A SOLUBLE ANTIGEN OF LYMPHOCYTIC CHORIOMENINGITIS : I. SEPARATION OF SOLUBLE ANTIGEN FROM VIRUS

LYMPHOCYTIC CHORIOMENINGITIS IN THE SYRIAN HAMSTER

The virus of lymphocytic choriomeningitis produces an intense systemic infection in Syrian hamsters with few if any clinical and pathological signs of disease. Specific soluble antigen is demonstrable in the spleen of infected animals until about the 14th day when antisoluble substance antibodies make their appearance. Circulating virus disappears after the 4th week and neutralizing antibodies are present in serum in detectable amounts shortly thereafter; both types of antibody persist for at least several months. The viruses of St. Louis encephalitis and lymphocytic choriomeningitis can be concurrently passed in series in the brains of hamsters. The resultant disease is indistinguishable from that caused by the virus of St. Louis encephalitis alone.

COMPLEMENT FIXATION WITH THE NEUROTROPIC VIRUSES

Antigens capable of fixing complement specifically with the appropriate antibodies have been prepared from brain tissue of hamsters and mice infected with the viruses of St. Louis, Japanese, Western, and Eastern encephalitis, and with the West Nile virus. The antigens were freed of the material which reacts with normal serum by means of centrifugation at relatively high speed. In addition, the infectivity of the preparation was destroyed by irradiation with ultraviolet light. Cross reactions were demonstrated by means of the complement-fixation technique with materials from animals infected with the viruses of Eastern and Western equine encephalitis. No relationship was detectable by this procedure between St. Louis, Japanese, and West Nile viruses. These findings emphasize the need for further investigation and correlation of the immunological reactions of the groups of neurotropic viruses, since the equine agents are apparently unrelated when studied by neutralization and cross-immunity tests while these methods provide evidence of the presence of common antigenic structures in the St. Louis, Japanese, and West Nile agents.

A SOLUBLE ANTIGEN OF LYMPHOCYTIC CHORIOMENINGITIS : II. CHARACTERISTICS OF THE ANTIGEN AND ITS USE IN PRECIPITIN REACTIONS

The soluble antigen of lymphocytic choriomeningitis which is readily separable from the virus is a relatively stable substance and appears to be of a protein nature. A specific precipitin reaction can be demonstrated when immune serum is added to solutions of antigen which have been freed of certain serologically inactive substances. The complement-fixation and precipitation reactions which occur in the presence of immune serum and non-infectious extracts of splenic tissue obtained from guinea pigs moribund with lymphocytic choriomeningitis seem to be manifestations of union of the same soluble antigen and its antibody. On the other hand, the antisoluble substance antibodies and neutralizing substances appear to be different entities.

Lymphocytic choriomeningitis virus. II. Characterization of extractable complement-fixing activity

Lymphocytic choriomeningitis (LCM) virus-specific complement-fixing (CF) antigen (ECFA) has been solubilized, concentrated, and partially purified. When inoculated together with Freund's adjuvant, ECFA induced CF antibody but not neutralizing antibody or protective immunity. By itself it boosted pre-existing CF antibody but no neutralizing antibody. In double diffusion tests one line developed between ECFA and its antiserum, and a corresponding line became visible when ECFA interacted with an antiserum directed against all LCM virus-specific antigens. Absorption of either serum with ECFA abolished all ECFA-precipitating qualities. Ouchterlony tests also revealed that ECFA prepared from cells and tissues of various species is immunologically identical. By a variety of procedures ECFA was not found to be represented on the surface of either the virion or the infected cell. When purified infectious LCM virus was disrupted, a CF antigen corresponding immunologically to ECFA was set free. In double diffusion tests this antigen gave a line of identity with ECFA. Thus, ECFA appears to be an internal component of the infectious LCM virus.

LCM virus infection of cells in vitro

Most mammalian cells cultivated in vitro can be infected with lymphocytic choriomeningitis (LCM) virus. In addition to infectious virus, the cells produce antigenic material that fixes complement in the presence of antibody and is precipitated by antiserum. Intracellular antigen can also be demonstrated by the immunofluorescence procedure. When infected cells are viewed with the electron microscope, viral structures are seen either budding from or in association with the cell membranes. Immunoelectron microscopy, immunofluorescence, and cytotoxicity tests reveal virus-specific antigens on the surface of intact cells. Virus multiplication may be succeeded by cytolysis. Two LCM virus-specific antigens (or antigenic groups) can at present be distinguished. One corresponds to the infectious virus; the other is the complement-fixing "soluble" antigen. This extractable complement-fixing activity is produced by infected cells and is also a structural component of the infectious virus. It is not represented on the surface of either the virion or the infected cell. The cytolytic potential of LCM virus varies and is dependent on its previous passage history. Cytolytic and "attenuated" variants are able to initiate persistent infection of Mus musculus.Together with infectious virus, particles are produced that temporarily protect cells against standard virus. They appear to be by-products of virus multiplication, not in the sense of deletion mutants but of virus structures insufficiently equipped for their own active or passive replication, though capable of interfering with infectious virus. No evidence has been found for the generation of "defective interfering" particles, though their presence has not yet been excluded.

Arenaviruses: purification and physicochemical nature

The arenaviruses are a group of enveloped viruses having as a unique morphological finding the presence in the virion of granules instead of a defined core. The viruses contain a single-stranded RNA genome, but appreciable amounts of ribosomal-like RNA and 4-6S RNA of host cell origin have been detected. Little information is available on the mode of replication of the viral nucleic acids. A virion-associated RNA-dependent RNA polymerase has been described and there is indirect evidence to suggest that host cell RNA or DNA participates in virus replication. However, the steps in viral RNA synthesis and expression have not yet been elucidated.Pichinde virus contains 2 glycoproteins and 2 polypeptides. Cells infected with Pichinde virus or LCM virus have been shown to produce 2 antigens detectable by immunodiffusion. Both antigens appear to be components of the virion, but the relation between the antigens detected by immunodiffusion and the polypeptides detected by polyacrylamide gel electrophoresis has not yet been clarified.

Lymphocytic choriomeningitis infection of mice as a model for the study of latent virus infection

Lymphocytic choriomeningitis virus has been used to set up a model of a latent virus infection in mice. It has been shown to be possible to induce reproducible virus infections in mice which remain completely symptom-free in spite of levels of viral growth equal to those found in the sick animal, by the inoculation of mice within a few hours of birth. This is a convenient method of producing a latent infection in the mice. The effect of the age of the mice at the time of intracerebral inoculation was studied with respect to the pattern of disease produced. Several methods were tried without success in order to induce overt disease in the latently infected animals. The virus did not cause any demonstrable cytopathogenic effect on mouse tissue and several other types of animal tissue. A slight cytopathogenic effect was observed in a strain of human cells in vitro. Virus persisted for weeks in some of the tissue cultures, without damaging the tissue but with the production of active virus. The bearing of the results obtained is discussed in relationship to current concepts of latent virus infection and particularly immunological tolerance. A concept of a special variety of latency is introduced and the name "vital" infection suggested for this.