JOURNAL OF CLINICAL MICROBIOLOGY, Jan. 1985, p. 82-870095-1137/85/010082-06$02.00/0Copyright © 1985, American Society for Microbiology

Sodium Dodecyl Sulfate-Polyacrylamide Gel ElectrophoresisImmunoblotting as a Serological Tool in the Diagnosis of Syphilitic

InfectionsULRIKE HENSEL,* HANS-JOBST WELLENSIEK, AND SUCHARIT BHAKDI

Institute of Medical Microbiology, University of Giessen, D-6300 Giessen, Federal Republic of Germany

Received 19 June 1984/Accepted 1 October 1984

The utility of sodium dodecyl sulfate-polyacrylamide gel electrophoresis immunoblotting as a serological toolin the diagnosis of human syphilitic infections was examined. In model experiments, rabbits were immunizedwith Treponema pallidum or T phagedenis, and the antisera were tested for'cross-reactivities with both sets ofantigçns. A major T. pallidum antigen with a molecular weight of ca. 17,000 appeared to be the most reliablespecific antigenic marker as assessed by the immunoblotting technique with peroxidase-labeled secondantibodies. Antibodies to this antigen were never detected in hyperimmune rabbit anti-T. phagedenis sera orin the sera of nonsyphilitic humans. In contrast, reactive antibodies were found in all syphilitic human sera andalso in liquor samples that were positive in the passive hemagglutination test. Differentiation betweenimmunoglobulin M and immunoglobulin G antibodies was directly possible by applying the respective specificsecond antibodies. Immunoblotting tests were performed with sera exhibiting low passive hemagglutination testtiters and equivocal-fluorescent treponemal antibody and rapid plasma reagin card reactions. In more than60% of these cases, immunoblot positivity with respect to the 17,000-molecular-weight antigen was found. Thesame results were obtained with partially purified 17,000-molecular-weight antigen. The immunoblot techniqueshould be useful as an additional diagnostic tool for differentiating between true and false-positive serologicalreactions.

The analysis of the humoral response to protein antigensof Treponema pallidum and T. phagedenis has made greatprogress through the recent application of immunoblottingand electroimmunoassay techniques (1, 2, 4-6, 8-12). West-ern blotting with 1251I-protein A led to the definition of 8 (8) to22 (6) T. pallidum antigens reacting both with rabbit andhuman antibodies (4, 5, 8). Cross-reactions between antibod-ies to T. phagedenis and several T. pallidum antigens havebeen previously reported (4, 8, 12). Although T. pallidum-specific antigens do seem to exist, general consensus has notyçt been reached regarding their number and molecular size.Hanff et al. (5) defined 14 T. pallidum-specific polypeptides,whereas Lukehart et al. (8) described 3 specific bands withmolecular weight (MW) of 12,000 (12K), 14K, and 48K.Three specific antigens were also identified by Pedersen etal. by electroimmunoassays (10, 12). In all cases, normalhuman sera were reported not to contain antibodies to thesemoieties.The objective of the present study was to examine the

potential usefulness of the sodium dodecyl sulfate-poly-acrylamide gel electrophoresis (SDS-PAGE) immunoblotsystem for aiding the diagnosis of human syphilitic infec-tions. We used peroxidase staining of immunoblots and firstexamined the cross-reactivities of rabbit antisera to T.pallidum and T. phagedenis obtained by experimental infec-tion or immunization. We found that a paired band of T.pallidum antigens with MW of ca. 14K and 17K probablyrepresent an easily purifiable, specific determinant for im-munoblot analyses. Antibodies to these antigens becamedetectable after day 10 of experimental syphilitic infectionsand persisted over the entire period of observation (1.5 to 2years). The 14K and 17K antigens were not recognized byhyperimmune rabbit antisera to T. phagedenis or by normal

* Corresponding author.

human sera. In contrast, the antigens were recognizedwithout exception by human antibodies in 130 syphilitic seraexamined. The immunoblot procedure is rapid and simpleenough to be performed in routine serological laboratoriesand readily permits differentiation between immunoglobulinM (IgM) and IgG antibodies in unfractionated sera. Its use isadvocated particularly in cases in which the "classical"passive hemagglutination test (TPHA) and fluorescent tre-ponemal antibody-absorbed (FTAabS) reactions yielded bor-derline, equivocal results.

