Specific and nonspecific cell-mediated resistance to influenza virus in mice

Proinflammatory cytokine response and viral replication in mouse bone marrow derived macrophages infected with influenza H1N1 and H5N1 viruses

The pathogenesis of human influenza H5N1 virus infection remains poorly understood and controversial. Cytokine dysregulation in human infection has been hypothesized to contribute to disease severity. We developed in vitro cultures of mouse bone marrow derived macrophages (BMDMΦ) from C57BL/6N mouse to compare influenza A (H5N1 and H1N1) virus replication and pro-inflammatory cytokine and chemokine responses. While both H1N1 and H5N1 viruses infected the mouse bone marrow derived macrophages, only the H1N1 virus had showed evidence of productive viral replication from the infected cells. In comparison with human seasonal influenza H1N1 (A/HK/54/98) and mouse adapted influenza H1N1 (A/WSN/33) viruses, the highly pathogenic influenza H5N1 virus (A/HK/483/97) was a more potent inducer of the chemokine, CXCL 10 (IP-10), while there was not a clear differential TNF-α protein expression pattern. Although human influenza viruses rarely cause infection in mice without prior adaption, the use of in vitro cell cultures of primary mouse cells is of interest, especially given the availability of gene-defective (knock-out) mice for specific genes.

The effect of Staphylococcus aureus phage lysate vaccine on a rabbit model of staphylococcal blepharitis, phlyctenulosis, and catarrhal infiltrates

PURPOSE

To evaluate the effect of Staphylococcus aureus phage lysate (SPL) vaccination on the development of blepharitis, corneal phlyctenules, and catarrhal infiltrates and on the development of antibodies and the delayed-type hypersensitivity response to S. aureus.

METHODS

Eighty rabbits received an intradermal immunization of cell wall-complete Freund's adjuvant followed by a booster immunization. Rabbits were given topical applications of viable S. aureus in both eyes and 40 rabbits received subcutaneous SPL vaccinations. Clinical observations were made weekly. An enzyme-linked immunosorbent assay was used to measure IgG, IgA, and IgM antibody levels to ribitol teichoic acid in sera, corneas, and tears. The delayed-type hypersensitivity response was evaluated by skin testing after subcutaneous injection of staphylococcal antigens.

RESULTS

In the SPL-vaccinated group, phlyctenules developed in eight of 40 rabbits while blepharitis developed in 13 of 40. In the nonvaccinated group, phlyctenules developed in three of 40 rabbits and blepharitis developed in five of 40. The number of rabbits with blepharitis was significantly higher in the SPL-vaccinated group than in the nonvaccinated group. In general, the antibody response to ribitol teichoic acid was enhanced, while the delayed-type hypersensitivity response to S. aureus was depressed.

CONCLUSIONS

Vaccination with SPL was not found to have a beneficial effect on the development of blepharitis, phlyctenules, and catarrhal infiltrates in our rabbit model.

Prophylactic activity against Sendai virus infection and macrophage activation with lipophilic derivatives of N-acetylglucosaminylmuramyl tri- or tetrapeptides

The efficacy of N-acetylglucosaminyl-beta (1----4)-N-acetylmuramyl tri- or tetrapeptides (GM) and the lipophilic derivatives for host augmentation against Sendai virus infection and for macrophage activation in vitro was examined. The anti-infectious activities of GM derivatives were shown to increase with the chain length of the fatty acid combined with the diaminopimelyl group. When the macrophages were activated with 1 U ml-1 murine interferon gamma (IFN-gamma) and 0.001 microgram ml-1 GM derivatives, the cytocidal ability of macrophages depended on the length of the side chain, and exhibited a positive relationship with the anti-infectious activity of GM derivatives against Sendai virus infection. These results indicated that the increment of lipophilicity of GM derivatives would play an important role in the anti-infectious activity and macrophage activation in vitro.

