Immunology Today, vol. 4, No. 11, 1983
319
antigens, which may behave in a TI-like fashion by crosslinking the B-cell receptors, would preferentially activate B cells and render them sensitive to helper signals. Thus, autoantigens may play a role in directing autoantibody production. In normal mice where there is no hyperresponsiveness to or over-production of accessory signals nor a genetic predisposition for autoimmune response, the levels of autoantibodies produced will be low. In contrast, in SLE mice where there is hyper-responsiveness and/or hyper-production of accessory signals as well as genetic predisposition for autoimmune response, autoantibodies will reach abnormal levels. As for possible analogies between murine and human SLE, the NZ mice parallel humans in the sex distribution and immunopathological characteristics of this disease, and BXSB males are the counterpart of a recently described SLE that afflicts primarily fathers and sons 2°. In contrast, no human correlate of SLE in the MRL-lpr/lpr mouse has been identified, although humans with SjSgren's syndrome and humans and mice undergoing graft-versus-host (GvH) reactions might qualify. Humans whose G v H response follows bone-marrow allotransplantation may develop autoantibodies, immune complex glomerulonephritis and immune deposits in the skin21. Certain non-autoimmune strains of mice .undergoing experimentally induced chronic G v H reactions have serological and pathological alterations that strongly resemble the MRL-lpr/lpr disease, including autoantibodies, glomerulonephritis and lymphoid proliferation 22. During the GvH response, recipient B cells, including autoreactive clones, stimulate alloreactive donor T cells via Ia determinants and thus induce helper signals by these T cells 23. Whether the helper signals include BCGFand BCDF-like molecules remains to be determined. Further analysis of the concepts outlined above should ultimately provide information that allows detailed understanding of systemic autoimmune diseases at the molecular level.
Acknowledgements This
is Publication
No.
2946
from
the
Department
of
Immunology, Scripps Clinic and Research Foundation, 10666 North Torrey Pines Road, LaJolla, C A 92037, USA. The authors' work cited in this article was supported by the National Institutes of Health, Grants AI-07007 and AG-01743, and the Cecil H. and Ida M. Green Endowment Fund.
References 1 Andersson, J., Coutinho, A., Melchers, F. and Watanabe, T. (1976) Cold Spring Harbor Syrup. Quant. Biol. 41,227-236 2 Howard, M., Farrar, J., Hilfdcer, M. eta/. (1982)J. Exp. Med. 155, 914-923 3 Parker, D. C., Wadsworth, D. C. and Schneider, G. B. (1980)J. EXP. Med. 152, 138-150 4 Zubler, R. H. and Glasebrook, A. L. (1982).f Exp. Med. 155, 666-680 5 Park, C. L., Balderas, R. S., Fieser, T. et al. (1983)J. Immunol. 130, 2161-2167 6 Prud'homme, G. J., Balderas, R. S., Dixon, F. J. and Theofdopoulos, A. N. (1983),J. Exp. Med. 157, 1815-1827 7 Isakson, P. C., Purd, E., Vitetta, E. S. and Krammer, P. H. (1982) .~ Exp. Med. 155, 734-748 8 Martinez-Alonso, C. and Coutinho, A. (1981) Nature (London) 290, 60-61 9 Theofdopoulos, A. N., Shawler, D. L., Eisenberg, R. A. and Dixon, F. J. (1980)J. Exp. Med. 151,446---466 10 Prud'homme, G. J., Park, C. L., Fieser, T. M..et a/. J. Exp. Meal. (in press) 11 Wofsy, D., Murphy, E. D., Roths, J. B. eta/. (1981)J. Exp. Mat. 154, 1671-1680 12 Ahman, A., Theofdopoulos, A. N., Weiner, R. et 02. (1981)J. Exp. Meal. 154, 791-808 13 Alcocer-Varela, J. and Alarcon-Segovia, D. (1982)J. Clin. Invest. 69, 1388-1392 14 Slack, J., Der-Balian, G. P., Wahm, M. and Davie, J. M. (1980)J. Exp. Med. 151,853-862 15 Smith, H. R., Chused, T. M., Smathers, P. A. and Steinberg, A. D. (1983) Fed. Pro¢. Fed. Am. So¢. Exp. Biol. (in press) (Abstract) 16 Taurog, J. D., Moutsopoulos, H. M., Rosenberg, Y. J. et 02. (1979)J. Exp. Med. 150, 31-43 17 Steinberg, B.J., Smathers, P. A., Frederiksen, K. and Steinberg, A. D. (1982).]. Clin. Invest. 70, 587-597 18 Singer, A., Asano, Y., Schigeta, M. eta/. (1982) Irtmmnol. Rev. 64, 137-160 19 Steinburg, E. B., Santoro, T. J., Chused, T. M., Smathers, P. A. and Steinburg, A. D. (1983)J. ImmunoL (in press) 20 Lahita, R. G., Chiorazzi, N., Gibofsky, A. eta/. (1983) Arthritis Rheum. 26, 39-44 21 Schulman, H. M., Sullivan, K. M., Weiden, P. L. eta/. (1980) Am.J. Med. 69, 204-217 22 Gleichman, E., Van Elven, E. H. and Vander Veen, J. P. w . (1982) Eur J. lmmunoL 12, 152-159 23 Vander Veen, F. M. R., Rolink, A. G. and Gleichmann, E. (1982)J. Exp. Med. 155, 1555-1560 24 Linker-Israeli, M., Bakke, A. G., Kitridou, R. C. eta/. (19ll3)J. lmmunol. 130, 2651-2655