MATERIALS AND METHODS

Antigen preparations. Adult male rabbits were used for thepassage of T. pallidum. All preimmune sera were found to befree of antibodies against treponemes. Rabbits were infectedintratesticularly with T. pallidum (Nichols) and treated withprednison (1 mg/kg) on day 3, 5, 7, and 9 postinfection. Whenorchitis developed, the rabbits were sacrificed with a lethalinjection ofpentobarbital sodium. Testes were removed asep-tically, sliced, and washed repeatedly with 10 ml of RPMI1640 medium (Flow Laboratories, Irvine KA 12 8 NB, Scot-land). Cellular debris was removed by centrifugation at 1,400x g for 30 min at 4°C. The supernatants were decanted,pooled, and centrifuged at 16,000 x g for 20 min at 4°C. Thepellets containing the treponemes were washed thrice with0.2 M phosphate-buffered saline (pH 7.4) containing 1 mMdithioerythrit. The treponemes were finally concentrated ca.100-fold to the primary volume by centrifugation.

Suspensions of T. phagedenis, biotype Reiter, were pre-

pared by cultivation in Spirolate broth (BBL Laboratories,Cockeysville, Md.) supplemented with 10% heat-inactivatednormal rabbit serum at 37°C for 5 days under anaerobicconditions in the dark. Organisms were harvested by cen-

trifugation at 16,000 x g for 20 min and washed thrice withphosphate-buffered saline-1 mM dithioerythrit. To all prep-

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DETECTION OF SPECIFIC HUMAN ANTIBODIES TO T. PALLIDUM 83

arations, 1 mM polymethylsulfonyl fluoride was finallyadded.

Rabbit immune sera. Syphilitic rabbit sera were obtainedfrom rabbits infected as described above at day 10 and laterpostinfection. These rabbits received no prednison. Somerabbits were boostered with washed treponemes (1 mg/ml)suspended in saline and admixed with 1 volume of Freundincomplete adjuvant (Difco Laboratories, Detroit, Mich.).

Antisera against T. phagedenis were obtained by repeatedimmunization with washed T. phagedenis (1 mg/ml) admixedwith 1 volume of incomplete Freund adjuvant. Antigen wasadministered intracutaneously every second day during week1 and a total of three booster injections were generally givenat time intervals of 2 to 3 weeks. Hyperimmune sera wereobtained on day 5 after the final boost.Human serum and liquor samples. Serum and liquor sam-

ples were obtained from the serological laboratory of ourinstitute. Rapid plasma reagin (RPR), FTAabS and TPHAtiters were determined by routine procedures. The results ofthe FTAabS assays were expressed as (+) to indicate border-line reactions or as +, + +, and + + + to indicate one, two,and three to four positive reactions, respectively. Serologi-cally negative sera were obtained from blood donors with noevidence or history of syphilis, and these sera were negativein all three tests.SDS-PAGE and immunoblotting. Treponemal samples were

given 2% SDS-25% sucrose, boiled for 30 s, and electropho-resed on 12.5% polyacrylamide gel slabs with a discontinu-ous gel and buffer system (7) as previously described (3).Gels were calibrated with the Pharmacia (Uppsala, Sweden)low-molecular-weight marker kit containing phosphorylaseb (94K), bovine serum albumin (67K), ovalbumin (43K),carbonic anhydrase (30K), soybean trypsin inhibitor (20.1K),and a-lactalbumin (14.4K). Gels were stained for 2 h at roomtemperature in 0.2% Coomassie blue R 250 in 50%ethanol-10% acetic acid and destained for 1 to 2 h in 50%ethanol-10% acetic acid, followed by several hours in 10%acetic acid.