Immunomodulation of intradermal mammary carcinoma using staphage lysate in a rat model

Staphage lysate (SPL), a preparation of Staphylococcus aureus obtained by bacteriophage lysis, is an interferon-inducer and stimulator of T and B lymphocytes. Does SPL, as an immunopotentiator, have an effect on the growth and metastases of an intradermal mammary carcinoma? To answer this question, a study using SPL in female Fischer rats injected with 7 x 10(6) viable 13762 mammary tumor cells on the midback were used. Four groups were created with 10 animals in each group. Group I was the control group. They received no treatment. Group II received 0.3 ml of medium in which SPL was carried on alternate days. Group III received 0.3 ml SPL on alternate days. Group IV were sensitized with dead staphylococcal organisms prior to SPL treatment as in Group III. Tumor diameters were recorded on days 10, 13, 17, and 21, and autopsies were performed to determine the extent of metastases. Histologic examination and serum antibody measurements were performed. The mean tumor diameters on day 21 were: Group I: 4.1 +/- 0.2 cm; Group II: 3.80 +/- 0.19 cm; Group III: 3.04 +/- 0.13 cm; and Group IV: 2.97 +/- 0.14 cm. Rats receiving SPL treatment in Groups III and IV had significantly smaller tumors (P less than .001). The incidence of axillary lymph node involvement was: Group I: 100%; Group II: 87.5%; Group III: 62.5%; and Group IV: 40%. Lung metastases were seen in all groups. Groups I and II had 100% incidence of grossly visible nodules, whereas Groups III and IV had 75% and 70% involvement. Gross findings were confirmed by microscopic examination.(ABSTRACT TRUNCATED AT 250 WORDS)

Stimulation of non-specific host resistance against Sendai virus and Escherichia coli infections by chitin derivatives in mice

The efficacy of chitin derivatives on non-specific host resistance to Sendai virus and Escherichia coli infections was studied in mice. Seventy percent deacetylated chitin (DAC-70) and N-trimethylated DAC-70 [DAC-70(Me)3] showed protective activity against Sendai virus infection; however, carboxymethyl-chitin (CM-chitin) did not. DAC-70 also showed protective activity against E. coli infection.

Staphage lysate: an immunomodulator of the primary immune response in mice

The immunological enhancing activity of staphage lysate on the primary immune response of mice as reflected by specific heteroantibody production to a single antigenic stimulus and on immunoglobulin synthesis was determined. Staphage lysate was administered at different periods in relation to time of sheep erythrocyte injection so that both the inductive and the productive phase of the immune response could be evaluated. Treatment with staphage lysate induced pronounced enhancement of the hemagglutinin response by day 14 regardless of the dose or the time of its administration. Significantly higher antibody levels in the test groups were observed when compared with the control animals, peak antibody titers being 21 days following antigenic challenge. Furthermore, staphage lysate administered without an accompanying antigen evoked a heightened serum immunoglobulin level in mice for a period of more than 14 days after injection. IgG1, IgG2a and IgG2b were significantly elevated as a direct result of this treatment. This increased output of immunoglobulin synthesis and heightened hemagglutinin titers demonstrate that staphage lysate is not only an immunomodulator of cell-mediated immunity, as previously reported, but also an effective immunoadjuvant of the humoral antibody response capability in the host.

Enhancement of natural resistance to influenza virus in lipopolysaccharide-responsive and -nonresponsive mice by Propionibacterium acnes

Lipopolysaccharide-responsive C3H/HeN mice were rendered resistant to a mouse-adapted strain of influenza (Aichi, H(3)N(2)) virus when Propionibacterium acnes was given either intranasally or intraperitoneally several days before virus infection. The time of P. acnes treatment was important since no protection was demonstrated when this agent was given either on the same day as or several days after virus challenge. In contrast, lipopolysaccharide-nonresponsive C3H/HeJ mice were not protected when P. acnes was administered intranasally at any time before infection; however, protection was demonstrated when P. acnes was given by the intraperitoneal route. Depending on the route of inoculation, P. acnes induced several distinctive immunological responses in the lungs of both C3H/HeN and C3H/HeJ mice. Intranasal inoculation was more effective in activating pulmonary macrophages in C3H/HeN than in C3H/HeJ mice. In contrast, intraperitoneal inoculation activated pulmonary natural killer cells in both mouse lines but did not activate pulmonary macrophages.

