Combined immunodeficiency in horses: characterization of the lymphocyte defect

Modelling Mutation in Equine Infectious Anemia Virus Infection Suggests a Path to Viral Clearance with Repeated Vaccination

Equine infectious anemia virus (EIAV) is a lentivirus similar to HIV that infects horses. Clinical and experimental studies demonstrating immune control of EIAV infection hold promise for efforts to produce an HIV vaccine. Antibody infusions have been shown to block both wild-type and mutant virus infection, but the mutant sometimes escapes. Using these data, we develop a mathematical model that describes the interactions between antibodies and both wild-type and mutant virus populations, in the context of continual virus mutation. The aim of this work is to determine whether repeated vaccinations through antibody infusions can reduce both the wild-type and mutant strains of the virus below one viral particle, and if so, to examine the vaccination period and number of infusions that ensure eradication. The antibody infusions are modelled using impulsive differential equations, a technique that offers insight into repeated vaccination by approximating the time-to-peak by an instantaneous change. We use impulsive theory to determine the maximal vaccination intervals that would be required to reduce the wild-type and mutant virus levels below one particle per horse. We show that seven boosts of the antibody vaccine are sufficient to eradicate both the wild-type and the mutant strains. In the case of a mutant virus infection that is given infusions of antibodies targeting wild-type virus (i.e., simulation of a heterologous infection), seven infusions were likewise sufficient to eradicate infection, based upon the data set. However, if the period between infusions was sufficiently increased, both the wild-type and mutant virus would eventually persist in the form of a periodic orbit. These results suggest a route forward to design antibody-based vaccine strategies to control viruses subject to mutant escape.

Genetics of Immune Disease in the Horse

Host defenses against infection by viruses, bacteria, fungi, and parasites are critical to survival. It has been estimated that upwards of 7% of the coding genes of mammals function in immunity and inflammation. This high level of genomic investment in defense has resulted in an immune system characterized by extraordinary complexity and many levels of redundancy. Because so many genes are involved with immunity, there are many opportunities for mutations to arise that have negative effects. However, redundancy in the mammalian defense system and the adaptive nature of key immune mechanisms buffer the untoward outcomes of many such deleterious mutations.

Molecular Pathology of Severe Combined Immunodeficiency in Mice, Horses, and Dogs

Severe combined immunodeficiency (SCID) is an inherited disorder of humans, mice, horses, and dogs, in which affected individuals are incapable of generating antigen-specific immune responses. It occurs when lymphocyte precursors fail to differentiate into mature lymphocytes because of mutations within recombinase-activating genes 1 and 2 or within the genes encoding deoxyribonucleic acid (DNA)-dependent protein kinase (DNA-PK). It also occurs when differentiated lymphocytes are incapable of completing signal transduction pathways because of defects in cell surface receptors for interleukins (IL). A spontaneous mutation in DNA-PKcs of BALB/c mice results in SCID, as do experimentally induced mutations in RAG1 and RAG2. SCID in horses results from a spontaneous mutation in DNA-PKcs. Two molecular mechanisms account for SCID in dogs. Jack Russell Terriers have a mutation within the DNA-PKcs gene, whereas Cardigan Welsh Corgi and Basset Hound have different defects in the gene encoding the γ chain that is common to the receptors for IL-2, −4, −7, −9, −15, and −21. The location of the mutation within target genes influences the spectrum of diseases observed in affected animals.

Experimental transmission of equine hepacivirus in horses as a model for hepatitis C virus

Equine hepacivirus (EHCV; nonprimate hepacivirus) is a hepatotropic member of the Flaviviridae family that infects horses. Although EHCV is the closest known relative to hepatitis C virus (HCV), its complete replication kinetics in vivo have not been described, and direct evidence that it causes hepatitis has been lacking. In this study, we detected EHCV in 2 horses that developed post-transfusion hepatitis. Plasma and serum from these horses were used to experimentally transmit EHCV to 4 young adult Arabian horses, two 1-month-old foals (1 Arabian and 1 Arabian-pony cross), and 2 foals (1 Arabian and 1 Arabian-pony cross) with severe combined immunodeficiency (SCID). Our results demonstrated that EHCV had infection kinetics similar to HCV and that infection was associated with acute and chronic liver disease as measured by elevations of liver-specific enzymes and/or by histopathology. Although most of these animals were coinfected with equine pegivirus (EPgV), also a flavivirus, EPgV viral loads were much lower and often undetectable in both liver and blood. Three additional young adult Arabian-pony crosses and 1 SCID foal were then inoculated with plasma containing only EHCV, and evidence of mild hepatocellular damage was observed. The different levels of liver-specific enzyme elevation, hepatic inflammation, and duration of viremia observed during EHCV infection suggested that the magnitude and course of liver disease was mediated by the virus inoculum and/or by host factors, including breed, age, and adaptive immune status.

