Fatal persistence of West Nile virus subtype Kunjin in the brains of flavivirus resistant mice

Flaviviruses cause febrile illnesses in humans that may progress to encephalitis and death. Both viral and host factors determine the level of virus replication and outcome of infection. In mice, genetically determined resistance conferred by the flavivirus resistance locus (Flv) is responsible for the restricted flavivirus replication and prevention of disease development. Majority of flaviviruses express significant virulence, replicate to high titers and cause high mortality in susceptible mice, while congenic resistant mice endure the infection, show significantly reduced levels of virus replication and remain healthy. In contrast, infection with West Nile virus subtype Kunjin (KUNV) causes morbidity and fatal outcomes even in mice that are naturally resistant to flaviviruses. There are two possible mechanisms that could account for such an unforeseen virulence of KUNV in resistant mice: (a) an abrogation of Flv-controlled natural resistance leading to high virus replication, or (b) massive virus-induced immunopathology in the brain. To identify the cause(s) of fatality of KUNV infection, disease progression, virus replication and brain histopathology were studied in parallel in resistant and congenic susceptible mice. While KUNV replicated to high titers causing early fatalities in susceptible mice, it showed only reduced replication associated with the delayed morbidity in resistant mice indicating no abrogation of the Flv resistance. No evidence of excessive immune cell infiltration and tissue damage following KUNV infection were found. However, incomplete KUNV clearance not previously described was perceived as an important source of pathogenesis in resistant mice.     

Analysis of age-dependent resistance to murine coronavirus JHM infection in mice.

Resistance to intraperitoneal murine coronavirus JHM infection in mice develops with age. C3H mice were found to be fully susceptible up to the age of 20 days and resistant after 23 days of age. Protection of susceptible animals from death due to infection could be achieved by maternal antibodies or by transfer of spleen cells from immunized, but not from nonimmunized, donor mice. Lack of protection by transfer of unprimed adult spleen cells was not related to immunosuppression by the host. Moreover, resistance of adult mice could not be abrogated by application of lymphocytes from suckling mice, although immune suppression by other means did affect the resistance of adult animals. On the other hand, spleen cells from nonimmunized mice could be primed with inactivated JHM virus in suckling mice and protected these mice from death due to a subsequent virus infection. Thus, the outcome of infection with JHM virus in suckling and adult mice can be influenced by immunological events, but is not exclusively due to the different stages of immune competence.     

Genetic resistance to lethal flaviviral encephalitis

Summary The replication of Banzi virus, a flavivirus, was comparedin vitro andin vivo in tissues of congenic mice genetically resistant (C3H/RV) or genetically susceptible (C3H/He) to lethal infection. Ultrastructural changes in brains of resistant and susceptible adult mice following intracerebral or intraperitoneal inoculation of virus also were compared.Banzi virus replicated equally well in monolayer cultures of infant and adult brain, stimulated and non-stimulated macrophages and embryonic cells from both strains of mice. Similarly, no significant differences were found between strains in virus growth in brain, spleen or thymus of peripherally-inoculated infant mice. In intracerebrally-inoculated adult mice, virus titers in brains of resistant mice were consistantly lower than in susceptible mice.Visualization of virus particles was dependent on virus concentration in tissues. The changes in brain tissues of both strains of mice were similar, differing only in the time of onset which was noted two days later in C3H/RV mice than in C3H/He mice.These results indicate that, in the case of Banzi virus, the phenotypic expression of genetically-determined resistance to lethal flavivirus infection cannot be attributed primarily to the ability of host cells to support virus replication.With 10 Figures     

Cellular basis for susceptibility to mouse cytomegalovirus: evidence from tracheal organ culture.

Tracheal organ cultures were prepared from strains of mice with known susceptibility to the lethal effects of mouse cytomegalovirus. When organ cultures from susceptible mice were infected with this virus, characteristic c.p.e. developed and virus titres greater than 10(5) p.f.u./ml were produced 2 to 3 weeks after infection. Identically infected tracheal organ cultures from resistant mice consistently failed to develop significant virus c.p.e. and produced 10- to 100-fold lower titres of virus. When fibroblast cultures were infected with mouse cytomegalovirus, however, no consistent differences between susceptible and resistant mouse strains were observed. Since cytomegalovirus replicates in epithelial cells both in vivo and in tracheal organ culture, these results suggest that resistance may be based in part upon innate genetic susceptibility of epithelial cells to cytomegalovirus infection.     

