Vol. 33, No. 3INFECTION AND IMMUNITY, Sept. 1981, p. 743-7490019-9567/81/090743-07$02.00/0

Recovery of Mice from Herpes Simplex Virus Type 2Hepatitis: Adoptive Transfer of Recovery with Immune

Spleen CellsS. C. MOGENSEN* AND H. KERZEL ANDERSEN

Institute ofMedical Microbiology, University ofAarhus, 8000 Aarhus C, Denmark

Received 9 February 1981/Accepted 19 May 1981

Young BALB/c mice inoculated intraperitoneally with herpes simplex virustype 2 develop focal necrotizing hepatitis. After infection, the livers of these miceshow increasing virus titers, which reach a maximum on day 3 after infection; thisis followed by a dramatic decrease in the amount of virus recovered on days 4 and5. This decrease in virus content is accompanied by a progressive infiltration ofthe lesions with mononuclear leukocytes and an apparent resolution ofthe lesions.Adoptive transfer of immune spleen cells from mice infected 6 days earlieraccelerated this process. When 50 x 106 to 100 x 106 immune spleen cells weretransferred 24 h after infection, the inflammatory response and the clearance ofvirus from the livers were advanced by almost 2 days. As few as 12 x 106 immunespleen cells accelerated the healing process, whereas fewer immune cells, dis-rupted immune cells, or normal spleen cells did not have an effect. The protectionconferred by herpes simplex virus type 2-sensitized immune spleen cells wasspecific since mouse cytomegalovirus- or vaccinia virus-sensitized immune spleencells had no effect on the course of infection with herpes simplex virus type 2,whereas some cross-reactivity was observed between herpes simplex virus types1 and 2. This model seems to be suitable for examining the immunologicalmechanisms that are active during recovery from visceral herpes simplex virusinfections.

A wide variety of host defense mechanismsare involved in resistance to and recovery frompnmary virus infections. Nonspecific host resist-ance factors represent the main line of defenseduring the first few days of infection. Some ofthese (for instance, barriers to virus penetrationat body surfaces [8] or the resident macrophagearmamentarium ofmost organs [13]) are presentin uninfected hosts and are thus ready to func-tion from the begiing of an infection. Others(for instance, the interferon response [2] and theinduction of NK-cell activity [22]) are inducedfairly rapidly in the course of an infection. Byinterfering with the early stages ofvirus invasionand spread in the body, the nonspecific hostdefense mechanisms may have important impli-cations for the course of an infection. However,the final elimination of the infectious process,leading to recovery, is probably mediated by thespecific immune response, which appears somedays after the initiation of infection.

Previously, we have analyzed the role of mac-rophages in natural, nonspecific resistance tovirus infections (10-15). This was done with amouse model of generalized herpes simplex virustype 2 (HSV-2) infection, in which focal necro-tizing hepatitis was a main feature of the non-

neural manifestation of the infection. With thismodel we found a correlation between the abilityof the virus to replicate in macrophages and theease with which the virus was able to establishhepatitis. However, even when focal necrotizinghepatitis was established fully, the infectiousprocess in the liver was more or less successfullyterminated after day 4 post-inoculation, asjudged by the lack of progression of the size ofinfectious foci and the decrease in the amountof virus isolated from the livers by this time.The only exception to this was an infection incongenitally athymic nude mice, which seemedunable to eliminate the infectious process de-spite the superior ability ofthese mice to restrainvirus multiplication in their livers during thefirst few days of infection (15). This indicatesthat a thymus-dependent cell-mediated mecha-nism is responsible for the final elimination ofthe infection in livers.

In this study we developed an adoptive trans-fer procedure with which the roles of immuno-logical mechanisms in the recovery from HSV-2-induced hepatitis can be studied further.

MATERIALS AND METHODSMice. A stock of inbred, specific-pathogen-free

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BALB/c/A/BOM mice was obtained from G. Bom-holtgaard Laboratory Animal Breeding and ResearchCenter, Ry, Denmark, and was bred further locallybehind specific-pathogen-free barriers. Donors of im-mune spleen cells were used when they were 8 to 12weeks old, whereas mice for experimental infectionswere used when they were 28 ± 1 days old. Eachmouse was inoculated intraperitoneally (i.p.) with 5x 105 plaque-forming units (PFU) of virus. In allexperiments mice of both sexes were used.