CONCLUSION

This work documents the complete infection kinetics and liver pathology associated with acute and chronic EHCV infection in horses and further justifies it as a large animal model for HCV.

Lymphocytes and macrophages are infected by Theileria equi, but T cells and B cells are not required to establish infection in vivo

Theileria equi has a biphasic life cycle in horses, with a period of intraleukocyte development followed by patent erythrocytic parasitemia that causes acute and sometimes fatal hemolytic disease. Unlike Theileria spp. that infect cattle (Theileria parva and Theileria annulata), the intraleukocyte stage (schizont) of Theileria equi does not cause uncontrolled host cell proliferation or other significant pathology. Nevertheless, schizont-infected leukocytes are of interest because of their potential to alter host cell function and because immune responses directed against this stage could halt infection and prevent disease. Based on cellular morphology, Theileria equi has been reported to infect lymphocytes in vivo and in vitro, but the specific phenotype of schizont-infected cells has yet to be defined. To resolve this knowledge gap in Theileria equi pathogenesis, peripheral blood mononuclear cells were infected in vitro and the phenotype of infected cells determined using flow cytometry and immunofluorescence microscopy. These experiments demonstrated that the host cell range of Theileria equi was broader than initially reported and included B lymphocytes, T lymphocytes and monocyte/macrophages. To determine if B and T lymphocytes were required to establish infection in vivo, horses affected with severe combined immunodeficiency (SCID), which lack functional B and T lymphocytes, were inoculated with Theileria equi sporozoites. SCID horses developed patent erythrocytic parasitemia, indicating that B and T lymphocytes are not necessary to complete the Theileria equi life cycle in vivo. These findings suggest that the factors mediating Theileria equi leukocyte invasion and intracytoplasmic differentiation are common to several leukocyte subsets and are less restricted than for Theileria annulata and Theileria parva. These data will greatly facilitate future investigation into the relationships between Theileria equi leukocyte tropism and pathogenesis, breed susceptibility, and strain virulence.

Review of equine piroplasmosis

Equine piroplasmosis is caused by one of 2 erythrocytic parasites Babesia caballi or Theileria equi. Although the genus of the latter remains controversial, the most recent designation, Theileria, is utilized in this review. Shared pathogenesis includes tick-borne transmission and erythrolysis leading to anemia as the primary clinical outcome. Although both parasites are able to persist indefinitely in their equid hosts, thus far, only B. caballi transmits across tick generations. Pathogenesis further diverges after transmission to equids in that B. caballi immediately infects erythrocytes, whereas T.equi infects peripheral blood mononuclear cells. The recent re-emergence of T.equi in the United States has increased awareness of these tick-borne pathogens, especially in terms of diagnosis and control. This review focuses in part on factors leading to the re-emergence of infection and disease of these globally important pathogens.

Protective effects of passively transferred merozoite-specific antibodies against Theileria equi in horses with severe combined immunodeficiency

Theileria equi immune plasma was infused into young horses (foals) with severe combined immunodeficiency. Although all foals became infected following intravenous challenge with homologous T. equi merozoite stabilate, delayed time to peak parasitemia occurred. Protective effects were associated with a predominance of passively transferred merozoite-specific IgG3.

