Identity of the residues responsible for the species-restricted complement inhibitory function of human CD59

Structural modelling of human complement FHR1 and two of its synthetic derivatives provides insight into their in-vivo functions

Human complement is the first line of defence against invading pathogens and is involved in tissue homeostasis. Complement-targeted therapies to treat several diseases caused by a dysregulated complement are highly desirable. Despite huge efforts invested in their development, only very few are currently available, and a deeper understanding of the numerous interactions and complement regulation mechanisms is indispensable. Two important complement regulators are human Factor H (FH) and Factor H-related protein 1 (FHR1). MFHR1 and MFHR13, two promising therapeutic candidates based on these regulators, combine the dimerization and C5-regulatory domains of FHR1 with the central C3-regulatory and cell surface-recognition domains of FH. Here, we used AlphaFold2 to model the structure of these two synthetic regulators. Moreover, we used AlphaFold-Multimer (AFM) to study possible interactions of C3 fragments and membrane attack complex (MAC) components C5, C7 and C9 in complex with FHR1, MFHR1, MFHR13 as well as the best-known MAC regulators vitronectin (Vn), clusterin and CD59, whose experimental structures remain undetermined. AFM successfully predicted the binding interfaces of FHR1 and the synthetic regulators with C3 fragments and suggested binding to C3. The models revealed structural differences in binding to these ligands through different interfaces. Additionally, AFM predictions of Vn, clusterin or CD59 with C7 or C9 agreed with previously published experimental results. Because the role of FHR1 as MAC regulator has been controversial, we analysed possible interactions with C5, C7 and C9. AFM predicted interactions of FHR1 with proteins of the terminal complement complex (TCC) as indicated by experimental observations, and located the interfaces in FHR1_1-2 and FHR1_4-5. According to AFM prediction, FHR1 might partially block the C3b binding site in C5, inhibiting C5 activation, and block C5b-7 complex formation and C9 polymerization, with similar mechanisms of action as clusterin and vitronectin. Here, we generate hypotheses and give the basis for the design of rational approaches to understand the molecular mechanism of MAC inhibition, which will facilitate the development of further complement therapeutics.

Bispecific mAb2 Antibodies Targeting CD59 Enhance the Complement-Dependent Cytotoxicity Mediated by Rituximab

Inhibition of complement activation via the overexpression of complement-regulatory proteins (CRPs), most notably CD46, CD55 and CD59, is an efficient mechanism of disguise of cancer cells from a host immune system. This phenomenon extends to counteract the potency of therapeutic antibodies that could lyse target cells by eliciting complement cascade. The manifold functions and ubiquitous expression of CRPs preclude their systemic specific inhibition. We selected CD59-specific Fc fragments with a novel antigen binding site (Fcabs) from yeast display libraries using recombinant antigens expressed in bacterial or mammalian cells. To produce a bispecific antibody, we endowed rituximab, a clinically applied anti-CD20 antibody, used for therapy of various lymphoid malignancies, with an anti-CD59 Fcab. This bispecific antibody was able to induce more potent complement-dependent cytotoxicity for CD20 and CD59 expressing Raji cell line measured with lactate dehydrogenase-release assay, but had no effect on the cells with lower levels of the primary CD20 antigen or CD20-negative cells. Such molecules are promising candidates for future therapeutic development as they elicit a higher specific cytotoxicity at a lower concentration and hence cause a lower exhaustion of complement components.

