THE IMMUNOLOGY OF SYPHILIS DANIEL M. MUSHER, M.D., RONALD F. SCHELL, Ph.D., AND JOHN M. KNOX, M.D. From the Departments of Medicine, Microbiology and Immunology and Dermatology, Baylor College of Medicine and the Veterans Administration Hospital, Houston, Texas

The clinical manifestations of syphilis have provided medical investigators with a fruitful field for research since the beginning of the 16th century. In recent years, however, syphilology has not advanced as rapidly as might have been anticipated, and immunologic studies of syphilis have remained outside the mainstream of modern immunology. As a result, the nature of the immune response of the human host to infection with Treponema paltidum remains poorly understood. This situation has resulted, to a great extent, from technical problems related to the nature of the infecting organism. Technical Problems in Experimentation 7. pallidum is a tiny organism with a transverse diameter that seldom exceeds 0.15 /im.'- 2 Visualization in wet preparations requires the use of a darkfield microscope and, in dry preparations, the use of special techniques such as silver impregnation. Although recent studies suggest that the metabolism of 7. pallidum may be investigated in vitro-^ and have shown prolonged survival in tissue culture,'* this fastidious pathogenic spiroSupporled in part by research funds from the Veterans Administration Hospital, Houston, Texas, and NIH Grants USPHS 5 RD 1 Al 11305 and 12618-02/05 P-1. Address for reprints: Daniel M. Musher, M.D., Veterans Administration Hospital, Houston, TX 77211. 566

chete has yet to be cultivated successfully in vitro. One investigator^ has described cultivation of 7. pallidum in vivo using an implanted chamber, but others have been unable to reproduce this result. Failure to grow 7. pallidum on artificial medium has several unfortunate consequences. The inability to determine colony-forming units makes it extremely difficult to study the onset and evolution of infection-induced immunity to syphilitic infection for which accurate counts of organisms in tissues are required. In addition, since 7. pallidum must be maintained in the laboratory by animal transfer and isolated from infected mammalian tissue, any attempt to characterize it immunologically raises the problem of distinguishing treponemal antigens from those of the mammalian host. Because of its fragility, vigorous attempts to obtain a pure suspension are likely to alter or damage substances, for example, by loss of cell envelope.'^' '' Immunologic studies are further limited by lack of animal models. With the exception of the guinea pig, which can be infected only with difficulty and the hamster, which develops an atypical infection,** the rabbit appears to be the only nonprimate that can be infected with 7. pallidum. Not only have rabbits been less well studied immunologically than mice, rats and guinea pigs, but they are expensive to buy and maintain, and i n bred strains which are so important in immunologic studies are not readily available. Mice do not appear to be infected,'J' 'o although they produce antibodies following injection of treponemes." It is conceivable that this model can be exploited, using more modern techniques and concepts.

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1. 2. 3.



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Evidence for Humoral Immunity in Syphilis

Nonspecific (VDRL) and specific (FTA-ABS, TPHA) antibodies are regularly present in the serum of patients who have syphilis. Antibodies which immobilize and inactivate T. pallidum are found with increasing frequency as syphilis progresses to latent and tertiary infection. Partial immunity can be conferred on rabbits by passive transfer of serum from syphilis-immune animals.

In addition to these formidable technical problems, the discovery of penicillin and the prediction, not borne out by subsequent events, that syphilis would rapidly be eliminated, must certainly have reduced the desire of scientists to work in this field. The investigations that have been carried out in the face of these difficulties have suggested that immunity in syphilis develops as a result of a complex interrelation between humoral and cellular factors. Observations indicate that humoral and cell-mediated immunity are activated during syphilitic infection. These, summarized in Tables 1 and 2 respectively, will now be examined in detail. Antibodies Although several kinds of antibodies have been demonstrated in the serum of patients with syphilis, the exact nature of their contribution to immunity remains Table 2.

poorly understood. Discovery of antibody to cardiolipin and its application to the detection of syphilitic infection^-'' i' represented a major advance in diagnosing syphilis. This serologic test supported the clinical diagnosis of primary or secondary infection. More importantly, it made it possible for the first time to diagnose latent syphilis with some degree of certainty.i^ The nonspecificity of these antibodies and their apparent lack of relation to antibodies directed against spirochetes makes it difficult to understand their relation to infection caused by 7. pallidum. They develop in several disease states such as malaria, infectious mononucleosis and systemic lupus erythematosus.^^'s These conditions appear to have nothing whatever to do with syphilis; therefore, it might be easy to dismiss these antibodies as being entirely unrelated to immunity. Moreover, the VDRL may

Evidence for Cell-Mediated

Itvmunily

in Syphilis

Syphilis progresses through primary and secondary stages, despite the presence of antibodies that immobilize the infecting organism. 2.