Cloning and large-scale expansion of epitope-specific equine cytotoxic T lymphocytes using an anti-equine CD3 monoclonal antibody and human recombinant IL-2

Cytotoxic T lymphocytes are involved in controlling intracellular pathogens in many species, including horses. Particularly, CTL are critical for the control of equine infectious anemia virus (EIAV), a lentivirus that infects horses world-wide. In humans and animal models, CTL clones are valuable for evaluating the fine specificity of epitope recognition, and for adoptive immunotherapy against infectious and neoplastic diseases. Cloned CTL would be equally useful for similar studies in the horse. Here we present the first analysis of a method to generate equine CTL clones. Peripheral blood mononuclear cells were obtained from an EIAV-infected horse and stimulated with the EIAV Rev-QW11 peptide. Sorted CD8+ T cells were cloned by limiting dilution, and expanded without further antigen addition using irradiated PBMC, anti-equine CD3, and human recombinant IL-2. Clones could be frozen and thawed without detrimental effects, and could be subsequently expanded to numbers exceeding 2 x 10(9)cells. Flow cytometry of expanded clones confirmed the CD3+/CD8+ phenotype, and chromium release assays confirmed CTL activity. Finally, sequencing TCR beta chain genes confirmed clonality. Our results provide a reliable means to generate large numbers of epitope-specific equine CTL clones that are suitable for use in downstream applications, including functional assays and adoptive transfer studies.

Severe combined immunodeficiency in a Fell pony foal

Five days after birth of a viable Fell pony filly, yellow watery diarrhoea appeared without any signs of systemic disease. Four days later the diarrhoea ceased. On 11th day, the animal showed apathy, and a few days later, the foal was very lethargic, suffered from muscular weakness and severe watery diarrhoea that reappeared. The illness did not respond to therapy. At the age of 21 days the filly spontaneously died under symptoms of intestinal colic and pneumonia. Haematological examinations revealed lower numbers of erythrocytes as well as non-selective lymphopenia. Phagocytic activity was slightly increased, lymphocyte activity was inhibited. Histopathology showed severe alteration of the lymphatic organs. T and B lymphocytes and antigen-presenting macrophages were not arranged in characteristic areas, and the quantity of these cells was lower than would be expected. Histopathological changes in lymphatic organs resembled those described in the literature as severe combined immunodeficiency.

Adaptive immunity is the primary force driving selection of equine infectious anemia virus envelope SU variants during acute infection

Equine infectious anemia virus (EIAV) is a lentivirus that causes persistent infection in horses. The appearance of antigenically distinct viral variants during recurrent viremic episodes is thought to be due to adaptive immune selection pressure. To test this hypothesis, we evaluated envelope SU cloned sequences from five severe combined immunodeficient (SCID) foals infected with EIAV. Within the SU hypervariable V3 region, 8.5% of the clones had amino acid changes, and 6.4% had amino acid changes within the known cytotoxic T lymphocyte (CTL) epitope Env-RW12. Of all the SU clones, only 3.1% had amino acid changes affecting potential N-linked glycosylation sites. In contrast, a much higher degree of variation was evident in SU sequences obtained from four EIAV-infected immunocompetent foals. Within V3, 68.8% of the clones contained amino acid changes, and 50% of the clones had amino acid changes within the Env-RW12 CTL epitope. Notably, 31.9% of the clones had amino acid changes affecting one or more glycosylation sites. Marked amino acid variation occurred in cloned SU sequences from an immune-reconstituted EIAV-infected SCID foal. Of these clones, 100% had amino acid changes within V3, 100% had amino acid changes within Env-RW12, and 97.5% had amino acid changes affecting glycosylation sites. Analysis of synonymous and nonsynonymous nucleotide substitutions revealed statistically significant differences between SCID and immunocompetent foals and between SCID foals and the reconstituted SCID foal. Interestingly, amino acid selection at one site occurred independently of adaptive immune status. Not only do these data indicate that adaptive immunity primarily drives the selection of EIAV SU variants, but also they demonstrate that other selective forces exist during acute infection.

Primary immunodeficiencies of horses

Primary immunodeficiency disorders are genetically determined failures of immune defense that increase susceptibility to infectious agents. This article reviews the salient features of equine primary immunodeficiency disorders, summarizes the molecular mechanisms of each disorder, and updates information that facilitates diagnosis and management of affected horses. The central theme is to encourage clinicians to ask, "I wonder if this horse has an underlying primary immunodeficiency disorder?" when caring for horses suffering from chronic and recurring infections and responding poorly to standard therapy.

