Characterization of mouse DAF on transfectant cells using monoclonal antibodies which recognize different epitopes

Depletion of complement does not impact initiation of xenobiotic-induced autoimmune disease

Deficiency in Daf1, a complement regulatory protein, has been shown to exacerbate development of various autoimmune diseases and recent studies have suggested that this may be explained by Daf1 acting to limit T-cell hyper-responsiveness. It has been suggested that the absence of Daf1 aggravates autoimmune disease in a complement-dependent manner, but others have shown that activation of T cells in the absence of Daf1 can be complement independent. However, the relationship between Daf1, complement components, lymphocyte activation, cytokine expression and antibody production remains to be determined in mice that are not Daf1 deficient. We have recently demonstrated, in murine mercury-induced autoimmunity (mHgIA), that an accumulation of CD44(high) Daf(low) CD4(+) T cells is associated with the development of autoimmunity. In this study we observed that complement depletion does not affect the accumulation of activated CD4(+) T cells, elevation of splenic interleukin-4 expression and autoantibody production in mHgIA. In addition, neither the accumulation of CD44(high) Daf(low) CD4(+) T cells nor the down-regulation of Daf1 expression on CD4(+) T cells was influenced by a lack of complement. In conclusion, these studies show that initiating events in xenobiotic-induced autoimmunity, including lymphocyte activation, cytokine expression and autoantibody production, are not dependent on complement.

PGAP1 knock-out mice show otocephaly and male infertility

A palmitate linked to the inositol in glycosylphosphatidylinositol (GPI) is removed in the endoplasmic reticulum immediately after the conjugation of GPI with proteins in most cells. Previously, we identified PGAP1 (post GPI attachment to proteins 1) as a GPI inositoldeacylase that removes the palmitate from inositol. A defect in PGAP1 caused a delay in the transport of GPI-anchored proteins (GPI-APs) from the endoplasmic reticulum to the cell surface in Chinese hamster ovary cells, although the cell-surface expression of GPI-APs in the steady state was normal. Nevertheless, in most cells, GPI-APs undergo deacylation. To elucidate the biological significance of PGAP1 in vivo, we established PGAP1 knock-out mice. Most PGAP1 knock-out mice showed otocephaly, a developmental defect, and died right after birth. However, some survived with growth retardation. Male knock-out mice showed severely reduced fertility despite the capability of ejaculation. Their spermatozoa were normal in number, motility, and ability to ascend the uterus, but were unable to go into the oviduct. In vitro, PGAP1-deficient spermatozoa showed weak attachment to the zona pellucida and a severely diminished rate of fertilization. Therefore, an extra acyl chain in GPI anchors caused severe deleterious effects to development and sperm function.

Reduced expression of decay-accelerating factor 1 on CD4+ T cells in murine systemic autoimmune disease

OBJECTIVE

Deficiency of decay-accelerating factor 1 (termed Daf1 in mice) has been shown to exacerbate autoimmunity, and recent studies have suggested that this may be explained by Daf1 acting as a regulator of T cell immunity. The aim of this study was to determine whether Daf1 expression on T cells is modulated during development of autoimmunity in mice.

METHODS

To test this hypothesis, we examined Daf1 levels in NZB, DBA/2, and B10.S mice before and after induction of murine mercury-induced autoimmunity (mHgIA). Daf1 was measured by real-time polymerase chain reaction and flow cytometry, and levels of Daf1 were correlated with markers of lymphocyte activation and cytokine production.

RESULTS

Autoimmune-prone NZB mice had low endogenous levels of Daf1 irrespective of the induction of mHgIA. Induction of autoimmunity reduced Daf1 expression in mHgIA-sensitive B10.S mice, particularly on activated/memory (CD44(high)) CD4+ T cells that accumulate as a result of exposure to mercury. Murine mercury-induced autoimmunity-resistant DBA/2 mice, which fail to accumulate CD44(high) T cells, showed no change in Daf1 expression. Modulation of Daf1 expression was found to require CD4+ T cell costimulation, since B10.S mice deficient in CD28 were unable to down-regulate Daf1 or accumulate activated/memory CD4+ T cells. In B10.S mice exposed to mercury, the production of interleukin-4 (IL-4), but not that of IL-2 or interferon-gamma, in the spleen was associated with CD44(high),Daf1(low),CD4+ T cells.

