Hydrophobic IgG-containing immune complexes in the plasma of autoimmune MRL/lpr mice, lactate dehydrogenase-elevating virus-infected mice, and pigs: association with transforming growth factor-beta and pH-dependent amplification

Persistent infection of mice with lactate dehydrogenase-elevating virus (LDV) is associated with polyclonal B cell activation, autoimmunity, and circulating hydrophobic IgG-containing immune complexes (ICs), which bind to the surfaces of uncoated ELISA plates in the presence of 0.05% Tween 20. We demonstrate here that hydrophobic IgG-containing ICs also appear naturally in the plasma of autoimmune MRL/lpr mice. These and the similar hydrophobic ICs of LDV-infected mice as well as pigs coincide on ELISA plate surfaces with TGF-beta, apparently in the form of an IgG-TGF-beta complex. Circulating hydrophobic IgG-containing ICs are also susceptible to considerable amplification in vitro by exposure to alkaline conditions. By this latter method, the fraction of in vivo hydrophobic IgG, relative to the maximum in vitro chemically inducible IgG, was found to be about 20% in the plasma of LDV-infected mice, 5% in normal mouse plasma, and less than about 2% in pig plasma. These results indicate the potential for both chemically induced and protein-binding contributions to the generation of hydrophobic IgG-containing molecules, and have implications for immunopathological mechanisms in autoimmunity and persistent virus infections.     

Immunoglobulins that bind to uncoated ELISA plate surfaces: appearance in mice during infection with lactate-dehydrogenase-elevating virus and in human anti-nuclear antibody-positive sera

Immunoglobulins present in the blood plasma of mice infected with lactate-dehydrogenase-elevating virus (LDV) were found to bind strongly in the presence of 0.05% Tween 20 to the uncoated surfaces of wells of certain ELISA plates with previously recognized high protein-binding capacity. The binding was readily distinguishable from non-specific background binding of immunoglobulins present in normal mouse plasma. The binding components absorbed to protein A and had molecular weights in the 150-300 kDa range. Binding of the purified IgG fraction was progressively inhibited by increasing the concentration of Tween 20 in the diluent and by preincubation of the fraction at pH 3-4 for 10 min. The appearance of plate-binding IgM and IgG during LDV infection corresponded approximately with previously reported time courses of appearance of IgM- and IgG-containing circulating immune complexes and of specific IgM and IgG anti-LDV antibodies in LDV-infected mice. We conclude that complexes of IgG and IgM with LDV antigens have a much higher affinity for ELISA plates with high protein-binding capacity than uncomplexed immunoglobulins. Immune complexes did not significantly bind to ELISA plates with low protein-binding capacity, which, therefore, are suitable for measuring specific antiviral antibodies. Preliminary experiments with human anti-nuclear antibody-positive serum samples demonstrated markedly elevated non-specific binding of immunoglobulins to high-binding-capacity ELISA plates.     

Polyclonal B cell activation of IgG2a and IgG2b production by infection of mice with lactate dehydrogenase-elevating virus is partly dependent on CD4+ lymphocytes

Concentrations of IgM and IgG isotypes were determined by capture ELISA in plasma of Swiss, BALB/c and C58/M mice. Plasma IgG isotype concentrations, especially of IgM, IgG1 and IgG2a, varied considerably between mouse strains, batches of mice of the same strain and individual mice and as a function of age. Infection of the mice with LDV, which is known to replicate primarily in a subpopulation of macrophages, consistently resulted in a rapid elevation of plasma IgG2a (or of IgG2b in some Swiss nu/+ mice), but no plasma IgG increases were observed in mice immunized with inactivated LDV. Plasma IgG2a elevation after LDV infection was greatly delayed and reduced by depletion of the mice of CD4+, but not of CD8+, T cells by administration of protein-G-purified anti-CD4 or anti-CD8 mAbs, and completely inhibited by repeated treatment of the mice with cyclophosphamide. Treatment with anti-CD4 mAbs, or cyclophosphamide also greatly reduced the production of anti-LDV antibodies, while not significantly affecting the replication of LDV in these mice. Nude Swiss mice also failed to produce anti-LDV antibodies, though supporting normal LDV replication. Plasma IgM, IgG1, IgG2a and IgG2b levels increased in LDV-infected nu/nu mice, but similar changes were observed in uninfected mice. The results indicate that the LDV-induced polyclonal activation of B cells requires productive LDV infection of mice and is, at least partly, dependent on functioning CD4+ cells. They suggest that productive infection of the LDV-permissive subpopulation of macrophages leads to the activation of CD4+ T lymphocytes of subset 1 and their Spleen cells from 5-day LDV-infected BALB/c mice incorporated [3H]thymidine 2-3 times more rapidly in vitro than spleen cells from companion uninfected mice, whereas their responses to concanavalin A and lipopolysaccharide were reduced 60-70%.     