Passive transfer of syphilis-immune serum is only partially protective and does not follow classic models of humoral immunity.

Delayed hypersensitivity to treponemal antigens is absent in primary and early secondary syphilis, but develops late in secondary infection and is regularly present in latent and tertiary syphilis. 4. Tertiary syphilis is characterized by granulomatous lesions. Immunization with killed organisms is usually unsuccessful, whereas hnmunization with live, 5. attenuated organisms has produced immunity. In vitro lymphocyte reactivity to treponemal and nontreponemal antigens is suppressed during 6. primary and secondary syphilis. 7.

Infecting rabbits with Treponema pallidum stimulates acquired cellular resistance to Listeria; this reaction is mediated by thymus-dependent lymphocytes.

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convert to nonreactivity in up to onethird of patients with certain types of tertiary syphilis.". 20 A number of observations suggest that VDRL reactivity has more than a coincidental association with infections caused by treponemes, particularly those due to 7. pallidum. VDRL reactivity is a consistent finding in late primary and secondary syphilis.2'' 22 VDRL antibodies appear in rabbits after repeated challenge with killed 7. pallidum, although this is not associated with immunity.2-'' 24 They may also be present in untreated rabbits, probably as a result of naturally acquired infection with 7. cuniculi.^- ^^ Rabbits which are naturally VDRL reactive have delayed appearance of chancres following intradermal challenge with 7. pallidum; we have found that those which naturally have reactions positive in 1:16 dilution are quite resistant to large inocula of 7. pallidum. Recent studies have shown that VDRL reactivity develops in chimpanzees which are infected experimentally with either 7. pallidum or 7. cun;CU//.26

Fluorescent treponemal antibody (FTA), especially when present following absorption with 7. reiter (FTA-ABS), is highly specific for infection with 7. pallidum.-^' 2« Both IgG and IgM have been shown to participate.2' 2fi. so ^ jj interesting to note that rheumatoid arthritis and systemic lupus erythematosus, which also cause falsely positive VDRL results, are occasionally associated with a false-positive FTA-ABS.-" -'-' The reaction produced by serum from patients with lupus is said to be distinguishable microscopically from that due to syphilis;-^' it may result from leakage of nuclear material from the treponeme during processing.-'2 FTA-ABS are detectable in nearly all syphilitic patients, except those whose primary chancre is in its earliest stages.^''^ ^^ Once present, these antibodies probably persist

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for life, although one recent study"" suggests that this may not be the case. Hemagglutinating antibodies (TPHA) have also been described and appear to have similar properties.-^^'^ ^^ Antibodies which immobilize (TPIA) and actually inactivate 7. pallidum in vitro are regularly present in the serum of patients who have syphilis, especially in late secondary and latent infection.-^** The mechanism by which they inactivate treponemes needs to be studied,2 especially in light of observations on inactivation of treponemes by normal human serum which has not been heat-treated.-'' Whether these are all the same or different antibodies remains to be elucidated. Humoral Aspects

Despite the presence of these antibodies in infected patients, passive immunization with immune serum does not fit the classic model for demonstrating humoral immunity. For example, when a mouse is injected with killed pneumococci, it produces antibodies which protect it against challenge with Streptococcus pneumoniae; transfer of 0.1 ml immune serum to another mouse confers protection against pneumococcal infection. By contrast, repeated injection of killed treponemes into rabbits leads to the development of antibodies but has usually been found not to confer protection.^J- 24 Transfer of large quantities of serum from syphilis-immune rabbits (rabbits which were infected with 7. pallidum, developed syphilitic lesions, recovered and were shown to resist subsequent rechallenge), produces an unusual state of partial immunity in which the appearance of syphilitic lesions is delayed but not altogether abolished.**• "'• 4i In addition, globulins have not been shown to be responsible. The most compelling evidence that humoral immunity does not protect