Viruses. The MS strain of HSV-2 used in thisstudy has been described previously (9, 16). This viruswas plaque purified before the virus pool used in theseexperiments was produced in human embryonic lungcell cultures. In addition, the following viruses wereused in experiments on the specificity of the effects oftransferred spleen cells: HSV-1 strain MacIntyre hasbeen described previously (16) and was produced inhuman embryonic lung cell cultures; the vaccine strainof vaccinia virus was produced as a calf lymph virus atthe State Serum Institute, Copenhagen, Denmark, andwas passaged once in human embryonic lung cell

cultures before use; and the Smith strain of murine

cytomegalovirus was obtained from the AmericanType Culture Collection and was produced in primarycultures of murine embryonic fibroblasts.Transfer experiments. Mouse donors of immune

spleen cells were immunized intravenously (i.v.) with2 x 105 PFU of HSV-2 in 0.1 ml. After 6 days thesemice were killed, and their spleens were removedunder aseptic conditions and placed in ice-cooled me-dium TC-199 supplemented with 5% fetal calf serum.These spleens were homogenized in glass homoge-nizers equipped with loose-fitting pestles (Bellco Glass,Inc., Vineland, N.J.), and the resulting preparationswere filtered through a double layer of sterile gauze toremove clumps of tissue. The cells were washed twicein medium TC-199 containing 5% fetal calf serum andthen filtered once more through a double layer ofgauze before counting. The viabilities of cell prepara-tions were determined by the trypan blue exclusiontest and generally were more than 95%. The desirednumber of viable cells was inoculated slowly i.v. intoeach recipient mouse in 0.5 ml of medium.

Disrupted spleen cell preparations were made byfreezing and thawing cell suspensions twice, followedby brief exposure to ultrasonic vibration. Several at-tempts to isolate HSV from disrupted spleen cell prep-arations yielded negative results.

HSV-1-, vaccinia virus-, and mouse cytomegalovi-rus-sensitized spleen cells were harvested from miceimmunized i.v. 6 days previously with 2 x 105 PFU ofHSV-1, 106 PFU of vaccinia virus, and 104 50% tissueculture infective doses of mouse cytomegalovirus, re-

spectively. These cells were processed for transfer torecipient mice as described above.Assay of organs for virus. Livers collected asep-

tically were frozen at -700C and were homogenizedindividually to make 10% (wt/vol) suspensions in Ea-gle minimal essential medium supplemented with 5%calf serum and antibiotics, using a Hannover homog-enizer (Ernst Schutt, Jr.). The suspensions were clar-ified by centrifugation at 4,000 x g and 4°C for 30 minand were tested for virus by a plaque method as

previously described (9), except that Vero cells wereused instead of human embryonic lung cells.

Histology. Liver specimens were fixed in 4% For-main. The fixed livers were embedded in paraffin, andhistological sections were prepared and stained withhematoxylin and eosin at the Institute of Pathology,University of Aarhus. Readings of the pathologicalchanges in the focal necrotic liver lesions were madeblindly, and the following features were evaluated: (i)virus-induced liver cell degeneration, (ii) infiltrationwith polymorphonuclear leukocytes, (iii) necrosis, and(iv) infiltration with mononuclear leukocytes. Individ-ual features were graded semiquantitatively from 0 to3 as follows: 0, absent; 1, sparsely represented; 2,present; 3, extensively represented.

RESULTS

Normal course of infection in livers. Tostudy the normal development of lesions in liv-ers, groups of 4-week-old mice were inoculatedi.p. with 5 x 105 PFU of HSV-2 and killed onthe following days. Macroscopically, liver lesionsdeveloped as previously described (16). Briefly,greyish white focal necrotic lesions just visibleto the naked eye appeared on day 2 post-inoc-ulation; these increased in size and number onthe following days and reached a maximum di-ameter of 0.5 to 1.0 mm on day 4. After day 4the lesions did not seem to expand further. Anassay of individual livers for virus content (Fig.1) showed increasing amounts of virus duringthe first days of the infection, with a maximumtiter of about 104 to 105 PFU/0.1 g on day 3.After this, the virus titers began to decrease, andthe most dramatic decrease occurred betweendays 4 and 5; only small amounts of virus or novirus at all was detected on days 5 and 6 post-inoculation. From these data, it appeared thatthe recovery mechanisms began to eliminate theinfectious process in the livers after day 3 andwere very efficient after day 4 of infection.Adoptive transfer of recovery mecha-

