Effect of immunosuppression on experimental Argentine hemorrhagic fever in guinea pigs.

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Effect

of

Immunosuppression

on

Experimental Argentine

Hemorrhagic

Fever

in

Guinea Pigs

RICHARD H. KENYON,* D. E. GREEN, AND C. J. PETERS

Department ofViralPathogenesis, Medical Division, U.S. Army MedicalResearch Institute of Infectious Diseases, Frederick, Maryland21701

Received27June 1984/Accepted 29September1984

Immunosuppression withcyclosporinA orcyclophosphamide had noapparenteffectonthe disease course ofguinea pigs infectedwith a virulent strain ofJuninvirus. Immunosuppression ofguinea pigsinfected with an attenuated strain ofJunin virusled to fulminating Argentine hemorrhagicfever. All immunosuppressed infected animals died. Virus distribution patterns in target organs, as determined by plaque assay and fluorescent antibody procedures, were similar to those from non-immunosuppressed animals infectedwith a virulent strain. Histopathological lesionsinimmunosuppressed guinea pigs infected withanattenuated strain of virus weresimilartothose innon-immunosuppressed guinea pigsinfected with a virulent strain.Histological changes attributabletotheimmunosuppressive dt-ug(s)wereregularlyobserved. Immunosuppressed animals infected withattenuatedJuninvirus andnon-immunosuppressedanimalsinfected with virulentvirus failedto developantibody or respondedat aminimal level.Virus-specific cytotoxic spleencellactivity, previouslyshown to be antibody dependent, failed to develop in the same animals. The presence of a competent immune response, probably serum antibody, determined whether Argentine hemorrhagicfeverinfection of theguinea pig was lethal or whether recovery ensued; no evidence for harmful effects of the immune response was obtained.

Junin virus causes Argentine hemorrhagic fever (AHF), an acute severe disease with a human mortality rate ofca. 15%. When infected with human pathogenic viral isolates,

guinea pigs manifest a disease similar to that expressed in humans; lethal disease in guinea pigs is a widely used

virulence markerfor Junin strains(6, 8).

AHFinhumansandguinea pigspossessescertain

similar-ities to thewell-studied murinemodelinfection withanother

member ofthe familyArenaviridae, lymphocytic

choriome-ningitis virus (LCMV). Bothvirusesreplicate tohightiterin reticuloendothelial organs, are immunosuppressive, and causetransienthematopoietic dysfunction(4, 7,24). In mice inoculated intracranially with LCMV, the onset of acute disease has beendefinitively linkedtodamage caused bythe hostimmune responsetothevirus, specifically by virus-spe-cificT-lymphocytes (13). Mice infectedwith Juninand other

arenaviruses often developahistologically similar encephali-tis which is ameliorated in congenitally athymic, neonatally thymectomized,orimmunosuppressedhosts(2, 9, 31). These

findings have led to speculation that immunopathological mechanismsmayunderlie disease induction inhuman AHF.

Sinceguinea pigs areclassically used as amodel for human

AHF, we examined the effects ofimmunosuppression on these animals infected with virulent or attenuated Junin strains to elucidate the role of an immune response in

precipitating the hemorrhagic fever syndrome. For

im-munosuppression weusedcyclophosphamide (CY), a global irhmunosuppressant (25, 27), and cyclosporin A (C-A), an endecapeptide selective forT-cells (3, 10, 19).

MATERIALSANDMETHODS

Animals. Hartleystrain, male guinea pigs (weight, 300 to 400g)(Buckberg Labs,TompkinsCove, N.Y.)wereused in allexperimentsexceptthecytotoxicityassays,in which male

*Corresponding author.

inbred strain 13 guinea pigs from the U.S. Army Medical

Research Institute ofInfectiousDiseases colonywere used. Theclinicalresponseofstrain 13guineapigstoRomeroand XJ-44 strains ofJuninandtoimmunosuppressive regimesis similartothat ofoutbred guinea pigs.