Frequency of memory cytotoxic T lymphocytes to equine infectious anemia virus proteins in blood from carrier horses

Horses with equine infectious anemia virus (EIAV) have episodes of viremia and disease; however, most eventually become inapparent carriers. A possible mechanism of control is cytotoxic T lymphocytes (CTL). To evaluate CTL in inapparent carriers with low viral loads, peripheral blood mononuclear cells (PBMC) were stimulated in vitro with autologous EIAV-infected PBMC and human IL-2 to detect memory CTL (CTLm). In initial studies, three carriers had CTLm and one of these had low-level effector CTL (CTLe). The CTLm were restricted by equine lymphocyte alloantigen-A (ELA-A) locus encoded MHC class I molecules on autologous equine kidney (EK) target cells. In addition, EK cells did not express MHC class II molecules. The CTLm frequency in PBMC from five inapparent carriers infected for 22 to 50 months was determined by limiting dilution analysis. PBMC were diluted, stimulated, and tested on EK cell targets infected with EIAV and recombinant vaccinia viruses expressing EIAV Env or Gag/Pr proteins. All five carriers had CTLm to EIAV-infected targets, while four had CTLm to targets expressing Env and four had CTLm to targets expressing Gag/Pr proteins. The CTLm frequency range was 60 to 468 per 10(6) PBMC to EIAV-infected targets, 4 to 286 to Env-expressing targets, and 25 to 190 to Gag/Pr-expressing targets. These results should facilitate the identification of epitopes recognized by predominant CTLm from horses controlling a lentivirus infection.

A morphometric study of bone marrow megakaryocytes in foals infected with equine infectious anemia virus

Morphometric evaluation of bone marrow core biopsies was used to determine megakaryocyte (MK) numbers and MK size in nine foals with equine infectious anemia virus (EIAV)-induced thrombocytopenia. Both immunocompetent normal foals and foals with severe combined immunodeficiency (SCID) were used. Platelet counts were made three times weekly following viral infection. Bone marrow core biopsies were taken from the ilium of each foal prior to experimental infection, immediately after the onset of thrombocytopenia, and at necropsy. All foals developed thrombocytopenia by 23 days postinfection. The bone marrow MK density did not change in response to the thrombocytopenia. MK area did not change significantly; however, the MK nuclear area at necropsy was significantly higher than that preinfection. The presence of thrombocytopenia in the SCID foals showed that immune-specific responses were not required for the production of EIAV-induced thrombocytopenia. Furthermore, the lack of a compensatory megakaryocytopoiesis in both SCID and normal foals was consistent with the theory that altered platelet production plays a role in the development of this thrombocytopenia.

Serum hypoxanthine and xanthine concentrations in horses heterozygous for combined immunodeficiency

SUMMARY

A group of diseases termed combined immunodeficiency (CID) results in a severe form of immunodeficiency. While CID in humans has two genetics bases, in Arabian it is inherited in an autosomal recessive manner. Kettler et al. (1989) determined that uric acid was significantly (p < .001) greater in the serum of carrier Arabian horses than in non-carrier horses. The current study measured serum levels of hypoxanthine and xanthine two other products of this pathway. There were no significant differences (p > 0.05) between carrier and non-carrier horse's serum levels of hypoxanthine or xanthine. These data, combined with our previous ones suggest that an enzymatic lesion in the purine salvage pathway may occur at the urate oxidase step. ZUSAMMENFASSUNG: Serum-Hypoxanthin- und -Xanthin-Spiegel in Pferden mit Heterozygotie für kombinierte Immundefizienz Kombinierte Immundefizienz (CID), die zu einer schweren Krankheit führt, hat beim Menschen zwei genetische Ursachen, wird aber bei Araberpferden autosomal rezessiv vererbt. Kettler u. Ma. (1989) fanden den Harnsäureserumspiegel in Überträgern signifikant höher als in freien Tieren. Hier wurden mit Hypoxanthin und Xanthin zwei andere Produkte des biochemischen Pfades bestimmt, jedoch keine statistisch signifikanten Unterschiede zu Nicht-Trägern der CID gefunden. Die Ergebnisse zusammen mit unseren früheren lassen vermuten, daß der Enzymmangel im Purinpfad auf der Harnsäureoxidationsstufe existiert.

Immune responses are required to terminate viremia in equine infectious anemia lentivirus infection

Six normal and four immunodeficient horses were injected with a cloned variant of equine infectious anemia virus (EIAV). The six normal horses had detectable EIAV in their plasma by 7 days postinjection. During their primary viremic episode, which was accompanied by fever and anemia, maximum titers of EIAV in plasma ranged from 10(3.8) to 10(4.8) 50% tissue culture infective doses per ml. All six normal horses cleared detectable virus from their plasma by 21 to 35 days after injection. Horses with combined immunodeficiency became viremic by 9 days postinjection and also developed anemia. In contrast to normal horses, foals with combined immunodeficiency did not eliminate the virus from their plasma.