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the human host against syphilis is that these variatious antibodies are regularly demonstrated late in primary syphilis, yet do not appear to alter the progression to, or the manifestations of, secondary infection. Cellular Factors

The fact that humoral mechanisms do not appear to abolish syphilitic infection naturally leads to an examination of cellular immune mechanisms. Several lines of evidence have supported a role for cell-mediated immunity. A skin test using a treponemal extract suggested that delayed hypersensitivity could be detected in latent or tertiary syphilis, especially if a second skin test was done.''2' 43 Using a relatively pure preparation of 7. pallidum, which was obtained by incubating fine slices of syphilitic rabbit testis in tissue culture medium and separating the treponemal-rich supernatant, 2 studies have shown that a delayed hypersensitivity reaction is not present in primary or early secondary syphilis but appears late in secondary syphilis and is regularly present in latency.'^' '^^ The absence of delayed hypersensitivity early in syphilis and its presence in latency suggests that cell-mediated immunity may be involved in bringing about the latent state. Further indirect evidence comes from the histologic appearance of syphilitic lesions. Although plasma cells predominate in the perivascular lesions of primary and secondary syphilis, tertiary infection is characterized by the formation of granulomas which cannot be distinguished from those that are seen in other granulomatous conditions.'"' Depletion of lymphocytes has also been shown to occur in thymus-derived areas of the spleen in neonates with congenital syphilitic infection and in the lymph nodes of patients with primary, secondary or early latent syphilis.'^ Likewise, infection of

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neonatal rabbits with 7. pallidum has been characterized as a progressive runting syndrome leading to death with depletion of the spleen and the thymus of lymphocytes.'"^ These features suggest that infection with 7. pallidum can induce a spectrum of immunologic reactivity which includes delayed hypersensitivity in adults and suppression of immune function in the immunologically immature host. A classic feature of infections that are mediated by cell-mediated immunity is the persistence of infecting organisms within macrophages.'"-""2 Treponemes have been found intracellularly both in rabbit and human tissue following infection with 7. patlidum.^'-^'^ However, there is little to suggest that these organisms are persisting in a viable state, much less replicating; in fact, they usually appear to be deteriorating, presumably due to enzymes and/or other substances with vacuoles. Moreover, there is a promiscuity to their intracellularity in that they can be found within lymphocytes, plasma cells and epithelial cells, although the predominant cell is the macrophage.-'^s-^^^ In addition, extracellular organisms predominate at all stages of infection.^s-ss These 2 features are not suggestive or classic intracellular infections. Another characteristic of infections that are mediated by cell-mediated immunity is that effective immunization can be achieved by inoculation with a live attenuated vaccine but not with killed organisms. Although Metzger-" has succeeded in immunizing rabbits by injecting them with killed 7. pallidum, others have been unable to do so.^. 23,24. eo Miller^" has attenuated 7. pallida by irradiation and has used them to protect rabbits against subsequent challenge with the virulent organism. It is possible that a highly labile immunogenic fraction is present only in living treponemes or in those that

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are killed in a particular fashion, as has been proposed in the case of M. tuberculosis.''^ This hypothesis would certainly seem to be applicable to a sensitive organism such as 7. pallidum and will be discussed further. However, one cannot exclude the possibility that some viable, virulent organisms survive both kinds of treatmenf'- '''' and are responsible for the observed immunity;f'2 this problem again relates to inability to cultivate and/or quantitate viable 7. pallidum in vitro. Laboratory Investigation

In our laboratory, 2 kinds of studies have provided further evidence that cellmediated immunity plays a role in syphilitic infection. We compared lymphocyte reactivity in patients who had primary or secondary syphilis with that in normal subjects who were matched for age, sex, race and approximate socioeconomic status. Lymphoblastic response, as determined by ^H-thymidine uptake following incubation with mitogens such as phytohemagglutinin, pokeweed mitogen and streptolysin, was identical in both groups. However, the response to treponemal antigens, including 7. refringens, 7. reiter and 7. pallidum, was suppressed as was the response to several other nontreponemal antigens such as Candida and trichophytins.''-^' '"' Friedmann and Turk*"^ have found that tuberculin reactivity was suppressed in syphilis but showed enhanced response to an antigenic preparation containing 7. pallidum. Although 2 preliminary studies suggested that syphilitic serum may contribute to this suppression,'''^^- ^'' one of these'''' has already been shown not to be reproducible.''^ The other has not been presented in final form at the time of this writing.