CONCLUSION

These findings demonstrate that reduction of Daf1 expression is closely associated with CD4+ T cell activation and the accumulation of CD44(high)(activated/memory),CD4+ T cells in both spontaneous and induced systemic autoimmune disease.

The complement inhibitory protein DAF (CD55) suppresses T cell immunity in vivo

Decay-accelerating factor ([DAF] CD55) is a glycosylphosphatidylinositol-anchored membrane inhibitor of complement with broad clinical relevance. Here, we establish an additional and unexpected role for DAF in the suppression of adaptive immune responses in vivo. In both C57BL/6 and BALB/c mice, deficiency of the Daf1 gene, which encodes the murine homologue of human DAF, significantly enhanced T cell responses to active immunization. This phenotype was characterized by hypersecretion of interferon (IFN)-γ and interleukin (IL)-2, as well as down-regulation of the inhibitory cytokine IL-10 during antigen restimulation of lymphocytes in vitro. Compared with wild-type mice, Daf1^−/− mice also displayed markedly exacerbated disease progression and pathology in a T cell–dependent experimental autoimmune encephalomyelitis (EAE) model. However, disabling the complement system in Daf1^−/− mice normalized T cell secretion of IFN-γ and IL-2 and attenuated disease severity in the EAE model. These findings establish a critical link between complement and T cell immunity and have implications for the role of DAF and complement in organ transplantation, tumor evasion, and vaccine development.

Inhibition of HIV-1 infection in cells expressing an artificial complementary peptide

TLMA2993 peptide (N'-TLMALELKGKLLLAGLAPSAFLPLSFPEGL-C') which was designed by a computer program (MIMETIC) inhibited the activity of HIV-1 reverse transcriptase in a cell-free system. Therefore, we constructed a TLMA2993 expression vector containing an artificial cDNA for TLMA2993 to generate the peptide in cells. The cell lysate of transfected U937 cells contained a detectable level of TLMA2993 peptide using competitive ELISA. The transfectants were resistant to HIV-1 infection due to expression of TLMA2993 peptide in the cells. The use of MIMETIC to design an inhibitory peptide to any intracellular target molecule, followed by transfection of the artificial cDNA for the peptide, could afford a new approach for treatment and/or prevention of viral infection.

Mouse Complement Receptor-Related Gene y/p65-Neutralized Tumor Vaccine Induces Antitumor Activity In Vivo

Two mouse tumor cell lines, Meth A (BALB/c mouse-derived fibrosarcoma) and MM46 (C3H/He mouse-derived mammary tumor), were shown to express high levels of complement receptor-related gene y/p65 (Crry/p65), a membrane-bound complement-regulatory protein. Inhibiting the complement-regulatory activity of Crry/p65 with mAb 5D5 induced high levels of C3 deposition on in vivo tumor-derived Meth A and MM46 cells. To determine the effect of Crry/p65 blockade and increased C3 deposition on in vivo tumor growth, Meth A and MM46 cells were treated with 5D5 mAb and injected into BALB/c and C3H/He mice, respectively. Pretreating MM46 cells with 5D5 mAb significantly suppressed their tumorigenicity when injected s.c. Pretreatment with 5D5 mAb had a modest effect on Meth A s.c. tumor growth. Because complement is involved in the induction of an immune response, we investigated the effect of Crry/p65 blockade and increased C3 deposition on the immunogenicity of the tumor cells in a vaccination protocol. Vaccination of mice with irradiated Meth A cells pretreated with 5D5 mAb protected mice from subsequent challenge. In contrast, vaccination with irradiated Meth A cells without pretreatment was not protective. Survival was correlated with a high titer IgM response and specific CTL activity. These data demonstrate that the functional inhibition of Crry/p65 on tumor cells affects tumor growth and immunogenicity, and that the complement deposition resulting from this inhibition can act in concert with antitumor effector mechanisms to elicit potent antitumor immunity in vivo.