Electrophoretic transfer of proteins from the gels to nitro-cellulose paper (13) (0.45 Ftm; Sartorius, Gottingen, FederalRepublic of Germany) was performed with a modification ofthe Western blotting technique as previously described (2a).The gel and nitrocellulose paper were sandwiched betweenScotch Brite pads and plastic grids. Electrophoretic transferwas performed for a minimum of 3 h at 0°C at a voltage of130 V. The buffer was 8 mM Tris-64 mM glycine-20%(vol/vol) methanol. Transfer was controlled by staining thepaper blots with amido black, and the blotted gels were alsostained subsequently with Coomassie blue. After proteintransfer, the blots were incubated for 30 min in Tris-bufferedsaline (TBS; 50 mM Tris, 150 mM NaCl [pH 8.5]) containing0.5% (vol/vol) Tween 20 to block unoccupied sites. The blotswere then transferred to TBS buffer containing 0.05% Tween20, and the serum to be tested was added in a final dilution of1:10 or 1:100. The total volume ofeach serum assay was 6 to8 ml. All incubations were performed at room temperatureunder gentle agitation. After overnight incubation with theantisera, the blots were washed four times in TBS-0.05%Tween 20 and then incubated with peroxidase-conjugatedsecond antibodies (90 min; dilution, 1:500). The followingantibodies were used: swine anti-rabbit IgG; polyvalentrabbit anti-human IgG, IgM, and IgA; rabbit anti-human IgG(-y-chain specific); and rabbit anti-human IgM (,u-chain spe-cific) (all from Dakopatts Immunglobulins, Copenhagen,Denmark). The immunoblots were then washed thrice withTBS-0.05% Tween and transferred to 10 mM Tris-hydrochlo-

ride (pH 8.0) containing no detergent. They were developedwith a solution of 20 mg of 3-amino 9-ethylcarbazole (dis-solved in 2 ml of dimethylformamide) in 50 ml of 0.1 MTris-hydrochloride (pH 8.0) given 30 jxl of 30% H202. Afterdevelopment, the blots were rinsed with TBS buffer andphotographed.

Partial purification of the 17K antigen from T. pallidum. T.pallidum suspensions in 0.2 M phosphate-buffered saline-1mM dithioerythrit-1 mM polymethylsulfonyl fluoride wereincubated for 1 h with 1% (vol/vol) Triton X-100. Thesuspensions were centrifuged at 30,000 x g for 15 min, andthe supernatants were recovered. The antigen solutionswere then equilibrated in 50 mM NaCl-25 mM Veronal (pH7.0) by a passage through Sephadex G-25 (PD 10 columns;Pharmacia) and applied to a DEAE-Sephacel column (5 by1.2 cm) equilibrated in this buffer. The 17K antigen passedthe column and was collected in a total of 8 ml after a columnwash with 10 ml of buffer. The antigen preparations wereconcentrated 10-fold with immersible CX-10 single-use ultra-filtration units (Millipore Corp., Bedford, Mass.) and thenchromatographed over an ACA 44 column (90 by 2.5 cm;LKB Laboratories, Bromma, Sweden) in 50 mM Tris-0.2 MNaCl (pH 9.0). Fractions of 1 ml each were collected, andthose containing the 17K antigen as detected by immunoblot-ting were pooled and used as the test antigen.

RESULTSSDS-PAGE immunoblotting of T. pallidum antigens devel-

oped with hyperimmune rabbit antiserum to T. pallidum andT. phagedenis. Figure 1 depicts SDS-PAGE patterns of T.pallidum antigens obtained by Coomassie staining (lane a)and by immunoblotting with hyperimmune rabbit antiserumto T. pallidum (lane b) and T. phagedenis (lane c). Theresults were similar to those reported by Hanff et al. (4) andLukehart et al. (8). However, the number of T. pallidumantigens cross-reacting with anti-T. phagedenis antibodies

90K - _MMU

60K -54K -

4-

-m

30K -

17.1K no14.4K _-

a b c

FIG. 1. (a) SDS-PAGE pattern of T. pallidum polypeptides afterelectrophoresis on a 12.5% gel and staining with Coomassie brilliantblue. The positions of the molecular weight markers are shown on

the left. (b) SDS-PAGE immunoblot pattern of T. pallidum antigensdeveloped with the homologous rabbit hyperimmune anti-T. pal-lidum serum. (c) SDS-PAGE immunoblot pattern of T. pallidumantigens developed with rabbit hyperimmune anti-T. phagedenisserum. The latter serum recognized a multitude of T. pallidumantigens but did not stain the 14K to 17K moieties.