nisms with immune spleen cells. Experi-ments were conducted to determine whether thenormal recovery process in the livers of HSV-2-infected mice could be accelerated by adoptivetransfer of spleen cells from HSV-2-sensitizedimmune mice. Figure 2 shows that 5 x 107 im-mune spleen cells administered from 24 h beforeto 24 h after infection eliminated the infectiousprocess in livers completely or almost com-pletely by day 4 of infection, as no virus or onlytraces of virus were detected in the livers on thisday, compared with about 103 PFU/0.1 g incontrol mice. Immune spleen cells administered48 h after infection also lowered the virus titerin the livers, but to a lesser and more variabledegree. On the other hand, disintegrated im-mune spleen cell preparations and normal spleen

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DAYS AFTER INFECTIONFIG. 1. Course of infection in the livers of 4-week-

old BALBIc mice inoculated i.p. with 5 x 105' PFU ofHSV-2. Each point represents the virus titer in theliver of one mouse. A line connects the means of thegroups.

cells had no effect. In subsequent experimentsimue spleen cells were administered 24 h

after infection to allow the establishment ofprogressive infections in the livers before celltransfer. Furthermnore, the data seemed to jus-tify an evaluation of the effect of immune spleencell transfer on the basis of the virus titers inmice that received no cell transfer at all.The minimnum number ofimmune spleen cells

able to transfer recovery mechanismis underthese conclitions was 12 x 106; this number ofcells lowered the virus titer on day 4 of theinfection by about 1 logio. Fewer cells had nodemonstrable effect, whereas a higher numberof cells was more effective, with 5 x 107 to 1 x10' cells being very efficient (Fig. 3).The specificity of the effect of immune spleen

cells was evaluated by transferring 5 x 107 spleencells from mice imnzd 6 days previouslywith mouse cytomegalovirus or vaccinia virus to4-week-old mice infected with HSV-2. As Fig. 4shows, neither mouse cytomegalovirus- nor vac-cinia virus-sensitized immune spleen cells wereable to confer recovery to HSV-2-infected mice,

as measured by the virus titers in the livers onday 4 of infection. On the other hand, HSV-1-and HSV-2-sensitized immune spleen cellsshowed some cross-reactivity in an experimentin which HSV-1- and HSV-2-infected mice weregiven spleen cells from mice immunized witheither virus (Fig. 5). However, even with theseclosely related virus types, homologous immunespleen cells were more effective in mediatingrecovery in both infections than heterologousimmune spleen cells, as measured by the de-creases in virus titers in the livers on day 3 ofinfection.Course of infection in mice receiving im-

mune spleen cells. To characterize the effectofimmune spleen cells on the course of infectionin HSV-2-infected mice further, two groups ofmice were inoculated i.p. with 5 x 105 PFU ofHSV-2, and 8 x 107 immune spleen cells weretransferred after 24 h of infection to one-half ofeach group. On the following days the mice were

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FIG. 2. Adoptive transfer of recovery from hepati-tis with immune spleen cells. Groups of 4-week-oldBALB/c mice were inoculated i.p. with 5 x 1O5 PFUof HSV-2 and were then given immune spleen cells(ISC) 1 day before infection (-1), on the day of infec-tion (0), or 1 (+1) or 2 (+2) days after infection. Othergroups ofmice received disintegrated immune spleencells 1 day after infection (DIS-ISC +1), normalspleen cells 1 day after infection (NSC +1), or no cellsat all (C). The spleen cells were from mice immunizedi.v. 6 days previously with 2 x 105 PFU of virus. Thenumber of cells transferred was 5 x 107 cells per 0.5ml. The points represent the titers of virus in thelivers of individual mice 4 days after infection.

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FIG. 3. Number ofimmune spleen cells required toaccelerate the recovery process. Groups of4-week-oldBALB/c mice were inoculated i.p. with 5 x 105 PFUof HSV-2, and after 24 h the mice received spleencells i.v. from mice immunized i.v. 6 days previouslywith 2 x 105 PFU of HSV-2. Each point representsthe virus titer of the liver of one mouse on day 4 ofinfection. A line connects the means of the groups.

killed, and the livers were examined for viruscontents and histopathology.

