Demonstration and partial characterization of parasite specific immunoglobulin A responses in human strongyloidiasis

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JOURNALOF CLINICALMICROÈIOLOGY, Aug. 1987, p. 1505-1510 0095-1137/87/081505-06$02.00/0

Demonstration and

Partial Characterization of Parasite-Specific

Immunoglobulin A Responses in Human Strongyloidiasis

ROBERT M. GENTA,l* DONALD F. FREI,' AND MICHAEL J. LINKE2

Departments of Pathology and Laboratory Medicinel andDivision of Infectious

Diseases,2

VeteransAdministration MedicalCenter and

University

of

Cincinnati

College

of

Medicine, Cincinnati,

Ohio 45220

Received 10November1986/Accepted 6 May 1987

By usinganenzyme-linkedimmunosorbentassay, serumimmunoglobulinA (IgA)responsesdirected against Strongyloides stercoralis larvae antigensweremeasuredin104 presumably immunocompetent individualswith

chronic uncomplicated strongyloidiasis and in 15immunocompromised patients with S. stercoralis infection. Fiftyhealthy North American adultsand18 patients with otherhelminthic parasites servedascontrols. All50

healthy controls were negative for antibody responses(mean absorbance standard deviation = 0.0724

0.040). Themeanabsorbance ofthe 18parasitized controlswas0.230± 0.087;twoindividuals parasitizedby

Ascaris lumbricoides showed positive antibody responses. The mean absorbance of the immunocompetent

patientswithstrongyloidiasiswas0.680±0.364,with91subjects (87.5%) havingapositivevalue(>0.300).Of

theimmunocompromised patients(meanabsorbance ± standarddeviation = 0.735 ± 0.538),11 (73%) hada

positive antibodyresponse test. WhentheIgA responsesof thesetwogroups werecompared, theywerenot significantlydifferent. Therewas nocorrelationbetweenthe levels of totalserumIgA and the concentrationof

specific IgAinthe infected patients. Both IgA andIgGimmunoreactive bandsweredetectedonimmunoblots ofsodiumdodecyl sulfate-polyacrylaniide gelelectrophoresis-separated larval antigen protein blots. Nineteen bandswere recognized by IgG, and 13wererecognized by IgA fromseraof infected patients. Several bands

displayedspecific IgGorIgA reactivity. Thepresentwork shows thatmostpatients with strongyloidiasismount specific IgA responses against filariform larval antigens. These responses are, for the most part, directed againstantigensthataredifferent from thoserecognized by IgG. The lack ofcorrelation betweenthe magnitude

of thespecificserumIgAresponsesand theclinicalaspectsof theinfection suggeststhat these antibodiesmay notplay acentral role inthe regulation ofthisparasitosis.

Theincreasinguseofimmunosuppressive drugs and cyto-toxic regimens and the present pandemic of the acquired immunodeficiency syndrome are changing the clinical and

epidemiological features of certain parasitic diseases. The concept of "opportunistic parasite" is now accepted, its

foremostrepresentative being Pneumocystis carinii,nowthe

leadingcause of deathinpatients with acquired immunode-ficiency syndrome (28).

Strongyloides stercoralis, because of its unique abilityto replicate within its host, is the only nematode capable of opportunistic behavior (5, 13).This insidiousparasite, which

may persist indefinitely as a silent guest in its human host (20),isnow oneofthemostcommonly diagnosed helminthic parasitesintheUnitedStates,both in rural(29)and in urban (27)areas. InEurope, wherehundreds ofimported,as well as autochthonous, cases have been reported, endemic foci

have beenidentified in several countries (4, 23).

As fatal disseminated strongyloidiasis begins to make its appearance in acquired immunodeficiency syndrome pa-tients (21; M. Wittner and H. B. Tanowitz, personal

com-munication), the need for a better understanding of the relationship between host and parasite is becoming more

pressing.

In an effort to clarify the immunological aspects ofthis relationshipand to providebettertools forthe diagnosis of this elusive nematode, wehave previously studied the

par-asite-specific cellular responses in individuals with

uncom-plicated strongyloidiasis (9)and theimmunoglobulinG(IgG)

*Correspondingauthor.