Virus. Attenuated XJ-44 strain of Junin virus represents the 44th successive suckling mouse brain passage of the

virulent

XJ parent. For

virulent

virus,the Romerostrain was

used. Romerostrain,isolated fromasevere, nonfatalhuman case, waspassed twiceinfetal rhesus lungcells andonce in Vero cells. L-15

buffer

with 2% heat-inactivated fetal calf serum wasusedfor all virusdilutions. Viruswasquantitated

byaplaqueassay system(28). Guineapigswere inoculated with virusbyintraperitoneal injection. On preselected days, organswereremoved fromanimals; sampleswere

weighed,

suspended in L-15

buffer,

processed intissue culture grind-ers, andassayed forvirus. Totestforreplication ofvirulent mutants or for a virulent subpopulation in immunosup-pressed, XJ-44-infected guinea pigs, a

spleen

cell homo-genate was prepared from a

moribund,

XJ-44-infected, CY-immunosuppressed guinea pig; 105 PFU of virus was inoculated intraperitoneally into both untreated and

immu-nosuppressed guineapigs.Thisprocedurewasusedfor three

sequential spleencell homogenatepassages.

Immunosuppression. A single dose (250 mg/kg) of CY (Cytoxan; Mead Johnson and Co., Evansville, Ind.) was administered to guinea pigs intraperitoneally 3 days before

inoculationwith virus. This single large dose given 3 days

before virus infection resembles that used by Turk and Parker (27) to abrogate

antibody

response and to enhance

delayed-typehypersensitivity.Itwasalsopossibletoproduce a lethal outcome with XJ-44 by giving a lower dose (150

mg/kg) of CY as late as 6 days postinfection. C-A (kindly

supplied by J. F. Borel, Sandoz Ltd., Basel, Switzerland) was dissolved in miglyol (Kay-Fries, Mont Vale, N.J.) and ethanol and administered daily intramuscularly on days -1 through 14postinfection at25 mg/kg perday. C-A

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76 KENYON, GREEN,ANDPETERS

tered in a single large dose to rodents before antigenic

challengeor in smalldoses afterantigenic challenge results in a marked suppression of T-cell responses, including

cytotoxic, helper, and delayed-type hypersensitively func-tions withrelatively lessimpressive direct effects on B-cell responses (reviewed in reference 26).

Histopathology and

immunohistochemnical

methods. Organ

samples from killed animals were fixed in Formalin, sec-tioned at 6 p.m, and stained with hematoxylin and eosin.

Samples were also frozen at -70°C in embedding medium

(Miles Scientific, Div. of Miles Laboratories, Inc.,

Naper-ville, Ill.), and cryostat sections were stained with

fluores-cein-conjugated mouse anti-Junin asciticfluid.

Antibody detection. (i) IFA. An indirect fluorescent

anti-body(IFA) assaywasperformed by theprocedureofPeters et al. (23). Briefly, Vero cells infected with XJ-44 strain

Junin virusorcontroluninfectedcells weredilutedto 2 x

104

cells per ml; 10 ,ul of this suspension was air driedon spot slides and fixed in cold acetone. Testserumwasallowed to react with the antigen, and after washing, the spots were

stainedwithfluorescein-labeled, goatanti-guinea pigserum.

Endpoints were recorded as the highest dilution exhibiting specific fltiorescence in the infected cells when examined

with a x25 oil immersion objective on a Zeiss microscope

(Oberkochen, Federal Republic ofGermany).

(ii) PRN. The plaque reduction neutralization (PRN) as-say, described by Webb et al. (28), consists of adding a constant amount of

virus,

ca. 100 PFU ofXJ-44 strain, to twofold dilutions of sera in microtiter plates. After an

overnight

incubation at 4°C,

samples

were titrated for live

virus,andendpointswererecordedas

the

highest dilution of serum

effecting

80% reduction.