Regulation of maternal-fetal virus transmission in immunologically reconstituted SCID mice infected with lactate dehydrogenase-elevating virus.

Immunodeficient SCID (C.B-17 scid/scid) mice with persistent lactate dehydrogenase-elevating virus (LDV) infection failed to produce IgG anti-LDV antibodies, and during chronic infection transmitted virus infection to 95% of their offspring. In contrast, normal mice infected 15 or more days prior to giving birth produced IgG anti-LDV antibodies and transmitted LDV infection to only 0-46% of their fetuses. Transplacental transmission of LDV infection was dependent on the timing of maternal infection. Adoptive transfer of immune competence to LDV-infected SCID mice resulted in fetal protection from maternally transmitted virus infection. Fetal protection correlated with the presence of maternal IgG anti-LDV but not with fetal levels of IgG anti-LDV, and the levels of viremia in nonimmune SCID mice did not affect transplacental virus transmission. These results demonstrate the importance of maternal immunity in protecting the fetus from infection, and validate the use of this mouse model for investigation of immune mechanisms of transplacental virus transmission.     

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.     

Immune complexes that bind to ELISA plates not coated with antigen in mice infected with lactate dehydrogenase-elevating virus: relationship to IgG2a- and IgG2b-specific polyclonal activation of B cells.

We have further investigated the nature of IgG-containing complexes of 150-300 kD that rapidly appear in the circulation of mice of various strains after infection with lactate dehydrogenase-elevating virus (LDV) and are recognized and quantitated by their binding in the presence of 0.05% Tween 20 to certain enzyme-linked immunosorbent assay (ELISA) plates with high protein affinity that have not been coated with protein antigen (5). These binding complexes have been found to contain primarily IgG2a or, in some mice, IgG2b. Their isotype specificity and time course of formation correlated with those of the polyclonal production of immunoglobulins in these mice, as measured by increases in total IgG2a or IgG2b in the circulation. In contrast, anti-LDV antibodies exhibited much broader isotype specificities in all mouse strains investigated. Depletion of BALB/c mice of CD4+T cells or lack of T cells in nude Swiss mice only partly reduced the polyclonal activation of B cells and the formation of ELISA plate-binding complexes, whereas anti-LDV antibody formation was completely blocked. Only a small proportion of the total IgG2a or IgG2b formed as a result of the LDV-induced polyclonal activation of B cells was recovered in plate-binding complexes, which sedimented in sucrose density gradients between 150 and 300 kD. Diverse monoclonal antibodies of different IgG isotopes did not bind to the plates at concentrations at which LDV-induced immune complexes exhibited binding activity. We suggest that the LDV-induced immune complexes do not contain anti-LDV antibodies, but are complexes of auto-antibodies and self-antigen(s). However, additional features must be responsible for the high affinity of these complexes for ELISA plates since various immune complexes formed in vitro failed to bind to the plates, and binding activity of the immune complexes formed in LDV-infected mice could not be regenerated in vitro once the complexes had been dissociated by a low pH treatment.     