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84 HENSEL, WELLENSIEK, AND BHAKDI

42$

17K- 1

14K--- r-'immmo.

a b c dFIG. 2. Antibody response of two infected rabbits to T. pallidum

polypeptides at day 10 (a and b) and month 3 (c and d) postinfection.The antisera had already recognized the 17K T. pallidum antigen atday 10 after infection.

exceeded those found previously (6, 8), and it is apparentthat such cross-reactions exist over the entire molecularweight region above ca. 30K.The 14K and 17K antigens appeared to be easily definable,

specific immunogens of T. pallidum, since they did not stainwith high-titered antisera to T. phagedenis (Fig. 1). Todetermine the stage at which antibodies to these moietieswould become detectable, we tested sera of rabbits aftervarious times postinfection. Whereas preimmune sera nevershowed 17K reactivity, antibodies to these antigens weredetectable already at day 10 postinfection (Fig. 2). Titersincreased thereafter and persisted throughout the entirecourse of our experiments (1 to 2 years). Preimmune rabbitsera sometimes recognized 90K, 60K, 42K, and 30K anti-gens of T. pallidum, and such nonspecific staining patternsresembled those frequently observed with nonsyphilitic hu-man sera (see below).

T. pallidum antigens recognized by syphilitic human sera. Atotal of 130 sera exhibiting TPHA titers of .1:256 andpositive FTAabS and RPR reactions were examined. The seracomprised seven groups, i.e., (i) 3 cases of treated, primarysyphilis, (ii) 6 cases of untreated secondary syphilis, (iii) 29cases of treated secondary syphilis, (iv) 10 cases of strongseropositivity without typical clinical symptoms of syphilis,(v) 1 case of tabes dorsalis, (vi) 1 case of neurosyphilis, and(vii) cases for which a precise clinical history could not beobtained.

Irrespective of the clinical stage of infection, the 17Kantigen was recognized by these sera without exception; the14K antigen showed some variation. Sera developed withanti-IgG second antibody that reacted strongly with the 14Kand 17K antigens generally showed strong accompanyingreactions, with two additional doublets of ca. 40K and 43Kand 54K and 60K and, more variably, with bands between43K and 54K MW. These results are basically consistentwith the report of Hanff et al. (5), although the differencesreported to exist among human sera at different stages ofsyphilitic infection were not so pronounced as to allow adistinct classification in our experience. The major emphasisof this study is placed on the recognition that the 17K moietyprobably represents the most reliable single antigenic markerin human syphilitic infections. The slight variations in appar-ent MW of other antigens compared with those given in

previous studies (4-6, 8) are likely to derive from methodol-ogy and are not regarded as basically relevant.The immunoblots of Fig. 3 were prepared with T. pallidum

as well as T. phagedenis antigens (lanes a and b, respec-tively). Many T. phagedenis antigens were recognized espe-cially by the high-titered syphilitic sera. Of these, one T.phagedenis antigen of ca. 14K and 15K MW also stainedoccasionally. This finding should not be taken as a contra-diction to the contention that the 14K and 17K T. pallidumantigens are species specific, since similar migration inSDS-PAGE cannot be equated with immunological identity.The 14K and 15K T. phagedenis antigen is, indeed, probablynot antigenically related to its counterparts in the T. pal-lidum system, since high-titered rabbit antisera prepared toT. phagedenis stain the homologous antigen (data not shown)but do not stain the respective T. pallidum antigens (Fig. 1).

Differentiation between IgG and IgM antibodies in unfrac-tionated sera were simply and directly achieved by using theappropriate commercially available second antibodies. Serapositive for IgM in the FTAabS test were also IgM positivefor the 17K antigen in the immunoblot; no exceptions wereever noted. Figure 4 depicts the identification of IgM anti-bodies in four different sera of patients with untreatedsecondary syphilis. Additional experiments were also per-formed in which the IgM fractions were recovered afterseparation for IgG by sucrose density gradient centrifuga-tion, and the same results were obtained (data not shown).

