New attenuation marker for junin virus based on immunologic responses of guinea pigs

Susceptible adult murine model for Junin virus

The adult mouse model had been considered resistant to Junin virus (JV) infection. However, we found that C3H/HeJ murine strain proved highly susceptible up to 5 months of age to intracerebral inoculation with the prototype XJ JV strain, showing neurological signs and 80-90% mortality within 13 days. Neutralizing antibodies (Nt Ab) were absent, but low immunofluorescent Ab levels (1:5) were detected as from day +7. The virus could only be rescued by coculture of brain samples with Vero cells. Histopathological findings were consistent with the suckling mouse model and with a delayed-type hypersensitivity reaction. XJ inoculation by extraneural routes failed to cause disease, however, it induced Nt Abs. Ic infection with XJCL3 strain of JV attenuated for man and guinea pig, but not for mouse, induced high Nt Ab levels but not mortality. In either case, mice resisted ic XJ challenge. Thus, C3H/HeJ is the first adult mouse model susceptible to JV.

Junin virus-induced delayed-type hypersensitivity suppression in adult mice

Junin virus (JV) infection of suckling mice leads to lethal meningoencephalitis consistent with a delayed-type hypersensitivity (DTH)-like immune response. In contrast, there are no central nervous system (CNS) alterations, and high antibody titers are induced in resistant adult mice. As a possible explanation, JV infection in adult mice may provoke DTH depression. Thus in this work we study the alterations induced by JV in the immune response of adult mice by using sheep red blood cells (SRBC) as an unrelated indicator antigen. JV infection was found to abrogate DTH significantly, regardless of SRBC priming time, virus strain attenuation, and viral route of inoculation. The effect proved viral dose-dependent and required live and infectious virus. However, the humoral response to SRBC, as determined by splenic "plaque-forming cell" count was found higher than controls. These results are consistent with adult mouse response to JV infection. In contrast to the guinea pig model, there is no destruction of immunocompetent cells. T-cell percentage in the spleen was high, suggesting involvement with DTH-suppressive action. We suggest that the immune response to SRBC in adult infected mice may be used for understanding the mechanisms involved in resistance.

Treatment of junin virus-infected guinea pigs with immune serum: development of late neurological disease

Guinea pigs infected with Argentine hemorrhagic fever virus (Junin) were treated with pooled, homologous convalescent sera. Use of 15,000 or 5,000 therapeutic units of immune sera prevented all signs of illness when administered within 24 hr of infection. We could also prevent illness and death in infected guinea pigs as late as 6 days after infection if we used more antisera (30,000 therapeutic units/kg). In some treatment groups, surviving animals developed a late neurological syndrome with prominent rear-limb paralysis. Treated animals typically expressed higher viral titers in the brain than in any other organ. There appeared to be no acute exacerbation of disease by antibody administration. Our data suggest that, after replicating peripherally, Junin virus infects the brain where circulating immunoglobulins may not eliminate viable virus. Subsequent replication of virus in the brain may generate a neurological phase of the illness. Histological examination of brains from guinea pigs in treatment groups favoring the neurological phase of illness showed encephalitis, meningitis, and swollen astrocytes, suggestive of neuronal degeneration. There is likely a delicate balance among presence of virus in the brain, the amount of antibody transported into the central nervous system, and the occurrence of this late neurological aspect of experimental Argentine hemorrhagic fever. Further study of this model may elucidate factors relevant in understanding the continuing problem of the late neurological syndrome seen in some human cases of Argentine hemorrhagic fever treated with immune plasma.

Effect of immunosuppression on experimental Argentine hemorrhagic fever in guinea pigs

Immunosuppression with cyclosporin A or cyclophosphamide had no apparent effect on the disease course of guinea pigs infected with a virulent strain of Junin virus. Immunosuppression of guinea pigs infected with an attenuated strain of Junin virus led to fulminating Argentine hemorrhagic fever. 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 infected with a virulent strain. Histopathological lesions in immunosuppressed guinea pigs infected with an attenuated strain of virus were similar to those in non-immunosuppressed guinea pigs infected with a virulent strain. Histological changes attributable to the immunosuppressive drug(s) were regularly observed. Immunosuppressed animals infected with attenuated Junin virus and non-immunosuppressed animals infected with virulent virus failed to develop antibody or responded at a minimal level. Virus-specific cytotoxic spleen cell activity, previously shown to be antibody dependent, failed to develop in the same animals. The presence of a competent immune response, probably serum antibody, determined whether Argentine hemorrhagic fever infection of the guinea pig was lethal or whether recovery ensued; no evidence for harmful effects of the immune response was obtained.