Interaction of influenza virus with swine alveolar macrophages: Influence of anti-virus antibodies and cytochalasin B

Interactions between swine influenza virus and swine alveolar macrophages were studied in vitro. Cell lysis occurred 24 h post-infection and was shown by a decreased neutral red uptake. UV inactivation of the influenza virus, or the addition of anti-influenza antibodies suppressed the virus-induced macrophage lysis. Inversely, pretreatment of swine alveolar macrophages by a phagocytosis inhibitor, namely cytochalasin B, increased the cell susceptibility to virus-induced lysis.

The present in vitro studies support the hypothesis that defects in pulmonary antibacterial mechanisms associated with influenza infections are partly due to a direct toxic effect of the virus on alveolar macrophages. Furthermore, the studies show that virus-induced macrophage lysis is influenced by specific humoral immune responses and by modifications in microfilament cell functions.

Dengue virus multiplication in cultures of mouse peritoneal macrophages: effects of macrophage activators

Dengue-2 virus multiplied in cultures of methylcellulose-induced peritoneal macrophages of BALB/c mice. The in vitro-cultivated macrophages from dengue-1 virus-immune mice produced larger amounts of dengue-2 virus than did those from nonimmune controls. The effect of macrophage activators was examined by using nonimmune macrophages. Enhanced virus production was demonstrated in cultures of macrophagges pretreated with phytohemagglutinin (PHA) or bacterial lipopolysaccharide (LPS). The number of virus-infected cells in the pretreated cultures was estimated to be about 0.01% or less of the total macrophages. Continuous treatment of macrophages with PHA before and after virus inoculation brought about the most marked enhancement of dengue-2 virus multiplication. On the other hand, treatment with concanavalin A or pokeweed mitogen showed little effect on the multiplication of the same virus. Treatment with carrageenan, a specific macrophage blocking agent, markedly suppressed dengue-2 virus production in both dengue-1 virus-immune macrophages and LPS-treated macrophages. The indirect fluorescent-antibody (FA) technique revealed dengue-2 viral antigen in the cytoplasm of infected macrophages, and the FA-positive macrophages were more numerous in PHA-treated cultures than in untreated controls. The results obtained are discussed in relation to a possible role of activated monocytes/macrophages in the pathogenesis of dengue hemorrhagic fever.

Effect of influenza virus on phagocytic cells

Many viral infections predispose to bacterial superinfection, and it has been suggested that the increased susceptibility to bacterial infections is at least in part due to the effect of virus on the phagocytic cell function. Since the mechanisms by which the viruses affect neutrophil function are not well understood, we studied the function of polymorphonuclear leukocytes (PMNs) after incubation with influenza virus. Phagocytosis was assayed by incubating influenza virus (strain type A-Texas-77 [H2N2] ) treated leukocytes with 3H-thymidine-labelled staphylococci. The oxidative metabolism of the PMNs was studied by measuring the chemiluminescence generated by virus-treated PMNs after incubation with zymosan. Chemotaxis was measured under agarose. After incubation with 10(7) EID50 units of influenza virus, PMNs ingested only 35% of the bacteria, whereas control leukocytes ingested over 80%. Influenza virus also reduced the mobility of the PMNs and markedly suppressed the generation of chemiluminiscence. UV-killed virus with intact neuraminidase produced similar effects but virus with heat-inactivated neuraminidase did not. Virus envelope-neuraminidase may be responsible for some of the effects of the virus on the PMNs.