Infection of SCID mice with lactate dehydrogenase-elevating virus stimulates B-cell activation

Mice of the C.B-17 strain homozygous for the scid mutation (SCID mice) were infected with lactate dehydrogenase-elevating virus (LDV), and plasma samples obtained at intervals up to 42 days postinfection were analyzed for total immunoglobulins, anti-LDV antibodies, virus-specific immune complexes, and viremia levels. The mice responded to LDV infection with transient increases in total blood IgM, production of IgM-antigen complexes and IgM anti-LDV, as well as increased blood IgG2a. However, SCID mice failed to make a specific IgG2a anti-LDV immune response, and their blood LDV levels were elevated about 100-fold relative to those of control mice. The results suggest a role for IgG antibodies in the regulation of viremia and demonstrate a viral pathway of B-cell differentiation in SCID mice.     

Immune response to lactate dehydrogenase-elevating virus: isolation of infectious virus-immunoglobulin G complexes and quantitation of specific antiviral immunoglobulin G response in wild-type and nude mice.

Lactate dehydrogenase-elevating virus (LDV) causes a normally benign persistent infection of mice, resulting in a life-long viremia characterized by the presence of circulating infectious immune complexes, impaired clearance of certain enzymes from the blood, and modification of the host immune response to various heterologous antigens. In this study, we isolated infectious immunoglobulin G (IgG)-LDV complexes in the plasma of persistently infected mice by adsorption to and elution from protein A-Sepharose CL-4B. We found that practically all infectious LDV in the plasma of persistently infected mice is complexed to IgG. LDV infectivity in these complexes was partially neutralized, but could be reactivated by treatment with 2-mercaptoethanol. We also quantitated total plasma IgG and anti-LDV IgG in wild-type and nude Swiss and BALB/c mice as a function of the time after infection with LDV by radial immunodiffusion and an enzyme-linked immunosorbent assay, respectively. Total plasma IgG levels nearly doubled in BALB/c mice during 150 days of infection. IgG levels in uninfected nude mice were only 20% of those in uninfected BALB/c mice, but during infection with LDV increased to approximately those found in uninfected BALB/c mice. Anti-LDV IgG levels were almost as high in nude mice as in normal BALB/c mice. Isoelectric focusing of purified IgG from BALB/c mice showed that LDV infection resulted in the enhanced synthesis of all 16 normal IgG fractions that we could separate by this method, which suggests that LDV infection results in polyclonal activation of IgG-producing lymphocytes.     

Persistent infection of mice by lactate dehydrogenase-elevating virus: effects of immunosuppression on virus replication and antiviral immune responses

Maximum plasma titers (10(9)-10(10) ID50/ml) of lactate dehydrogenase-elevating virus (LDV) in mice are observed one day after infection, but then decrease 4-5 log during the next 5 weeks to attain a persistent steady-state level for the remainder of the life of the animal. The decrease in plasma LDV level during the first 5 weeks after infection and long-term viremia were not affected by lethal X-irradiation of the mice, daily injections of cyclosporin A or depletion of the mice of T cells by treatment with anti-CD4, anti-CD8, or anti-Thy1.2 monoclonal antibodies, although these treatments inhibited the formation of anti-LDV antibodies. LDV viremia was also the same in nu/nu and nu/+ Swiss mice, though the former did not mount an anti-LDV immune response, while the latter did. The appearance of anti-LDV neutralizing antibodies in infected mice 1-2 months after infection or the injection of infected mice with high doses of anti-LDV neutralizing monoclonal antibodies also did not affect the level of LDV viremia. Repeated treatments of infected mice with either cyclophosphamide or dexamethasone caused 1-2 log increases in plasma LDV titers. Although cyclophosphamide treatment prevented the formation of anti-LDV antibodies, dexamethasone caused an increase in plasma LDV levels without affecting anti-LDV antibody formation. We conclude that an anti-LDV immune response does not play a significant role in controlling LDV replication in mice. The data support the view that within 1 day after infection of a mouse, all LDV-permissive macrophages, which appear to be the only cells supporting LDV replication in the mouse, are destroyed as a result of a cytocidal infection by LDV. Subsequently, LDV replication is limited by the rate of generation of new permissive macrophages. The steady-state viremia attained about 5 weeks after infection reflects a balance between LDV replication in permissive macrophages as they arise and LDV inactivation and clearance.     

Immune response to lactate dehydrogenase-elevating virus: serologically specific rabbit neutralizing antibody to the virus.

