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Intrathymic injections of various donor alloantigens, such as MHC I or MHC II peptides [ 1,2 ], 3M KCl cell extracts [ 3 ], or of donor cells, such as bone marrow [ 4 ], splenocytes [ 5 ], islet cells [ 6 ], have been shown to induce tolerance of grafted organs, by negative selection, in the medulla of the thymus, of the CD 4+8- and CD 4- 8+ T cells having high affinity for MHC-injected antigen peptide complexes. However, few studies have been devoted to the effect of injection of antibodies in the thymus. The intrathymic injection in mice of antibodies to MHC I or MHC II molecules has been shown to interfere clearly with the thymocyte development of the corresponding MHC specificity [ 7 ]. In the same way, the intrathymic injection of anti CD 4 was found to inhibit the deletion of Vβ 17a- bearing cells from the CD 4-8+ thymocyte subset [ 8 ]. In the case where a foreign antigen does not induce an immune response ( because of the deletion in the thymus of the T cells recognizing its peptide-MHC complexes ), an intrathymic injection of antibodies against this antigen, obtained from a foreign species, could prevent the formation of the peptide-MHC complexes and the negative selection in the medulla of the corresponding T-cells. On the other hand, in autoimmune diseases, the circulating autoantibodies could block in the thymus the corresponding epitopes of the self, preventing the negative selection of the T cells that normally recognize these self antigens in complexes with MHC molecules. In the case where the antigen responsible for the autoimmune disease is known, it would be possible to immunize an animal with it and to prepare the corresponding antiidiotypic antibody, which mimics the antigen and recognizes the binding site of the autoantibodies. It is expected that it would be able, when injected into the thymus, to displace the autoantibodies bound to the antigens of the self, allowing the formation of the peptide-MHC complexes and the deletion of the corresponding T cells.
Improvement of the immune response
When a pathogen shares most of its antigenic epitopes with the proteins of the host, few antibodies are formed. It is the case of HIV, the envelop proteins of which ( gp120 and gp 41 ) share regions of homology with human proteins ( HLA class I and II antigens, secreted IL2, IgG constant domains and T cell receptors ) [ 9-13 ]. As a consequence, the thymocytes recognizing the epitopes of the virus identical to those of the self expressed in the thymus, are submitted to negative selection.
To improve the immune response, the following protocol is proposed [ 14 ]: antibodies against the pathogen would be prepared from a species sufficiently distant from humans, and purified by chromatography on affinity column. A part of these antibodies would be directed against epitopes of the self, since the pathogen is assumed to share many antigens with the host. By injecting these antibodies in the medulla of the thymus, it is expected that the formation of some peptide-MHC complexes will be prevented and that the developing thymocytes that would recognize them will escape from negative selection ( Figure I )
 Figure 1 : Proposition to improve the immune response. The epitope sets are represented by the surfaces of the different circles. a) The pathogen shares most of its antigenic epitopes ( small circle ) with the host ( great circle ), only a small number is outside the set of the host epitope set and leads to the production of antibodies ( in grey ). b) The animal species epitope set is different from the human epitope set and antibodies are produced in greater quantity ( in grey ). They are purified by affinity chromatography and injected into the thymus of the patient. c) The set of the self epitopes of the patient is reduced by blocking with the antibodies of the animal. A greater number of epitopes of the pathogen is recognized as foreign to the host and elicits antibodies ( in grey ).
The thymus decreases in size in adults and is often involuted, one may think that the treatment could be difficult to be applied. However, it has been shown that the adult thymus still contributes to T cell reconstitution [ 15 ]. The choice of the species for the preparation of antibodies against the pathogen is important : the animal must be immunologically distant from humans to induce numerous different antibodies, but not too much, to avoid important immune response against the xenoantibodies. When the treatment would be stopped, the human circulating antibodies would replace the xenoantibodies from their binding sites. A constant watching of the titre of the neutralizing antibodies should be ensured during a sufficient time. Besides adverse immunological responses, such as urticaria, the repeated injection of antibodies will induce the formation of antiidiotypic antibodies. As the latter bear an internal image of the antigens, they could compete with them and displace the injected xenoantibodies from their complexes with the self epitopes, which would lessen the efficacy of the treatment. It must also be borne in mind that idiotype-antiidiotyptype reactions may precipitate autoimmune diseases.
In addition, the ability of certain pathogens such as human immunodeficiency, hepatitis C, influenza viruses or Plasmodium falciparum, to escape from host immune response is generally ascribed to be due to the high mutation rate of their genome. In the cases where there is similarity of the pathogen proteins with those of the host ( HIV, Plasmodium falciparum ), a small number of mutations shifting the virus epitope set ( Figure 2 ) can reduce dramatically the number of epitopes recognized by the antibodies of a previous immunization. On the contrary, when the epitope sets of the pathogen and of the host are sufficiently different, the influence of punctual mutations would be lesser. It is expected to be the case for the treatment we propose.