A novel hemolytic complement-sufficient NSG mouse model supports studies of complement-mediated antitumor activity in vivo

Monoclonal antibodies (mAbs) have emerged as a mainstream therapeutic option against cancer. mAbs mediate tumor cell-killing through several mechanisms including complement-dependent cytotoxicity (CDC). However, studies of mAb-mediated CDC against tumor cells remain largely dependent on in vitro systems. Previously developed and widely used NOD-scid IL2rγ^null (NSG) mice support enhanced engraftment of many primary human tumors. However, NSG mice have a 2-bp deletion in the coding region of the hemolytic complement (Hc) gene, and it is not possible to evaluate CDC activity in NSG mice. To address this limitation, we generated a novel strain of NSG mice-NSG-Hc¹-that have an intact complement system able to generate the membrane attack complex. Utilizing the Daudi Burkitt's human lymphoma cell line, and the anti-human CD20 mAb rituximab, we further demonstrated that the complement system in NSG-Hc¹ mice is fully functional. NSG-Hc¹ mice expressed CDC activity against Daudi cells in vivo following rituximab treatment and showed longer overall survival compared with rituximab-treated NSG mice that lack hemolytic complement. Our results validate the NSG-Hc¹ mouse model as a platform for testing mechanisms underlying CDC in vivo and suggest its potential use to compare complement-dependent and complement-independent cytotoxic activity mediated by therapeutic mAbs.

Molecular and functional characterization of CD59 from Nile tilapia (Oreochromis niloticus) involved in the immune response to Streptococcus agalactiae

CD59, the major inhibitor of membrane attack complex, plays a crucial role in regulation of complement activation. In this paper, a CD59 gene of Nile tilapia, Oreochromis niloticus (designated as On-CD59) was cloned and its expression pattern under the stimulation of Streptococcus agalactiae was investigated. Sequence analysis showed main structural features required for complement-inhibitory activity were detected in the deduced amino acid sequence of On-CD59. In healthy Nile tilapia, the On-CD59 transcripts could be detected in all the examined tissues, with the most abundant expression in the brain. When immunized with inactivated S. agalactiae, there was a clear time-dependent expression pattern of On-CD59 in the skin, brain, head kidney, thymus and spleen, with quite different kinetic expressions. The assays for the complement-inhibitory activity suggested that recombinant On-CD59 protein had a species-selective inhibition of complement. Moreover, our works showed that recombinant On-CD59 protein may possess both binding activities to PGN and LTA and inhibiting activity of S. agalactiae. These findings indicated that On-CD59 may play important roles in the immune response to S. agalactiae in Nile tilapia.

An ultra scale-down analysis of the recovery by dead-end centrifugation of human cells for therapy

An ultra scale-down method is described to determine the response of cells to recovery by dead-end (batch) centrifugation under commercially defined manufacturing conditions. The key variables studied are the cell suspension hold time prior to centrifugation, the relative centrifugal force (RCF), time of centrifugation, cell pellet resuspension velocities, and number of resuspension passes. The cell critical quality attributes studied are the cell membrane integrity and the presence of selected surface markers. Greater hold times and higher RCF values for longer spin times all led to the increased loss of cell membrane integrity. However, this loss was found to occur during intense cell resuspension rather than the preceding centrifugation stage. Controlled resuspension at low stress conditions below a possible critical stress point led to essentially complete cell recovery even at conditions of extreme centrifugation (e.g., RCF of 10000 g for 30 mins) and long (∼2 h) holding times before centrifugation. The susceptibility to cell loss during resuspension under conditions of high stress depended on cell type and the age of cells before centrifugation and the level of matrix crosslinking within the cell pellet as determined by the presence of detachment enzymes or possibly the nature of the resuspension medium. Changes in cell surface markers were significant in some cases but to a lower extent than loss of cell membrane integrity. Biotechnol. Bioeng. 2015;112: 997–1011. © 2014 Wiley Periodicals, Inc.