Infectivity titrations of individual liversshowed that immune spleen cells exerted a clearantiviral activity 24 h after transfer. This effectwas accentuated further on the following days,so that the clearance of virus from the livers wasaccelerated by almost 2 days (Fig. 6).This impression was supported by microscopic

examinations of the livers. The histologies of theliver lesions in the two groups of mice werefundamentally alike. Thus, there was no signifi-cant difference between the numbers of lesionsin the two groups, nor was there any differencebetween the sizes of the individual foci in thetwo groups during the first days of infection;however, at the end of the study period the sizesof the lesions in the mice receiving immunespleen cells were only one-half the sizes of thelesions in the control mice.The development and healing of liver lesions

are shown in Fig. 7. This figure shows that themorphological events in the two groups werefundamentally alike, although the time coursesof the appearance or disappearance of theseevents differed greatly. Virus-induced liver celldegeneration, polymorphonuclear leukocyte in-

filtration, and residual necrotic material werenot present to the same extent or for as long atime in mice receiving immune spleen cells as incontrol mice. In contrast, migrating mononu-clear cells arrived earlier and more densely pop-ulated the lesions in the adoptively immunizedmice than in the mice that did not receive im-mune spleen cells.Even though immune spleen cells were able

to mediate an accelerated clearance ofvirus fromthe livers of mice infected with 5 x 105 PFU ofHSV-2, these cells had only a minimal effect onthe final outcome of the infection, which wasdeath from myelitis and encephalitis. In mortal-ity control groups of 20 mice infected with 5 x105 PFU of virus, adoptive transfer of 5 x 107immune spleen cells 24 h post-inoculation didnot protect the mice from death, even though aslight delay of about 1 day in the time of deathwas observed in mice that received immunespleen cells. This was also true when the virusdose was reduced to 105 PFU. When the virusdoses were reduced further to 104 and 103 PFU

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C MCMV Vac. HSV-2FIG. 4. Specificity of immune spleen cells. Groups

of4-week-old BALB/c mice were inoculated i.p. with5 x 10i PFUofHSV-2. After24 h each mouse received5 x 10' spleen cells i.v. from mice inoculated i.v. 6days previously with either 4.0 50% tissue cultureinfective doses of mouse cytomegalovirus (MCMV),106 PFU of vaccinia virus (Vac), or 2 x 105 PFU ofHSV-2. Each point represents the virus titer of theliver ofone mouse 4 days after infection. C, Control.

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FIG. 5. Specificity of immune spleen cells. Groupsof4-week-old BALB/c mice were inoculated i.p. with5 x 105 PFU of either HSV-1 or HSV-2. After 24 heach mouse received 5 x 107 spleen cells i.v. frommice inoculated i.v. 6 days previously with 2 x 105PFUofeitherHSV-1 orHSV-2. Eachpoint representsthe virus titer of the liver of one mouse 3 days afterinfection. C, Control.

(which still killed all control mice), survival ratesof 20 and 30%, respectively, were obtained in thegroups receiving immune spleen cells.

DISCUSSIONSpecific immunological reactions are generally

considered to be responsible for recovery fromprimary virus infections, as well as for resistanceto subsequent infection with the same agent. Inmost infections the recovery process may beascribed to an intense interaction among anti-bodies, complement components, and differentcell types (1). However, recently substantial ev-idence has indicated that cell-mediated immu-nity plays a major role in the elimination of theinfectious process in some virus infections, no-tably those caused by members of the herpes-virus and poxvirus groups (1, 4).

In this paper we describe a mouse model forexning the role of immunological mecha-nisms in recovery from HSV infections, in whichthe transfer of immune spleen cells as describedby Blanden (3, 4) was used as the experimentalapproach.The hallmark of our animal model is the focal

necrotizing hepatitis which develops in youngmice after i.p. inoculation of HSV-2. The normalcourse of infection in a liver is characterized byan initial phase lasting for 3 or 4 days, in whichthe virus titer increases, followed by a phase ofresolution, in which the virus is more or lesssuccessfully eliminated from the liver. Duringthe first days of infection, the necrotic lesionsincrease in size, and they are characterized byabundant necrotic tissue with predominantlypolymorphonuclear cell infiltration and acutelydegenerating liver cells in the periphery of thenecrotic foci. The regression ofthe lesions, whichis characterized by the disappearance of thedegenerating liver cells and the resolution of thenecrotic material, is accompanied by a progres-

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FIG. 6. Course of infection in the livers of 4-week-old BALB/c mice inoculated i.p. with 5 x 105 PFU ofHSV-2 with (0) and without (0) i.v. transfer of 8 x

107 spleen cells from mice immunized i.v. 6 dayspreviously with 2 x 105 PFU of HSV-2. isc, Immunespleen cells.