Analysis of liquor samples. We analyzed a total of 30different liquor samples. Nine were from nonsyphilitic pa-tients and served as controls. Six samples were from pa-tients exhibiting seropositivity in the FTAabS, TPHA, andimmunoblot tests, but whose liquor samples were negativein the former assays. No positivity in the staining of the 14Kand 17K moieties was found in any of these 15 cases with theimmunoblot technique (data not shown). In contrast, 12liquor samples with TPHA titers ranging from 1:64 to 1:512all recognized the 14K and 17K antigens, and the respectiveantibodies were identified as IgG (Fig. 5). IgM antibodieswere not detected in any of these samples. Three final liquorsamples that were judged positive by the immunoblot assaywere of particular interest, because these samples had been

A

60K-54K--b-

42 >-:Z ,. .,--:F

17K_-o mm

a bRPR undil.+TPHA 1:128FTAabs (+)

By

a1:41:512

c oe

-ou1

tt-;l

10 '}>

IM_Su .,.I _s_

b a b1 8

1:102444+

a b1: 64

1:32000

FIG. 3. Typical immunoblot patterns of human syphilitic sera (Ato D) that showed different reactions in the TPHA, FTAabs, andRPR tests. Serum was from a patient with primary syphilis (A) andsera from patients with secondary syphilis (B to D) are shown. T.pallidum extracts (lanes a) and T. phagedenis extracts (lanes b) wereapplied. All syphilitic sera recognized the 17K T. pallidum antigen.The sera also reacted with some antigens of T. phagedenis.

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DETECTION OF SPECIFIC HUMAN ANTIBODIES TO T. PALLIDUM 85

A

t

B c ...D

4

17K ---- -

14K--o -

a b a b a b a bFIG. 4. Detection of IgM antibodies to T. pallidum antigens in

unfractionated human sera. T. pallidum extracts were electropho-resed, and immunoblots were developed with the sera of fourpatients (A to D) exhibiting positive IgM titers. The second antibod-ies used were rabbit anti-human immunoglobulin (IgG, IgM, andIgA; (lanes a) and specific rabbit anti-human IgM (lanes b). Note thedetection of IgM antibodies present against certain T. pallidumantigens, including the 17K moiety.

graded negative in the classical TPHA, FTAab,, and RPRtests. One sample was from a patient with tabes dorsalis, onewas from a patient with lues latens seropositiva, and one wasfrom a lues seropositiva acquired immunodeficiency syn-

drome patient.Reactions of serologically negative human sera. Hanff et al.

(6) have reported the reaction of nonspecific human antibod-ies present in the sera of healthy individuals with fourcommon antigens of T. pallidum and T. phagedenis (MW of30K, 33K, 40K, and 45K), of which two may represent axialfilament proteins. We examined a total of 30 different seraand a serum pool from 100 donors that were negative inTPHA and FTAab, tests. In some cases, we found noreactivities with T. pallidum antigens; the immunoblots wereentirely blank. In other cases, reactions with 43K and 54Kantigens occurred; slight staining was also sometimes de-tected on 30K and 90K bands (Fig. 6). In contrast to Hanff(5) and Lukehart (8), we often found a remarkably strongreaction with the 60K antigen (Fig. 6, lanes e, f. and g).Thus, the 60K antigen, although strongly reactive with all

a b c d e

17K

14K

FIG. 5. T. pallidum antigens recognized by antibodies present infive liquor samples of syphilitic patients (a to e). Second antibodieswere rabbit anti-human immunoglobulin. The 17K antigen was

recognized in all cases, and isolated staining of this moiety was

found in one case (d).

17

14K---_

e

-o60K_-54K

_*-42K-40K

-17K

: -a 14K

h i k

FIG. 6. T. pallidum antigens detected by nonspecific antibodiespresent in 10 different negative human sera (a to k). The reaction ofa positive serum is shown for comparison (i). None of the negativesera recognized the 17K antigen. The 60K antigen was stainedheavily in three cases (e, f. and g).

syphilitic sera tested, does not appear to represent a specificantigenic marker for T. pallidum. The 17K antigen was, incontrast, never detected by any of the negative sera.