Mapping the intermedilysin-human CD59 receptor interface reveals a deep correspondence with the binding site on CD59 for complement binding proteins C8alpha and C9

CD59 is a glycosylphosphatidylinositol-anchored protein that inhibits the assembly of the terminal complement membrane attack complex (MAC) pore, whereas Streptococcus intermedius intermedilysin (ILY), a pore forming cholesterol-dependent cytolysin (CDC), specifically binds to human CD59 (hCD59) to initiate the formation of its pore. The identification of the residues of ILY and hCD59 that form their binding interface revealed a remarkably deep correspondence between the hCD59 binding site for ILY and that for the MAC proteins C8α and C9. ILY disengages from hCD59 during the prepore to pore transition, suggesting that loss of this interaction is necessary to accommodate specific structural changes associated with this transition. Consistent with this scenario, mutants of hCD59 or ILY that increased the affinity of this interaction decreased the cytolytic activity by slowing the transition of the prepore to pore but not the assembly of the prepore oligomer. A signature motif was also identified in the hCD59 binding CDCs that revealed a new hCD59-binding member of the CDC family. Although the binding site on hCD59 for ILY, C8α, and C9 exhibits significant homology, no similarity exists in their binding sites for hCD59. Hence, ILY and the MAC proteins interact with common amino acids of hCD59 but lack detectable conservation in their binding sites for hCD59.

Application of a novel inhibitor of human CD59 for the enhancement of complement-dependent cytolysis on cancer cells

Many monoclonal antibodies (mAbs) have been extensively used in the clinic, such as rituximab to treat lymphoma. However, resistance and non-responsiveness to mAb treatment have been challenging for this line of therapy. Complement is one of the main mediators of antibody-based cancer therapy via the complement-dependent cytolysis (CDC) effect. CD59 plays a critical role in resistance to mAbs through the CDC effect. In this paper, we attempted to investigate whether the novel CD59 inhibitor, recombinant ILYd4, was effective in enhancing the rituximab-mediated CDC effect on rituximab-sensitive RL-7 lymphoma cells and rituximab-induced resistant RR51.2 cells. Meanwhile, the CDC effects, which were mediated by rituximab and anti-CD24 mAb, on the refractory multiple myeloma (MM) cell line ARH-77 and the solid tumor osteosarcoma cell line Saos-2, were respectively investigated. We found that rILYd4 rendered the refractory cells sensitive to the mAb-mediated CDC effect and that rILYd4 exhibited a synergistic effect with the mAb that resulted in tumor cells lysis. This effect on tumor cell lysis was apparent on both hematological tumors and solid tumors. Therefore, rILYd4 may serve as an adjuvant for mAb mediated-tumor immunotherapy.

The good and evil of complement activation in HIV-1 infection

The complement system, a key component of innate immunity, is a first-line defender against foreign pathogens such as HIV-1. The role of the complement system in HIV-1 pathogenesis appears to be multifaceted. Although the complement system plays critical roles in clearing and neutralizing HIV-1 virions, it also represents a critical factor for the spread and maintenance of the virus in the infected host. In addition, complement regulators such as human CD59 present in the envelope of HIV-1 prevent complement-mediated lysis of HIV-1. Some novel approaches are proposed to combat HIV-1 infection through the enhancement of antibody-dependent complement activity against HIV-1. In this paper, we will review these diverse roles of complement in HIV-1 infection.

No effects of a synonymous variant within the CD59 gene on its protein product in duodenal biopsies of coeliac individuals

A novel rare variant within the CD59 gene was linked with coeliac disease in a family with high incidence of disease. Functional analyses of this variant were performed using complementary DNA analysis and protein analysis in paraffin-embedded duodenal biopsies from affected individuals and controls. No effects on pre-mRNA or size of linear protein were observed, although these results do not exclude the possible effects of this variant on co-translational protein folding.

Variants within protectin (CD59) and CD44 genes linked to an inherited haplotype in a family with coeliac disease

Coeliac disease (CD) is an autoimmune disorder characterised by inflammation, villous atrophy and hyperplasia of the small intestinal mucosa that affects genetically susceptible individuals. A genome-wide scan was performed in 17 family members with high incidence of CD. Highest nonparametric linkage (NPL) and logarithm of odds (LOD) scores were of 6.21 (P = 0.0107) and 2.57, respectively, to a region on chromosome 11p13-12. Following fine mapping, NPL and LOD scores did not change, but the linkage interval on chromosome 11 was narrowed to a region that is approximately 50.94 cM from pTer. Two inherited haplotypes on chromosomes 11p13-12 and 9q21 were observed in all affected members but not in the majority of clinically normal individuals. Sequencing of genes at region 11p13-12 showed a number of sequence variants, two of which were linked with the inherited haplotype. One of these variants in the CD59 gene was found at a very low frequency in the population and could possibly affect pre-messenger RNA splicing. This study is of particular importance for the identification of novel genes that might be responsible for CD other than human leukocyte antigen.