New directions in research
T lymphocytes in viral clearance Early studies by Blanden I with ectromelia virus suggested that T cells mediating delayed-type hypersensitivity (DTH) were ideal candidates for viral clearance. T cells were shown to localize in foci of infection and to be followed by macrophage infiltration which eliminated virus. Subsequent studies indicated that cytotoxic T cells (Tc) alone were sufficient for clearance 2. A D T H mechanism seemed not to be involved because delayedtype hypersensitivity T cells (Td) were defined as Lyl ÷ and 1-region-restricted, while the Tc conferring viral clearance were Ly2 ÷3 + and K,D-region-restricted. The more recent findings that both K,D-restricted Tc and 1restricted T cells can mediate D T H 3,~ have therefore
raised questions about the roles of D T H and cytotoxicity in viral clearance. In-vitro-generated influenza-virus-specific secondary effector cells adoptively transferred to mice inoculated with a lethal dose of influenza virus reduced virus titres in the lungs, and protected the mice from death 5. The protective T cells were identified as Ly2÷3 + and K,Dregion-restricted. In contrast, T cells exhibiting 1-regionrestricted D T H failed to protect infected mice and the recipients had a significantly higher mortality rate than controls ~. Similar results were obtained with Sendal virus 6. A requirement for K,D-restricted T cells in herpes
320
Immunology Today, vol. 4, No. 11, 1983
simplex virus (HSV) infections has also been shown. Nash et al. 7 demonstrated that I-A compatibility was sufficient for transferring D T H , but that K, I-A and D were necessary for viral clearance. In subsequent studies the same group s concluded that Lyl + and not Ly2 +3 ÷ cells are directly involved in viral clearance. However, in these experiments the existence of K,D-restricted Ly2+3 ÷ T cells was not demonstrated. The increase in virus titre following treatment with anti-Lyl.1 sera and complement which was used as evidence for the direct involvement of an/-restricted T cell can be explained by the need of an/-restricted helper cell for antibody or other T-cellmediated responses. This would accord with the hypothesis of Howes et al. 9 concerning T help in antibody responses, based on the result that/-restricted but not K,D-restricted cells conferred long-lasting protection against H S V infection. Involvement of help could be demonstrated by transferring irradiated cells. Recent studies by Sethi et al. lo using a cloned culture of HSVmemory T-enriched cells demonstrated protection in vivo. These authors mention in their discussion that L y l ÷ and Ly2+3 + cells are involved, but this work is not yet published. It m a y be significant with respect to defining the cells involved in protection following H S V infection that K,D-restricted Ly2 ÷3 + Tc 11and T d (V. Sinickas, unpublished observations) are difficult to detect. Thomsen et al. 12 have now proposed that lymphocytic choriomeningitis virus ( L C M V ) elimination correlates with D T H rather than cytotoxicity. Again, however, the, findings are inconclusive because the restriction and phenotype of the cells mediating the response were not characterized. The proposal is based on a correlation between increased viraemia and decreased D T H following cyclophosphamide pretreatment of LCMV-infected mice. In order to exclude involvement of Tc in these experiments, it will be necessary to establish the dose effect of cyclophosphamide for K,D-restricted Ly2÷3 ÷ and/-restricted L y l ÷ T cells. The strong evidence that K,D-restricted Ly2 +3 ÷ Tc are involved in viral clearance raises the question: is the D T H component of Tc important? This question is relevant for three reasons. First, K,D-restricted T cells mediate both cytotoxicity and D T H . Unequivocal evidence that the same cell mediates both functions has been obtained from an influenza-specific T-cell clone which lysed influenzavirus-infected targets, transferred D T H , and reduced infectious titres in the lungs 13. Second, autoradiographic studies of Leung and A d a demonstrated decreased mono-