(11)andIgE (19)responsesinpatients with chronic,

uncom-plicated strongyloidiasis and in a small group of

immuno-compromised patients withorwithout dissemination (7). Because ofits intestinal localization and its continuous recycling through the lungs, S. stercoralis is likely to elicit local and systemic IgA responses. To ourknowledge,

how-ever, noattempthadbeenmadeto measure orcharacterize these responses. In the present study, we focused on the characterization of serum IgA against larval antigens in a

large number ofpatients with differentforms of strongyloi-diasis. These data were used to explore the relationship between the magnitudeof these responses, thepresence of otherclasses ofparasite-specific antibodies, and theclinical

courseof the infection.

MATERIALS ANDMETHODS

Study population. The sera of individuals from three

groupswerestudied.Group 1 (infected patients)consistedof 119 adults with parasitologically documented S. stercoralis infection. Theirdemographic characteristics aredepicted in Table 1. In this group 104 patients were apparently im-munocompetent, and 15 were immunocompromised. Of

these 15immunocompromised patients, 10 had autoimmune diseases forwhichtheywerereceivingcorticosteroids,2had untreated disseminated cutaneous leishmaniasis, 1 had re-ceived chemotherapyfor cancer, 1 had idiopathic acquired agammaglobulinemia, and 1 had the acquired immunodefi-ciency syndrome. Seven of these patients had evidence of

hyperinfection (defined bythe identificationof S. stercoralis larvae in a site otherthan the digestive tract); three of the 1505

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1506 GENTA ET AL.

TABLE 1. Demographic characteristicsof the 119patients with strongyloidiasis studied

Characteristic& Totalno.of %

with

individuals

characteristic

S.stercoralis infection 119 100

Sex

Male 78 65

Female 41 35

Area of residence

Latin America 56 47

Southeast Asia 28 23

United States 35 30

aMedian age was 32 years (range, 17 to 84years).Individuals settledinthe

United States for less than 3 years at the timie ofdiagnosis are listed as

residents of their originalgeographicarea.

patients died as a consequence of disseminated strongyloi-diasis. Ofthe 15immunocompromised patients,7havebeen described previouslyinastudydesignedtoanalyze parasite-specific IgG and IgE responses (7). Group 2 (parasitized controls) consisted of18 adults infected with one or more

parasiteother than S.stercoralis(Ancylostoma

duodenale,

8 patients; Ascaris lumbricoides, 3 patients; Trichuris trich-iura, 2patients; Schistosoma mansoni, 1 patient; intestinal protozoa, 8 patients). Group 3 (noninfected controls) was composed of 50 healthy North American blood donors presumably not infected with any parasite. This study was approved by the University of Cincinnati Human Research Committee.

Immunologic studies. (i) Antigens. Somatic S. stercoralis larval antigen was prepared as described elsewhere

(9).

Briefly, third-stage filariform larvae were collected from fecalculturesobtainedfrombeagles

experimentally

infected withahumanstrainofS.stercoralis.Thesedogs, free ofany other parasites, are maintained in our

laboratory

as larval donors (10). After several washings in deionized waterand one 5-min washing ina 0.25% solution of

Na3ClO

in

phos-phate-buffered saline (PBS) fordisinfection,the larvaewere suspended in sterile PBS andfragmentedbya2-min sonica-tion at 4°C in wet ice (Sonifier; Branson Sonic PowerCo., Danbury, Conn.). The suspension with fragmented larvae wasextracted overnight at4°Cin PBS and thencentrifuged

athigh speed (18,000 x g)for 30min. The supernatantwas then passed through a 0.45-,um-pore-size membrane filter (Millipore Corp., Bedford, Mass.), divided into equal por-tions, andfrozen at -70°C for storage.