Replication of herpes simplex virus type 1 in macrophages from resistant and susceptible mice.

Studies were carried out to determine whether the in vitro capacity of adherent peritoneal cells to replicate herpes simplex virus type 1 (HSV-1) might correlate with the in vivo susceptibility of mice genetically resistant, moderately susceptible, or very susceptible to HSV-1 infection. Unstimulated and proteose peptone-stimulated monolayers restricted viral replication when infected immediately, but replicated HSV-1 when infected after 3 to 7 days of culture. Macrophages from resistant C57Bl/6 mice restricted HSV-1 replication significantly better than cells from susceptible mice. This function did not segregate with resistance, since macrophages from resistant F1 mice failed to restrict HSV-1 replication. Induction of peritoneal exudate cells with thioglycolate yielded cells capable of replicating HSV-1 when infected immediately after plating and after 4 days of culture.     

Resistance to vaginal or systemic infection with herpes simplex virus type 2

Summary Mortality due to vaginal or intravenous infection of female BALB/c mice with herpes simplex virus type 2 (HSV-2) was significantly reduced by treatment of mice with the immunomodulator pyran. Following intravaginal inoculation with virus, the incidence of vaginal infection and titers of virus present in the vaginal secretions were significantly reduced in pyran treated as compared with control mice. Vaginal infection did not elicit neutralizing antibody activity in either untreated or pyran treated mice. Intravenous HSV-2 infection did elicit virus specific humoral and delayed type hypersensitivity responses. However, pyran treatment markedly reduced the incidence of these immune responses as compared with those in control intravenously infected mice. Moreover, mice surviving the initial vaginal or intravenous infection with HSV-2 were resistant to subsequent rechallenge by the same route. Pyran treatment did not enhance resistance to vaginal rechallenge, while pyran treated mice were not resistant to intravenous rechallenge. Thus, the antiviral protection mediated by pyran does not appear to involve enhancement of specific humoral or cellular immune responses. Nonspecific host mediated resistance may be involved in the protective effect of the immunomodulator, as suggested by the ability of pyran activated peritoneal cells to inhibit HSV-2 growthin vitro and to transfer resistance to recipient mice challenged with the virus.With 1 Figure     

The pregnant ferret as a model for studying the congenital effects of influenza virus infection in utero: infection of foetal tissues in organ culture and in vivo.

Organ cultures of ferret foetal tissues showed a similar pattern of susceptibility to influenza virus to that already observed for human foetal tissues (Rosztoczy et al., 1975); respiratory, alimentary and urogenital tissues supported the replication of influenza virus but nervous and lymphopoietic tissues (those which, in man, are associated with foetal or postnatal abnormalities) were insusceptible. In contrast to corresponding human tissues, ferret foetal placenta and amnion readily supported viral replication although both human and ferret umbilical cord were susceptible. In limited experiments, neither the membranes nor the susceptible foetal tissues became infected after intranasal inoculation of pregnant ferrets of various gestational ages. However, after intracardial inoculation of pregnant ferrets with high titre virus (ca 10(9) EBID50) virus was isolated from both foetal membranes and foetuses. The membranes became infected at early, middle and late gestation, but virus appeared to cross the placental barrier to infect foetal tissues only in late gestation. At this stage virus could be isolated not only from those foetal tissues (respiratory, alimentary and urogenital) susceptible in organ culture, but also in small amounts from tissues which were insusceptible in organ culture (heart, lymphopoietic and nervous tissue). Virus was also isolated from foetal membranes and foetuses of late gestation ferrets following intracardial inoculation with a one hundred-fold lower dose of virus which, unlike the higher dose, did not induce a maternal febrile response. The pregnant ferret appears to be a suitable model for investigating the effects on development of foetal infection with influenza virus but it may have disadvantages with regard to the nature and strength of the placental barrier.     