Decay-accelerating factor induction by tumour necrosis factor-alpha, through a phosphatidylinositol-3 kinase and protein kinase C-dependent pathway, protects murine vascular endothelial cells against complement deposition

We have shown that human endothelial cells (EC) are protected against complement-mediated injury by the inducible expression of decay-accelerating factor (DAF). To understand further the importance of DAF regulation, we characterized EC DAF expression on murine EC in vitro and in vivo using a model of glomerulonephritis. Flow cytometry using the monoclonal antibody (mAb) Riko-3 [binds transmembrane- and glycosylphosphatidylinositol (GPI)-anchored DAF], mAb Riko-4 (binds GPI-anchored DAF) and reverse transcription-polymerase chain reaction (RT-PCR), demonstrated that murine EC DAF is GPI-anchored. Tumour necrosis factor-alpha (TNF-alpha) increased EC DAF expression, detectable at 6 hr and maximal at 24-48 hr poststimulation. DAF upregulation required increased steady-state DAF mRNA and protein synthesis. In contrast, no increased expression of the murine complement receptor-related protein-Y (Crry) was seen with TNF-alpha. DAF upregulation was mediated via a protein kinase C (PKC)alpha, phosphoinositide-3 kinase (PI-3 kinase), p38 mitogen-activated protein kinase (MAPK) and nuclear factor-kappaB (NF-kappaB)-dependent pathway. The increased DAF was functionally relevant, resulting in a marked reduction in C3 deposition following complement activation. In a nephrotoxic nephritis model, DAF expression on glomerular capillaries was significantly increased 2 hr after the induction of disease. The demonstration of DAF upregulation above constitutive levels suggests that this may be important in the maintenance of vascular integrity during inflammation, when the risk of complement-mediated injury is increased. The mouse represents a suitable model for the study of novel therapeutic approaches by which vascular endothelium may be conditioned against complement-mediated injury.

Disruption of mouse CD46 causes an accelerated spontaneous acrosome reaction in sperm

Human membrane cofactor protein (MCP, CD46) is a ubiquitously expressed protein known to protect cells from complement attack. Interestingly, when we examined the expression of mouse CD46, which we recently cloned, the message was found only in testis and the protein was found on the inner acrosomal membrane of sperm. In order to elucidate the function of CD46, we produced mice carrying a null mutation in the CD46 gene by using homologous recombination. Despite the absence of CD46, the mice were healthy and both sexes were fertile. However, to our surprise, the fertilizing ability of males appeared to be facilitated by disruption of the CD46 gene, as the average number of pups born from CD46(-/-) males was significantly greater than that of wild-type males. It was also revealed that the incidence of the spontaneous acrosome reaction doubled in CD46(-/-) sperm compared to that in wild-type sperm. It was assumed that this increase caused the heightened fertilizing ability found in CD46(-/-) sperm. These data suggest that CD46 may have some role in regulating sperm acrosome reaction.

Complement-mediated clearance of erythrocytes: mechanism and delineation of the regulatory roles of Crry and DAF. Decay-accelerating factor