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Groups offour or five 4-week-old BALB/c mice wereinoculated i.p. with 5 x 105 PFU of HSV-2. After 24h one-half of the groups received 8 x 107 spleen cellsfrom mice immunized i.v. 6 days previously with 2 x105 PFU of HSV-2. (A) Virus-induced liver cell de-generation. (B) Infiltration with polymorphonuclearleukocytes. (C) Necrosis. (D) Infiltration with mono-nuclear leukocytes. Grading of the types of lesionswas as follows: 0, absent; 1, sparsely represented; 2,present; 3, extensively represented. Symbols: 0, recip-ients of immune spleen cells (isc); *, control mice.

sive infiltration of the lesions by mononuclearcells.

Conceivably, this mononuclear inflammatoryresponse may play a major role in the clearanceof HSV-2 from the infectious foci of the livers.This hypothesis was supported by the findingthat passive transfer of immune spleen cells toinfected mice accelerated the inflammatory re-

sponse. Thus, when 50 x 106 to 100 X 106 im-mune spleen cells were given 24 h after infection(i.e., at a time when hepatic foci were well estab-lished [14]), most lesions were densely populatedwith mononuclear cells on day 3 post-inocula-tion, a situation not reached until day 5 or 6 incontrol mice. Concomitantly, the progression ofthe infectious process was terminated, as judged

from the size of the focal lesions and from thedisappearance ofacutely degenerating liver cells,and the virus was cleared very rapidly from thelivers. The antiviral activity of the passivelyadministered immune cells was immunologicallyspecific, which is in agreement with previousfindings with ectromelia (3), Sindbis (7) andinfluenza (24) virus infections in mice. A highdegree of cross-reactivity between HSV-1 andHSV-2 was observed, which is not surprising inview of the close antigenic relationship of thesetwo virus types (17-19).

Previously, adoptive transfer of immunespleen cells has been found to confer antiviralactivity to mice infected with HSV (5, 6, 20, 21,23). However, these studies were conducted withHSV-1 and differ from our study in a number ofways. First, in most of these studies (5, 6, 20) theimmune spleen cells were administered from 12to 72 h before the virus was administered, pro-viding an opportunity for the cells to interferewith the establishment of the infection. Second,the recipient mice were immunosuppressed orotherwise immunologically manipulated to in-crease the severity of the normal course of infec-tion (6, 20, 21, 23). Third, all of the above-de-scribed studies focused on the ability of trans-ferred cells to protect the mice from death,which is usually caused by encephalitis. How-ever, Rager-Zisman and Allison (21) also re-ported that immune spleen cells exerted anti-viral activity in the livers of cyclophosphamide-treated, HSV-1-infected mice on day 6 of infec-tion, with reconstitution of the mononuclear cellinfiltrate of infectious foci.The mechanism by which transferred immune

spleen cells facilitate recovery from hepatitis wasnot determined in this study. However, the find-ing that the morphological events in the necroticfoci during a natural infection were fundamen-tally the same as those found after adoptivetransfer of immune spleen cells (except for tim-ing) suggests that the recovery mechanismstransferred with the immune spleen cells are thesame as those that are operative during thenormal course of infection. Therefore, our modelseems to be suitable for a closer examination ofthe recovery mechanisms that are operative dur-ing visceral HSV infections. Such studies are inprogress in our laboratory.

ACKNOWLEDGMENTSWe thank Aase Hvergel and Eln Jakobsen for their excel-

lent technical assistance.This work was supported by grant 512-20475 from the

Danish Medical Research Council and by grant 7131-01-2from the Aarhus University Research Foundation.

LITERATURE CITED1. Allison, A. C. 1974. Interactions of antibodies, comple-

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ment components and various cell types in immunityagainst viruses and pyogenic bacteria. Transplant. Rev.19:3-55.

2. Baron, S. 1973. The defensive and biological roles of theinterferon system, p. 267-293. In N. B. Finter (ed.),Interferons and interferon inducers. American ElsevierPublishing Co., Inc., New York.