(ii) ELISA. IgA directed against S.stercoralislarval anti-genswas measuredby using an enzyme-linked immunosor-bent assay (ELISA) adapted from a method previously described for the measurement ofparasite-specific IgG (6). Briefly, alternating rows of flat-bottom microtiter plates (Dynatech Laboratories, Inc., Alexandria, Va.) were sensi-tized with S. stercoralis larval antigen at a concentration of 5,ug/mlof PBS. Each serum was tested at the single dilution of 1:8 both in a well with antigen and in a well without antigen. This dilution was selected because, in preliminary experiments, it provided the best discrimination between infectedpatients and controls. The test was then completed by using goat anti-human IgA conjugated with alkaline phosphatase (Kirkegaard and Perry Laboratories, Gaithers-burg, Md.) at a dilution of 1:1,000 (determined to be optimal by blocktitration). The specificity of this conjugated antise-rum wasconfirmed by immunodiffusion and by the ELISA.

p-Nitrophenylphosphate

disodium saltwas usedasthe

sub-strate. The net

absorbancy

of each serum was determined with an automatic ELISA reader

(Artek Systems,

Farmingdale, N.Y.)

programmed

tocalculate thenet

absorb-ance

by substracting

the absorbance of the well without

antigen from that ofthe sensitized well. Three

highly

posi-tive sera, one

positive

poolof sera, and three

negative

sera

were

repeatedly

testedtomonitorthe

interassay

variability,

which didnotexceed 7%.

(iii)

SDS-polyacrylamide gel

electrophoresis.

S. stercoralis

antigen

was

prepared

for sodium

dodecyl

sulfate

(SDS)-polyacrylamide gel electrophoresis by diluting

1.0 ml of

antigen (at

a concentration of 1.0

mg/ml)

with 1.0 ml of

sample

buffer

(4% SDS,

0.1 M Tris

[pH 6.8],

2%

glycerol,

10%

2-mercaptoethanol)

and

boiling

the

preparation

for 3

min. After

cooling,

150 ,ug ofthis

preparation

was

electro-phoresed

in a 0.75-mm-thick

preparative

discontinuous

SDS-15%

polyacrylamide

gel (15).

Molecular

weight

mark-ers were runinaseparatewelloneach

gel.

Electrophoresis

wascarriedout at 25 mA per

gel

for3 h.

(iv) Detection of

SDS-polyacrylamide gel

electrophoresis-separated antigens. Protein

blotting

and

immunoblotting

were

performed by

amodification ofthe transfer

technique

described

by

Towbin et al.

(26).

Separated proteins

were

transferred from the

gel

to nitrocellulose paper

(0.45-ptm

pore

size)

inaTrans-Blotcell

(Bio-Rad

Laboratories,

Rich-mond,

Calif.)

containing

transfer buffer

(0.192

M

glycine,

0.025 MTris

[pH

8.3],

20%

[vol/vol] methanol) overnight

at 30

V,

followed

by

1 hat60V. Asectionof the

resulting

blot

containing

the molecular

weight

markers anda

portion

of the

separated

antigens

wasstained with fast green

(22).

The rest

oftheblotwasincubatedfor 1 h in 3.0%

gelatin

toblock the unreacted sites. The blocked blotswerethencut onto0.5-cm

strips.

Each

strip

was incubated

overnight

with a 1/10

dilution ofthe serum to be tested. Blots were then rinsed with deionized water and washed twice in Tris-buffered saline withTween 20. After anotherdeionized-water

rinse,

the blots were incubated for 2 h with

alkaline

phosphatase-conjugated

goat anti-human

IgA

or anti-human

IgG

(Kirkegaard

and

Perry

Laboratories).

The blots were then

rinsed and washed. The washed blots were

developed

in a

Nitro Blue

Tetrazolium-5-bromo-4-chloro-3-indolyl

phos-phate

development

system

(Kirkegaard

and

Perry

Laborato-ries). Apparent

molecular

weights

of the immunoreactive

bands were determined

by using

the method described

by

Shapiro

et al.

(24).

