Commentary

Polyreactive antibodies and their association withxenotransplantation

A wide range of studies, described briefly in thisreview, suggests that xenoreactive antibodies canbe divided into at least two classes: anti-carbohy-drate and polyreactive. Taken together, thesestudies point to the conclusion that, although thereare relatively few antigen binding sites for polyre-active antibodies on cultured porcine cells, thereare more polyreactive, xenoreactive antibodiesthan anti-Gala1-3Gal antibodies in the blood ofnormal individuals. Thus, although the predomin-ant xengeneic antigen is Gala1-3Gal, the predom-inant xenoreactive antibody is likely polyreactive.Here, the data underlying this conclusion arebriefly reviewed.A polyreactive antibody can be defined as an

antibody able to bind a definable, restricted set ofantigens, not just a single antigen [1–7]. As oneexample, Moller’s group examined the specificityof a monoclonal antibody that bound to a numberof seemingly unrelated molecules including cellu-lose, nucleic acids, bovine thyroglobulin, transfer-rin, and cytochrome c [8]. They found that theantibody bound, at least in part, to the carbohy-drate on bovine thyroglobulin, but not to Gala1-3Gal.Polyreactive antibodies are thought to provide

the primary defense against pathogens in primitiveinvertebrates [9] and perhaps provide some defensein the mucosal barriers of humans [10]. At the timeof birth, the peripheral antibody repertoire pro-duced by the newborn human consists mainly ofpolyreactive immunoglobulin M (IgM) [7]. Severalinvestigators have found that polyreactive anti-bodies recognize the same set of antigens through-out the life of a given individual [1–7]. Polyreactiveantibodies are predominately IgM [1], are pro-duced by more than 10% of Epstein-Barr virus(EBV)-transformed human B-cells [11], and bind toa number of antigens including tubulin, actin,myosin, ssDNA, a variety of unrelated haptens,and thyroglobulin [2, 12, 13].Although polyreactive antibodies are known to

recognize both autoantigens and xenogeneic anti-gens, only recently have the xenoreactivity andautoreactivity of the antibodies been compareddirectly. Lee et al. [14] evaluated the specificity ofpolyreactive, affinity purified anti-ssDNA IgM

from adult human serum and polyreactive IgM inhuman cord serum to compare the xenoreactive vs.autoreactive nature of polyreactive human anti-bodies. Using competitive and direct bindingassays, human polyreactive IgM were found to begenerally more reactive with foreign (xenogeneic)proteins than with self or allogeneic proteins.Using a novel approach to address the xenoreac-tive nature of polyreactive antibodies, Sigounaset al. [3] injected human polyreactive and mono-reactive IgM, IgA, and IgG antibodies into severecombined immune deficient (SCID) mice. Exam-ination of mouse tissues revealed substantialdeposition of human polyreactive antibodies, butnot monoreactive antibodies, in the organs of themice. This finding is consistent with the idea that,at least when murine antigens are the target,human polyreactive antibodies are substantiallymore xenoreactive than autoreactive.The interpretation of some studies, involving the

polyreactive nature of xenoreactive antibodies, hasbeen complicated by the presence of Gala1-3Galon a wide variety of proteins of non-human origin.Further complications arise from aggregation ofpurified antibodies or other technical problemsassociated with purification of antibodies [15].Despite these hurdles, compelling evidence nowexists that at least some and probably a substantialamount of polyreactive antibodies may be xenore-active in the pig-to-human model. Turman et al.[16] utilized Western blotting to describe thespecificity of human monoclonal antibodies thatare polyreactive and bind to porcine antigens.These human antibodies bound to a variety ofautoantigens, in addition to xenogeneic antigens,but, based on a lack of reactivity with Gala1-3Galcontaining proteins [16], did not bind toGala1-3Gal. Geller et al. [17], using a monoclonalanti-idiotype antibody specific for a polyreactiveantibody, demonstrated that polyreactive antibod-ies are deposited on xenografts.In a study by Gaca et al. [18], investigating the

xenoreactivity of polyreactive antibodies, carefulattention was paid to the adsorption of polyreac-tive antibodies from the serum of neonatal pri-mates receiving pulmonary xenografts. This modelprovided some advantages in that the vascular bedof the lung is large, presenting substantial amountof antigen, and in that the majority of the IgMantibodies in neonatal blood are polyreactive.

Xenotransplantation 2003: 10: 542–544Printed in UK. All rights reserved

Copyright � Blackwell Munksgaard 2003

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Gaca found that >90% of the polyreactive IgM inneonatal baboons was adsorbed by pulmonaryxenografts. No changes in albumin levels wereobserved, indicating that the adsorption was spe-cific for IgM. Further, Gaca found that polyreac-tive antibodies were approximately twofold morereactive with pig lung extracts than with humanlung extracts. Finally, Gaca found that the major-ity of polyreactive antibodies from normalbaboons were adsorbed by pulmonary xenografts,consistent with the idea that most polyreactiveantibodies are xenoreactive.The serum of patients with impaired ability to

produce monospecific antibodies has provided auseful tool to investigate the xenoreactivity ofpolyreactive antibodies. Some patients with hyper-IgM syndrome produced xenoreactive antibodieseven when they were unable to produce affinitymaturated antibodies or anti-Gala1-3Gal antibod-ies [19]. The majority of xenoreactive antibodiesproduced by these patients were apparently poly-reactive, as binding to cultured porcine cells wassubstantially inhibited by human insulin, humanFc, human albumin, and human thyroglobulin[19]. The dose-dependent binding of these antibod-ies to cultured porcine endothelial cells afterenzymatic removal of Gala1-3Gal epitopes [19]indicates that the polyreactive antibodies are moreabundant than anti-Gala1-3Gal antibodies andrecognize ��minor�� antigens on the porcine cells.Although mounting evidence described above