A rabbit was immunized with large amounts of the lactate dehydrogenase-elevating virus (LDV) over a 9-month period. The plasma from this rabbit possessed an anti-LDV IgG titer of 1:80,000 as measured by the enzyme-linked immunosorbent assay (ELISA) method and a neutralizing titer of 1:1,000 for the homologous strain of LDV. LDV neutralization at 4 degrees C followed single-hit kinetics. In contrast, mouse anti-LDV IgG in plasma of chronically LDV-infected mice failed to neutralize LDV at 4 degrees C and neutralization at 37 degrees C was slow, biphasic, and inefficient compared with the neutralization caused by rabbit anti-LDV IgG, even though high levels of anti-LDV IgG were detectable in mouse plasma by the ELISA method. Rabbit anti-LDV IgG neutralized one heterologous strain of LDV as rapidly as it did the homologous strain, but failed to significantly neutralize five other strains of LDV, all of which were originally isolated from different mouse strains bearing transplantable tumors. The results indicate clear serological differences between LDV strains. Cross-reactions between the strains, however, were observed by ELISA, using the antibody induced during persistent infection of mice with each LDV strain. Immunoglobulin M (IgM) from mice infected for 15 days with the various strains bound equally to our LDV strain. IgG obtained from 2-month-infected mice also cross-reacted, but to a varying extent which partly correlated with the specificity detected by neutralization. Both rabbit and mouse anti-LDV IgG enhanced the infectivity of LDV at a low multiplicity of infection for primary cultures of peritoneal mouse macrophages.     

Glucocorticoid regulation of lactate dehydrogenase-elevating virus replication in macrophages

Lactate dehydrogenase-elevating virus (LDV) is a macrophage-tropic arterivirus which generally causes a persistent viremic infection in mice. LDV replication in vivo seems to be primarily regulated by the extent and dynamics of a virus-permissive macrophage population. Previous studies have shown that glucocorticoid treatment of chronically LDV-infected mice transiently increases viremia 10-100-fold, apparently by increasing the productive infection of macrophages. We have further investigated this phenomenon by comparing the effect of dexamethasone on the in vivo and in vitro replication of two LDV quasispecies that differ in sensitivity to immune control by the host. The single neutralizing epitope of LDV-P is flanked by two N-glycans that impair its immunogenicity and render LDV-P resistant to antibody neutralization. In contrast, replication of the neuropathogenic mutant LDV-C is suppressed by antibody neutralization because its epitope lacks the two protective N-glycans. Dexamethasone treatment of mice 16 h prior to LDV-P infection, or of chronically LDV-P infected mice, stimulated viremia 10-100-fold, which correlated with an increase of LDV permissive macrophages in the peritoneum and increased LDV infected cells in the spleen, respectively. The increase in viremia occurred in the absence of changes in total anti-LDV and neutralizing antibodies. The results indicate that increased viremia was due to increased availability of LDV permissive macrophages, and that during a chronic LDV-P infection virus replication is strictly limited by the rate of regeneration of permissive macrophages. In contrast, dexamethasone treatment had no significant effect on the level of viremia in chronically LDV-C infected mice, consistent with the view that LDV-C replication is primarily restricted by antibody neutralization and not by a lack of permissive macrophages. beta-Glucan, the receptor of which is induced on macrophages by dexamethasone treatment, had no effect on the LDV permissiveness of macrophages.     

Clearance of lactate dehydrogenase by SJL/J mice infected with lactate dehydrogenase-elevating virus

The plasma level of lactate dehydrogenase (LDH) activity rises to about ten times the normal level by 4 days after infection of mice with lactate-dehydrogenase elevating virus (LDV). The levels of seven other enzymes are also increased, but to a lesser degree. SJL/J mice demonstrate a unique, genetically controlled 20-fold increase in the plasma level of LDH enzyme after LDV infection, as well as enhanced levels of the other plasma enzymes elevated by LDV infection. Comparison of virus infection in SJL/J and Swiss mice as well as in cultures of peritoneal exudate cells made from them indicated that the time course and extent of virus replication was similar for the two strains of mice. The rate of clearance of intravenously injected rabbit or mouse LDH was found to be impaired to a similar extent in LDV-infected SJL/J and Swiss mice. The effect of LDV infection on the levels of endogenous LDH released as a result of injection of carbon tetrachloride or tumor growth was also similar in the two strains of mice. These results suggest that LDV infection may specifically induce a greater influx of LDH into the plasma of SJL/J mice from an as-yet-unknown source than in other strains of mice.     