Depletion of CD8(+) T cells in vivo impairs host defense of mice resistant and susceptible to pulmonary paracoccidioidomycosis

Using a pulmonary model of infection, we demonstrated previously that A/Sn and B10.A mice are, respectively, resistant and susceptible to Paracoccidioides brasiliensis infection. Employing the same experimental model, we examined herein the role of CD8(+) T cells in the course of paracoccidioidomycosis. Treatment with anti-CD8 monoclonal antibodies caused a selective depletion of pulmonary and splenic CD8(+) T cells in both mouse strains. The number of pulmonary CD4(+) T cells and immunoglobulin-positive cells was independent of the number of CD8(+) T cells. In susceptible mice, the loss of CD8(+) T cells by in vivo treatment with anti-CD8 monoclonal antibodies impaired the clearance of yeasts from the lungs and increased the fungal dissemination to the liver and spleen. The same treatment in resistant mice increased fungal dissemination to extrapulmonary tissues but did not alter the pulmonary fungal load. Furthermore, CD8(+) T-cell depletion did not modify delayed-type hypersensitivity reactions of A/Sn mice but increased these reactions in B10.A mice. The production of P. brasiliensis-specific antibodies by resistant and susceptible mice depleted of CD8(+) T cells was similar to that of mice given control antibody. Histopathologically, depletion of CD8(+) T cells did not disorganize the focal granulomatous lesions developed by both mouse strains. These results indicate that CD8(+) T cells are necessary for optimal clearance of the fungus from tissues of mice infected with P. brasiliensis and demonstrate more prominent protective activity by those cells in the immune responses mounted by susceptible animals.     

Response of genetically susceptible and resistant mice to intranasal inoculation with mouse hepatitis virus JHM

Mouse hepatitis virus (MHV)-JHM infection was studied in genetically susceptible (BALB/cByJ) and resistant (SJL/J) mice following intranasal inoculation at 1, 3, 6 or 12 wk of age. Markers of infection included histology, immunohistochemistry, virus quantification and virus serology. All BALB mice developed severe disseminated disease with high mortality due to encephalitis and hepatitis. Peak MHV titers appeared in brain, liver, spleen and intestine on days 3 or 5. Age at inoculation did not influence virus titers in brain, spleen or intestine, but virus titers in liver were inversely proportional to age at inoculation. In 6-wk-old BALB mice, virus was cleared from spleen, intestine and liver by day 30 and from brain by day 60. In intestine, MHV was localized to lymphoid tissue, without fecal excretion. SJL mice of all ages developed remarkably milder disease with low mortality occurring only among mice inoculated at 1 wk of age. SJL mice inoculated at 1 wk had disseminated infection at day 3, but lesions and antigen were cleared from most organs by day 5. Mice inoculated at 3 and 6 wk of age had minimal or no involvement of peripheral organs, and mice inoculated at 12 wk of age had infections restricted to the nose. At day 5, MHV titers in brain, liver, spleen and intestine were significantly lower or undetectable in SJL mice of all ages compared to age-matched BALB mice. In 6-wk-old mice, MHV was cleared from all organs by day 10. Serum antibody titers to MHV were many-fold higher in BALB mice, compared to SJL mice, which mounted only a modest response.     

Resistance to Yersinia pestis infection decreases with age in B10.T(6R) mice

We demonstrate that 2-month-old female B10.T(6R) mice are highly resistant to systemic infection with the KIM5 strain of Yersinia pestis and that B10.T(6R) mice become susceptible to Y. pestis infection by the age of 5 months. In this study, young (2-month-old) and middle-aged (5- to 12-month-old) B10.T(6R) mice were infected with equal CFU counts of Y. pestis. The 50% lethal dose (LD(50)) for young B10.T(6R) mice was ～1.4 × 10(4) CFU, while middle-aged B10.T(6R) mice exhibited an LD(50) of ～60 CFU. Elevated bacterial burdens were found in the spleens of middle-aged mice at 24 and 60 h and in the livers at 60 h postinfection. Immune cell infiltration was greater in the livers of resistant young mice than in those of middle-aged mice and mice of the susceptible C57BL/6N strain. Unlike susceptible mice, young B10.T(6R) mice did not develop necrotic lesions throughout the liver. Instead, livers from young B10.T(6R) mice contained granuloma-like structures. Immunohistochemical staining of liver sections from these mice at 60 h postinfection revealed that the majority of immune cells present in these structures were neutrophils. These findings suggest that resistance to plague in B10.T(6R) mice correlates with early formation of neutrophilic lesions in the liver.     

Resistance to highly virulent mouse hepatitis virus acquired by mice after low-virulence infection: enhanced antiviral activity of macrophages.