Immunoblotting results obtained with sera exhibiting bor-derline serological reactions. The above results led us tosuspect that the 17K antigen probably represents the mostreliable single antigenic marker for diagnosis of humansyphilitic infection. Clear positivity with respect to thismoiety was tentatively taken to indicate the presence oftruly specific antibodies to T. pallidum. Applying this crite-rion, we then examined 147 sera that exhibited borderlineserological reactions. Cases included sera that showed iso-lated TPHA titers of 1:64 and 1:128 (negative FTAab, andRPR titers), and cases exhibiting equivocal RPR, FTAab,, orboth reactions. The sera were grouped into five categories(Table 1). More than 60% of the sera developed withpolyvalent anti-IgA, -IgM, and -IgG second antibodies clearlyrecognized the 17K antigen; this generally coincided withfaint recognition of other bands, in particular the 54K and60K doublet, that constituted the typical pattern of positivesyphilitic sera (Fig. 7; cf. Fig. 3). These almost certainlyrepresented true syphilitic sera. The other sera did notrecognize the 17K antigen and showed varying recognitionof other bands (Fig. 7, lane a). These sera were provisionallyclassified as false-positive. In one case, a series of serum

TABLE 1. Immunoblot reactivity of 147 serum samplesexhibiting borderline serological reactions with the 17K antigen of

T. pallidumNo. (%) showing

Results with: reactivity withGroup No. of 17K antigen

TPHA samplesRPR (tier) FTAabs Positive Negative

A + 1:64/1:128 + 27 26 1(96.3%) (3.7%)

B + 1:64/1:128 (+) 6 6 0(100%)

C 0 1:64/1:128 + 38 32 6(84.2%) (15.8%)

D 0 1:64/1:128 (+) 42 18 24(42.9%) (57.1%)

E 0 1:64/1:128 0 34 12 22(35.3%) (64.7%)

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86 HENSEL, WELLENSIEK, AND BHAKDI

A B,.

g2.Immetml4K i

40K--O

17K----

14 Y, --* -"_*im -

RPRTPHAFTAabs

a bB1:64

a b

1:64*r

FIG. 7. Immunoblotting patterns obtained with two sera (A andB) showing equivocal serological reactions (see Table 1) selected toillustrate the clear differences observed by the immunoblot tech-nique. T. pallidum antigens (a) and T. phagedenis antigens (b) wereapplied. Both sera were negative in the RPR and FTAab, tests, andboth exhibited TPHA titers of 1:64. However, one serum (A) was

clearly negative with respect to 17K immunoblot reactivity, whereasthe other serum (B) was positive. Both sera reacted with antigens ofT. phagedenis.

samples was obtained over a period of 3 years from a youngfemale with no history of syphilis, but with a constant TPHAtiter of 1:64. The serum of this patient also never recognizedthe 17K antigen in the immunoblots. A closer inspection ofthe results with immunoblotting (Table 1) indicates that, as

might be expected, 17K positivity was found in virtually allcases in which all three serological reactions (TPHA,FTAabs, and RPR) were weakly positive. Because one ortwo of these classical reactions became negative or equivo-cal, the percentage of cases exhibiting 17K negativity rose.It appears significant that in group E which comprises caseswith isolated, low TPHA titers, immunoblot positivity wasstill clearly detected in 35% of the tested sera.A clinical correlation could not be obtained for the major-

ity of these patients, because the sera had been selected outof a random collection spanning 4 years solely on the basis oftheir low seropositivity which were generally accidentalfindings. However, there were a few interesting exceptions.In groups A and C, two patients were retrospectively foundto represent cases of sufficiently treated primary syphilis;their sera contained no IgM antibodies. In groups B and C,there were two cases of old, treated infections (without IgMantibodies) and two cases of fresh, untreated primary syph-ilis that had been diagnosed clinically. These latter two sera

were indeed found to contain specific IgM antibodies by theimmunoblotting technique.Immunoblotting with isolated 14K and 17K antigen. A