Figure 2 : Influence of mutations on the efficiency of the immune response. The epitope sets are represented by the surfaces of the different circles. a) The pathogen shares most of its epitopes with the host. Few antibodies are produced ( in grey ),and a reduced part of them ( in dark grey ) are active against a mutant ( dotted circle ). b) Many epitopes of the pathogen are foreign to the host and elicit multiple antibodies ( in grey ). Even after mutation of the pathogen, a sufficient number of different antibodies will react with its epitopes ( in dark grey ). Treatment of autoimmune diseases
Autoimmune diseases are characterized by the production of antibodies against proteins of the self. The origin of this production is attributed in some cases to a previous immunization by an organism sharing common epitopes with the host [16 ]. The circulating autoantibodies are supposed to block in the thymus the corresponding epitopes of the self, preventing the negative selection of the T cells that recognize these self epitopes. These T cells help then B cells to synthesize additional autoantibodies, the production of which would be self-sustained. Many experiments [ 17-20 ] have shown that intrathymic injections of antigens induce tolerance toward them and prevent the appearance of autoimmune diseases ( diabetes, lupus, encephalitis, uveoretinis, etc. ). In the cases where the antigen responsible of the immune disease is identified, we propose to immunize an animal with it ( the species of which to be defined ) and to prepare the corresponding antiidiotypic antibodies. It is expected that the latter, which resembles the antigen ( internal image) , will be able, when injected into the medulla of the thymus, to displace the autoantibodies bound to the epitopes of the self and to provoke the negative selection of the T cells and the break in the production of autoantibodies.
Hypothesis of a germline origin of the antibody diversity
The antibody diversity is attributed, according to the classical theory [ 21 ], to the recombination of the V, D and J gene regions, creating a population of cells that vary widely in their specificities, from which a few cells are selected by any foreign antigen. Then, the mutational mechanism is called in action during the proliferation of the selected clones, giving genes that produce antibodies with maximum fitting of the antigen. However, although it has been demonstrated that the VDJ recombinations and the somatic mutations do effectively bring diversity among immunoglobulins of a given specificity, no direct proof has been given that different specificities could be created, from the same V genes, by recombinations and somatic mutations. They only lead to modifications of the binding constants with antigens, the configurations with the highest antigen binding values being selected. Moreover, some antibodies ( Mabs and polyclonal antibodies ) raised against a particular antigen are less fitted to it than to another one, generally related to it. This phenomenon is called heteroclisis and cannot be explained by the classical theory [ 22 ]. In the same way, mutations introduced in the second complementary determining region of anti-phosphocholine antibodies have been shown to often decrease their antigen binding capacity [ 23 ]. We propose an hypothesis of a germline origin of the antibody diversity, which accounts for the main characteristics of the immune response: response against any foreign proteic antigen and specificity [ 24 ]. The antibody reaction is supposed to be due to the combination of a limited number of different antibodies against the epitopes of the antigen. In addition, the efficient part of the epitopes of the proteins is assumed to have the size of dipeptides. The 400 different dipeptides would be distributed in two sets, those which are foreign to the host, against which would be directed the antibodies, and those which are present in the proteins of the host, which would normally elicit no reaction. Each chain of the antibodies would recognize one aminoacid and 40 genes would be sufficient ( 20 for the light chains and 20 for the heavy chains ) to code for the variable parts of the immunoglobulins against proteic epitopes. With these hypotheses, the combination of antibodies against at least four different epitopes of the size of dipeptides can account for the main characteristics of the antibody response : antigenicity and specificity toward any foreign substance. In fact, the epitopes have been found to be larger than dipeptides, several additional aminoacids are involved in the antibody binding sites. Somatic mutations and VDJ recombinations are selected to give the maximal affinity for the antigens. The phenomenon of heteroclisis and the decrease, due to mutations, of antigen binding capacity of antibodies, sometimes observed, are better explained by the model exposed , than by the classical theory. Moreover, the positive selection of the developing thymocytes in the thymus cortex is better conceivable if they must bind to a finite number of peptide-MHC complexes. This model is compatible with the size of the variable gene region repertoire ( several hundred VH and VL segments ). Enough genes would remain available for other antibody specificities ( anti-sugars, anti-nucleotides etc.).