As early as 1 to 2 days after intranasal inoculation with a mouse hepatitis virus of low virulence, MHV-S, susceptible DDD mice became fully resistant to a normally lethal challenge with a highly virulent MHV-2. The resistance of MHV-S-pretreated mice was correlated with significantly decreased MHV-2 multiplication in the liver, spleen, and brain. Infection with MHV-S did not induce a high level of interferon in DDD mice, and no neutralizing antibody against MHV-2 was detected in the sera of mice until day 6 of MHV-S infection. The multiplication of MHV-2 was suppressed in peritoneal cells (PC) in vivo and peritoneal adherent cells (PAC) in vitro of MHV-S-pretreated mice was compared with those of normal mice. This suppression of virus multiplication was demonstrated in PAC collected during days 1 to 3 of infection but not in PAC collected from day 5 on. PC from MHV-S-pretreated mice were also suppressive to MHV-2 growth in DK cells as compared with PC from normal mice. By treatment of MHV-S-pretreated mice with silica, suppression of virus growth in the liver was partially diminished. These findings suggest that increased suppression of MHV-2 growth in PAC (mostly macrophages) of MHV-S-pretreated mice is responsible for resistance.     

Sendai virus infection in genetically resistant and susceptible mice.

The pathogenesis of Sendai virus infection in genetically resistant (C57Bl/6) and susceptible (DBA/2) nonimmune adult mice was investigated. Rising serum complement-fixation (CF) antibody titers were delayed in DBA/2 mice as compared with C57Bl/6 mice. C57Bl/6 mice developed descending desquamative endobronchiolitis, and DBA/2 mice developed descending proliferative endobronchiolitis and bronchogenic alveolitis. Peribronchiolar lymphoid cuffs that formed in C57Bl/6 mice were thicker and more densely populated than those of DBA/2 mice. Immunofluorescence demonstrated viral antigens confined to the epithelial lining of conducting airways in C57Bl/6 mice but extending to alveolar corner cells in DBA/2 mice. Studies with a transmission electron microscope confirmed that Type II pneumocytes were infected only in DBA/2 mice. IgG-containing cells selectively accumulated along the airways of both strains, but fewer were recruited by DBA/2 mice. These results suggest that genetic resistance to Sendai virus is expressed through the immune system.     

Mouse hepatitis virus and host determinants of vertical transmission and maternally-derived passive immunity in mice

Summary Transmission of mouse hepatitis virus (MHV) in utero following oronasal inoculation of pregnant mice was found to depend upon MHV strain and host genotype. Virulent, polytropic MHV-JHM was recovered from multiple maternal tissues, including liver and uterus, as well as placenta and fetus in susceptible BALB/cByJ mice. Fetuses were infected during all 3 trimesters of pregnancy. Low virulence, polytropic MHV-S infected fetuses in a low percentage of susceptible BALB/cByJ dams. Infection of resistant CD-1 mice with MHV-JHM was limited, with no fetal infection. Enterotropic MHV-Y was largely restricted to intestine of BALB/cByJ and CD-1 dams, with minimal dissemination and no fetal infection. Maternally-derived MHV IgG antibody was detectable in pup sera through 4 weeks of age. Antibody titers were generally lower in second litters of the same dam. Cross-fostering experiments showed that antibody was transferred via colostrum and not in utero, and that pups were capable of absorption through 2 weeks of age. Pups nursing immune dams were protected against MHV challenge at 1 and 2 weeks of age, compared to pups nursing naive dams. Immunity to MHV challenge was cross-protective against both antigenically homotypic and heterotypic strains of MHV.     

Macrophage activity in resistant and susceptible mouse strains infected with Mycobacterium lepraemurium.

The level of activation of peritoneal macrophages following subcutaneous inoculation of resistant (C57BL) and susceptible (BALB/c) mice was assessed by monitoring superoxide anion and hydrogen peroxide production and also tumour cell cytostasis. The level of systemic macrophage activation appeared to correlate with bacterial load, rather than resistance to infection. It was observed that the more susceptible (BALB/c) strain developed higher and more sustained levels of systemic macrophage activation, whereas the more resistant (C57BL) strain showed only low transient levels of macrophage activation. In contrast, in vivo challenge of subcutaneously infected C57BL mice, via the intra-peritoneal route, with heat-killed Mycobacterium lepraemurium and thioglycollate resulted in a high level of macrophage activation compared with similarly treated uninfected mice. Similar treatment of susceptible BALB/c mice, however, did not result in enhanced macrophage activation. It was also observed that high levels of macrophage activation occurred in T-cell deprived C57BL mice following infection with M. lepraemurium.     