Molecular and functional characterization of a CD59 analogue from large yellow croaker Pseudosciana crocea

CD59 is a widely distributed membrane-bound inhibitor of the cytolytic membrane attack complex (MAC) of complement. Here, the cDNA of a CD59 analogue was cloned from large yellow croaker (Pseudosciana crocea), a marine fish (LycCD59), by expressed sequence tags (EST) and RACE techniques. The open reading frame (ORF) of 351 nucleotides (nt) of LycCD59 encodes a polypeptide of 117 amino acids (aa), which includes a putative 20-aa NH(2)-signal peptide and a 97-aa coding region with a putative GPI-anchoring site at Asn(71). The deduced LycCD59 protein shared the structural feature of mammalian CD59, including a conserved cysteine skeleton responsible for the formation of disulfide bonds, and a similar pattern of hydrophobic termini. RT-PCR analysis showed that LycCD59 mRNA was broadly expressed in various tissues examined, except for intestine. And Northern blot analysis revealed a single LycCD59 transcript of approximately 1.0kb. LycCD59 expression in blood, spleen, and kidney was significantly up-regulated during 24h of induction with poly(I:C) or inactivated trivalent bacterial vaccine as determined by a relative quantitative real-time PCR analysis, and a coordinated up-regulation of LycCD59 and complement C3 and C7 mRNA was also found in these three tissues post-induction although their up-regulation pattern and extent were somewhat different in various tissues with poly(I:C) or bacterial vaccine. The recombinant protein of LycCD59 produced in E. coli was shown to significantly inhibit the erythrocyte lysis of tilapia (Oreochromis niloticus) in an in vitro hemolytic system, which was mediated by serum from large yellow croaker and tilapia, respectively, but not from mouse and chicken, suggesting that LycCD59 has a species-selective inhibition of complement activation. These results represent the first functional identification of a CD59 analogue in teleost fish, strongly suggesting the presence of regulatory mechanism for terminal complement pathway in teleost fish.

Plethodontid modulating factor, a hypervariable salamander courtship pheromone in the three-finger protein superfamily

The soluble members of the three-finger protein superfamily all share a relatively simple 'three-finger' structure, yet perform radically different functions. Plethodontid modulating factor (PMF), a pheromone protein produced by the lungless salamander, Plethodon shermani, is a new and unusual member of this group. It affects female receptivity when delivered to the female's nares during courtship. As with other plethodontid pheromone genes, PMF is hyperexpressed in a specialized male mental (chin) gland. Unlike other plethodontid pheromone genes, however, PMF is also expressed at low levels in the skin, liver, intestine and kidneys of both sexes. The PMF sequences obtained from all tissue types were highly variable, with 103 unique haplotypes identified which averaged 35% sequence dissimilarity (range 1-60%) at the protein level. Despite this variation, however, all PMF sequences contained a conserved approximately 20-amino-acid secretion signal sequence and a pattern of eight cysteines that is also found in cytotoxins and short neurotoxins from snake venoms, as well as xenoxins from Xenopus. Although they share a common cysteine pattern, PMF isoforms differ from other three-finger proteins in: (a) amino-acid composition outside of the conserved motif; (b) length of the three distinguishing 'fingers'; (c) net charge at neutral pH. Whereas most three-finger proteins have a net positive charge at pH 7.0, PMF has a high net negative charge at neutral pH (pI range of most PMFs 3.5-4.0). Sequence comparisons suggest that PMF belongs to a distinct multigene subfamily within the three-finger protein superfamily.