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Our results have suggested that suppressed responsiveness results entirely from factors inherent in the lymphocytes themselves, independent of serum factors.''-^ Following treatment for syphilis, lymphocyte responsiveness returned to normal.''^ Two studies have shown leukocyte migration to be stimulated by treponemal extract early in syphilis and suppressed late in the course of infection/'S. 69 These observations are difficult to interpret because of the relatively small differences and the unusual nature of the observed response. All the aforementioned studies provide circumstantial evidence that cell-mediated immunity is involved in syphilis. Suppressed lymphocyte reactivity has been observed in a number of infections in which the role of cell-mediated immunity is well-established, such as cryptococcosis,'" tuberculosis'' and histoplasmosis.'^ However, it has not been demonstrated in acute bacterial infections.'^^ ''' The mechanism for the observed suppression has been thought to be related to antigen overload.'^' '^ Certainly the persistence of large numbers of treponemes in secondary lesions is consistent with this hypothesis. An alteration in the ratio of circulating thymus-dependent and bursa-dependent lymphocytes may be partially responsible; this has not yet been studied in syphilitic subjects. A second set of studies in our laboratory has shown that infection due to 7. pallidum stimulates cell-mediated immunity. Ability to suppress the growth of Listeria monocytogenes has been shown in other animal systems to be dependent upon cellular factors. It is also a good index for assessing the degree of stimulation of cellular immune mechanisms by antigenically unrelated organisms.-''2 We found that the growth of

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Listeria in rabbit livers was suppressed between 3 and 6 weeks after intravenous infection with 7. pallidum.'^^ Transferring spleen cells from syphilitic rabbits along with 7. pallidum conferred resistance to Listeria upon recipients; transferring syphilitic spleen cells without 7. pallidum alone did not confer resistance."' '^ Adoptive transfer of immunity using sensitized lymphocytes and the sensitizing antigen closely followed the model established by Mackaness''^ and appears to support the hypothesis that cell-mediated immunity develops in response to syphilitic infection. Further proof was obtained when a highly specific antithymus serum was developed and used to abolish the transfer of immunity, thereby showing that thymus-dependent lymphocytes are responsible for the observed phenomena.'** Additional evidence for the role of cell-mediated immunity was obtained by Metzger (personal communication). He showed that beginning several weeks after infection with 7. pattidum, rabbit lymphocytes produce substances in vitro which inhibit the migration of leukocytes. If cell-mediated immunity plays a central role in immunity to infection with 7. pallidum it should be possible to show that activation of cellular immunity protects rabbits against syphilis and that immunity to 7. pattidum can be transferred using thymus-dependent lymphocytes. Studies in both these areas have been negative to date. Rabbits infected with BCG and shown to suppress the growth of Listeria failed to demonstrate immunity to intradermal or intravenous challenge with 7. patlidum.^° Preincubating 7. pattidum in heat-inactivated, syphilis-immune rabbit serum before challenging BCG-infected rabbits just before challenge did not alter the course of syphilitic infection.^O' ^^

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These results suggest that, although infection with 7. patlidum induces nonspecific acquired cellular resistance, this nonspecific resistance may not play a major role in controlling syphilitic infection. Although it has generally been thought that infections which stimulate cell-mediated immunity are brought under control by these same nonspecific mechanisms,-''2 recent studies'^2, 83 have questioned this concept. Syphilis may be another infection that does not follow the more "classic" view.^-^ Finally, our recent experiments have failed to induce immunity to 7. pallidum by adoptive transfer of lymphocytes from syphilis-immune rabbits. It is possible that in outbred New Zealand rabbits lymphocytes do not survive the 2-3 weeks that are necessary to cover the incubation period of syphilis; these experiments are being repeated using inbred rabbits. Metzger claims to have successfully transferred immunity to 7. paltidum using lymphocytes from litter mates. In this kind of experiment, immunity will need to be transferred using thymus-dependent spleen cells in order to support a prominent role for an effective cell-mediated immune response in syphilis. Clinical Investigation Important observations on immunity to syphilitic infection in human beings were made in studies carried out at Sing Sing Prison after the Second World War.^^ Prisoners who volunteered for the study were challenged intradermally with 2 X 10' 7. pallidum. Of 11 who had previously received treatment for early syphilis, 9 developed typical chancres which contained 7. pattidum and 2 developed darkfield-negative lesions. By contrast, 5 subjects with latent syphilis who had