Invasion of mouse brain by Mount Elgon bat virus

Mount Elgon bat virus killed mice up to 13 days of age when given intranasally. Virus reached the brain of these mice via the olfactory nerve route without obvious multiplication in any tissues other than the nasal mucosa of 1- to 6-day-old mice and in the absence of viraemia or circulating virus neutralizing antibody. Large amounts of interferon were, however, synthesized in brain where virus grew to high titres. In mice older than 13 days virus did not multiply in brain but it reached the olfactory bulbs and persisted until virus neutralizing antibody appeared in the nasopharynx. No antibody was detected in blood of the resistant mice, nor was interferon detected in their brains or nasal mucosa. Immunosuppression of the resistant mice with cyclophosphamide resulted in moderate virus growth in mid- and hind-brain accompanied by interferon synthesis and death of the mice. The local immune response prevented invasion of mid- and hind-brain in the resistant mice.     

Diabetes-prone NOD mice are resistant to Mycobacterium avium and the infection prevents autoimmune disease.

It was recently proposed that the diabetes genes of non-obese diabetic (NOD) mice are linked to the Bcg gene that is associated with resistance to infection by mycobacteria; however, it has not been established whether NOD mice are resistant or susceptible to the infection, although there are previous investigations on response of NOD mice to other intracellular parasites (e.g. Kaye et al., Eur. J. Immunol. 22: 357-364). We have investigated here this question, as well as the consequences of mycobacterial infection on the natural history of murine diabetes. Female NOD mice were intraperitoneally infected with 10(8) viable bacilli of Mycobacterium avium at 2 months of age, i.e. before the mice show diabetes; they were studied up to the sixth month of age (when more than half of untreated female NOD mice show glycosuria). To determine whether NOD mice were susceptible or resistant to M. avium infection, we have compared the kinetics of bacterial growths in liver and spleen of the mice with those determined in M. avium-susceptible (BALB/c) and resistant (C3H) strains of mice. NOD mice were able to control the M. avium infection, following a pattern similar to that observed in infected C3H mice. The mycobacterial infection prevented the expression of diabetes in all of the infected NOD mice and it also decreased the incidence of proteinuria in the treated mice. The infected NOD mice showed a marked enhancement in antibodies against the 65,000 mycobacteria antigen (heat-shock protein (hsp) 65) up to the second month of infection and these elevated titres slowly decreased in the following months; anti-hsp 65 antibodies were not detected in age-matched controls. This is the first demonstration that NOD mice are naturally resistant to mycobacterial infection, and we reinforce evidence on the role of immune response triggered by mycobacteria and its hsp 65 antigen in prevention of diabetes in NOD mice.     

In Vivo and In Vitro Models of Demyelinating Disease: Endogenous Factors Influencing Demyelinating Disease Caused by Mouse Hepatitis Virus in Rats and Mice

Intracerebral inoculation of JHM virus (JHMV), the neuropathic strain of mouse hepatitis virus, into Wistar Furth, Wistar Lewis, and Fischer 344 rats at various ages indicated that Wistar Furth rats are more susceptible to the virus than are the other strains. Fischer 344 and Wistar Lewis rats were more resistant to inoculation at 2 and 5 days of age and completely resistant by 10 days of age. In contrast, Wistar Furth rats which were very susceptible at both 2 and 5 days of age remained susceptible until 21 days of age. Intracerebral challenge of an F1 cross between Wistar Furth and Wistar Lewis rats at 10 days of age indicated that resistance to JHMV infection is dominant. Cyclophosphamide treatment 28 days after intracerebral inoculation exacerbated an inapparent infection, leading to paralysis in eight of nine and death in six of nine Wistar Furth test rats. In such immunosuppressed animals, grey- and white-matter lesions were noted throughout the central nervous system, in contrast to the purely demyelinating lesions noted previously. Since rats, unlike mice, were not susceptible to disease after intracerebral injection with the serorelated viscerotropic strain MHV-3, we wished to extend our understanding of the neurological disease process elicited by the two viruses in rodents. For this reason, various mouse strains, including some with recognized immunodeficiencies, were challenged by different routes of inoculation. Intraperitoneal infection of nude and beige mice with JHMV indicated that lack of natural killer cell functions does not markedly enhance the susceptibility to virus, whereas T-cell activity appears to be essential for resisting infection. JHMV and MHV-3 replication in peritoneal macrophages from highly resistant A/J mice was reduced in comparison with that noted in macrophages from susceptible C57BL6/J mice. An initial intraperitoneal inoculation of JHMV was able to protect C57BL6/J mice against fatal intracerebral challenge within 3 days, whereas A/J mice remained susceptible beyond day 3. The protective effect did not appear to result from increased levels of circulating interferon, preceded elevation in serum JHMV-neutralizing antibody titers, and persisted for at least several weeks after intraperitoneal inoculation. Based on the combined studies described here and on previous work by us and others, it appears that the factors influencing the outcome of coronavirus disease in rodents are age at inoculation, route of challenge, genetic constitution of the virus and host, and competence of the immune system, particularly cellular immunity involving T-cells.     