nuclear infiltration in viral lesions following transfer of K, D-restricted influenza-immune T cells, indicating that D T H may not be an absolute requirement for protection 14. Third, /-region-restricted T cells capable of mediating D T H are not directly involved in protection*. The obvious advantage of a D T H component is amplification of the response by recruitment of macrophages. The beneficial effect of Tc lysis is apparent only before replication of virus, therefore macrophage recruitment would be important if live virus were released from infected cells. Similarly, there are obvious reasons as to why/-region-restricted T cells mediating D T H would be an ineffective means of viral clearance. These cells recognize Ia antigens which, unlike K , D antigens, are not present on most cell types, so only the limited numbers of infected cells expressing Ia antigens would be recognized. In addition, these cells are unable to discriminate between live virus presented on the surface of infected cells and dead or shed virus. The weight of present experimental evidence, therefore, indicates that K,D-restricted Ly2+3 ÷ T cells mediating cytotoxicity and D T H appear to be important in viral clearance.
Acknowledgements I would like to thank ProfessorG. L. Ada and Dr C. R. Parish for their help and for criticallyreading the manuscript. MARY BRENAN
Department of Microbiology, John Curtin &hool of Medical Research, Australia National Universi~, Canberra 2601, Australia.
References 1 2 3 4 5 6 7 8 9 10 11 12 13 14
Blanden, R. V. (1970)J. Exp. Med. 132, 1035-1054 Blanden, R. V. (1971)J. Exp. Met/. 133, 1074-1089 Zinkernagel, R. M. (1976)J. Exp. Meal 144, 776-787 Ada, G. L., Leung, K.-N. and Ertl, H. (1981) Immunol. Rev. 58, 5-24 Leung, K. N. and Ada, G. L. (1980) Stand. J. ImmunoL 12, 129-139 Ertl, H. (1981) Immunogenetics 12, 579-586 Nash, A. A., Phelan, J. and Wildy, P. (1981)J. ImmunoL 126, 1260-1262 Nash, A. A. and Cell, P. G. H. (1983) Cell. Immunol. 75, 348-355 Howes, E. L., Taylor, W., Mitchison, N. A. and Simpson, E. (1979) Nature (London) 277, 66-68 Sethi, K. K., Omata, Y. and Schneweis, K. E. (1983)J. Gen. Virol. 64, 443-447 Pfizenmaier, K. H., Jung, H. and Starzinski-Powitz, A. eta/. (1979)J. Immunol. 119, 937-944 Thomsen, A. R., Volkert, M. and Bro-J~rgensen, K. (1983) Stand. J. Iramunol. 17, 489-495 Lin, Y.-L. and Askonas, B. A. (1981)J. Exp. Med. 154, 225-234 Leung, K. N. and Ada, G. L. (1982) Cell. Immunol. 67, 312-324
Polymorphism and monomorphism in class-I MHC antigens In man, the mouse and the rat, the class-I histocompatibility antigens have many different alleles. In the Syrian or golden hamster, however, according to a series o f studies b y J . W. Streilein and his colleagues (see Ref. 1), the corresponding class-I genes have just one monomorphic product. This striking observation has prompted some recent reflections on the evolution of major histocompatibility loci and their products' role in directing T-cell function 1'2. Years ago, one peculiar oddity struck some of us who
used electrophoretic variants of various enzyme loci as a means to study genetic polymorphism of populations and species. The autosomally inherited 6-phosphogluconate dehydrogenase (6-PGD) locus is an example of this peculiar phenomenon. This locus in most vertebrate species remains in either a monomorphic or a dimorphic state, but in the Japanese quail (C0turnix coturnixjaponica) seven or more alleles coexist at this locus 3. It was to explain this oddity that the principle of polymorphism generating more polymorphism was proposed in 1969L