(v) Absorption studies. A

panel

of

positive

sera was

preabsorbed

with S. stercoralis with S. stercoralis soluble

antigens (at

concentrations of

20, 100,

and 200

ptg

of

antigen

per ml of

serum),

live filariform and rhabditiform larvae

(5,000

larva per

ml),

or the Ascaris serum-soluble

antigen

A671-901-185

(National Institutes

ofHealth research reagent

Asc-1)

(at concentrations of

20,

100, and 200

ptg

of

antigen

permlof

serum).

(vi)

Determination of

parasite-specific IgG

and IgE. The levels of

Strongyloides-specific IgG

were measured in all

patients by

the

ELISA,

as

previously

described

(7).

Results

are

expressed

asthenet

absorption,

a

figure

correlated with

theamount of

antibody

presentin the serum.

Specific-IgE

levelsweremeasured

by

a

previously

described

solid-phase

radioallergosorbent

assay(19). Resultsareexpressed asthe

percent

binding,

a value that

reflects

in alinear fashion the amountof

parasite-specific IgE

in theserumtested.

(vii)TotalserumIgAdetermination. Total serumIgAwas measured

by using

theBeckman

IgA

Reagentin

conjunction

with a Beckman

Immunochemistry

AutoanalyzerII

(Beck-J. CLIN.MICROBIOL.

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HUMAN STRONGYLOIDIASIS PARASITE-SPECIFIC IgA RESPONSES man Instruments, Inc., Brea, Calif.). Results are expressed

ingrams per deciliter of serum.

(viii) Statistical analysis. Datawere analyzed by usingthe analysis of variance, the chi-squaretest, Fjsher's exact test, the Wilcoxon nonpaired rank-sum test, linear regression, and thetwo-tailed Student t test, as appropriate.

RESULTS

Specific IgA. The absorbance values of all of the sera tested aredepicted in Fig. 1. Theabsorbance values of the 50 healthy controls ranged beween 0.02 and 0.216 (mean +

standard deviation [SDI = 0.0724 + 0.040). Patients with

other parasites only had absorbance values between 0.119 and 0.483 (mean ± SD = 0.230 + 0.087). Ofthese patients, 16 were negative. Two Latin American individuals parasi-tized by A. lumbricoideshadpositive values (absorbance = 0.483 and 0.378). The 104 immunocompetent patients with chronic uncomplicated S. stercoralis infection had absorb-ancevalues ranging from 0.041 to 1.780 (mean± SD=0.680 ± 0.364). Theabsorbance values ofthe 15 immunocompro-mised patients with strongyloidiasis ranged from 0.0490 to 1.845 (mean + SD = 0.735 ± 0.538). When the 104 im-munocompetent individuals with chronic, uncomplicated strongyloidiasis were compared with the 15 immunosup-pressedpatients for their parasite-specific IgA responses, no significant difference between the two groups was detected

(P

= 0.3054).

SpecificIgA values weresignificantly differentin the three geographic subgroups of immunocompetent patients (Table 2). BothLatin Americans and Southeast Asians had signifi-cantly higher specific IgA values than a group ofinfected patients who either were North Americans or had been residents oftheUnitedStatesfor at least 3 years (P < 0.001 by the analysis of variance). However, no difference was detectable when Latin Americans and Southeast Asians were compared with each other.

Positivity. The mean absorbance value ofall 68 controls (50 nonparasitized and 18 with other parasites) was 0.114, with an SD of 0.09.This mean plus 2 SD (0.294) was used as theupper negativelimit. Thus, an absorbance value of 0.300 orhigher wasconsidered positive.

Specificity studies. Preabsorptionwith S. stercoralis solu-ble antigens of sera from patients with strongyloidiasis resultedinanaverage82%decrease in theabsorbancevalue. Six of these sera werepreabsorbed with livefilariform and rhabditiformlarvae; the averagedecreasein theabsorbance valuewas

21%.

Preabsorption ofthe same serawithAscaris suum antigens did not significantly alter the absorbance values. When the sera oftwo

individuals

with A.

lumbri-coides infectionswerepreabsorbed withA. suum antigens, no significant variation ofthe absorbance value occurred. Because Asc-1 is an affinity-purified preparation and does not contain all Ascaris antigens, these results can only be interpreted as indicating that Asc-1 antigens are not major componentsofS. stercoralis.