3. Blanden, R. V. 1971. Mechanisms of recovery from ageneralized viral infection: mousepox. II. Passive trans-fer of recovery mechanisms with immune lymphoidcells. J. Exp. Med. 133:1074-1089.

4. Blanden, R. V. 1974. T cell response to viral and bacterialinfection. Transplant. Rev. 19:5648.

5. Ennis, F. A. 1973. Host defense mechanisms againstherpes simplex virus. H. Protection conferred by sensi-tized spleen cells. J. Infect. Dis. 127:632-638.

6. Howes, E. L, W. Taylor, N. A. Mitchison, and E.Simpson. 1979. MHC matching shows that at least twoT-cell subsets determine resistance to HSV. Nature(London) 277:67-68.

7. McFarland, H. F., D. E. Griffin, and R. T. Johnson.1972. Specificity of the inflammatory response in viralencephalitis. I. Adoptive immunization of immunosup-pressed mice infected with Sindbis virus. J. Exp. Med.136:216-226.

8. Merigan, T. C. 1974. Host defenses against viral disease.N. Engl. J. Med. 290:323-329.

9. Mogensen, S. C. 1976. Biological conditions influencingthe focal necrotic hepatitis test for differentiation be-tween herpes simplex virus types 1 and 2. Acta Pathol.Microbiol. Scand. Sect. B 84:154-158.

10. Mogensen, S. C. 1977. Role of macrophages in hepatitisinduced by herpes simplex virus types 1 and 2 in mice.Infect. Immun. 15:686-691.

11. Mogensen, S. C. 1977. Genetics ofmacrophage-controlledresistance to hepatitis induced by herpes simplex virustype 2 in mice. Infect. Immun. 17:268-273.

12. Mogensen, S. C. 1978. Macrophages and age-dependentresistance to hepatitis induced by herpes simplex virustype 2 in mice. Infect. Immun. 19:46-50.

13. Mogensen, S. C. 1979. Role of macrophages in natural

resistance to virus infections. Microbiol. Rev. 43:1-26.14. Mogensen, S. C., and H. K. Andersen. 1977. Effect of

silica on the pathogenic distinction between herpessimplex virus type 1 and 2 hepatitis in mice. Infect.Immun. 17:274-277.

15. Mogensen, S. C., and H. K. Andersen. 1978. Role ofactivated macrophages in resistance of congenitallyathymic nude mice to hepatitis induced by herpes sim-plex virus type 2. Infect. Immun. 19:792-798.

16. Mogensen, S. C., B. Teisner, and H. K. Andersen.1974. Focal necrotic hepatitis in mice as a biologicalmarker for differentiation of Herpesvirus hominis type1 and type 2. J. Gen. Virol. 25:151-155.

17. Nahmias, A. J., and B. Roizman. 1973. Infection withherpes-simplex viruses 1 and 2. I. N. Engl. J. Med. 289:667-674.

18. Nahmias, A. J., and B. Roizman. 1973. Infection withherpes-simplex viruses 1 and 2. II. N. Engl. J. Med. 289:719-725.

19. Nahmias, A. J., and B. Roizman. 1973. Infection withherpes-simplex viruses 1 and 2. III. N. Engl. J. Med.289:781-789.

20. Oakes, J. E. 1975. Role for cell-mediated immunity in theresistance of mice to subcutaneous herpes simplex virusinfection. Infect. Immun. 12:166-172.

21. Rager-Zisman, B., and A. C. Allison. 1976. Mechanismof immunologic resistance to herpes simplex virus 1(HSV-1) infection. J. Immunol. 116:35-40.

22. Welsh, R. M., and R. M. Zinkernagel. 1977. Heterospe-cific cytotoxic cell activity induced during the first threedays of acute lymphocytic choriomeningitis virus infec-tion in mice. Nature (London) 268:646648.

23. Worthington, K, M. A. Conliffe, and S. Baron. 1980.Mechanism of recovery from systemic herpes simplexvirus infection. I. Comparative effectiveness of antibodyand reconstitution of immune spleen cells on immuno-suppressed mice. J. Infect. Dis. 142:163-174.

24. Yap, K. L, and G. L Ada. 1978. The recovery of micefrom influenza virus infection: adoptive transfer of im-munity with immune T lymphocytes. Scand. J. Immu-nol. 7:389-397.

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