Maturation of suckling rat response to SRBC.

The immune response of the Buffalo/Sim rat to heterologous sheep red blood cells (SRBC) was studied here. The earliest time of response to 10(9) SRBC, the most suitable inoculation route and the behavior to challenge were determined. The intraperitoneal (ip) proved more effective than the subcutaneous (sc) route, since serum agglutinins became detectable in low titers in animals inoculated at 6-7 days of life by the former route and at 12 days by the latter. No splenic plaque-forming cells (PFC) were found in rats immunized ip at 2 days of age, and strong inhibition developed on challenge at day 14 post-inoculation (pi) (agglutinin titers at day 7 pi: 0.71 +/- 0.47 vs 4.6 +/- 0.51 in unprimed controls; PFC/10(7) cells at day 5: 122.21 +/- 36.17 vs 3,977.38 +/- 777.5 in unprimed controls). Serum agglutinin formation was also decreased, though to a lesser degree, when: a) animals were challenged at 30 or 60 days of age; b) both priming and challenge took place by sc route; or c) antigen dose was lowered to 10(7) or 10(5) SRBC. Mechanisms interpreting observed behavior are discussed.

Abrogation of Junin virus encephalitis by critical cyclophosphamide timing and dosage.

Junin virus-induced encephalitis in suckling mouse is a delayed-type hypersensitivity reaction, whose immunopathologic nature has been proven by suppressing the thymus-dependent response. Cyclophosphamide (CY) given at day +6 post-infection (p.i.) has been shown to modulate infection, presumably by TDTH lymphocyte inactivation. To determine critical timing and i.p. drug dose, brain histology and survival were studied in 3-day-old Balb/c mice, inoculated i.c. with Junin virus. Optimal protection was achieved with a non-toxic, 50 mg/kg CY dose at day 6 p.i. (+6): no brain tissue damage was detected in animals killed at day +12, when the necropsied controls exhibited widespread lesions. Other timings (day +3, +4, +5) proved less effective. As regards alternative dosage at day +6, 30 mg was useless, and severe leptomeningitis was evident, whereas 40 mg significantly lowered mortality, and lesions were much milder and less constant. It seems that the 50 mg/kg CY dose must be administered at a critical time p.i. to inactivate sensitized TDTH lymphocytes and to reduce mortality and CNS pathology significantly.

Studies of cell-mediated immunity to Junín virus.

Junín virus is uniformly lethal in newborn mice, but fails to kill adult animals. But there is direct evidence that in older mice the immunological mechanism occurs when certain conditions are fulfilled. The present results showed that in Junín virus infection of mice, the development of the immunological mechanism occurs irrespectively of age. The induction of a regular cell-mediated immunity appears to be intimately related to multiple injections of the virus. These results were confirmed by the 51Cr release assay and by the effect of the transplanted sensitized cells. The extent of infection in the brain is another variable to be considered. Only when the virus has reached high titres and has been concentrated in the brain, the damage done by the immune attack is lethal. On the other hand the data obtained suggest that Junín virus-sensitized spleen cells do not possess the ability to transfer significant antiviral effects to recipient preinfected newborn mice.

Further experiments on the action of antithymocyte serum in experimental Junín virus infection.

Rabbit anti-mouse thymocyte serum (ATS) administrated as late as 7 days after infection suppressed host cell-mediated responsiveness to intracerebrally injected Junín virus, thereby diminishing the morbility and mortality of this infection. It did not affect either the humoral antibody response or the virus titer in brain. This findings suggest that: a) in mouse brain cells, Junín virus infection is basically non-cytopathic: b) cell-mediated immunity is responsible for morbility and mortality and does not clear up virus from brain as in other viral encephalitides; and c) ATS may be of therapeutic interest by suppressing or diminishing the cell-mediated response to Junín virus.

Passive immunity against Junín virus in mice.

Passive immunity, naturally acquired from immune mothers or artificially induced by the administration of homologous hyperimmune serum, conferred on suckling mice a high degree of resistance against infection with Junín virus. Maternal antibodies in the circulating blood of the young were not detectable in the first days after birth, but rised rapidly from the 8th to the 20th day of lactation. By cross-foster nursing experiments it was shown that the greater part of the transmission of passive immunity occurred after birth, although there was transmission of a significant, though small part, before birth. The virus passage from mothers to offspring was excluded, since Junín virus was not recovered from brains, livers, spleens and kidneys of uninfected young, born from infected mothers.

Development of specific immune response in mice infected with Junin virus.