Specific IgG and IgE. The results ofspecific IgG andIgE tests are summarized in Table 3. Specific IgG values were

similar in the 104immunocompetent patients and in the 15 immunocompromised individuals with

strongyloidiasis.

However, thelevels of

parasite-specific

IgE (as measured

by

theradioallergosorbent assay)were

significantly

lower in the immunocompromisedpatients thanin the

immunocompetent

patients (percent binding = 15.0 ± 16.1% versus 34.2 +

12.4%;

P<0.001). No

significant

differences weredetected

among the three

geographic

groups when their

parasite-i.8 1.7[

1.6 1.5 1.4 1.3 1.2 1.1 : 1.0

n_I2 .9

e .8

.7 .6 .5 .4 .3 .2 .1

+1

l..

j

*1::4

.I-.

,.

uninfected otherparasite

controls patients

S. stercoralis S.stercoralis

patients immunocompromised patients FIG. 1. Absorbance valuesobtained with a serum dilution of 1:8 for allindividuals tested. Barsindicate mean + SE ofthemean.

specific IgG and IgE responses were compared by the one-way analysisof variance.

Aweakcorrelation existed between the levels ofspecific IgGand specificIgA in theinfected patients(r2=0.2304); no correlation existed between the levels of specific IgA and

specific

IgE

(r2

= 0.0121).

Total IgA. The levels of total IgA were measured in all patients with strongyloidiasis, in the parasitized controls, and in randomly selected nonparasitizedcontrols. The IgA levels ofthe immunocompetent patients ranged from 7 to 1,326g/dl (mean ± SD = 227 ± 190). These values did not differfrom standard normal values obtained in our labora-tory(222± 129g/dl).Theimmunocompromised patients had totalIgA levels between 9 and 334g/dl(mean + SD = 121± 99g/dl).This valuewassignificantlylower than thatobtained in healthy persons and that of the immunocompetent pa-tients (P <0.05). Theparasitizedcontrols had values within the normal range (from87to 345g/dl;mean ± SD = 205 +

143). The total IgA levels of 23 apparently immunocom-petentU.S. residentswere significantly lower than those of all ofthe other groups (125 ± 85 g/dl;P < 0.05), with the

exception

ofthe

immunocompromised patients.

The mean

concentration of total IgAinagroupof18randomlyselected healthycontrolswaswithinnormal limits (235 + 149g/ml).

There was nocorrelation between the levels oftotal IgA and theconcentration of

specific

IgAin the infected

patients

(r2

= 0.1444).

Immunoblotting studies. IgA and IgG

immunoreactive

bands were detected on immunoblots of

SDS-polyacryl-amide gel electrophoresis-separated larval antigen

protein

blots. Nineteen distinct bandswere

recognized by

IgG, and 13distinct bands were

recognized

by IgA from the seraof infected patients (Fig. 2). Five bands were cross-reactive with IgG and IgAfrom the sera of infected

patients.

These bands had apparent molecular

weights

of

78,300,

62,200,

43,000, 21,900, and

18,200

(Table 3,

bands

2, 8, 13,

26,

and 28, respectively). Bands 13, 26, and 28

appeared

to havea

greateraffinityforIgA. Bands 2 and 8werealso

recognized

by IgAandIgGfromcontrolsera

(Fig. 2).

Another band with anapparent molecular

weight

of

46,400

(Table 3,

band

12)

was detected by control serum antibodies but not

by

anti-Voi- 25, 1987 1507

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TABLE 2. Parasite-specificIgG,IgE, and IgA responses in the different groups of subjects studied Parasite-specific response'

TotalIgA (g/dl) Patient group (n) IgG net absorption IgE(%binding) IgA net absorption (mean+ SD)

(mean+ SD) (mean +SD) (mean +SD)