(iii) Cytotoxicity assay. Methods detecting cell killing by spleen cells are described elsewhere (Kenyon and Peters, submitted for publication). Briefly, a strain 13 guinea pig kidney cell line was infected with Junin XJ-44 strain; in-fected cells were labeled with

51Cr

and planted in 96-well

plates for use as target cells. Single cell suspensions were

prepared from XJ-44-infected strain 13 guinea pig spleens and wereused as effector cells. Percent

51Cr

released after

overnight

incubation was used as an indication of

cytotoxi-city

and was

calculated

asfollows: percent

51Cr

=

(experi-mental 51CR released - spontaneous

51Cr

released)/(total

51Cr

with sodium

dodecyl

sulfate released - spontaneous

51Cr

released) x 100.

RESULTS

Theeffect of

immunosuppression

onguinea pigs infected with an attenuated (XJ-44) or virulent (Romero) strain of Junin virus is shown(Table 1).Therewere nosignsof illness in untreatedXJ-44-infected

animals,

and these animals con-tinued to

gain

weight.

However, when

immunosuppressed

with either C-A or CY, all

XJ-44-inoculated

animals lost weight beginningonday 12, and all diedby day25. Untreat-ed Romero-infected guinea pigs began to lose weight after

day 7,and allanimals died

by

day15. TreatmentwithC-Aor

CY appeared to have no

sparing

effect on the outcome of

virulent disease. Although asingle CY dose administered 3 daysbefore viruswasusedinthese

experiments,

all animals

receiving150

mg/kg

6daysafterinoculation with XJ-44virus

died.

To determine whether the lethality seen in

immunosup-pressed,XJ-44-infectedguinea pigs might,infact,be due to

the presence of virulent tnutants or to the

expansion

of a

virulentviral subpopulation, weserially passagedthe XJ-44 strainthree times in

CY-immunosuppressed

animals.

Spleen

TABLE 1. Effects of immunosuppressive agents on Junin virus infection ofguineapigs

Virus Daysof No.of Dyo et

strrauin

Treatment' treatment deaths/no. Day of death ofanimals (mean SD)

XJ-44 None 0/10

C-A -1to14 10/10 20 ± 4

CY -3 10/10 22 + 3

Romero None 6/6 15 ± 1

C-A -1 to14 6/6 16 1

CY -3 6/6 13±1

None C-A 0/6

CY 0.6

a Concentrations: C-A,25mg/kg;CY, 250mg/kg.

cell-grown virus from each of the three passages failed to cause illness or death when inoculated into normal guinea pigs.

To confirm the immunosuppressive effects of drug treat-ment in XJ-44-infected guinea pigs, anti-Junin antibody

production was examined (Fig. 1). In XJ-44-infected,

non-immunosuppressed animals, significant antibodywas present

by day 13, as measured by IFA assay, and by day 21, as measured by PRN assay. However, when animals were

immunosuppressed with either CY orC-A, PRN antibody failed todevelop, and fluorescent antibody was undetectable orwas detectedat very reduced levels.

In XJ-44-infected guinea pigs, the effect of

immunosup-pression on cytotoxicity of spleen cells for Junin virus-in-fected target cells is shown in Fig. 2. Spleen cells from infected untreated guinea pigs effected up to 40% 51Cr release. However, spleen cells from infected, immunosup-pressedguineapigs demonstrated nearly the same

cytotoxi-city (CY animals)asoronlyslightlymorecytotoxicity(C-A

animals) than spleen cells from normal uninfected guinea pigs. Responses atdays 12 and 16 were similar. Later time

points were not measured due to the increasing severity of the disease.

Virus titers in samples from Romero-infected animals

(Fig. 3A) resembled those previously reported with the

virulentJunin XJ strain(5). Infectivitywasfirstapparent in

spleen and lymphnodes by day 5,

reaching

levelsof5.5 to 6.5

log1o

PFU/gin these organs and bone marrow. Virenlia

z

-a

2048-1024.

512.

256*

128.

64

-32 .

16

-

8-ce8

0 0 o 0- 0o

0 0 0

0/ , r 8 --- A--XJ-44(PRN)

'0° /A XJ-44(IFA)

8'o d CY+XJ-44(IFA)

."/A /0

O,,"~ ~

0*'

.

