Antibody response of mice to lactate dehydrogenase-elevating virus during infection and immunization with inactivated virus.

BALB/c and Swiss mice were infected with lactate dehydrogenase-elevating virus (LDV) or immunized with glutaraldehyde-inactivated or ether-extracted virus and their plasma was monitored for anti-LDV IgG and IgM levels by ELISA and indirect fluorescent antibody staining, for neutralizing antibodies, for sensitized antibody-virus complexes, for immune complexes, and for total plasma IgG and IgM. In infected mice, anti-LDV IgM was transiently formed during the first 2 weeks post infection (p.i.) but only at a low level. Anti-LDV IgG was produced in a biphasic manner with an initial peak at about 10 days p.i. and a secondary rise reaching a maximum level 30-80 days p.i. which was retained throughout the persistent phase of infection. The concomitant appearance of comparable levels of low molecular weight immune complexes suggests that most anti-LDV IgG was complexed with LDV proteins. Also, as early as 10 days p.i., infectious antibody-LDV complexes developed, which were neutralizable by rabbit anti-mouse IgG, whereas antibodies that neutralize the infectivity of exogenously added LDV appeared only 1-2 months p.i. Throughout infection, most of the anti-LDV IgG was directed to VP-3, the envelope glycoprotein of LDV, which was found to exist in at least 10 distinct forms ranging in molecular weight from 24 to 42 kDa. Anti-LDV IgG levels as high as those observed in infected mice developed in mice immunized with inactivated LDV. Antibodies to glutaraldehyde-inactivated LDV were also mainly directed to VP-3, but exhibited no neutralizing activity. The polyclonal B cell activation associated with a persistent LDV infection and the formation of immune complexes were not observed in mice immunized with inactivated virus.

Replication of lactate dehydrogenase-elevating virus in C58 mice and quantification of antiviral antibodies and of tissue virus levels as a function of development of paralytic disease.

Infection with the lactate dehydrogenase-elevating virus (LDV) triggers a generally fatal paralytic disease in old immunosuppressed C58 mice, but not in comparable mice of many other strains. We have compared the replication of LDV and the humoral immune response to it in C58 mice and mice of various resistant strains. Plasma LDV titres of persistently infected C58 mice were about tenfold higher than in other strains of mice and the proportion of LDV-permissive macrophages in peritoneal exudates of C58 mice was about twice as high as that observed in other mouse strains. C58 mice developed normal levels of anti-LDV IgG, as measured by ELISA, and normal levels of IgG that sensitized LDV to neutralization by rabbit anti-mouse IgG. C58 mice also developed normal IgM and IgG responses to human gamma-globulin and sheep erythrocytes. The antibody responses to LDV were similarly inhibited by cyclophosphamide in C58 and resistant strains of mice, which enhanced the incidence of signs of paralysis only in C58 mice. Thus, the sensitivity of C58 mice to LDV-induced paralytic disease is not due to an inherent inability of the mice to mount a humoral antibody response to LDV, and a suppression of the antibody response by cyclophosphamide is not the only prerequisite for development of the disease. We have quantified LDV in various tissues of immunosuppressed and non-immunosuppressed, 8- or 9-month-old C58 mice as a function of time after LDV infection and in relation to the development of paralytic disease. Changes in tissue LDV titres as a function of time after infection paralleled those found in the plasma; LDV titres were highest 1 day post-infection, and then decreased to a lower persistent level during the next 1 to 2 weeks. Tissue LDV titres, including those of the spinal cord, were lower than those in the plasma, and our results indicate that most of the LDV in tissue homogenates may be attributable to blood contamination, even though the animals were extensively perfused before removal of the tissues.

Cell surface receptors for lactate dehydrogenase-elevating virus on subpopulation of macrophages.

We examined the binding and internalization of unlabeled and 125I-labeled, purified lactate dehydrogenase-elevating virus (LDV) by peritoneal macrophages cultured in vitro. Upon incubation of the cells at 4 degrees C with greater than 100 ID50/cell, the virus was surface-bound on a small subpopulation of macrophages (about 5% of the total cells) as determined by electron microscopy, fluorescent antibody staining, and autoradiography of cells incubated with 125I-labeled LDV. At 37 degrees C, LDV particles were seen in intracellular endocytic vesicles also in about 5% of the cells, and the proportion of cells with virus-containing vesicles correlated with the proportion of cells which became productively infected with LDV as assessed by determining LDV RNA synthesis in individual cells and by fluorescent antibody staining. Pretreatment of the resident peritoneal macrophages with trypsin inhibited the binding of 125I-labeled LDV and the productive infection of the cells with the virus. After removal of the trypsin and incubation in complete medium, permissiveness for LDV reappeared after an 8-12 h lag, whereas Fc and C3 receptors reappeared more rapidly after trypsin treatment. Populations of resident peritoneal macrophages, starch-elicited peritoneal macrophages, splenic macrophages, and bone marrow macrophages contained a similar proportion of cells that could be productively infected with LDV. Little, if any, LDV replication was detected in cultures of lung, liver and peripheral blood macrophages as well as in thioglycollate-elicited and BCG-activated macrophages. We conclude that the permissiveness for LDV of resident peritoneal macrophages correlates with the presence of a trypsin-sensitive receptor present on a subpopulation of these cells. The identity of the receptor has not been definitively established. Treatment of macrophages with neuraminidase or various sugars had no significant effect on LDV replication. Lysis of I-A-positive macrophages with a monoclonal antibody and complement reduced the number of macrophages which could be productively infected by 50%, which suggests that macrophages lacking surface Ia can be productively infected with LDV in vitro.