Role of macrophage activation and interferon in the resistance of alveolar macrophages from infected mice to influenza virus

Lung macrophages from uninfected CD1 mice support the replication of influenza viruses (H1N1 and H0N1), but the cells from influenza-infected mice do not. The possible mechanisms of this resistance were investigated. Murine macrophages were "activated" in vitro with lipopolysaccharide and lymphokines, and in both cases activation was associated with resistance of cells to infection with influenza virus. Exposure of alveolar macrophages in vitro to 500 U of purified type I interferon per ml enhanced cell spreading and Fc receptor-mediated phagocytosis, suggesting macrophage activation, and protected the cells against infection with influenza virus. Alveolar macrophages were also protected by a soluble factor in the bronchoalveolar washings from influenza-infected mice. This effect was not virus specific and was abolished by anti-interferon serum.

Depressed macrophage functions in dengue virus-infected mice: role of the cytotoxic factor

Dengue virus Type 2 (DV) infection causes immunosuppression in mice. Since macrophages are crucial for immune response, we have studied their functions in this condition and report our findings here. It was observed that in DV-infected mice the phagocytosis of neutral-red and latex particles by splenic and peritoneal-cavity macrophages was significantly reduced (P less than 0.001) from Days 3 to 10 after inoculation. Similarly the migration of splenic and peritoneal macrophages on a glass surface was reduced significantly (P less than 0.001) from Days 4 to 10 after inoculation. Pre-treatment of normal mouse spleen cells with DV-induced cytotoxic factor (CF) inhibited the phagocytic and migratory functions in the same way as observed in DV-infected mice. Higher dilutions of CF (10(-3) and 10(-3.7)) did not kill the cells but affected their functions. It was concluded that macrophage functions are affected by killing and metabolic changes in these cells by DV-induced CF, thus producing immunosuppression.

Different effects of influenza virus, respiratory syncytial virus, and Sendai virus on human lymphocytes and macrophages

Influenza virus, respiratory syncytial virus, and Sendai virus depress human cell-mediated immune responses, such as mitogen-induced lymphocyte transformation, but differ in their ability to induce other immune defense mechanisms, such as interferon production. Exposure to the different viruses resulted in depressed transformation responses to the mitogen phytohemagglutinin by affecting the function of lymphocytes, or macrophages, or both cell types.

Interaction of influenza virus with mouse macrophages

Mouse peritoneal and alveolar macrophages differed substantially in their response to influenza in vitro. Immunofluorescent and infectious-center techniques showed that viral proteins were produced in only a small subpopulation (17%) of peritoneal macrophages and that these infected cells were removed from culture by 3 days postinfection. In contrast, alveolar macrophages were highly susceptible to influenza, and viral antigens were produced in all cells. This was accompanied by a cytopathic effect and cell death. However, no infectious virus was released and the infection was considered abortive. With mouse cytomegalovirus, however, both alveolar and peritoneal macrophages were equally restrictive, and viral antigens were produced in only 1 to 5% of either cell population. No significant differences were observed between mouse-virulent and -avirulent strains of influenza in their interaction with macrophages either in vitro or in vivo. In vivo, both strains induced an influx of cells to the alveolar spaces by 3 to 4 days postinfection, and this was reflected by a 5- to 10-fold increase in the number of "macrophages" in harvest fluids at this time. Many of these cells had an altered morphology compared with alveolar macrophages from uninfected mice, and the cell population as a whole was not susceptible to influenza. However, this resistance was lost by 7 days of in vitro culture.