single passage of a Triton X-100 treponemal extract over

DEAE-Sephacel at pH 7 led to the recovery of the 14K and17K antigens in satisfactorily pure form as assessed byimmunoblotting. We used this antigen preparation to exam-

ine negative and positive sera and consistently found correctreactions (Fig. 8). The amount of purified antigen to beapplied was approximated from comparative stainings withdefined positive sera, and overloads were avoided to circum-vent any danger of nonspecific staining reactions with neg-

ative sera. Under these conditions, none of the tested 20seronegative sera yielded a positive immunoblot, whereas

positivities were observed with all tested positive samples (n= 20).

DISCUSSIONOur investigations were launched with the primary intent

of examining the possible utility of SDS-PAGE immunoblot-ting in the serological diagnosis of human syphilitic infec-tions. As this work was underway, a number of reportsappeared that were, on the whole, consistent with the dataobtained here. We utilized peroxidase-labeled second anti-bodies rather than 1'25-protein A since the antibodies allowmore complete detection of human IgG antibodies and alsopermit straightforward differentiation ofthe antibody classes.The possibility of occasional artifacts generated by rheuma-toid factor in the detection of IgM antibodies has not beenexcluded and may require further investigation. At present,the immunoblot method appears reasonably simple to per-form, avoids the use of radioactivity, and is more rapid andprobably at least as sensitive as the protein A method. Thereare distinct advantages over the use of internally labeledTreponema antigens (9). The latter method requires thepreparation of radiolabeled antigen, which is cumbersome.Moreover, preferential incorporation of a given label intocertain antigens would obviously generate serious interpre-tational difficulties, and sensitivity would be markedly re-duced with regard to those antigens which incorporate littlelabel. Such considerations most probably explain the dis-crepancies between this and the cited study (9). In particu-lar, the dominance of antibodies to the 14K and 17K antigenswas not noted with the radioprecipitation assay. We note,however, that our results are in very good accord with thoseof Hanff et al. (5), who also consistently found a strongantibody response to a 15.5K antigen of T. pallidum inhuman sera during all stages of syphilitic infections with theimmunoblot method.The collective data obtained from a study with a large

number of syphilitic sera now lead us to conclude thatantibodies to the low-MW 14K and 17K T. pallidum antigensindeed represent most reliable immunological markers forhuman syphilitic infections, irrespective of clinical stage.Thus, hyperimmune rabbit antisera to T. phagedenis, al-though cross-reacting with a multitude of at least 14 antigensof T. pallidum, did not react with the 17K antigen. Second,rabbits experimentally infected with T. pallidum showed

l 2 3 4 5 6 7 8 9 O

RPR 4 4 1:2 1:2 1.2 udit.+ f 1:64 1:64 1:128TPHA & 1:64 1:128 1:256 1:256 1:512 1:2048 1:20481:4096 1:4096FTAaÉ e& .# + *+++ +++ + 4- ++ .+ + 4++

FIG. 8. Reaction of eight syphilitic sera (lanes 3 to 10) and twononsyphilitic sera (lanes 1 and 2) with partially purified 14K and 17Kantigens of T. pallidum.

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DETECTION OF SPECIFIC HUMAN ANTIBODIES TO T. PALLIDUM 87

positive reactivity (defined as the appearance of a clearlyvisible band on the nitrocellulose blots) already commencingat day 10 postinfection and persisting throughout the courseof our studies. Moreover, sera obtained from seven patientswith treated or untreated primary syphilis also exhibitedpositive reactions. Third, all human syphilitic sera tested, aswell as 15 syphilitic liquor samples, reacted with this anti-gen. Positivity to the 17K moiety was accompanied by atypical pattern comprising four bands of ca. 40K, 43K, 54K,and 60K MW. Data of Hanff et al. (5) have previouslysuggested an existing correlation between the immune re-sponse to these antigens and the stage of syphilitic infection.Our present analyses of the IgG responses have not yieldedsuch clear-cut patterns, and this issue has not been pursuedfurther in our laboratory. Finally, 17K reactivity was notfound in any of the 30 serologically negative sera examined.The usefulness of immunoblotting in the routine serologi-