319
antigens, which may behave in a TI-like fashion by crosslinking the B-cell receptors, would preferentially activate B cells and render them sensitive to helper signals. Thus, autoantigens may play a role in directing autoantibody production. In normal mice where there is no hyperresponsiveness to or over-production of accessory signals nor a genetic predisposition for autoimmune response, the levels of autoantibodies produced will be low. In contrast, in SLE mice where there is hyper-responsiveness and/or hyper-production of accessory signals as well as genetic predisposition for autoimmune response, autoantibodies will reach abnormal levels. As for possible analogies between murine and human SLE, the NZ mice parallel humans in the sex distribution and immunopathological characteristics of this disease, and BXSB males are the counterpart of a recently described SLE that afflicts primarily fathers and sons 2°. In contrast, no human correlate of SLE in the MRL-lpr/lpr mouse has been identified, although humans with SjSgren's syndrome and humans and mice undergoing graft-versus-host (GvH) reactions might qualify. Humans whose G v H response follows bone-marrow allotransplantation may develop autoantibodies, immune complex glomerulonephritis and immune deposits in the skin21. Certain non-autoimmune strains of mice .undergoing experimentally induced chronic G v H reactions have serological and pathological alterations that strongly resemble the MRL-lpr/lpr disease, including autoantibodies, glomerulonephritis and lymphoid proliferation 22. During the GvH response, recipient B cells, including autoreactive clones, stimulate alloreactive donor T cells via Ia determinants and thus induce helper signals by these T cells 23. Whether the helper signals include BCGFand BCDF-like molecules remains to be determined. Further analysis of the concepts outlined above should ultimately provide information that allows detailed understanding of systemic autoimmune diseases at the molecular level.
Acknowledgements This
is Publication
No.
2946
from
the
Department
of
Immunology, Scripps Clinic and Research Foundation, 10666 North Torrey Pines Road, LaJolla, C A 92037, USA. The authors' work cited in this article was supported by the National Institutes of Health, Grants AI-07007 and AG-01743, and the Cecil H. and Ida M. Green Endowment Fund.
References 1 Andersson, J., Coutinho, A., Melchers, F. and Watanabe, T. (1976) Cold Spring Harbor Syrup. Quant. Biol. 41,227-236 2 Howard, M., Farrar, J., Hilfdcer, M. eta/. (1982)J. Exp. Med. 155, 914-923 3 Parker, D. C., Wadsworth, D. C. and Schneider, G. B. (1980)J. EXP. Med. 152, 138-150 4 Zubler, R. H. and Glasebrook, A. L. (1982).f Exp. Med. 155, 666-680 5 Park, C. L., Balderas, R. S., Fieser, T. et al. (1983)J. Immunol. 130, 2161-2167 6 Prud'homme, G. J., Balderas, R. S., Dixon, F. J. and Theofdopoulos, A. N. (1983),J. Exp. Med. 157, 1815-1827 7 Isakson, P. C., Purd, E., Vitetta, E. S. and Krammer, P. H. (1982) .~ Exp. Med. 155, 734-748 8 Martinez-Alonso, C. and Coutinho, A. (1981) Nature (London) 290, 60-61 9 Theofdopoulos, A. N., Shawler, D. L., Eisenberg, R. A. and Dixon, F. J. (1980)J. Exp. Med. 151,446---466 10 Prud'homme, G. J., Park, C. L., Fieser, T. M..et a/. J. Exp. Meal. (in press) 11 Wofsy, D., Murphy, E. D., Roths, J. B. eta/. (1981)J. Exp. Mat. 154, 1671-1680 12 Ahman, A., Theofdopoulos, A. N., Weiner, R. et 02. (1981)J. Exp. Meal. 154, 791-808 13 Alcocer-Varela, J. and Alarcon-Segovia, D. (1982)J. Clin. Invest. 69, 1388-1392 14 Slack, J., Der-Balian, G. P., Wahm, M. and Davie, J. M. (1980)J. Exp. Med. 151,853-862 15 Smith, H. R., Chused, T. M., Smathers, P. A. and Steinberg, A. D. (1983) Fed. Pro¢. Fed. Am. So¢. Exp. Biol. (in press) (Abstract) 16 Taurog, J. D., Moutsopoulos, H. M., Rosenberg, Y. J. et 02. (1979)J. Exp. Med. 150, 31-43 17 Steinberg, B.J., Smathers, P. A., Frederiksen, K. and Steinberg, A. D. (1982).]. Clin. Invest. 70, 587-597 18 Singer, A., Asano, Y., Schigeta, M. eta/. (1982) Irtmmnol. Rev. 64, 137-160 19 Steinburg, E. B., Santoro, T. J., Chused, T. M., Smathers, P. A. and Steinburg, A. D. (1983)J. ImmunoL (in press) 20 Lahita, R. G., Chiorazzi, N., Gibofsky, A. eta/. (1983) Arthritis Rheum. 26, 39-44 21 Schulman, H. M., Sullivan, K. M., Weiden, P. L. eta/. (1980) Am.J. Med. 69, 204-217 22 Gleichman, E., Van Elven, E. H. and Vander Veen, J. P. w . (1982) Eur J. lmmunoL 12, 152-159 23 Vander Veen, F. M. R., Rolink, A. G. and Gleichmann, E. (1982)J. Exp. Med. 155, 1555-1560 24 Linker-Israeli, M., Bakke, A. G., Kitridou, R. C. eta/. (19ll3)J. lmmunol. 130, 2651-2655