Specific and nonspecific immune resistance enhancing activity of staphage lysate

The immunopotentiating activity of staphage lysate (SPL) was evaluated in terms of its immune protection against lethal bacterial infection and its antitumor activity. Mice were pretreated weekly with 10(8) viable, Staphylococcus aureus, strain 18Z for 3 weeks (Induction), followed by intraperitoneal SPL injections (Elicitation) at various times in relation to infectious challenge or tumor implantation. Induction without elicitation, or elicitation alone failed to provide protection against Klebsiella pneumoniae infection and resulted in only 30-40% survival against homologous infection with pathogenic S. aureus type III, whereas combined induction and elicitation produced enhanced resistance induction and elicitation regimens resulted in 50% and 80-100% survival in mice inoculated with K. pneumoniae and S. aureus, respectively. SPL had no antitumor effect in mice implanted with median survival time resulting from induction and elicitation in animals implanted which Ehrlich's ascites. This enhancement of immune resistance may possibly be related to activation of thymus-modulated lymphocytes and macrophages by SPL.

Histopathological changes in the lungs of influenza-infected mice superinfected with Staphylococcus aureus

Histopathological changes in the lung were assessed in a model infection of mice using swine influenza virus and Staphylococcus aureus. Virus preinfection markedly enhanced both the persistence of S. aureus and the extent and duration of the inflammatory response. Lymphocytic infiltration was increased and regeneration and squamous metaplasia were delayed, suggesting an enhancement of an immunopathological response to the virus. Mice with the dual infection showed a decrease in phagocytic cell infiltration of the lung compared to that observed in those with the bacterial infection only, and this may reflect a decline in alveolar macrophage function after influenza infection.

Role of interferon in mice in protection against influenza A virus by bacterial ribosomes together with membranal glycoproteins of Klebsiella pneumoniae as adjuvant

Nonspecific protection against infectious aerosols of influenza A virus was obtained in Swiss mice after vaccination by aerosols of bacterial ribosomes together with membranal glycoproteins extracted from Klebsiella pneumoniae as the adjuvant. It was shown that repeated stimulant aerosols were necessary to obtain this protection. Routine estimation of serum interferon levels after administration of the association of ribosomes plus membranal glycoproteins to the animals by aerosol or intravenous route showed that there was no correlation between protection and the presence of serum interferon. It was shown that the serum interferon-inducer activity was due to ribosomes. No induction of serum interferon was obtained with membranal glycoproteins used separately. Local liberation of interferon in the mucous membrane of the upper respiratory tract was not investigated.

Effect of influenza infection on the phagocytic and bactericidal activities of pulmonary macrophages

The effect of mouse-adapted influenza A/PR/8/34 virus on pulmonary macrophage function was evaluated by using an in vitro system which allowed direct virus interaction with macrophages and then separate analysis of the steps required for bacterial clearance by macrophages. Infection of macrophages with this virus resulted in the appearance of a hemagglutinating activity on the macrophage surface; expression of this activity was inhibited by amantadine, 2-deoxyglucose, and cycloheximide and by pretreatment of the virus inoculum with ultraviolet light and specific antiserum. Since there was no release of extracellular virus, this growth cycle appeared to be incomplete (abortive). After influenza infection, net ingestion of viable Staphylococcus aureus by macrophage monolayers was unaltered and there was no change in the fraction of the monolayer which ingested cocci over a wide range of bacterial inputs. Influenza-infected macrophages also inactivated intracellular S. aureus at a rate indistinguishable from controls. Therefore, these in vitro studies do not support the hypothesis that the defect in pulmonary antibacterial mechanisms associated with influenza infections results from a direct effect of virus infection on either the phagocytic or bactericidal activity of resident pulmonary macrophages.