Insights into the Human CD59 Complement Binding Interface Toward Engineering New Therapeutics

CD59 is a 77-amino acid membrane glycoprotein that plays an important role in regulating the terminal pathway of complement by inhibiting formation of the cytolytic membrane attack complex (MAC or C5b-9). The MAC is formed by the self assembly of C5b, C6, C7, C8, and multiple C9 molecules, with CD59 functioning by binding C5b-8 and C5b-9 in the assembling complex. We performed a scanning alanine mutagenesis screen of residues 16-57, a region previously identified to contain the C8/C9 binding interface. We have also created an improved NMR model from previously published data for structural understanding of CD59. Based on the scanning mutagenesis data, refined models, and additional site-specific mutations, we identified a binding interface that is much broader than previously thought. In addition to identifying substitutions that decreased CD59 activity, a surprising number of substitutions significantly enhanced CD59 activity. Because CD59 has significant therapeutic potential for the treatment of various inflammatory conditions, we investigated further the ability to enhance CD59 activity by additional mutagenesis studies. Based on the enhanced activity of membrane-bound mutant CD59 molecules, clinically relevant soluble mutant CD59-based proteins were prepared and shown to have up to a 3-fold increase in complement inhibitory activity.

Human CD59 is a receptor for the cholesterol-dependent cytolysin intermedilysin

Cholesterol is believed to serve as the common receptor for the cholesterol-dependent cytolysins (CDCs). One member of this toxin family, Streptococcus intermedius intermedilysin (ILY), exhibits a narrow spectrum of cellular specificity that is seemingly inconsistent with this premise. We show here that ILY, via its domain 4 structure, binds to the glycosyl-phosphatidylinositol-linked membrane protein human CD59 (huCD59). CD59 is an inhibitor of the membrane attack complex of human complement. ILY specifically binds to huCD59 via residues that are the binding site for the C8alpha and C9 complement proteins. These studies provide a new model for the mechanism of cellular recognition by a CDC.

Glycation inactivation of the complement regulatory protein CD59: a possible role in the pathogenesis of the vascular complications of human diabetes

Micro- and macrovascular diseases are major causes of morbidity and mortality in the diabetic population, but the cellular and molecular mechanisms that link hyperglycemia to these complications remain incompletely understood. We proposed that in human diabetes, inhibition by glycation of the complement regulatory protein CD59 increases deposition of the membrane attack complex (MAC) of complement, contributing to the higher vascular risk. We report here 1) the generation and characterization of an anti-glycated human CD59 (hCD59) specific antibody, 2) the detection with this antibody of glycated hCD59 colocalized with MAC in kidneys and nerves from diabetic but not from nondiabetic subjects, and 3) a significantly reduced activity of hCD59 in erythrocytes from diabetic subjects, a finding consistent with glycation inactivation of hCD59 in vivo. Because hCD59 acts as a specific inhibitor of MAC formation, these findings provide a molecular explanation for the increased MAC deposition reportedly found in the target organs of diabetic complications. We conclude that glycation inactivation of hCD59 that leads to increased MAC deposition may contribute to the extensive vascular pathology that complicates human diabetes.

Evolution and comparative genomics of odorant- and pheromone-associated genes in rodents

Chemical cues influence a range of behavioral responses in rodents. The involvement of protein odorants and odorant receptors in mediating reproductive behavior, foraging, and predator avoidance suggests that their genes may have been subject to adaptive evolution. We have estimated the consequences of selection on rodent pheromones, their receptors, and olfactory receptors. These families were chosen on the basis of multiple gene duplications since the common ancestor of rat and mouse. For each family, codons were identified that are likely to have been subject to adaptive evolution. The majority of such sites are situated on the solvent-accessible surfaces of putative pheromones and the lumenal portions of their likely receptors. We predict that these contribute to physicochemical and functional diversity within pheromone-receptor interaction sites.