Analysis of immunoglobulin isotype blood levels, splenic B-cell phenotypes, and spleen cell immunoglobulin gene expression in mice infected with lactate dehydrogenase-elevating virus

B-lymphocyte activation was studied in mice infected with lactate dehydrogenase-elevating virus (LDV). ELISA determinations of blood total immunoglobulin levels demonstrated that, at 10 days post-infection (p.i.) with LDV, only the IgG2a isotype was elevated. DNA-excess dot-blot hybridization showed that RNA specific for IgG2a and IgA immunoglobulin isotypes was increased in the spleens of mice at 10 days p.i. with LDV. Immunoglobulin surface phenotype analysis of spleen cells at 8-10 days p.i. with LDV revealed that there was no alteration in immunoglobulin isotype-bearing cell proportions, although total spleen mass and number of cells increased during LDV infection. When blood immunoglobulins from LDV-infected mice were analyzed by two-dimensional isoelectric focusing gels, followed by specific immunoblotting for immunoglobulin isotype, the presence of new IgG2a species was observed at 10 days p.i.     

Lactic dehydrogenase virus infection enhances parasite egg production and inhibits eosinophil and mast cell responses in mice infected with the nematode Nippostrongylus brasiliensis.

The effects of lactic dehydrogenase virus (LDV) infection on the protective immune responses to the nematode Nippostrongylus brasiliensis were studied. Mice with chronic LDV infection showed significantly higher levels of parasite egg production than non-LDV-infected (control) mice after N. brasiliensis infection. Concurrent LDV infection also suppressed peripheral blood eosinophilia and the lung mastocytosis induced by this nematode. LDV infection showed higher expression levels of the interferon-gamma (IFN-gamma) mRNA in lymph nodes compared with control mice before N. brasiliensis infection. In addition, the IgG2a production in LDV-infected mice was higher than that in control mice before and after N. brasiliensis infection. These results suggest that LDV infection modulates protective immune responses against N. brasiliensis infection by the activation of T-helper type 1 cells.     

Lactic dehydrogenase virus infection reduces the expulsion of the nematode <Emphasis Type="Italic">Nippostrongylus brasiliensis</Emphasis>

The interleukin (IL)-13-mediated goblet cell response is the major host effector system involved in the expulsion of Nippostrongylus brasiliensis. Lactic dehydrogenase virus (LDV) induced higher levels of N. brasiliensis egg production compared with controls, but the effect of LDV infection on worm expulsion of, and goblet cell and IL-13 responses to, N. brasiliensis have not been studied. In this study, the effects of LDV infection on these host responses against N. brasiliensis were examined. Mice with chronic LDV infection showed significantly lower worm expulsion rates than non-LDV-infected mice after N. brasiliensis infection, and there were no significant differences in the ratio of female versus male adult worms between control and LDV-infected mice. The number of goblet cells in LDV-infected mice was significantly lower than that in controls. In addition, the levels of IL-13 gene expression in lymph nodes were significantly lower in LDV-infected mice compared with controls. These results suggest that LDV infection reduces the protective immune responses against N. brasiliensis infection by the suppression of IL-13 production.     

Migration inhibition of lymph node lymphocytes as an assay for regional cell-mediated immunity in the intestinal lymphoid tissues of mice immunized orally with ovalbumin.

A Sepharose-IgG-Clq sorbent has been successfully used to isolate circulating virus-antibody (virus Ab) complexes from serum of lactic dehydrogenase virus (LDV)-infected mice. The chronological study demonstrated that although circulating LDV in persistently infected mice was complexed with IgG antibody, the virus-Ab complexes bound to the Clq sorbent only early during the infection. Inactivation of these Clq-isolated virus Ab complexes was accomplished with rabbit anti-mouse IgG but not anti-IgM. The critical time period for the demonstration of Clq-binding LDV-Ab complexes in serum was between 10 and 18 days postinoculation. The reasons for the apparent shift from Clq-binding virus-Ab complexes to non-Clq binding are not clear, however, possible explanations and implications in immune complex tissue injury are discussed.     