The role of complement in the pathogenesis of autoimmune hemolytic anemia (AIHA) has been controversial and may depend on a number of factors, including the affinity and isotype of the pathogenic antibodies involved. We have recently shown that mouse erythrocytes deficient in the membrane C3 regulatory protein, complement receptor 1-related gene/protein y (Crry), but not decay-accelerating factor (DAF), were spontaneously eliminated in vivo by complement. Here, by generating a mouse deficient in both DAF and Crry, we further delineated the roles of Crry and DAF in regulating alternative and classical pathway C3 activation. By using immunoglobulin-, Fcgamma receptor (FcgammaR)-, C3-, C4-, and C5-deficient mice, we also determined the mechanism by which membrane C3 regulator-deficient erythrocytes are cleared from the circulation. Finally, we evaluated the relative importance of the Fc receptor versus the complement pathway in disposing antibody-opsonized DAF/Crry-deficient erythrocytes. We conclude that (1) Crry plays a more dominant role than DAF in regulating the alternative pathway of complement, whereas DAF and Crry are equally effective in preventing antibody-induced runaway complement activation on mouse erythrocytes; (2) DAF/Crry-deficient erythrocytes are eliminated by the alternative pathway of complement via complement receptor-mediated erythrophagocytosis in the spleen; and (3) when opsonized with an immunoglobulin G2a (IgG2a) autoantibody, Crry/DAF-deficient erythrocytes are eliminated more rapidly by complement than by the Fc receptor pathway. These results shed new light on the relative activities of Crry and DAF and underscore the critical roles of membrane C3 regulators in preventing spontaneous and antibody-induced erythrocyte damage in vivo.

Deletion of decay-accelerating factor (CD55) exacerbates autoimmune disease development in MRL/lpr mice

Decay-accelerating factor (DAF, CD55) is a glycosylphosphatidylinositol-anchored membrane protein that restricts complement activation on autologous cells. It is also a ligand for CD97, an activation-associated lymphocyte antigen with seven transmembrane domains. It is widely expressed on cells of both the hematopoietic and nonhematopoietic lineages. Although deficiency of DAF on human erythrocytes is associated with the hemolytic anemia syndrome paroxysmal nocturnal hemoglobinuria, the in vivo biology of DAF is still poorly understood. We addressed the in vivo function of DAF in a knockout mouse model and describe here that deletion of DAF exacerbates autoimmune disease development in MRL/lpr mice, a model for human systemic lupus erythematosus. Compared to DAF-sufficient littermate controls, DAF-deficient female MRL/lpr mice developed exacerbated lymphadenopathy and splenomegaly, higher serum anti-chromatin autoantibody levels, and aggravated dermatitis. Consistent with the phenotype of aggravated dermatitis in DAF-deficient mice, Northern and Western blots and immunofluorescence studies showed DAF to be expressed abundantly in the mouse skin, suggesting that it may play a particularly important role in this tissue. Histology and immunostaining demonstrated inflammatory infiltrate and focal C3 deposition in early skin lesions, mostly along the dermal-epidermal junction. These results reveal a protective function of DAF in the development of a systemic autoimmune syndrome and suggest that dysfunction or down-regulation of DAF may contribute to autoimmune disease pathogenesis and manifestation.

Crry, but not CD59 and DAF, is indispensable for murine erythrocyte protection in vivo from spontaneous complement attack

Decay-accelerating factor (DAF) and CD59 are 2 glycosylphosphatidylinositol-anchored membrane proteins that inhibit complement activation at the C3 and C5b-9 step, respectively. CD59 is considered critical for protecting erythrocytes from spontaneous complement attack, as deficiency of CD59 or CD59/DAF, but not of DAF alone, on human erythrocytes renders them sensitive to complement lysis in paroxysmal nocturnal hemoglobinuria syndrome. To evaluate the relative roles of CD59 and DAF in vivo, we have generated and studied a CD59 knockout and a CD59/DAF double-knockout mouse. CD59-deficient and CD59/DAF-double-deficient mouse erythrocytes were highly sensitive to antibody-induced complement lysis in vitro, yet neither CD59 knockout nor CD59/DAF double-knockout mouse developed spontaneous hemolytic anemia. Consistent with the latter observation, erythrocytes from the 2 strains of mutant mice were shown to have a normal lifespan in vivo. In contrast, mouse erythrocytes deficient in complement receptor 1 (CR1)-related gene y (Crry), a membrane C3 inhibitor with DAF and membrane cofactor protein activities, were rapidly eliminated from the circulation by a complement-dependent mechanism. Compared with DAF-deficient erythrocytes, Crry-deficient erythrocytes incurred higher levels of spontaneous C3 deposition in vivo. These findings demonstrate that CD59 and DAF are not indispensable on murine erythrocytes. Rather, effective C3 regulation on the cell surface, provided by Crry rather than DAF, is necessary for mouse erythrocytes to resist spontaneous complement attack. Our results raise the possibility that proper control of C3 activation may also be critical on human erythrocytes, where CR1 but not DAF could be the principal regulator of spontaneous C3 activation.