Immunologic reconstitution of foals with combined immunodeficiency

Thirty-eight foals with combined immunodeficiency (CID) received transplanted fetal liver cells, fetal liver and thymus cells, histocompatible bone marrow cells, or equine lymphocyte antigen (ELA) haploidentical bone marrow cells in an attempt to reconstitute their deficient immune systems. Engraftment was infrequent, partial, and unpredictable when fetal cells were employed. Three of five CID foals receiving ELA haploidentical bone marrow cells demonstrated partial reconstitution, but engraftment was only temporary. Administration of histocompatible bone marrow cells resulted in rapid, full and sustained engraftment.

Defects in cell-mediated immunity

Defects in cell-mediated immunity result in remarkable susceptibility to opportunistic infection in contrast to the recurrent pyogenic infections observed in children with defects in antibody-mediated immunity. The major congenital defects in T lymphocytes are presented.

Equine alternative pathway activation by unsensitized rabbit red blood cells

The equine alternative complement pathway has been partially characterized and compared to the equine classical activation pathway. A dose-dependent lysis of RbRBC was observed with peak lytic values noted within 10 minutes at 37 degrees C when rabbit red blood cells (RbRBC) were used as an alternative pathway activator. Sheep red blood cells (SRBC) sensitized with rabbit hemolysin or partially purified equine IgM antibodies were equally sensitive to lysis. Dilution of the commercial hemolysin by 1/5 reduced lysis from 90% to 38% in the presence of constant cell numbers. Hemolysis of SRBC peaked at 10 minutes and the majority of lysis occurred within 10 minutes. Dilution of equine sera by as little as 1/5 decreased hemolytic activity for SRBC to 21.5% from greater than 90% with undiluted sera. The alternative pathway protein, equine factor B, was tested using RbRBC and monitored by its differential susceptibility to heat treatment at 50 degrees C. This treatment led to almost complete inactivation after a 15-minute incubation. An apparent heat-dependent decay of certain classical pathway components was also observed after 50 degrees C treatment. This sensitivity was indicated by a reduction in the lytic activity for sensitized SRBC. Treatment for 15 minutes at 56 degrees C with either RbRBC or SRBC was sufficient to abolish hemolytic activity in all equine sera tested. Chelation of cations with 0.04 M EDTA blocked expression of alternative and classical pathway activation; however, chelation of Ca++ ions with 10 mM EGTA containing 1 mM Mg++ ions permitted lysis of the RbRBC but not the SRBC. A dose-related Mg++-ion dependence for RbRBC hemolytic activity was observed as the concentration of Mg++ was increased to 1.0 mM. In addition, our results obtained with pre-colostral foal serum strongly suggest that natural antibody to RbRBC was of little importance in the lysis observed with these cells. These results also show that the equine alternative pathway activation may require Ca++ ions. If Ca++ ions are required, the equine alternative pathway is quite different from any other mammalian complement system so far described. Our results suggest that the alternative pathway of activation is of major importance in the equine complement system. Confirmation of this hypothesis requires both purification of the components involved as well as further characterization.

Increased susceptibility of fibroblasts from horses with severe combined immunodeficiency to growth inhibition by 2'-deoxyadenosine

The effect of adenosine, deoxyadenosine, guanosine, and deoxyguanosine on the growth rate of fibroblasts derived from normal horses, horses heterozygous for the severe combined immunodeficiency (SCID) trait (heterozygotes), and horses with SCID was studied. All four purines were found to inhibit growth in a dose-dependent manner, but only adenosine and deoxyadenosine were inhibitory at concentrations of less than 100 microM. No statistical difference in sensitivity to adenosine was detected between normal and SCID fibroblasts. Fibroblasts from SCID horses were, however, more sensitive to the growth inhibitory effects of deoxyadenosine than were fibroblasts from normal horses. Furthermore, following 24 hr of incubation with radiolabeled deoxyadenosine, radiolabeled deoxyATP concentrations were found to be twofold higher in SCID fibroblasts compared to those concentrations measured in normal fibroblasts cultured under identical conditions. Adenosine deaminase and S-adenosylhomocysteine hydrolase activities were normal in SCID fibroblasts. These findings suggest that SCID horses may have a defect in either transport or phosphorylation of deoxyadenosine, or in the utilization of deoxyATP.