An aberrant form of CD59 derived from HeLa cells

We isolated a CD59 cDNA from a HeLa cell library which encoded a mutated form of CD59, having a single base substitution (G to T) that changed Arg55 to Met. Since this mutation occurred in the vicinity of the putative active site of CD59, we expressed the aberrant form of the protein in Chinese hamster ovary cells in order to test for effects upon function. We found that the mutation did not influence complement inhibitory activity of CD59. However, the epitopes recognised by the function-blocking CD59 monoclonal antibodies BRIC229 and YTH 53.1 were significantly affected. The G to T substitution caused loss of an Mnl I restriction site which permitted PCR-RFLP analysis. All of 52 human subjects studied, and our in-house HeLa cells, were homozygous for the normal CD59 sequence, indicating that the altered sequence was not due to normal variation in the general population. Therefore this mutation probably arose spontaneously in the HeLa cell line used to generate the commercially obtained cDNA library.

Human complement protein C8 gamma

Human C8 gamma is a 22 kDa subunit of complement component C8, which is one of five components (C5b, C6, C7, C8, C9) that interact to form the cytolytic membrane attack complex (MAC) of complement. C8 contains three nonidentical subunits (alpha, beta, gamma) that are products of different genes. These subunits are arranged asymmetrically to form a disulfide-linked C8 alpha-gamma dimer that is noncovalently associated with C8 beta. C8 alpha and C8 beta are homologous to C6, C7 and C9 and together these proteins comprise what is referred to as the 'MAC protein family'. By comparison, C8 gamma is distinct in that it belongs to the lipocalin family of small, secreted proteins which have the common ability to bind small hydrophobic ligands. While specific roles have been identified for C8 alpha and C8 beta in the formation and function of the MAC, a function for C8 gamma and the identity of its ligand are unknown. This review summarizes the current status of C8 gamma structure and function and the progress made from efforts to determine its role in the complement system.

Genomic structure and chromosome location of the gene encoding mouse CD59

The gene encoding the mouse analogue of the human complement regulator CD59 was cloned using a combination of long range PCR and genomic library screening. Sequence obtained showed that its genomic structure closely resembled that of the human CD59 gene, comprising 4 exons, each separated by a long intron region. The sizes of introns and exons were comparable to those of the human gene with the exception of the third intron which is 2.5 kb in the mouse compared to 7 kb in the human gene. All exon/intron boundaries conformed to the GT-AG rules for splicing. Radiation hybrid mapping localised mouse Cd59 between D2Mit333 and D2Mit127 on chromosome 2, a region homologous with human chromosome 11p13 where the human CD59 gene is localised. These data have permitted the construction of a gene targeting vector for the generation of transgenic mice deficient in CD59.

Identification and functional characterization of a new gene encoding the mouse terminal complement inhibitor CD59

CD59 is a 18- to 20-kDa, GPI-anchored membrane protein that functions as a key regulator of the terminal step of the complement activation cascade. It restricts binding of C9 to the C5b-8 complex, thereby preventing the formation of the membrane attack complex (C5b-9 of complement). A single human CD59 gene has been identified, and corresponding genetic homologues from rat, mouse, and pig have been characterized in previous studies. In this study, we report the discovery and functional characterization of a separate cd59 gene in the mouse (referred to as cd59b, the previously characterized mouse cd59 gene as cd59a). Mouse cd59b is 85% and 63% identical to cd59a at the nucleotide and amino acid level, respectively. In cDNA transfection experiments with Chinese hamster ovary cells, peptide-tagged cd59b was detected on the cell surface by flow cytometry and was shown to be susceptible to phosphatidylinositol-specific phospholipase C cleavage. Chinese hamster ovary cells expressing cd59b were significantly more resistant than control cells to human and mouse complement-mediated lysis. These results suggest that cd59b encodes a GPI-anchored protein that is functionally active as a membrane attack complex inhibitor. Northern blot analysis revealed that cd59b is expressed selectively in the mouse testis. In contrast, the major transcript of cd59a was shown to be expressed at high levels in the heart, kidney, liver, and lung, but only minimally in the testis. These results revealed the existence of two distinct cd59 genes in the mouse that are differentially regulated and that may have nonoverlapping physiological functions in vivo.