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never received treatment were completely refractory to this rechallenge. Of 31 with late latent or congenital syphilis, 14 developed neither lesions nor change in VDRL titer. Three developed atypical chancres that contained no treponemes and had developed no rise in VDRL. Twelve had darkfield-negative lesions but exhibited a rise in VDRL; the histologic changes in 2 cases were those of a gumma. Only 2 subjects developed chancres that contained 7. patlidum. These results show that although patients who have been treated for early syphilis are susceptible to reinfection, syphilitic infection which goes untreated, or for which treatment is given only late in its course, produces immunity to rechallenge with 7. pallidum. The investigators assumed that this is not the case for early, untreated syphilis, although they did not specifically investigate this problem. Despite the immunity to rechallenge and the fragile nature of the organism, 7. paltidum can sometimes be recovered from human and rabbit tissues years after seemingly adequate antibiotic therapy for late latent disease.^^"'^^ Future Research

An area that needs to be examined further is the one that deals with the relation between syphilis, yaws (caused by 7. pertenue) and pinta (caused by 7. carateum).^ The organisms that cause these 3 infections cannot be distinguished by morphologic or immunologic criteria, and the antibodies which are produced in response to infection have not been separated.** Some investigators**'-'^ believe that these organisms are identical and that differences in expression result from varying host response which may be mediated in part by climatic conditions. Others'2 have proposed that these species have evolved from a common precursor,

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although they now are, in fact, different. This may be saying the same thing, and Willcox'-* believes that the apparent differences due to climate and socioeconomic factors can be altered by antibiotics and changing social factors. It is well known that patients who have yaws are protected against developing syphilis, although the reverse relation has not received attention.**' ^^ *" Pinta has not been shown to protect humans against syphilis.'^'* We believe that it would be most useful to repeat immunologic studies—for example, the ones described previously from our own laboratory—using 7. pertenue and 7. carateum along with 7. pallidum in order to clarify the relation among these organisms. Is there some way of putting together all these observations to formulate a unifying hypothesis?2' s. 48,95 infection caused by 7. paltidum stimulates both humoral and cellular immune responses. Immobilizing antibodies may appear early in the infection, but are found most regularly in latency. Delayed hypersensitivity to 7. patlidum is found only in latency. Only human subjects with latent syphilis appear to be uniformly immune to rechallenge with 7. pallidum. It is reasonable to conclude that delayed hypersensitivity and immobilizing antibodies together may be necessary to bring about latency. Further support comes from demonstrating that in animals neither transfer of immune serum nor activation of nonspecific acquired cellular resistance produces an immune state. The demonstration of suppressed lymphocyte reactivity and the possibility that this reflects immunosuppression in human syphilis may explain why early syphilis is a prolonged infection with waxing and waning of symptoms until immunity ensues.

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Wby should it take so long for buman subjects to develop bumoral and cellular immunity? Immunosuppression caused by syphilitic infection may be, in part, responsible. However, an important factor migbt be found in tbe treponeme itself. Tbe mucoid envelope of tbe treponeme is tbought to render it resistant to phagocytosis.'•' As noted previously, electron microscopic studies of buman syphilitic lesions bave sbown the majority of treponemes to lie extracellularly, altbougb intracellular localization has also been documented. Turner'^'' has proposed tbat only after tbe treponemes remain in the buman bost for a prolonged time is tbe mucoid coat broken down enough to allow for phagocytosis. Processing of antigen could tben begin to take place, followed by stimulation of bumoral and cellular immune mechanisms. In tbe buman host tbis process may take so long tbat treponemal proliferation produces a condition of antigenic overload wbicb could account for the observed immunosuppression. Rabbits migbt bave a sbort-lived infection because their bigher body temperature leads to more rapid breakdown of tbe treponeme witb processing of antigen before overload can occur. A number of observations on tbe growtb of T. pallidum in rabbits and cultivatable treponemes in vitro as well as tbose made in otber experimental infections are consistent with exquisite temperature sensitivity.^ Anotber possibility is tbat the treponeme is presented to the host in such a way tbat antibodies are rapidly produced wbicb serve to "block" a further cell-mediated immunity response. Evidence for early coating of 7". pallidum with rabbits globulin has recently been presented.'^ Tbere remains much to be done in tbe field of sypbilis immunology. An old clinical adage used to be tbat be wbo

