-lymphocytes and erythrocytes. Recent evidence indicates that H19 also participates in T-cell activation. CD59 is a widely distributed 18,000-25,000 Mr protein anchored to the cell membrane by phosphatidylinositol (PI). The function of CD59 is unknown. Affinity-purified H19 incorporates into cell membranes and inhibits channel formation by human C5b-9 on guinea pig erythrocytes. Significant inhibition is achieved with picogram quantities of H19, corresponding to approximately 600 molecules per erythrocyte. H19 is most effective when C9 is limiting but quite active when C5b-7 or C8 are limiting, indicating that it may interact with several of the structurally related terminal complement components. The inhibitory activity is blocked by mAbs to either CD59 or to H19. H19 is PI-anchored: it is released from the cell membrane by treatment with PI-specific phospholipase C, and it is absent from cells from a patient with paroxysmal nocturnal hemoglobinuria (PNH). Analysis of PNH erythrocytes after treatment with terminal complement proteins shows that the H19-negative erythrocytes are more susceptible to C5b-9-mediated lysis. Treatment of normal human erythrocytes with either anti-H19 or anti-CD59 renders them more susceptible to lysis by human C5b-9. We conclude that H19 and CD59 are probably the same molecule and are identical or closely related to the recently described inhibitors of C5b-9 channel formation

Trypanosoma cruzi releases into the culture medium heat-labile, trypsin-sensitive molecules which lyse erythrocytes from various animal species. Production of the hemolysin is abolished by removal of glucose from the medium or by addition of the metabolic inhibitors sodium azide, 2-deoxy-D-glucose or puromycin. Sieving experiments with erythrocyte ghosts indicate that large channels are formed on the target membranes. The activity of the hemolysin is maximal at pH 5.5 and undetectable at neutral pH, indicating that it functions in acidic intracellular compartments. This agent could be involved in promoting the escape of T. cruzi into the cytoplasm of the host cell, by mediating the lysis of the membrane of the phagosome in which the parasite resides at early times after invasion