Appearance of rosette-forming macrophages in the lungs of influenza virus-infected mice

Approximately one fifth of the macrophages obtained from the lungs of mice infected 2 to 5 days with influenza A/HK virus were found to rosette well with either unmodified human, chicken, or guinea pig erythrocytes, but not with erythrocytes from hamsters, sheep, or mice. Rosette-forming macrophages were seldom seen in suspensions from uninfected mice (3+/-3%) or mice infected 24 h previously (3+/-3%). Rosette formation was not due to virus hemadsorption, as indicated by the failure of specific antiserum to influenza virus to block rosette formation; by the induction of comparable levels of rosette-forming macrophages in the lungs of mice infected with herpes simplex virus type 2, a nonhemadsorbing virus; and by the inhibition of rosette formation at 4 degrees C. Instead, rosette formation appeared to be directly related to macrophage elicitation or activation since nonstimulated macrophage populations such as peripheral blood monocytes, macrophages from uninfected lungs, or noninduced peritoneal macrophages were not observed to rosette to any significant extent. Furthermore, peritoneal macrophages induced with filter-sterilized normal horse serum rosetted at levels comparable to that observed with cells from infected lungs. These results indicate that hemadsorption alone can not be used as a criterion of virus infection of macrophages. However, rosette formation may serve to identify macrophage subpopulations which are active in host defense against viral infections.

Interaction between 6/94 virus, a parainfluenza type 1 strain, and mouse macrophages

The 6/94 virus, a type 1 parainfluenza virus recovered from multiple sclerosis brain cells after lysolecithin-induced fusion of these cells with African green monkey kidney cells (CV-1), has been found to grow in splenic and peritoneal macrophages obtained from outbred and different strains of inbred mice. Macrophages from C57BL animals were least susceptible to infection, a resistance apparently partially age related. The virus also has been found to replicate in IC21 cells, a line of simian virus 40 virus-transformed mouse macrophages. Viral growth was detected by development of hemadsorption in infected cells, followed by the appearance of infectious virus. The growth of 6/94 virus had different kinetics in mouse macrophages, in the standard continuous cell lines L, 3T3, and CV-1, and in primary mouse kidney and mouse embryo cells. The virus produced in macrophages could be passed in series to other macrophage cultures. In addition, once infected, the cultures continued to produce virus, and permanently infected cell lines were thus obtained. Macrophages from immunized mice with high titers of humoral neutralizing antibodies were found variably able to support virus growth.

Role of macrophages in natural resistance to virus infections

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Host defense mechanisms against influenza virus: interaction of influenza virus with murine macrophages in vitro

The interaction of mouse macrophages with influenza virus was examined as part of a study into the defense mechanisms against influenza infection. Macrophages exposed to A/Port Chalmers/1/73 virus produced infectious foci on susceptible indicator cell monolayers. Sampling of supernatant fluids and cells from infected macrophage cultures showed release of virus adsorbed to the cell surface. Active virus replication in macrophages could not be demonstrated. Exposing macrophages to specific antibody before or after virus infection resulted in a significant decrease in the number of infectious macrophages. The results suggest that although macrophages are not the source of replicating influenza virus, they are able to spread the infection by having virus attaching to their surface. This activity is interfered with by the presence of specific antibody.

Cellular changes in lungs of mice infected with influenza virus: characterization of the cytotoxic responses

Transpleural lavage of lungs from uninfected C3H mice yielded an average of 300,000 leukocytes per mouse. This number increased eightfold within 6 days after intranasal inoculation with virulent influenza A/Hong Kong/68 (H3N2) virus. Macrophages and lymphocytes in approximately equal numbers comprised 90% or more of the leukocytes both before and during infection. B, T, and null lymphocytes comprised, respectively, 9, 21, and 18% of the leukocytes before infection and 7, 26, and 5% by day 6. In absolute numbers, macrophages and T lymphocytes provided the major increments during infection. Cytotoxic activity of mononuclear cells from lung lavages was compared in a chromium release assay using syngeneic L929 target cells with the activity of mediastinal lymph nodes, spleens, and peripheral blood of uninfected and infected C3H mice. Nonspecific cytotoxicity for target cells infected with H3hkNeq1 or B/Lee influenza virus was found with mononuclear cells from uninfected mice. This activity tended to be highest with lavage leukocytes and was associated with adherent cells, presumably macrophages. Increased virus-specific cytotoxicity was detected with lavage cells by day 6 and persisted through day 9, the period of maximal pneumonia. Similar cytotoxic activity also appeared in cells from the nodes and spleen at this same time but was not detected in peripheral blood cells. The virus-specific cytotoxicity of lavage cells was due largely to a nonadherent cell possessing Fc receptors and theta antigen but lacking C3 receptors; these properties are compatible with actively cytotoxic T lymphocytes. The cytological characteristics of the infiltrating leukocytes and the cytotoxicity data suggest that the local T cell response to influenza virus infection in the lung is a major contributor to the pneumonia observed in this mouse model.