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knows sypbilis knows disease. Tbis migbt be modified sligbtly in tbe modern era to indicate tbat be wbo truly knows the immunology of syphilis knows immunology. Acknowledgment: Tbe autbors are indebted to Evan Kerr and Maxi Denneby of tbe Medical Library Service, Houston VA Hospital, for bibliograpbic assistance and to Mona Tbomas for secretarial assistance.

References 1. Swain, R. H. A., Electron microscopic studies of tbe morpbology of patbogenic spirocbaetes. J. Patbol. Bacteriol. 49:117, 1955. 2. World Healtb Organization. Treponematoses Researcb. Tecbnical report no. 455. Geneva, Switzerland, 1970. 3. Baseman, J. B., and Hayes, N. S., Protein syntbesis by Treponema pallidum extracted from infected rabbit tissue. Infect. Immunity 10:1350, 1974. 4. Fitzgerald, T. J., Miller, J. N., and Sykes, J. A., Treponema pallidum (Nichols strain) in tis.sue culture: cellular attachment, entry, and survival. Infect. Immunity 11:1133, 1975. 5. Rathlev, T., Cultivation of patbogenic Treponema pallidum in cbambers surgically implanted in experimental animals. Acta Patbol. Microbiol. Scand. 81:269, 1973. 6. Jones, R. H., Nevin, T. A., Guest, W. J., and Logan, L. C, Lytic effect of trypsin, lysozyme, and complement on Treponema pallidum. Br. J. Vener. Dis. 44:193, 1968. 7. Tbomas, M. L., Clark, J. W., Jr., Cline, G. B., Anderson, N. G., and Russell, H., Separation of Treponema pallidum from tissue substances by continuous-flow zonal centrifugation. Appl. Microbiol. 23:714, 1972. 8. Turner, T. B., and Hollander, D. H., Biology of tbe treponematoses. Geneva, Switzerland, World Healtb Organization, 1957. 9. Gueft, B., and Rosabn, P. D., Experimental mouse sypbilis, a critical review of tbe literature. Am. J. Sypbilol. Gon. Vener. Dis. 32: 59, 1948. 10. Scbell, R. F., Musber, D. M., Jacobson, K., and Schwetbelm, P., New evidence for noninfectivity of Treponema paltidum for mice. Br. J. Vener. Dis. 51:19, 1975. 11. Obta, Y., Treponema pallidum antibodies in syphilitic mice as determined by immunofluorescence and passive bemagglutinatfon techniques. J. Immunol. 108:921, 1972.

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12. Wasserman, A., Neissen, A., and Brivick, C , Eine seradiagnostiche reaktion bei syphilis. Dtscb. Med. Wochenschr. 32:745, 1906. 13. U. S. Department of Health, Education and Welfare. Manual of serologic tests for syphilis. 1969. USPHS Publ. no. 988. Washington, D. C, US Government Printing Office, 1969. 14. riaken, J. L., Syphilis. Practitioner 209:60.5613, 1972. 15. Hoagland, R. J., False-positive serology in mononucleosis. JAMA 185:783, 1963. 16. Carpenter, G. M., Boak, R. A., LeGlair, R. A., and Nickerson, ]., Serum abnormalities In patients with biologic false-positive tests for syphilis. Amer. ). Glin. Pathol. 43-146, 1965. 17. Putkonen, R., Jokinen, E. J., Lassus, A., and Mustakallio, K. K., Ghronic biologic falsepositive seroreactions for syphilis as a harbinger of systemic lupus erythematosus. Acta Derm. Venereol. 47:83, 1967. 18. LeGlair, R. A., Montague, T. S., and Keitin, L. H., Biologic false-positive reactions for syphilis as measured by tbe Treponema pallidum immobilization test. J. Med. 3:264, 1972. 19. Olansky, S., and Norins, L. C, Gurrent diagnosis and treatment of syphilis. JAMA 198: 165, 1966. 20. Hoosbmand, H. Escobar, M. R., and Opf, S. W., Neurosyphilis, a study of 241 patients. JAMA 219:726, 1972. 21. Wende, R. D., Mudd, R. L., Knox, J. M., and Holder, W. R., The VDRL slide test in 322 cases of darkfield positive primary syphilis. South. Med. J. 64:633, 1971. 22. Moore, M. B., Jr., and Knox, ). M., Sensitivity and specificity in syphilis serology: clinical implications. South. Med. J. 58:963, 1975. 23. McLeod, G. P., and Magnuson, H. J., Production of immobilizing antibodies unaccompanied by active immunity to Treponema pallidum as sbown by active injecting rabbits and mice with the killed organism. Am. J. Syph. 36:9, 1953. 24. Izzat, N. N., Knox, J. M., Davis, W. C, and Smith, E. B., Resistance and serological changes in rabbits immunized with virulent Treponema pallidum sonicate. Acta Derm. Venereol. 51:157, 1971. 25. Small, J. D., and Newman, B., Venereal spirochetosis of rabbits (rabbit syphilis) due to Treponema cuniculi: a clinical, serological, and hostopathological study. Lab. Animal Sci. 22:77, 1972. 26. Duncan, W. P., and Kuhn, U. S. G., Treponemal IgG and IgM response in experimen-