New directions in research
T lymphocytes in viral clearance Early studies by Blanden I with ectromelia virus suggested that T cells mediating delayed-type hypersensitivity (DTH) were ideal candidates for viral clearance. T cells were shown to localize in foci of infection and to be followed by macrophage infiltration which eliminated virus. Subsequent studies indicated that cytotoxic T cells (Tc) alone were sufficient for clearance 2. A D T H mechanism seemed not to be involved because delayedtype hypersensitivity T cells (Td) were defined as Lyl ÷ and 1-region-restricted, while the Tc conferring viral clearance were Ly2 ÷3 + and K,D-region-restricted. The more recent findings that both K,D-restricted Tc and 1restricted T cells can mediate D T H 3,~ have therefore
raised questions about the roles of D T H and cytotoxicity in viral clearance. In-vitro-generated influenza-virus-specific secondary effector cells adoptively transferred to mice inoculated with a lethal dose of influenza virus reduced virus titres in the lungs, and protected the mice from death 5. The protective T cells were identified as Ly2÷3 + and K,Dregion-restricted. In contrast, T cells exhibiting 1-regionrestricted D T H failed to protect infected mice and the recipients had a significantly higher mortality rate than controls ~. Similar results were obtained with Sendal virus 6. A requirement for K,D-restricted T cells in herpes
320
Immunology Today, vol. 4, No. 11, 1983
simplex virus (HSV) infections has also been shown. Nash et al. 7 demonstrated that I-A compatibility was sufficient for transferring D T H , but that K, I-A and D were necessary for viral clearance. In subsequent studies the same group s concluded that Lyl + and not Ly2 +3 ÷ cells are directly involved in viral clearance. However, in these experiments the existence of K,D-restricted Ly2+3 ÷ T cells was not demonstrated. The increase in virus titre following treatment with anti-Lyl.1 sera and complement which was used as evidence for the direct involvement of an/-restricted T cell can be explained by the need of an/-restricted helper cell for antibody or other T-cellmediated responses. This would accord with the hypothesis of Howes et al. 9 concerning T help in antibody responses, based on the result that/-restricted but not K,D-restricted cells conferred long-lasting protection against H S V infection. Involvement of help could be demonstrated by transferring irradiated cells. Recent studies by Sethi et al. lo using a cloned culture of HSVmemory T-enriched cells demonstrated protection in vivo. These authors mention in their discussion that L y l ÷ and Ly2+3 + cells are involved, but this work is not yet published. It m a y be significant with respect to defining the cells involved in protection following H S V infection that K,D-restricted Ly2 ÷3 + Tc 11and T d (V. Sinickas, unpublished observations) are difficult to detect. Thomsen et al. 12 have now proposed that lymphocytic choriomeningitis virus ( L C M V ) elimination correlates with D T H rather than cytotoxicity. Again, however, the, findings are inconclusive because the restriction and phenotype of the cells mediating the response were not characterized. The proposal is based on a correlation between increased viraemia and decreased D T H following cyclophosphamide pretreatment of LCMV-infected mice. In order to exclude involvement of Tc in these experiments, it will be necessary to establish the dose effect of cyclophosphamide for K,D-restricted Ly2÷3 ÷ and/-restricted L y l ÷ T cells. The strong evidence that K,D-restricted Ly2 +3 ÷ Tc are involved in viral clearance raises the question: is the D T H component of Tc important? This question is relevant for three reasons. First, K,D-restricted T cells mediate both cytotoxicity and D T H . Unequivocal evidence that the same cell mediates both functions has been obtained from an influenza-specific T-cell clone which lysed influenzavirus-infected targets, transferred D T H , and reduced infectious titres in the lungs 13. Second, autoradiographic studies of Leung and A d a demonstrated decreased mono-