/.

" ° 4o' Yt C-A.XJ-44(IFA)

_ ~~~~~oO

10 15 20

DAYS POST INFECTION

25 75

FIG. 1. Antibody developmentinimmunosuppressedor

non-im-munosuppressed XJ-44-infectedguinea pigs. Antibody was meas-ured by IFA and PRNtechniques. Novirus-neutralizingantibody

wasdetected inimmunosuppressed, infectedguinea pigs.

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EFFECT OF IMMUNOSUPPRESSION ON AHF IN GUINEA PIGS 77 so5

4c

w

0

40

30

20

10-XJ-44

v

_T

T-,

iC-A+XJ-44

CY + XJ-44

6 1 0 20 30 40 50

EFFECTOR:TARGET RATIO

FIG. 2. Effect ofimmunosuppression of Junin-infected guinea pigs on cytotoxicity of spleen cells for Junin-infected target cells. Thecytotoxicity of spleen cells from guinea pigsatdays 16(shown) and 12 postinfection was similar. These spleen cell suspensions effected less than 6%

51Cr

release on uninfected targets. Normal

spleen cells effected lessthan5%

51Cr

releaseoninfected targets. wasfirstdetected on day 7 and reached levels in excess of 4

log10 PFU/ml by day 11. High virus titers persisted until

death, around day15. Incontrast, XJ-44viruswasdetected

for only a brief time span (days 5 through 7), in limited

distribution (lymph nodes and spleen) and in lowtiter(less

than 3.5log10 PFU/g) (Fig 3B).

CY-immunosuppressed guinea pigs infected with XJ-44 had detectable organ virus titers by days 5 to 7, rising to

°, 6-o

J 5. w

M 4-(n

CD

3-u.

2-1. 'c1.2 5

levels of 3.4 to 5.2 log10 PFU/g (Fig. 3C). Viremia was detected atlow levels ondays 7 to 16. C-A-treatedanimals

had similar XJ-44 titers, except at later time points when organvirus titers were even higher (5.1 to 6.4 log10PFU/g),

and there was clear evidence for brain invasion (Fig. 3D).

Brain virus exceeded 4.0 log10 PFU/g in the absence of detectable viremia in most animals examined after day 20 and was often associated with clinical findings of hind-limb paralysis.

Histopathological lesions in samples from

Romero-in-fected animals (Table 2) also resembled those previously reported with the Junin XJ strain (5, 7). The primary lesions consisted of necrosis of bone marrow, lymphoid depletion of lymph nodes and spleen, and, terminally in some cases,

lymphoid necrosisof the spleen. Lesionsincreasedin

sever-ity asthe disease progressed. Although sparse antigen was detected in brain tissue (cortex) by day 11, no significant

lesions were observed in either medulla or cortex, even in moribund animals at day 15. In contrast, samples from guinea pigs infected with XJ-44 virus demonstrated very limited viral antigen ondays 8 and 14 in bone marrow, lymph nodes, and spleen. Minimal lesions consisting of lymphoid depletion were found in lymph nodes and spleen, but no

lesions were observed in bone marrow. Occasionally,

non-suppurative, multifocal encephalitis and gliosis were ob-served in themedulla, but viral antigen was not observed in the brain by fluorescence, although the medulla was not

included in the brain section examined by this technique. Whenobserved in any organ by IFA assay, viral antigen was

randomly distributed and not confined to asingle cell type.

CY-immunosuppressed guinea pigs infected with XJ-44 had detectable viral antigen in spleen, bone marrow, and

A

*SERUM oLYMPH NODE

oSPLEEN

u B &BRAIN

4. &~AMARROW

'3- ,?

0"

U.~ 2- !o

0.1.25L d

2 4 6 810 DAYS POST

INFECTION

C

d

. sr-...t>..,...,...o..,

I.~~ ~ ~ ~ ~ ~ ~~ .