References
1. Chowdhury N., Saborio D., Garrovillo M., Chandraker A., Magee C., Waaga A., Sayegh M., Jin M., Oluwole S.: Comparative studies of specific acquired systemic tolerance induced by intrathymic inoculation of a single synthetic Wistar-Furth (RT1U) allo class I (RT1.AU) peptide or WAG(RT1U) – derived class I peptide. Transplantation 66: 1059-1066, 1998. 2. Hancock W., Khoury S., Carpenter C., Sayegh M.: Differential effects of oral versus intrathymic administration of polymorphic major histocompatibility complex class II peptides on mononuclear and endothelial cell activation and cytokine expression during a delayed type hypersensitivity response. Am. J. Pathol. 144: 1149-1158, 1994. 3. Oluwole S., Jin M., Chowdhury N., Engelstad K., Ohajekwe O.,James T.: Induction of peripheral tolerance by intrathymic inoculation of soluble alloantigens : Evidence for the role of host antigen presenting cells and suppressor cell mechanism. Cell Immunol. 162: 33-41, 1995. 4. Remuzzi G.: Cellular basis of long term organ transplant acceptance: a pivotal role of intrathymic clonal deletion and thymic dependence of bone marrow microchimerism associated tolerance. Am. J. Kidney Dis. 31: 197-212, 1998. 5. Furukawa M., Fukuda Y., Tashiro H., Ohdan H., Hoshino S., Shintaku S., Itou H., Kiyohiko D.: Analysis of PCR microchimerism induced by intrathymic inoculation of donor alloantigens in rats. Cell Transplant 5 (suppl.1): 75-77, 1996. 6. Posselt A., Barker C., Tomaszewski J., Markmann J., Choti M., Naji A. Induction of donor specific unresponsiveness by intrathymic islet transplantation. Science 249: 1293-1295, 1990. 7. Marusic-Galesic S., Stephany D., Longo D., Kruisbeek A.: Development of CD4- CD8+ cytotoxic T cells requires interactions with class I MHC determinants. Nature 333:180-183, 1988. 8. Fowlkes B., Schwartz R., Pardoll D.: Deletion of self reactive thymocytes occurs at a CD4+ CD8+ precursor stage. Nature 334: 620-623, 1988. 9. Golding H., Robey F., Gate III F., Linder W., Beining P., Hoffman T., Golding B.: Identification of homologous regions in human immunodeficiency virus I gp 41 and human MHC class II β 1 domain. J.Exp. Med. 167: 914-923, 1989. 10. Grassi F., Meneveri R., Gullberg M., Lopalco L., Rossi G.B., Lanza P., De Santis C., Brattsand G., Butto S., Ginelli E., Beretta A., Siccardi A.: Human immunodefficiency virus type I gp 120 mimics a hidden monomorphic epitope borne by Class I Major Histocompatibility Complex heavy chains. J. Exp. Med. 174: 53-62, 1991. 11. Bost K., Hahn B., Saag M., Shaw G., Weigent D., Blalock J.: Individual infected with HIV possess antibodies against IL2. Immunology 65: 611-615, 1988. 12. Solder B., Marschang P., Wachter H., Dierich M., Navyar S., Levin IV, Stanworth D.: Anti viral antibodies in HIV infection possess autoantibody activity against a CH1 domain determinant in human IgG: possible immunological consequences. Immunol.Lett. 23: 9-19,1989. 13. Marchalonis J., Lake D;, Schluter S. Dehghanpisheh K., Watson R., Ampel N., Galgiani J.: Autoantibodies against peptide-defined epitopes of T cell receptors in retrovirally infected humans and mice. Adv. Exp. Med. Biol. 383: 211-222, 1995. 14. Berger G.: Proposition of treatment to improve the immune response: possible application to AIDS. Med. Hypotheses 58: 416-421, 2002. 15. Douek D., Koup R. : Evidence for thymic function in the elderly. Vaccine 18: 1638-1641, 2000. 16. Roitt I., Hutchings P., Dawe K., Sumar N., Bodman K., Cooke A.: The forces driving autoimmune disease. J. Autoimmun. 5 (suppl 5A) :11- 26,1992. 17. Gerling I., Serreze D., Christianson S., Leiter E.: Intrathymic islet cell transplantation reduces beta cell autoimmunity and prevents diabetes in NOD/Lt mice. Diabetes 41: 1672-1676, 1992. 18. Duncan S., Rubin R., Burlingame R., Sinclair S., Pekny K., Theofilopoulos A.: Intrathymic injection of polynucleosomes delays autoantibody production in BXSB mice. Clin. Immunol. Immunopathol. 79 : 171-181, 1996. 19. Khoury S., Sayegh M., Hancock W., Gallon L., Carpenter C., Weiner H.: Acquired tolerance to experimental autoimmune encephalomyelitis by intrathymic injection of myelin basic protein or its major encephalitogenic peptide. J. Exp. Med. 178: 559-566, 1993. 20. Koevary S., Caspi R.: Prevention of experimental uveoretinis by intra thymic S-antigen injection.Ocul. Immunol. Inflamm. 5: 165-172, 1997. 21. Tonegawa S.: Somatic generation of antibody diversity. Nature 302: 575-581, 1983. 22. Liu J., Minnerath J., Nelson R., Mueller C. Jemmerson R.: Kinetic and genetic bases for the heteroclitic recognition of mouse cytochrome c by mouse anti pigeon cytochrome c monoclonal antibodies. J. Immunol. 37: 847-859, 2000. 23. Chen C., Roberts V., Stevens S., Brown M., Stenzel-Poore M., Rittenberg M.: Enhancement and destruction of antibody function by somatic mutation : unequal occurrence is controlled by V gene combinatorial associations. EMBO J. 14: 2784-2794, 1995. 24. Berger G.: Hypotheses on a germline origin of antibody diversity. Possible applications: improvement of the efficiency of immune response and autoimmune disease treatment. Med. Hypotheses 63: 847-854, 2004.
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