Inborn resistance of mice to myxoviruses: macrophages express phenotype in vitro

A strain of avian influenza A virus was adapted to grow in mouse peritoneal macrophages in vitro. The adapted strain, called M-TUR, induced a marked cytopathic effect in macrophages from susceptible mice. Mice homozygous (A2G) or heterozygous (F1 hybrids between A2G and several susceptible strains) for the gene Mx, shown previously to induce a high level of resistance towards lethal challenge by a number of myxoviruses in vivo, yielded peritoneal macrophages which were not affected by M-TUR. Peritoneal macrophages could be classified as resistant or susceptible to M-TUR without sacrificing the cell donor. Backcrosses were arranged between (A2G X A/J)F1 and A/J mice. 64 backcross animals could be tested individually both for resistance of their macrophages in vitro after challenge with M-TUR, and for resistance of the whole animal in vivo after challenge with NWS (a neurotropic variant of human influenza A virus). Macrophages from 36 backcross mice were classified as susceptible, and all of these mice died after challenge. Macrophages from 28 mice were classified as resistant, and 26 mice survived challenge. We conclude that resistance of macrophages and resistance of the whole animal are two facets of the same phenomenon.

Nonspecific resistance to Escherichia coli in mice

Nonspecific cell-mediated immunity to a relatively virulent strain of Escherichia coli was studied in mice infected with Staphylococcus aureus and elicited with specific antigens. The infected and elicited mice were protected against as intraperitoneal challenge by E. coli for an observation period of 7 days, whereas normal mice, given the same number of bacteria, died within 18 to 24 h. However, the amount of time elapsing between elicitation and challenge greatly affected the rate of protection. Little or no protection was observed in mice injected with S. aureus but not elicited or in mice injected with staphylococcal antigens but not infected with staphylococci.

Specific macrophage immunity to Sendai virus: macrophage aggregation in vitro with Sendai virus by cytophilic antibodies

When a 24-h tube culture of rabbit alveolar macrophages was infected with Sendai virus, the rate of infected cells was found to be limited. Even at a multiplicity of infection (MOI) of 500 plaque-forming units per cell, an average of 63% cells was found to synthesize viral antigens stainable by direct immunofluorescence. When the macrophages obtained from rabbits hyperimmunized by an intravenous injection of Sendai virus were infected under the same in vitro conditions, the rate of antigen synthesis averaged a low as 23%. At the time of infection of alveolar macrophages from immunized rabbits (immune macrophages), cell aggregation at an MOI 50 and cell fusion at an MOI 500 were found 24 h after infection, and these reactions were never encountered after the infection of nonimmune macrophages. When the immune macrophages were either pretreated by trypsin or incubated in medium at pH 4.0, the infection no longer caused the aggregation. The supernatant fluid obtained after incubation at pH 4.0 contained neutralizing antibody to Sendai virus. Conversely, when nonimmune macrophages were incubated in the presence of rabbit anti-Sendai virus serum or purified immunoglobulin G, the same aggregation reaction occurred after virus infection. Ultraviolet light-killed Sendai virus could be used as the counterpart of alive virus in the same aggregation reaction. These results suggest that the aggregation reaction of the immune macrophages could be attributed to the presence of specific cytophilic antibodies on their surface.