cal laboratory became apparent when sera exhibiting bor-derline reactivities in the classical reactions were examined.These comprised five categories (Table 1). Immunoblotpositivity with respect to the 17K antigen was found invirtually all cases in which three classical reactions wereslightly positive. If RPR or RPR and FTAabs reactions werenegative, the percentage of serum samples not recognizingthe 17K antigen increased. Negativity with regard to thisantigen was generally accompanied by nonreactivity to-wards the 40K, 43K, 56K, and 60K pattern typically recog-nized by antibodies of syphilitic patients. It is for theevaluation of such sera showing equivocal TPHA, RPR, andFTAabs reactions that we would advocate the use of theimmunoblotting method. Whole extracts of T. pallidum canbe used for these analyses. However, it may turn out to bemore convenient to work with isolated antigen preparations,e.g., in an immunodot enzyme-linked immunosorbent assay.Initial results with partially purified 14K and 17K antigenpreparations have been encouraging. Nevertheless, use ofthe whole SDS extracts may be required in cases of weak17K positivity. In such cases, positivity with respect to the54K and 60K and the 40K and 43K doublet should be takenas an additional criterion in the evaluation. Interpretation ofthe results should be unidirectional until otherwise provenby an extended series of analyses. Thus, we provisionallygrade 17K positivity as reflecting the presence of specificantibodies to T. pallidum, but do not yet conclude thatnegativity proves their absence. Based on this criterion, apositive reaction in 64 of the 147 borderline cases was found.Of these, two patients had just been clinically diagnosed ashaving primary syphilis, and four were retrospectively foundto have been treated for syphilis in the past. It is clear that anassay for IgM antibodies is essential for diagnosing freshinfections and, indeed, these antibodies were found in thetwo cases of fresh infections cited above. In conclusion, webelieve that the application of the immunoblot technique

provides a significant extension over existing serologicalmethods for the diagnosis of human syphilitic infections.

ACKNOWLEDGMENTSWe thank Heike Welle for excellent technical assistance.These studies were supported in part by funds from the Deutsche

Forschungsgemeinschaft (Bh 2/1-3,4) and the Verband der Chemi-schen Industrie.

LITERATURE CITED1. Baker-Zander, S. A., and S. A. Lukehart. 1983. Molecular basis

of immunological cross-reactivity between Treponema pallidumand Treponema pertenue. Infect. Immun. 42:634-638.

2. Baughn, R. E., C. B. Adams, and D. M. Musher. 1983. Circu-lating immune complexes in experimental syphilis: identificationof treponemal antigens and specific antibodies to treponemalantigens in isolated complexes. Infect. Immun. 42:585-593.

2a.Bhakdi, S., M. Muhly, and R. Füssle. 1984. Correlation betweentoxin binding and hemolytic activity in membrane damage bystaphylococcal a-toxin. Infect. Immun. 46:318-323.

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5. Hanff, P. A., T. E. Fehniger, J. N. Miller, and M. A. Lovett.1982. Humoral immune response in human syphilis to polypep-tides of Treponema pallidum. J. Immunol. 129:1287-1291.

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9. Moskophidis, M., and F. Muller. 1984. Molecular analysis ofimmunglobulins M and G immune response to protein antigensof Treponema pallidum in human syphilis. Infect. Immun.43:127-132.

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11. Pedersen, N. S., N. H. Axelsen, and C. S. Petersen. 1981.Antigenic analysis of Treponema pallidum: cross-reactions be-tween individual antigens of T. pallidum and T. reiter. Scand. J.Immunol. 13:143-150.

12. S. Petersen, C., N. S. Pedersen, and N. H. Axelsen. 1982.Purification of a Reiter treponemal protein antigen that isimmunologically related to an antigen in Treponema pallidum.Infect. Immun. 35:974-978.

13. Towbin, H., T. Staehelin, and J. Gordon. 1979. Electrophoretictransfer of proteins from polyacrylamide gels to nitrocellulosesheets: procedure and some applications. Proc. Natl. Acad. Sci.U.S.A. 76:4350-4354.

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