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tally infected chimpanzees. Br. J. Vener. Dis. 50:257, 1974. 27. Deacon, W. F., Hunter, E. F., Treponemal antigens as related to identification and syphilis serology. Proc. Soc. Exp. Biol. Med. 110:352, 1962. 28. Hunter, E. F., Deacon, W. E., and Meger, P. E., An improved test for syphilis, the absorption procedure (FTA-ABS). Public Health Reports 79:410, 1964. 29. Septjian, M., Guerraz, F. T., Monier, J. G., Nivelon, ]. L., and Thivolet, J., Investigation of a specific IgM antibody test in neonatal congenital syphilis. Br. J. Vener. Dis. 46:18, 1970. 30. O'Neil, P., and Nicol, G. S., IgM class antitreponemal antibody in treated and untreated syphilis. Br. J. Vener. Dis. 48:460, 1972. 31. Kraus, S. j . , Haserick, J. R., and Lantz, M. A., Fluorescent treponemal antibody-absorption test reactions in lupus erythematosus. N. Engl. J. Med. 282:1287, 1970. 32. Strobel, P. L., and Kraus, S. J., An electron microscopic study of the FTA-ABS "beading" pbenomenon witb lupus erythematosus sera, using ferritin-conjugated anti-human IgG. J. Immunol. 108:1152, 1972. 33. Wright, D. J. M., The significance of tbe fluorescent treponemal antibody {FTA-ABS) test in collagen disorders and leprosy. J. Glin. Pathol. 26:968, 1973. 34. Hunter, E. F., Norins, L. G., and Falcone, U. H., The fluorescent treponemal antibody absorption (FTA-ABS) test: development, use, and present status. Bull. World Health Organization 39:873, 1968. 35. Knox, J. M., Short, D. H., and Wende, R. D., The FTA-ABS test for syphilis: performance in 1033 patients. Br. J. Vener. Dis. 42:16, 1966. 35a. Burnes, R. E., Spontaneous reversion of FTAABS test reactions. JAMA 234:617, 1975. 36. Garner, M. F., Backhouse, ]. L., Daskalopoulos, G., and Walsh, J. L., Treponema pallidum hemagglutination test for syphilis. Gomparison with the TPI and FTA-ABS tests. Br. J. Vener. Dis. 48:470, 1972. 37. Lesinski, J., Krach, J., and Kadziewicz, E., Specificity, sensitivity, and diagnostic value of the TPHA test. Br. J. Vener. Dis. 50:334, 1974. 38. Wood, R. M., Inouye, Y., and Argonza, W., Gomparison of the fluorescent treponemal antibody absorption and Treponema pallidum immobilization test on serum from

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The immunology of syphilis.

THE IMMUNOLOGY OF SYPHILIS DANIEL M. MUSHER, M.D., RONALD F. SCHELL, Ph.D., AND JOHN M. KNOX, M.D. From the Departments of Medicine, Microbiology and...
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