Properties of exogenously added GPI-anchored proteins following their incorporation into cells

Isolated glycosylphosphatidylinositol (GPI)-anchored proteins, when added to cells in vitro, incorporate into their surface membranes and, once incorporated, exert their native functions. Virtually any protein of interest, if expressed as a GPI-reanchored derivative, can be modified to acquire this capacity. Such transfer of proteins directly to cells, termed "protein engineering" or "painting" constitutes an alternative to conventional gene transfer for manipulating cell surface composition that has many potential applications. Previous studies with incorporated GPI-anchored proteins have focused almost entirely on their extracellular functions. In this study, biotinylated human erythrocyte (E(hu)) decay accelerating factor, E(hu) acetylcholinesterase, and GPI-reanchored murine B7-1 and B7-2 were used as GPI-anchored reporters to characterize their plasma membrane organization and cell signalling properties following addition to Hela or Chinese hamster ovary cells. For each reporter, three types of cell-association were documented; (1) nonphysiological attachment and/or incomplete insertion, (2) uncomplexed membrane integration, and (3) organization into TX-100-resistant microdomains. Transit from the first two compartments into the third, i.e., microdomains, progressed slowly, continuing even after 24 to 36 h and was associated with the acquisition of cell signalling capacity. All four reporters, incorporated in two different detergents, behaved similarly. When organized in microdomains, caveolin and other GPI proteins co-isolated with the incorporated reporter. These results have implications for protein engineering of cells in general, and in particular, for cells such as modified tumor cell immunogens administered to patients for therapeutic purposes.

Characterization of glycosylphosphatidylinositol-anchored decay accelerating factor (GPI-DAF) and transmembrane DAF gene expression in wild-type and GPI-DAF gene knockout mice using polyclonal and monoclonal antibodies with dual or single specificity

Decay-accelerating factor (DAF, CD55) is a glycosylphosphatidylinositol (GPI)-linked membrane inhibitor of complement activation. While human and other mammalian species contain only one DAF gene, two distinct DAF genes, referred to as GPI-DAF and transmembrane (TM)-DAF, respectively, have been identified in the mouse. Using several independently generated monoclonal and polyclonal antibodies, either with dual or single specificity for GPI-DAF and TM-DAF gene products, we have examined the expression of the two DAF genes in tissues of the wild-type and a strain of knockout mouse whose GPI-DAF gene has been inactivated. By fluorescence-activated cell sorting (FACS) analysis, we found that DAF protein is present on the wild-type mouse erythrocytes and lymphocytes but no signal was detectable on the same cells of GPI-DAF gene knockout mice. Both T and B lymphocytes and splenic macrophages express the GPI-DAF gene but the expression level is higher on B lymphocytes than on T lymphocytes. Within the T cell population, both CD4+ and CD8+ T cells are positive. DAF protein was detected by immunohistochemistry at high levels on wild-type mouse spermatids and mature sperm. In contrast, only mature sperm stained positive in the GPI-DAF gene knockout mouse testis, suggesting that GPI-DAF but not the TM-DAF gene is expressed on spermatids. Examination of the fetoplacental unit at the day 7.5 stage revealed that GPI-DAF but not the TM-DAF gene is expressed in the maternal decidua cells surrounding the trophoectoderm of the embryo. No DAF expression was detected on trophoblast or the embryo proper. These findings suggest that although the TM-DAF gene is irrelevant on mouse blood cells, the two DAF genes may have different roles in germ cell development and/or mature sperm function. Because complement receptor 1-related gene/protein y (Crry) has been shown to be expressed on early mouse embryos, the complete lack of GPI-DAF and TM-DAF gene expression in early mouse development may explain the observed sensitivity of Crry-deficient embryos to maternal complement attack.