Suppression of development of diabetes in NOD mice by lactate dehydrogenase virus infection

It has been reported that lactate dehydrogenase virus (LDV) selectively infects a subpopulation of macrophages, thereby affecting the immune system. We studied the effects of LDV infection on the development of diabetes in non-obese diabetic (NOD) mice. Five-week-old female NOD mice were infected with LDV (10(8) ID50/mouse) and observed until 23 weeks of age. None of the 21-LDV-infected mice developed diabetes, whereas 10/14 (71.4%) uninfected mice did. Although the subpopulations of T cells and the percentage of Mac1-positive cells in the NOD murine spleen and the number of harvested peritoneal macrophages were unaffected by LDV infection, the proportions of Ia-positive peritoneal macrophages were significantly decreased in LDV-infected compared with uninfected mice (1.1 +/- 0.2%, 6.5 +/- 2.9%; P < 0.01). In LDV-infected NOD mice, insulitis of the same grade as that seen in uninfected NOD mice was observed. In another experiment, 3, 5, 10 or 16-week-old female NOD mice were infected with LDV. None of the mice infected with LDV at 3, 5 or 10 weeks of age developed diabetes and only one of six infected at 16 weeks of age did. These findings indicate that LDV infection suppresses the development of diabetes in female NOD mice by reducing the capacity of Ia-positive macrophages, and suggest that the development of human type 1 diabetes may be suppressed by certain viral infections.     

Detection of the binding of IgG2a and IgG2b on the surface of macrophages from mice chronically infected with lactic dehydrogenase virus.

This study showed that sera from mice chronically infected with lactic dehydrogenase virus (LDV) contained virus-antibody complexes (IC). IgG2a and IgG2b, but not IgG1, IgG3, IgM or IgA, were demonstrated on the surface of macrophages from chronically infected mice. These results suggest that IC in the circulation may bind to Fc receptors for IgG2a and IgG2b on the surface of macrophages and lead to the modulation of macrophage function seen in chronically LDV-infected mice.     

Immune responses in mice infected with lactic dehydrogenase virus. I. Antibody response to DNP-BGG and hyperglobulinaemia in BALB/c mice.

The humoral immune response to DNP-BGG of BALB/c mice acutely infected with lactic dehydrogenase virus (LDV) has been investigated. Virus-infected mice injected with antigen in saline exhibit a greater anti-DNP response than uninfected controls. When this antigen is presented in Freund''s complete adjuvant (FCA) the anti-DNP response is greater than obtained with antigen in saline, but significant differences between infected and uninfected controls are not observed. These data are consistent with the view that acute LDV infection can have an adjuvant-like effect when this T-dependent antigen is introduced in saline. In addition, the effect of viral infection on plasma Ig class and subclass levels has been investigated. LDV infection leads to a gradual increase in plasma Ig concentration. This effect is restricted to the IgG2a subclass in most animals, but occasionally is restricted to IgG1. The mechanisms responsible for these changes have not been delineated.     

Immune responses in mice infected with lactic dehydrogenase virus. IV. Functional status of the macrophage during acute LDV infection.

Macrophages from uninfected and lactic dehydrogenase virus (LDV)-infected mice were compared with respect to the affinity and number of their Fc receptors for IgG2a; no differences were found regarding these parameters. When the uptake of DNP-BGG by macrophages from uninfected and acutely LDV-infected mice was compared, again no differences were observed. However, when the per cent membrane-bound DNP-BGG was determined as a function of time after antigen uptake in these two groups, more DNP-BGG was found membrane-bound on the macrophages from the LDV-infected mice, than on uninfected macrophages. In view of the fact that humoral immunity is enhanced during acute LDV infection, these data provide a positive correlation between increased retention of membrane-bound antigen and enhanced humoral immune responses.