:trc

ul o

e

-r3 E

M

IL

X_cO.25/ml 1.

D

.

-0.

:>'0.25

5 10 15 20 25 5 10 15 20 25 30

DAYS POST INFECTION DAYS POST INFECTION

FIG. 3. Virus titers inguinea pigsinfected withJuninvirus. (A) Romero strain. (B) XJ-44 strain. (C) CY treated,XJ-44strain infected.(D) C-Atreated, XJ-44 straininfected.

6-0

0 5-0

'Li

:

4-cn

;

3-M

2-U.

0.

-c1. 2 5 VOL. 53,1985

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lymph nodes by day 7. Virus-specific antigen increased in these organs, and by day 24, nearly every spleen cell contained antigen. The most significant histological change observed in these animals was pancellulardepletion of the

bone marrow and extensive loss oflymphocytes from the spleen. The brains of these animals rarely demonstrated virus antigen. Minimal nonsuppurative encephalitis in the brain and thalamus was histologically observed only in the guinea pigs examined at days 18 and 24. Controls with CY onlyshowednosignificant drug-related lesions in the critical period ofdays 12 to 24. C-A-treated animals infected with XJ-44 virus also showed virus antigen in bone marrow, lymph nodes, and spleen. Lymph nodes and spleen exhib-ited a moderate degree of diffuse lymphoid depletion, and one guinea pig (day 25) exhibited severe active pancellular necrosis of bone marrow. Controls receiving only C-A exhibited similarlesionsin lymph nodesandspleen through the experiment, making discernment of virus lesions over drug lesions impossible atcritical times of infection.

DISCUSSION

The arenaviruses are a family of viruses that survive through chronic infection of their natural host, often a viremic rodent. They may, however, infect other species and induce acute disease. This situation has often been modeled by intracranial inoculation of mice, which may result in no disease or in the development of encephalitis.

The outcome canbe explained by the extent of viral repli-cation at the time ofonsetof the immuneresponse. If viral spread isnotextensive, the immune responseis protective;

however, if critical brainstructuresareinvolved, the host's attempt to eradicate virus is responsible for disease induc-tion (16,21).

Suppression of the immune response by global measures

(CY or radiation) or by specific modalities directed at

thymus-dependent functions spares infected rodents from acute disease (15). LCMV inoculated intracranially in the adult mouse is the classical model and probably shares similarpathogenesiswith LCMV-inducedasepticmeningitis

in humans(22). Junin virus in mice andratsalsoappearsto

induceanimmunopathologicalencephalitis afterintracranial inoculation(17, 18). However, viral hemmorhagic fevers in humans to not present this pattern; in particular, cellular infiltrates are scant, and virus titers do not necessarily decline with theonsetof disease(22a). Realistic models for thesehemorrhagic fevers have been developed in nonhuman primates and guinea pigs (22), but the effects of

immunosup-pression have notbeencarefully studied. We examined the effects oftwopotentimmunosuppressive drugsoninfection with a low-passage guinea pig-virulent human isolate (Romero strain ). Incontrasttotheestablishedaction of CY inameliorating immunopathological encephalitis in mice and

rats,there was noeffect intheguinea pig viral AHF model. Animals died in the same time after inoculation and with

[image:4.612.60.553.383.649.2]

similar histopathological lesions as unmanipulated infected

TABLE 2. Distribution of Juninantigensand pathology in organs" ofguinea pigs infectedwithJuninvirus Distributionin:

Strain Dayof Bonemarrow Lymph node Spleen

(treatment) death

Virus Lesions" Virus Lesions Virus Lesions

antigen'b antigen antigen

XJ-44 8 +1 0 +1 d +1 0

14 +1 0 +1 +2 +1 +2

17 0 0 0 +1 0 +1

28 0 0 0 0 0 +1

XJ-44(C-A) 14 +1 0 +3 +2e +2 +2

18 +1 +3 +2

21 - 0 +3 +2 +2 +2

23 +3 +3 0 +2

25 +3 +3 +3 +2 +3 +2

XJ-44(CY) 7 +1 0 +1 +1 +4

9 +1 +1 +1 +3

10'f +3 +2

12 +1 +1 +2 0 +2 0

18 +2 +2 +2 +2 0

24 +1 +3 +2 +3 +2

Romero 5 +1 0 +1

7 +1 +1 0

9 +1 - +1 +1

11 +3 +1 - +2

13 +3 - 0 +3

15 +3 +2 +2

aBothbrain cortex and medulla were examined for lesions;onlycortex wasexamined forantigen. Significantlesions were notconsistentlyobserved in either cortex ormedulla of any of theexperimentalanimals.Antigen was observed only inimmunosuppressed,XJ-44-infected animals and inRomero-infected animals and was alate event usually involving only a few cells.

b0,Antigennotobserved; +1,slight(up to10%); +2,moderate (10 to25%); +3heavy(>25%).

cObservedbyhistological studyand evaluated herebya scalesimilar to thatfor viral antigens. d-, Notexamined.

eControlswith C-A only showed approximately the sameseverityof lesions inlymphnode andinspleenatdays14 to 25 as wasseenin C-AplusXJ-44in this

time frame.

fControls with CY only showednosignificantlesions inlymphnode,marrow,spleen,orbrain atdays12 to 24.

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controls. Wealso studied theeffects ofimmunosuppression oninfectionswith Junin strain (XJ-44) attenuated for rhesus monkeys and guinea pigs (J.Barrera Oro and R. H. Kenyon, unpublished data). This virus in unmodified adult outbred guinea pigs kills <1% and usuallyresults in mildencephalitis orlate"no-lesion" disease, neverflorid hemorrhagicfever. The virus resembles its cousin XJ-clone 3 in quantitative virulencetitrations in immatureguinea pigs and mice.

How-ever, wefound thatdrug-treatedanimalsinfected with XJ-44 all died. Replication of XJ-44, detectable only atlow levels in unmanipulated guinea pigs, was almost as extensive as seen inRomero infections. Although virusfirst appeared in bothgroupsby day 5,ingeneral,viralantigendevelopment, histopathological lesions, and mortality were all delayed in thedrug-treated XJ-44-infected animals in comparison with Romero-infected animals. Although typicalAHFdeveloped inboth the Romero-infected andimmunosuppressed, XJ-44-infected guinea pigs, the Romero animals developed more fulminating disease with little or no brain involvement. In

contrast, the XJ-44 group had an unrelenting progressive course,withmanysurvivors pasttheacutephaseof disease and intoalatephase characterized bybrain-associated virus

andoccasional rear-limbparalysis. Romero-infected guinea pigs showed fewhistological lesionsinlymph nodes, spleen, andbonemarrow ondays5to9(despitevirusreplicationat

these sites as determined by PFU and IFA techniques).

Severe lesions were observed in these organsfrom day 11 until deathandwerecharacterized bylymphoid necrosis and depletion of nodes and spleen as well as necrosis and degeneration of bone marrow. Such lesions eventually de-veloped in CY- and C-A-immunosuppressed guinea pigs infectedwithXJ-44Juninvirus, but similarlesions found in C-Aonlycontrol animalsconfoundedtheinterpretationwith thisgroup.

Signs ofnervous system dysfunction or brain lesions or

evidence forviralreplicationin the brainwereonlyseen2to

3 weeks postinfection or later. By this time, Romero-in-fectedanimals had died withtypicalviralhemorrhagicfever. They had detectable viral antigen in the brain but had no

clinical signs or pathological changes in that organ. Immu-nosuppressed XJ-44-infected animals dying in the fourth week ofinfection often had relatively high brain virus titers, foci ofencephalitisandgliosis,andhind-limbparalysis.This

courseofeventsmayhave differentpathogenesisand deter-minantsthan moreacute diseaseassociatedwith viral repli-cation in otherviscera.