Tissue distribution of products of the mouse decay-accelerating factor (DAF) genes. Exploitation of a Daf1 knock-out mouse and site-specific monoclonal antibodies

Decay-accelerating factor (DAF) is a membrane regulator of C3 activation that protects self cells from autologous complement attack. In humans, DAF is uniformly expressed as a glycosylphosphatidylinositol (GPI)-anchored molecule. In mice, both GPI-anchored and transmembrane-anchored DAF proteins are produced, each of which can be derived from two different genes (Daf1 and Daf2). In this report, we describe a Daf1 gene knock-out mouse arising as the first product of a strategy for targeting one or both Daf genes. As part of the work, we characterize recently described monoclonal antibodies against murine DAF protein using deletion mutants synthesized in yeast, and then employ the monoclonal antibodies in conjunction with wild-type and the Daf1 knock-out mice to determine the tissue distribution of the mouse Daf1 and Daf2 gene products. To enhance the immunohistochemical detection of murine DAF protein, we utilized the sensitive tyramide fluorescence method. In wild-type mice, we found strong DAF labelling of glomeruli, airway and gut epithelium, the spleen, vascular endothelium throughout all tissues, and seminiferous tubules of the testis. In Daf1 knock-out mice, DAF labelling was ablated in most tissues, but strong labelling of the testis and splenic dendritic cells remained. In both sites, reverse transcription-polymerase chain reaction analyses identified both GPI and transmembrane forms of Daf2 gene-derived protein. The results have relevance for studies of in vivo murine DAF function and of murine DAF structure.

Bulk production and functional analyses of mouse CD55's native and deglycosylated active domains

We report the use of methylotrophic yeast Pichia pastoris as a host to efficiently express complement control protein repeats (CCPs) 1-4 of mouse decay accelerating factor (DAF, CD55) as a soluble protein. With this system, the mouse DAF CCP1-4-active-domain-containing module linked to a 6x His tag at its C terminus was secreted into the culture supernatant at 15 mg/L after 24 h of induction with methanol. A mouse DAF CCP1-4 mutant protein in which its two potential N-glycosylation sites were deleted by changing Asn(187) and Asn(262) to Gln was also produced. Using Ni(2+)-immobilized agarose affinity chromatography, the recombinant mouse DAF modules with their 6x His tags could be one-step isolated to SDS-PAGE purity. Polyclonal antibody against native mouse DAF CCP1-4 was raised by immunizing NZW rabbits with the purified product. Measurements of the bioactivities of the wild-type and mutant mouse DAF proteins in C3b uptake assays showed no differences in regulatory activities in either the classical or the alternative pathways. With the use of the mutant DAF protein, small rod-shaped crystals were produced and preliminary data obtained. The production of large quantities of functional recombinant mouse DAF CCP1-4 modules and their antibody offers the opportunity to study DAF structure and DAF function in vivo.

Membrane complement regulatory proteins: insight from animal studies and relevance to human diseases

The complement system plays an important role in host defense. However, if not properly regulated, activated complement can also cause significant damage to host tissues. To prevent complement-mediated autologous tissue damage, host cells express a number of membrane-bound complement regulatory proteins. These include decay-accelerating factor (DAF, CD55), membrane cofactor protein (MCP, CD46) and CD59. Recent studies of membrane complement regulatory proteins from various animal species have revealed similarities as well as significant differences from the corresponding human proteins. In this review, we summarize recent advances in this area and contrast the structure, function and tissue distribution of membrane complement regulatory proteins in human and nonprimate mammalian species. We also discuss how the characterization of the animal proteins has provided important clues and might continue to show relevance to the pathogenesis and therapeutics of a number of human diseases.

Functional domains, structural variations and pathogen interactions of MCP, DAF and CR1

The Regulators of Complement Activation (RCA) are a fascinating group of proteins that play important roles in innate and acquired immunity. In this review, we examine structure-function aspects of three membrane-bound RCA proteins and discuss the unique impact of their genetic organization on their evolution.