Equine immunology 4: vaccines and antisera

This paper attempts to relate the practicalities of vaccine development to the ideals which should be aimed for in a new vaccine. The type of immune response induced is dependent upon the nature of the antigen in the vaccine and the site and timing of its presentation to the immune system. In this respect the influence of age, maternal immunity and antigenic competition are discussed. The possible side effects associated with vaccination are defined and vaccines which are currently available for horses are reviewed. These vaccines are mostly for the prevention of respiratory disease. Finally, the possible uses for antisera are considered.

A severe combined immunodeficiency mutation in the mouse

The most debilitating human lymphoid deficiency disease, known as severe combined immunodeficiency (SCID), impairs the differentiation of both T and B lymphocytes. Affected infants are highly susceptible to recurring infections of viruses, fungi and bacteria and invariably die within 2 yr of birth. Inheritance of this congenital syndrome may show X-linked or autosomal recessive control. To date autosomal recessive inheritance of SCID has been observed in Arabian foals which represent the only known animal model of this disease syndrome but here we report an autosomal recessive mutation in mice that severely impairs lymphopoiesis. Mice homozygous for this mutation have few if any lymphocytes; consequently they are hypogammaglobulinaemic and deficient for immune functions mediated by T and B lymphocytes. These mice, therefore, represent a new model for investigating how lymphoid differentiation may be impaired in the disease state and regulated in the normal state.

Equine immunology: an introductory review

This article attempts to relate some of the more recently accepted concepts of immunology to an understanding of the mechanisms of immunity in the horse. The cellular mechanisms involved in the immune response are outlined, with an indication of their likely role in humoral and cell-mediated immunity. In describing the humoral immune response, the structure and function of the different equine immunoglobulins are reviewed. The significance of humoral and cell-mediated immune responses are considered in relation to actively and passively acquired immunity.

An evaluation of contribution derived from investigations of equine immunodeficiencies

Following the descriptions of immunodeficiencies in horses beginning in 1973, there has been considerable effort to develop methods for differential diagnosis and to determine the cause and prevalence of the disorders. In addition, the equine immunodeficiencies, especially combined immunodeficiency, have been studied from a comparative viewpoint with the goal of finding information applicable to similar diseases of children. Coincident with the development of knowledge about the immunodeficiencies per se, considerable information about several aspects of immunology has been obtained. It is the purpose of this review to focus on findings from experiments with equine immunodeficiencies concerning prenatal and neonatal immunology, lymphocyte function, secretory immunity, immunoreconstitution, graft-versus-host reactions and previously unrecognized diseases.

Primary, severe, combined immunodeficiency disease of Arabian foals

Set in a context of immunodeficiency diseases in general this paper provides a brief, illustrated review of a primary, severe, combined immunodeficiency (PSCID) disease of Arabian foals. Affected foals are clinically normal at birth but beginning at about 10 days of age they develop a range of clinical signs particularly bronchopneumonia and diarrhoea with which adenoviruses are peculiarly associated. Despite intensive therapy foals invariably die by about 3 months of age. Affected foals are profoundly lymphopagenic (greater than 1000 lymphcoytes per mm3). There is thymic and lymph node hypoplasia and all lymphoid tissues are profoundly depleted of both T and B lymphocytes. The depletion of both T and B lymphocytes suggests that the primary defect is at the level of bone marrow stem cells which are the precursor cells for both lymphocyte populations. PSCID of Arabian foals is inherited as a simple, autosomal, recessive gene. Some 2 to 3% of all such foals may be born with PSCID, this frequency corresponds to a gene frequency of about 30% in parents. The syndrome is, therefore, an important cause of economic wastage. It also represents the only occurrence of the syndrome in an animal species other than man and as such has considerable comparative interest.

Evaluation of adenosine deaminase and other purine salvage pathway enzymes in horses with combined immunodeficiency

Foals with combined immunodeficiency had normal levels of purine salvage pathway enzymes, including adenosine deaminase, nucleoside phosphorylase, and xanthine oxidase.