Identification of the complement decay-accelerating factor (DAF) on epithelium and glandular cells and in body fluids

Decay-accelerating factor protects human tumor cells from complement-mediated cytotoxicity in vitro

The disialoganglioside GD2 is expressed on a wide spectrum of human tumor types, including neuroblastomas and melanomas. Upon binding of 3F8, a murine monoclonal antibody (MAb) specific for GD2, neuroblastomas and some melanomas are sensitive to killing by human complement, whereas some melanomas are not. To investigate the mechanism underlying these differences in complement mediated cytotoxicity, complement-insensitive melanoma cell lines were compared with respect to expression of the decay-accelerating factor (DAF), a membrane regulatory protein that protects blood cells from autologous complement attack. While DAF was undetectable among neuroblastomas, it was present in complement-insensitive melanomas. When the function of DAF was blocked by anti-DAF MAb, C3 uptake and complement-mediated lysis of the insensitive melanoma lines were markedly enhanced. F(ab')2 fragments were as effective in enhancing lysis as intact anti-DAF MAb. The DAF-negative and DAF-positive melanoma cell lines were comparably resistant to passive lysis by cobra venom factor-treated serum. The data suggest that in some tumors, DAF activity accounts for their resistance to complement-mediated killing. The ability to render these cells complement-sensitive by blocking DAF function may have implications for immunotherapy.

Normal polymorphic variations and transcription of the decay accelerating factor gene in paroxysmal nocturnal hemoglobinuria cells

In paroxysmal nocturnal hemoglobinuria (PNH), an acquired hemolytic anemia, deficiency of decay accelerating factor (DAF) renders blood cells susceptible to increased deposition of autologous complement activation fragments (C3b) and complemented-mediated injury. To investigate the mechanism of the DAF defect, DNA and mRNA from normal and PNH leukocytes were compared in blot hybridization assays by using DAF cDNA and oligonucleotide probes. Southern analyses of DNA from normal cells revealed a single gene spanning approximately equal to 35 kilobases of DNA. Six HindIII banding patterns were distinguishable among normal individuals. In family studies, the patterns segregated as three homozygous and three heterozygous genotypes deriving from three haplotypes: A, B, and C with frequencies of 0.47, 0.36, and 0.17, respectively. Oligonucleotide mapping localized the polymorphic HindIII sites to two noncoding regions in the vicinity of exons encoding (i) the protein oligosaccharide-rich domain and (ii) the mRNA 3'-untranslated region. Analyses of DNA from DAF-negative leukocytes of eight PNH patients demonstrated restriction fragment profiles identical to those of normal individuals for all enzymes studied. Three patients had the BC (normals = 3/32), three patients had the AA (normals = 6/32), and two patients had the AC (normals = 8/32) HindIII genotype. Of the three PNH patients exhibiting the BC genotype, family studies of two demonstrated the expected inheritance patterns, and RNA gel blot analyses of two showed mRNA transcripts indistinguishable from those in normal cells. The absence of DAF gene or mRNA alterations in affected PNH cells that lack other glycolipid-anchored proteins as well as DAF argues that the lesion underlying PNH cells resides in the glycolipid-anchor pathway.

Developmentally regulated expression by Trypanosoma cruzi of molecules that accelerate the decay of complement C3 convertases

We recently showed that culture-derived metacyclic trypomastigotes (CMT), but not epimastigotes (Epi), of the Miranda 88 strain of Trypanosoma cruzi evade lysis by the human alternative complement pathway because of inefficient binding of factor B to complement component C3b on the parasite surface. These results suggested that CMT and tissue-culture-derived trypomastigotes (TCT), which also activate the alternative pathway poorly, might produce a molecule capable of interfering with factor B binding to C3b. We now demonstrate that CMT and TCT lysates, as well as molecules spontaneously shed from CMT and TCT but not Epi, accelerate decay of 125I-labeled factor Bb from the alternative-pathway C3 convertase (C3bBb) assembled on zymosan or Epi and also accelerate decay of the classical-pathway C3 convertase (C4b2a) on sheep erythrocytes. Parasites metabolically labeled with [35S]methionine spontaneously shed a limited number of radioactive components ranging in molecular mass from 86 to 155 kDa for trypomastigotes and 25 to 80 kDa for Epi. Decay-accelerating activity within supernatants is inactivated by papain and is coeluted with 35S-containing polypeptides on FPLC anion-exchange chromatography, suggesting that the active constituents are protein molecules. Molecules with decay-accelerating activity may explain the developmentally regulated resistance to complement-mediated lysis in infective and vertebrate stages of the T. cruzi life cycle.

A glycan-phosphatidylinositol-specific phospholipase D in human serum

A group of proteins anchored to the cell by phosphatidylinositol (PI) has recently been identified. The significance of this new class of membrane anchor is unknown; one possibility is that it facilitates release of the molecule by phospholipases. In fact, phospholipase C enzymes specific for the complex carboxyl-terminal glycolipids of these proteins have been isolated from African trypanosomes and from hepatocyte plasma membranes. This study reports the discovery of a glycan-PI-specific phospholipase D in human serum that cleaves both the membrane form of the variant surface glycoprotein of African trypanosomes and its glycolipid precursor, but not phosphatidylethanolamine, phosphatidylcholine, or phosphatidylinositol. Decay-accelerating factor, another PI-anchored molecule, is also cleaved by the enzyme and converted from a hydrophobic to a soluble protein. The enzyme is Ca2+-dependent, heat labile, and not affected by the inhibitor of serine proteases, phenylmethylsulfonylfluoride. Its function is not known, but the present findings indicate that it participates in the metabolism of glycolipid-anchored membrane proteins.

Relationship between decay accelerating factor deficiency, diminished acetylcholinesterase activity, and defective terminal complement pathway restriction in paroxysmal nocturnal hemoglobinuria erythrocytes

Paroxysmal nocturnal hemoglobinuria (PNH) erythrocytes exhibit abnormalities in decay accelerating factor (DAF), acetylcholinesterase, and resistance to autologous C5b-9 attack. To investigate the nature of the lesion underlying PNH cells, we examined the relationship of these abnormalities to one another. Analyses of DAF in acetylcholinesterase-negative erythrocytes revealed that these two abnormalities involve functionally independent molecules, coincide precisely in the same cell populations, and are similarly expressed in PNH II and more complement-sensitive PNH III erythrocytes. The DAF and acetylcholinesterase deficiencies contrast with the C3b/C4b receptor (CR1) deficit, which is less profound and similarly distributed in complement-insensitive cell populations. Hemolytic studies showed that defective resistance to autologous C5b-9 attack is mediated by another mechanism. Whereas reconstitution of PNH II erythrocytes with DAF completely corrected their complement sensitivity, DAF reconstitution of PNH III erythrocytes restored their ability to circumvent C3b uptake but had no effect on their heightened susceptibility to reactive lysis. Assays of complement-insensitive (PNH I) erythrocytes surviving after reactive lysis disclosed partial DAF and acetylcholinesterase deficits. These findings indicate that the PNH lesion involves multiple membrane components and that PNH I erythrocytes are also abnormal.