Healthycontrols(50) 0.085 + 0.015 2.1 ± 0.4 0.072 ± 0.040 205± 149b

Parasitized controls(18) 0.169 + 0.188 5.2 ± 8.1 0.230± 0.088 195 ± 126

AllS. stercoralis patients (119) 0.795 + 0.341 31.9 + 14.1 0.687 ± 0.388 216 ± 185 ImmunocompetentS. stercoralis patients (104) 0.787 + 0.346 34.2 + 12.4 0.681 ± 0.365 227 ± 191

Latin Americans(53) 0.841 + 0.390 35.6 + 9.8 0.756± 0.398 277 ± 220

Southeast Asians (28) 0.735 ± 0.390 35.1 + 14.8 0.724± 0.349 222 + 158

U.S. residents(23) 0.723 + 0.284 29.7 + 16.4 0.473 ± 0.197 125 + 85

Immunocompromised S.stercoralis patients (15)C 0.797 + 0.460 15.0+ 16.1 0.735 ± 0.538 121 + 99

aAnabsorption value of0.300 or greateris consideredpositive. Apercentbindingof6%orgreateris consideredpositive.

bMean ± SD of 18 randomly selectedhealthy controls.

Ofthe15immunocompromised patients,12 werefromtheUnitedStatesand3werefrom LatinAmerica.

bodies from infected serum. Serum IgA from healthy indi-viduals reacted witha

22,400-molecular-weight

band and a

70,700-molecular-weight

band (Table 3, bands 5 and 25, respectively) notdetected by serum IgG fromhealthy indi-viduals. However,band5 wasreactivewithserumIgG from infected patients.

The remaining bands recognized by sera from infected patients were not reactive with antibodies fromthe seraof

TABLE 3. Apparent

molecular

weightsof

immunoreactive

bandsfromFig. 2

Mol wtofbandsrecognized inserafrom: Band no. Noninfected controlsby: Infectedpatients by:

IgG IgA IgG IgA

1 86,700

2 78,300 78,300 78,300 78,300

3 74,400

4 72,500

5 70,700 70,700

6 66,300

7 65,500

8 62,200 62,200 62,200 62,200

9 54,100

10 53,400

il 49,400

12 46,400 46,400

13 43,000 43,000

14 38,300

15 36,400

16 33,700

17 31,200

18 30,400

19 29,700

20 28,200

21 26,800

22 26,500

23 24,800

24 23,300

25 22,400

26 21,900 21,900

27 20,200

28 18,200 18,200

29 16,700

30

healthy individuals. These bands displayed IgA reactivity.

specific IgG or

DISCUSSION

The presence ofIgA has been noted in the serum, milk, and intestinal secretion in many parasitic infections(1), but

its functional significance is largely unknown. A limited

protectiveroleforsecretoryIgAhasbeen demonstratedina murinemodel ofTaeniataeniaeformis (16). Inexperimental

-2

a

2-

8-

12-i.G.

IgG

IgA

b

-2 -4

-s

5-6 7

-8 8-_--8

-12

13- P13

14-

15-

16-17- -18

19-

20-21- -22

23--25~~~~~~

-25 26-- -26

-27 28-- '-28

--29

gGIgA

FIG. 2. Immunoblot analysis offilariformlarval antigens react-ingwith humanIgGand IgA.Sera fromhealthy (a) and infected (b) patients were used to probe the protein blots. Antibody-binding bands were detected with anti-human IgG and IgA conjugated to alkaline phosphatase. Twenty-nine bands were visualized upon developmentin Nitro BlueTetrazolium-5-bromo-4-chloro-3-indolyl phosphate. The band numbers are listed on the basis of their molecularweightsin Table 3.

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HUMAN STRONGYLOIDIASIS PARASITE-SPECIFIC IgA RESPONSES

mutine giardiasis, secretory IgA interacts in vitro with antitrophozoite IgG andmousephagocytic cells to promote parasite clearance, and it is believedto represent a

biologi-cally relevant protective response(14). The participationof

mucosal IgA in the intestinal expulsion of experimental murine Nippostrongylus brasiliensis infections has been extensively investigated (17, 25, 30), and the modulation of theseresponses by mucosal T-cells has been partially

eluci-dated (18). Incontrast tothegrowing volume of experimen-tal work with animal models, the function of IgA in human parasitic infections has received little attention. Yet it is likely that local defenses playacritical role in host resistance

against intestinalhelminths (1, 2).