Interaction between 6/94 virus, a parainfluenza type 1 strain, and mouse macrophages.

The 6/94 virus, a type 1 parainfluenza virus recovered from multiple sclerosis brain cells after lysolecithin-induced fusion of these cells with African green monkey kidney cells (CV-1), has been found to grow in splenic and peritoneal macrophages obtained from outbred and different strains of inbred mice. Macrophages from C57BL animals were least susceptible to infection, a resistance apparently partially age related. The virus also has been found to replicate in IC21 cells, a line of simian virus 40 virus-transformed mouse macrophages. Viral growth was detected by development of hemadsorption in infected cells, followed by the appearance of infectious virus. The growth of 6/94 virus had different kinetics in mouse macrophages, in the standard continuous cell lines L, 3T3, and CV-1, and in primary mouse kidney and mouse embryo cells. The virus produced in macrophages could be passed in series to other macrophage cultures. In addition, once infected, the cultures continued to produce virus, and permanently infected cell lines were thus obtained. Macrophages from immunized mice with high titers of humoral neutralizing antibodies were found variably able to support virus growth.     

Delayed hypersensitivity to murine cytomegalovirus and its depression during pregnancy.

A delayed hypersensitivity skin test for murine cytomegalovirus is described, in which ear swelling is measured after injection of heat-killed virus. The response appeared within 4 days of infection, peaked at 8 days, and remained at high levels for at least 100 days. When live virus was inoculated into the ear of previously uninfected mice, a much greater degree of ear swelling was seen, maximal at 7 days, but without growth of virus in the inoculated ear. Mice infected with avirulent (cell culture-passaged) virus gave greater delayed hypersensitivity responses than with those infected with virulent (salivary gland-passaged) virus. One of the strains of mice that is susceptible to murine cytomegalovirus (C57BL) developed greater delayed hypersensitivity responses than did the resistant strain (C3H). The delayed hypersensitivity response of neonatally infected mice was greatly depressed during pregnancy and lactation.     

Differential susceptibility and resistance of immunocompetent and immunodeficient mice to fatal Hantaan virus infection

Summary Susceptibility and resistance to fatal Hantaan virus meningoencephalitis were studied in immunocompetent (nu/+) and congenitally T-cell deficient (nu/nu) CD-1 mice of different ages. Susceptibility of nu/+ mice to fatal infection was age-dependent, as evidenced by 100 percent mortality in mice inoculated intracerebrally with Hantaan virus (strain 76–118) during the first week of life, 60 percent mortality in mice inoculated at 9 days of age, and no disease or death in mice inoculated at 14 to 42 days of age. Deaths occurred significantly earlier in nu/+ mice inoculated at 5 and 7 days of age than in nu/+ mice less than 24 hours old. Nu/nu littermates of the same age did not exhibit a similar inverse relationship between age and survival times. Moreover, nu/nu mice 14 days or older remained susceptible, albeit with delayed onset of illness and time of death. Virus titers in brain tissues of nu/+ mice inoculated at 7 days and of nu/nu nice inoculated at 7, 9 and 14 days of age were nearly identical. In older nu/+ mice, peak virus titers were comparatively lower, but they did not seem to be influenced by the magnitude of the neutralizing antibody response. The more rapidly fatal course in nu/+ mice inoculated at a week of age than at birth, and the differential resistance between weanling nu/+ and nu/nu mice to Hantaan virus meningoencephalitis suggest that cell-mediated immunity may be responsible for both enhancement of disease and recovery from infection.With 5 Figures