Thepathophysiological changes leadingtoafataloutcome

and the mechanismsby which virus infection induces them remainspeculative forall thearenavirushemorrhagicfevers (22, 22a). Nevertheless, the prominent lymphoid and bone marrow necrosis seen in these fatalJunin guinea pig

infec-tions isnotimmunemediated. Immunosuppression doesnot protectagainst disease induction byavirulentstrain, and the adequacy of immunosuppression was vouchsafed by the

results with an attenuated strain. The same treatment that failedtoprotectagainsttheguineapig-lethalRomero isolate resulted in a fatal outcome with the XJ-44 strain and with almost as rapid and severe a sequence of pathogenetic consequences.This raisestwocritical issues: (i)what is the mechanismof diseaseinductionbytheserelatively

non-cyto-cidal viruses, and (ii) how are nonfatal Junin-infections controlled? We have no data relevant to the first question (reviewed inreference 22a)but have measured the immune

response relevant to the second. In the XJ-44-infected guinea pig, virus abruptly decreases in critical sites of replication in lymphoid tissues between days 7 and 10. We

initially detected fluorescent antibodies at this time, but

circulating serum-neutralizing antibodies, often acandidate

for limiting viral spread in other systems, was not demon-stratedfor another week. TwomechanismsforlysingJunin

virus-infected cells becameoperative during the critical 7- to 10-day time period: (i) cytolysis by antibody plus

comple-ment (Kenyon and Peters, manuscript in preparation), and

(ii)cytolysisbyspleen cells (Kenyon and Peters,submitted).

Indeed, the second mechanism was shown to be antibody-dependentcell-mediated cytotoxicity(nodiminution oflysis

after treatment of spleen cells with monoclonal antibody generatedagainst guinea pig T-cells, blocked by aggregated human gammaglobulin, still measureable at 30 days postin-fection, response with B-cell-enriched fractions). Thus, all the measured immune responses seem to be antibody de-pendent, and thecommondenominatorisanti-infected cell,

not particularly antivirus. Since, however, the absolute

sensitivity of assays in measuring the immune response and the exact antibody species are unknown, we cannot estab-lish with certainty which is first operative, but there is no

clear support for an overriding role for neutralizing anti-body.

Immunosuppression with CY or C-A unleashes the poten-tial of XJ-44 but delays or inhibits PRN and fluorescent

antibodies as well as spleen cell cytotoxicity responses. The two drugs used can have profound effects on both T- and B-cell responses. It is probably relevant that no immune responseisdetected in the virulent virus infection andpoints to the importance of immunosuppression by Junin virus itself. Guineapigs infected with virulent Junin strains appear tobeimmunosuppressed withrespect toboth viral and other

antigens (1, 20). This may not be surprising in light ofthe extentofmacrophage infectionandlymphoidpathology but may be relevant to lifeordeath outcome.

Thesedatafromamodel oflethal arenavirus hemorrhagic

fever contrast with those classically obtained in mice inoc-ulated intracraniallywith LCMV in twoprominent ways: (i) the lackof immunopathology, and (ii) the predominance of

evidence supporting B-cell products as the most important

determinant ofrecovery. Our studies, as well as those of

others, havewellestablishedthesimilarities of hemorrhagic

fever infections in humans to those expressed by theguinea

pig model(11, 12, 29)ratherthan tothose expressed by the

encephaliticmousemodel (22a). Although the argument that

immunoglobulin is responsible for recovery is circumstan-tial, antibody in the form of convalescent plasma aborts AHFinboth guinea pigs (30) and humans (14). In part, these

observationsmayreflectinterspecies differencesin

eradicat-ing cells with foreign surface antigens. The relative role of

antibodyin Old World arenavirus infections, suchasLassa fever and LCMV, also seems to be much less than in the

South American hemorrhagicfevers (22).Finalresolutionof

theseproblems is anecessityfor further advances in therapy andvaccine development for these viruses.

ACKNOWLEDGMENTS

WethankDavid Brantley and Bernard Kellyfor technical assist-ance.

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