S. stercoralis, with its long permanence in the host's

intestine and the continuous recycling of the filariform larvae, subjects the hostto a constant exposure to parasite

antigens. The results ofour work show that most patients with strongyloidiasis mount specific IgA responses against

larvalantigens. As shown by the recognitionpatterns of the immunoblots, these responses are, for the most part, di-rectedagainst antigens that are different from those

recog-nizedby IgG.

Itremainstobe determined whether theIgA demonstrated inthe infectedsubjects' serareflectsparallel mucosal

secre-toryresponses and what its functional significance may be.

The relationship between the primarily monomeric serum

IgA and the dimeric form found in secretions (IgA2) is stillan

unanswered question (12). However, the biological role of secretoryIgAas afirst line of defenseatthe mucosal surface isreceiving wideacceptance(3), andanefforttoexplore this problem inacaninemodel of strongyloidiasis is underwayin ourlaboratory.

Whereas the immunocompromised patients in this study hadsignificantly lower total IgA levels than infectedpersons

with an apparently intact immune system, there was no

correlation between the titers of parasite-specificserum IgA

and the clinical form ofstrongyloidiasis. Furthermore,

ab-sence of detectable circulating specific IgA was not

neces-sarily associated with immunosuppression ordissemination.

This may suggest that these antibodies, directed against filariform larval antigens, do not play a central role in the

regulation of the infection. A similar situation appears to exist in thecase of anti-filariform larvaIgG inboth humans (7) and animal models (8, 10). Conversely, we previously

reported an association between lower levels of

parasite-specific IgE and immunosuppression, with or without dis-semination (7). The present study, which included eight additionalimmunocompromised patients,confirmedour

pre-viousfindings, suggesting that the possible protective roleof IgE in strongyloidiasis may deserve further attention. The lower levels of both total and specific IgAfound in infected U.S. residentsarenoteasily explained. It ispossiblethat the infections in these individualsweremorechronicthan in the two groups from endemic areas, who could be exposed to theparasite constantly. Also, althoughmostof the Southeast Asians and Latin Americans had other intestinalparasites, S.stercoraliswastheonlyonefound in the North American

group. Little is known about the clinical significance of

serum IgA levels; it is possible thatsignificant correlations

may emerge as more data are gathered from parasitized

patients.

Anotheraspectof S.stercoralis infectiontobe considered with regard to local responsesis the fact that the parasitic formsmostintimatelyassociatedwith the intestinalmucosa arethe adult females and the rhabditiformlarvae. The rate of intraintestinaltransformation of rhabditiformlarvae into the

tissue-penetrating

filariform larvae may, atleast in part, be

regulated by

local cellular and secretory responses. In

preliminary experiments using

indirect immunofluorescence

(data

not

shown),

wehave detected

IgA

directed

against

the

surface ofrhabditiformlarvae in theseraofmost, butnot

all,

patients

who had

high

titers of anti-filariform larva

IgA.

It

has notyet been

possible, however,

to determine whether

they

represent

specific

responses to this larval stage or reflect the presence of shared surface

antigens

on rhab-ditiform and filariformlarvae. Immunochemical

analysis

of

the

antigenic

makeup

ofthedifferent stages of this

parasite

may

eventually

enhance our

understanding

ofthe

immuno-regulatory

mechanisms

modulating

S.stercoralisinfections.

ACKNOWLEDGMENTS

We thank Theresa Huitger-O'Connor for her skilled technical help. H. Tanowitz, M. Wittner, J. Arroyo, and A. Cohen kindly

donatedsomeoftheserafromimmunocompromised patients.Peter D.Walzerprovidedconstructive criticism.BettyGuttridge compe-tently typedthemanuscript.

This workwassupported bythe MedicalResearchServiceofthe VeteransAdministration, Washington, D.C.

LITERATURE CITED

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