Ly-6I, a new member of the murine Ly-6 superfamily with a distinct pattern of expression

A new member of the mouse Ly-6SF, designated Ly-6I, has been isolated as a gene homologous to a segment of the Ly-6C gene. A single allelic difference in the mature protein sequence was identified, which is similar to other Ly-6SF members. Ly-6I mRNA has been detected in a wide range of tissues and cell lines, and a rabbit polyclonal Ab has been used to determine that Ly-6I protein is present at a low constitutive level on cell lines from several different lineages. In contrast to Ly-6C and Ly-6A/E, the Ly-6I gene is only weakly responsive to IFNs. Expression in vivo is most abundant on bone marrow populations and is coexpressed with Ly-6C on granulocytes and macrophages. However, Ly-6I is also expressed on immature B cell populations that do not express Ly-6C. Expression on mature B cells in spleen is uniformly low. Similarly, Ly-6I is expressed on TCRlow/int, but not TCRhigh, thymocytes. Ly-6I is re-expressed on Ly-6Chigh T cells in the periphery. Thus, Ly-6I may be a useful marker to define maturation stages of both T and B lymphocytes as well as subsets of monocytes and granulocytes.

Identification of mutations in rat CD59 that increase the complement regulatory activity

Formation of the membrane attack complex (MAC) of complement on host cells is inhibited by the glycosylphosphatidylinositol- (GPI-) anchored glycoprotein CD59. Published data on the active site of human CD59 are confusing. To clarify these data, we set out to elucidate the active site of a nonprimate CD59 molecule by site-directed mutagenesis. We also undertook to investigate a region of potential species selectivity, and to this end rat CD59 was chosen for all mutations. Our investigations confirmed the proposal that the active site of CD59 is the major hydrophobic groove, with mutations Y36A, W40A, and L54A ablating complement inhibitory function of CD59. Other mutations reducing the function of rat CD59 were I56E, D24A, and D24R. Importantly, mutations at one residue increased the function of rat CD59. The K48E mutation significantly increased function against human rat or rabbit serum, whereas the K48A mutation increased function against human serum alone. A similar mutation in human CD59 (N48E) had no effect on activity against human or rat serum but completely abolished all activity against rabbit serum. These findings suggest that the alpha-helix of human CD59, adjacent to the hydrophobic groove, influences the interaction between human CD59 and rabbit C8, C9, or both.

Biochemical background of paroxysmal nocturnal hemoglobinuria

Paroxysmal nocturnal hemoglobinuria (PNH) is an acquired disorder characterized by paroxysms of intravascular hemolysis. A considerable part of erythrocytes in patient blood is susceptible to autologous complement activation because of the deficiency of CD59, which is a glycosylphosphatidylinositol (GPI)-anchored protein and inhibits the formation of the membrane attack complex (MAC) of complement. The deficiency of CD59 is derived from the inability of GPI-anchor synthesis. Although more than 10 proteins are involved in the GPI-anchor synthesis, the mutation of only one protein, PIG-A, causes the defect in about 200 patients with PNH who have been analyzed. The reason why only PIG-A causes the deficiency of GPI anchor is due to the location of its gene on X chromosome. The clonal stem cell mutated with PIG-A gene in the bone marrow loses the capability of the synthesis of GPI-anchor. The mutation of PIG-A gene alone, however, seems to be insufficient to account for the survival of the PIG-A-deficient cells in the bone marrow. Thus, a fraction of the mutant stem cells probably gain a survival advantage by some additional changes, either additional mutations or changes in immunological circumstances. The release of the surviving cells into blood stream results in a clinical syndrome with PNH.