Different parameters of specific immune response involved in the resistance to intracerebral Junin virus (JV) infection were studied in adult BALB/c mice. The relationship of virus replication to production of antiviral antibodies, to occurrence of cytotoxic T cells and to development of delayed-type hypersensitivity response was evaluated. Spleen cytotoxic T cells were assayed by 51Cr-release method on virus-infected H2 compatible targets. Effector T cells were detected on day 2, reached peak concentrations by day 6 and declined on day 10. These cells seemed responsible, at least in part, for virus clearance from the infected target organ, since virus could not be recovered from the brain in any sample taken on days 2, 5, 6, 8, 10, 15 and 20 post infection (p.i.). All three main antibody classes common in viral infection were present. Serum antibodies appeared later than the T cell cytotoxic response. Neutralizing antibodies and those detected by immunofluorescence prevailed in the IgG fraction, whereas the IgM antibody class was reactive in complement fixation assay. Challenge of infected mice with JV did not result in production of delayed-type hypersensitivity as measured by footpad swelling irrespective of the route of sensitization. The possible interpretations of these findings are discussed in connection with the resistance of adult mice to JV infection.

[Effect of various schedules of cyclophosphamide administration on the mortality of the adult mouse infected with Junín virus]. / Efecto de distintos esquemas de administración de ciclofosfamida en la mortalidad del ratón adulto infectado con virus Junín.

The object of this paper was to determine the influence of cyclophosphamide immunosuppression on the mortality of 40-45 day old Balb/c mice infected intracerebrally with a pathogenic strain of Junin virus, using different administration schedules. Up to 200 mg/kg of cyclophosphamide were not toxic. Results show that, unlike in other experimental models, three or four 50 mg/kg cyclophosphamide doses given both before and after viral infection were required to break-down resistance to Junin virus (90-96.5% mortality vs. 8% in controls). Taking into account the effect of cyclophosphamide on the cell populations involved in the immune response, causes likely to lead to the greater susceptibility of the suppressed adult mouse to Junin virus are discussed.

[Experimental infection of guinea pigs with Tacaribe virus: effect on the functioning of the immunocompetent system]. / Infección experimental del cobayo con virus Tacaribe: acci n sobre lafunción del aparato inmunocompetente.

Tacaribe virus is the member most closely related to Junín virus within the Tacaribe complex. It has been demonstrated that both viruses are indistinguishable by complement-fixation, due to the high cross-reactivity. However, adult guinea pigs are highly sensitive to infection with the XJ pathogenic strain of Junín virus whereas Tacaribe virus is nonpathogenic for this species. Furthermore this last virus protects them against Junín virus. The XJ strain reduces the immune response of guinea pigs to many antigens. Both the humoral response and the hypersensitivity of the Arthus type have been reduced in infected animals. Considering that Tacaribe virus could be used as vaccine antigen, the purpose of the present study was to investigate the effect of Tacaribe infection on the immune system of guinea pigs. The data reported here supports earlier findings showing that the XJ strain of Junín virus suppresses humoral immune response as indicated by lower precipitating antibody titers to ovoalbumin (which contributed to milder Arthus cutaneous reactivity) and a significant depression of plaque-forming cells to sheep erythrocytes. In contrast, Tacaribe-infected guinea pigs did not show detectable immunosuppression employing the same models. Similar results were found when the cell-mediated immunity was investigated. Tacaribe-infected guinea pigs had a normal immune response to contact sensitivity to 2-4 dinitro-1-fluorobenzene as demonstrated by measuring ear swelling and unmodified tuberculin reaction, after injection with complete Freund's adjuvant. Our results and those of previous investigations justify the consideration of Tacaribe immunization as an approach to the prophylaxis of Argentine Hemorrhagic Fever.

[Variations of the delayed hypersensitivity reaction in mice inoculated with different lots of sheep red blood cells]. / Variaciones de la reacción de hipersensibilidad retardada en ratones inoculados con diferentes lotes de glóbulos rojos de carnero.

Delayed-type-hypersensitivity (DTH) response "in vivo" is commonly evaluated by the footpad swelling test (FPST). High doses of Sheep Red Blood Cells (SRBC) are known to produce negligible DTH, while low doses lead to optimal sensitization. As expected FPST values obtained in Balb/c mice using 10(6) or 10(8) SRBC as sensitizing doses, showed that in 9 out of 10 batches from individual rams, the former dose resulted in higher values than the latter. However, only 3 out of the above 9 exhibited statistically significant differences between immunizing doses (Table 1). Therefore, in our hands, the accuracy of FPST is highly dependent on the SRBC source. We suggest the need of testing individual SRBC batches at both dilutions before use.