indicates that many if not most polyreactiveantibodies are xenoreactive and bind to xenografts,the importance of polyreactive antibodies in xen-otransplantation remains unclear. Studies addres-sing the issue are lacking and design of such studiesfaces some hurdles. For example, selective elimin-ation of polyreactive antibodies in experimentalmodels, a potential tool to probe the role of theantibodies in xenograft rejection, will likely provedifficult to achieve. Thus, the role of polyreactiveantibodies in xenograft rejection remains specula-tive: polyreactive antibodies are presumably not aspotent mediators of damage to xenografts as areanti-Gala1-3Gal antibodies. The human comple-ment regulatory proteins currently expressed bytransgenic pig lines may be sufficient to alleviateany potential problem imposed by these antibod-ies. On the other hand, polyreactive natural anti-bodies have been identified in immune complexesof a wide variety of patients, including those withsystemic lupus erythematosus, and may be in-volved in slowly developing pathological processes.Further, it remains unknown whether polyreactiveantigen-binding B-cells might be stimulated byporcine xenografts to produce more or higher

affinity antibodies. Thus, while we hope that nonew natural humoral barriers to xenotransplanta-tion will be encountered, it may be worthwhile tokeep an eye on polyreactive antibodies.

William ParkerDepartment of Surgery,

Duke University Medical Center,Durham, NC, USA

(E-mail: bparker@duke.edu)

References

1. Lacroix-Desmazes S, Kaveri SV, Mouthon L et al.Self-reactive antibodies (natural autoantibodies) in healthyindividuals. J Immunol Meth 1998; 216: 117.

2. Logtenberg T. Properties of polyreactive natural anti-bodies to self and foreign antigens. J Clin Immunol 1990;10: 137.

3. Sigounas G, Harindranath N, Donadel G, Notkins

AL. Half-life of polyreactive antibodies. J Clin Immunol1994; 14: 134.

4. Bouvet JP, Dighiero G. From natural polyreactiveautoantibodies to a la carte monoreactive antibodies toinfectious agents: is it a small world after all? Infection andImmunity 1998; 66: 1.

5. Stahl D, Lacroix-Desmazes S, Mouthon L et al. Ana-lysis of human self-reactive antibody repertoires by quan-titative immunoblotting. J Immunol Meth 2000; 240: 1.

6. Lydyard PM, Quartey-Papafio R, Broker B et al. Theantibody repertoire of early human B cells. I. High fre-quency of autoreactivity and polyreactivity. Scand JImmunol 1990; 31: 33.

7. Mouthon L, Lacroix-Desmazes S, Nobrega A et al.The self-reactive antibody repertoire of normal humanserum IgM is acquired in early childhood and remainsconserved throughout life. Scand J Immunol 1996; 44: 243.

8. Al-Balaghi S, Abedi-ValugerdiM,Moller E. Bindingspecificities of a polyreactive and a monoreactive humanmonoclonal IgG rheumatoid factor: role of oligosaccha-rides. Scand J Immunol 1996; 44: 470.

9. Gonzalez R, Charlemagne J, Mahana W, Avrameas

S. Specificity of natural serum antibodies present in phy-logenetically distinct fish species. Immunology 1988; 63: 31.

10. Quan CP, Berneman A, Pires R et al. Natural polyre-active secretory immunoglobulin A autoantibodies as apossible barrier to infection in humans. Infection andImmunity 1997; 65: 3997.

11. Notkins AL. Polyreactive antibodies and polyreactiveantigen-binding B (PAB) cells. Curr Topics MicrobiolImmunol 2000; 252: 241.

12. Ailus K, Palosuo T. IgM class autoantibodies in humancord serum. J Reprod Immunol 1995; 29: 61.

13. Guilbert B, Dighiero G, Avrameas S. Naturallyoccurring antibodies against nine common antigens inhuman sera. J Immunol 1982; 128: 2779.

14. Lee W, Gaca JG, Edwards LA et al. Binding of poly-reactive antibodies to self versus foreign antigens. Immu-nobiology 2002; 205: 95.

15. Mcmahon MJ, O’kennedy R. Polyreactivity as anacquired artefact, rather than a physiologic property, ofantibodies: evidence that monoreactive antibodies may

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gain the ability to bind to multiple antigens after exposureto low pH. J Immunol Meth 2000; 241: 1.

16. Turman MA, Casali P, Notkins AL et al. Polyreactivityand antigen specificity of human natural xenoreactivemonoclonal and serum natural antibodies. Transplanta-tion 1991; 52: 710.

17. Geller RL, Bach FH, Turman MA et al. Evidence thatpolyreactive antibodies are deposited in rejected discord-ant xenografts. Transplantation 1993; 55: 168.

18. Gaca JG, Lee W, Aksoy O et al. Evidence for polyreac-tive xenoreactive antibodies in the repertoire of humananti-swine lung antiboidies. Transpl Immunol 2002; 9: 19.

19. Parker W, Yu PB, Holzknecht ZE et al. Specificity andfunction of ��natural�� antibodies in immunodeficient sub-jects: clues to B cell lineage and development. J ClinImmunol 1997; 17: 311.

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