Characterization of the human antiporcine immune response: a prerequisite to xenotransplantation

Successful xenotransplantation necessitates solving problems of hyperacute rejection and understanding the cellular immune responses that occur. Considerable progress has been made in our understanding of the molecular genetic basis of the rapid hyperacute antibody-mediated rejection mechanisms that occur in xenogeneic organ rejection. In parallel, strategies involving the use of transgenic animals expressing complement inhibitors are beginning to offer encouraging evidence that hyperacute rejection can be overcome. A greater understanding of cell-mediated immune interactions is now required to achieve long-term xenograft survival. Current studies are focused on T cell receptor (TCR)/major histocompatibility complex (MHC) and costimulatory signals that activate human CD4 and CD8 T cells.

Identification of the individual residues that determine human CD59 species selective activity

Formation of the cytolytic membrane attack complex of complement on host cells is inhibited by the membrane-bound glycoprotein, CD59. The inhibitory activity of CD59 is species restricted, and human CD59 is not effective against rat complement. Previous functional analysis of chimeric human/rat CD59 proteins indicated that the residues responsible for the species selective function of human CD59 map to a region contained between positions 40 and 66 in the primary structure. By comparative analysis of rat and human CD59 models and by mutational analysis of candidate residues, we now identify the individual residues within the 40-66 region that confer species selective function on human CD59. All nonconserved residues within the 40-66 sequence were substituted from human to rat residues in a series of chimeric human/rat CD59 mutant proteins. Functional analysis revealed that the individual human to rat residue substitutions F47A, T51L, R55E, and K65Q each produced a mutant human CD59 protein with enhanced rat complement inhibitory activity with the single F47A substitution having the most significant effect. Interestingly, the side chains of the residues at positions 47, 51, and 55 are all located on the short single helix (residues 47-55) of CD59 and form an exposed continuous strip parallel to the helix axis. A single human CD59 mutant protein containing rat residue substitutions at all three helix residues produced a protein with species selective activity comparable to that of rat CD59. We further found that synthetic peptides spanning the human CD59 helix sequence were able to inhibit the binding of human CD59 to human C8, but had little effect on the binding of rat CD59 to rat C8.

Structure/function characterization of porcine CD59: expression, chromosomal mapping, complement-inhibition, and costimulatory activity

BACKGROUND

Complement regulatory proteins have become important targets to potentially modulate inflammatory reactions or transplant rejection. Since pig into human xenotransplantation could potentially overcome the enormous shortage of donor organs and tissues, characterization of porcine complement regulatory proteins is critical.

METHODS

The porcine CD59 cDNA has been isolated from porcine aortic endothelial cells and its structure determined. In addition, a molecular genetic analysis of the gene and its transcriptional properties and a functional analysis have been performed utilizing the transfected cDNA.

RESULTS

The most prominent mRNA species is 1.8 kilobases but cloned reverse transcriptase polymerase chain reaction products suggest that multiple polyadenylation sites are utilized. Gene mapping was performed utilizing a polymorphism identified in the 3' UT, and the gene was localized to within 3 cM of follicle-stimulating hormone, beta polypeptide in the middle of the chromosome 2 linkage map. RNA expression was equivalent in endothelial, kidney, and testis cell lines. Comparisons have been made with CD59 sequences from other species to identify possible important domains of the protein. The cDNA has been utilized to express an epitope-tagged or wild-type protein either transiently on COS-7 cells or stably in Chinese hamster ovary cells. The porcine CD59 protein effectively inhibited the antibody-mediated lytic activity of both porcine and human complement. In contrast to human CD59, porcine CD59 is incapable of providing costimulation to human T cells.

CONCLUSIONS

These data suggest that overexpression of porcine CD59 might be more effective than human CD59 in prolonging xenograft survival with transgenic pig organs because of reduced immunoreactivity.