nuclear infiltration in viral lesions following transfer of K, D-restricted influenza-immune T cells, indicating that D T H may not be an absolute requirement for protection 14. Third, /-region-restricted T cells capable of mediating D T H are not directly involved in protection*. The obvious advantage of a D T H component is amplification of the response by recruitment of macrophages. The beneficial effect of Tc lysis is apparent only before replication of virus, therefore macrophage recruitment would be important if live virus were released from infected cells. Similarly, there are obvious reasons as to why/-region-restricted T cells mediating D T H would be an ineffective means of viral clearance. These cells recognize Ia antigens which, unlike K , D antigens, are not present on most cell types, so only the limited numbers of infected cells expressing Ia antigens would be recognized. In addition, these cells are unable to discriminate between live virus presented on the surface of infected cells and dead or shed virus. The weight of present experimental evidence, therefore, indicates that K,D-restricted Ly2+3 ÷ T cells mediating cytotoxicity and D T H appear to be important in viral clearance.
Acknowledgements I would like to thank ProfessorG. L. Ada and Dr C. R. Parish for their help and for criticallyreading the manuscript. MARY BRENAN
Department of Microbiology, John Curtin &hool of Medical Research, Australia National Universi~, Canberra 2601, Australia.
References 1 2 3 4 5 6 7 8 9 10 11 12 13 14
Blanden, R. V. (1970)J. Exp. Med. 132, 1035-1054 Blanden, R. V. (1971)J. Exp. Met/. 133, 1074-1089 Zinkernagel, R. M. (1976)J. Exp. Meal 144, 776-787 Ada, G. L., Leung, K.-N. and Ertl, H. (1981) Immunol. Rev. 58, 5-24 Leung, K. N. and Ada, G. L. (1980) Stand. J. ImmunoL 12, 129-139 Ertl, H. (1981) Immunogenetics 12, 579-586 Nash, A. A., Phelan, J. and Wildy, P. (1981)J. ImmunoL 126, 1260-1262 Nash, A. A. and Cell, P. G. H. (1983) Cell. Immunol. 75, 348-355 Howes, E. L., Taylor, W., Mitchison, N. A. and Simpson, E. (1979) Nature (London) 277, 66-68 Sethi, K. K., Omata, Y. and Schneweis, K. E. (1983)J. Gen. Virol. 64, 443-447 Pfizenmaier, K. H., Jung, H. and Starzinski-Powitz, A. eta/. (1979)J. Immunol. 119, 937-944 Thomsen, A. R., Volkert, M. and Bro-J~rgensen, K. (1983) Stand. J. Iramunol. 17, 489-495 Lin, Y.-L. and Askonas, B. A. (1981)J. Exp. Med. 154, 225-234 Leung, K. N. and Ada, G. L. (1982) Cell. Immunol. 67, 312-324
Polymorphism and monomorphism in class-I MHC antigens In man, the mouse and the rat, the class-I histocompatibility antigens have many different alleles. In the Syrian or golden hamster, however, according to a series o f studies b y J . W. Streilein and his colleagues (see Ref. 1), the corresponding class-I genes have just one monomorphic product. This striking observation has prompted some recent reflections on the evolution of major histocompatibility loci and their products' role in directing T-cell function 1'2. Years ago, one peculiar oddity struck some of us who
used electrophoretic variants of various enzyme loci as a means to study genetic polymorphism of populations and species. The autosomally inherited 6-phosphogluconate dehydrogenase (6-PGD) locus is an example of this peculiar phenomenon. This locus in most vertebrate species remains in either a monomorphic or a dimorphic state, but in the Japanese quail (C0turnix coturnixjaponica) seven or more alleles coexist at this locus 3. It was to explain this oddity that the principle of polymorphism generating more polymorphism was proposed in 1969L
