Previous Article | Next Article ![]()
Journal of Virology, August 2002, p. 8110-8117, Vol. 76, No. 16
0022-538X/02/$04.00+0 DOI: 10.1128/JVI.76.16.8110-8117.2002
Copyright © 2002, American Society for Microbiology. All Rights Reserved.
U892 INRA, Domaine de Vilvert, 78352 Jouy-en-Josas, France,1 Department of Microbiology and Immunology, Stanford University School of Medicine, Stanford, California 94305,2 Palo Alto V.A. Health Care System, Palo Alto, California 94304,3 Section of Pediatric Infectious Diseases, University of Chicago Children's Hospital, Chicago, Illinois 606374
Received 29 January 2002/ Accepted 17 May 2002
|
|
|---|
|
|
|---|
Several successful vaccination strategies against RV involving laboratory scale experiments and clinical trials have been used. Vaccination with heterologous RV (virus isolated from a different species) (42), with live heterotypic RV (virus with a distinct serotype) (12), or with heterologous virus-like particles (VLP) (30) have conferred either total or partial protection. These findings suggest that common antigenic structures in different viral isolates generate a protective immunity. A Jennerian approach using rhesus or bovine RV against a murine RV challenge (ECw) indicated that protection was correlated with fecal immunoglobulin A (IgA) levels to the antigenically conserved group-specific VP6 protein, and not with serum IgG responses (12). Since antibodies to the inner capsid protein VP6 are not neutralizing (4, 34), the mechanism by which they would exert an antiviral effect is unclear. Burns et al. reported that two murine hybridomas producing an IgA directed to the VP6 protein and implanted in a backpack model completely protected adult mice from a murine RV challenge (4). The authors suggested that the anti-VP6 IgA probably blocks crucial steps of the viral cycle inside the infected enterocyte during the transcytosis of dimeric IgA via the polymeric Ig receptor (pIgR). However, Ruggeri et al. reported findings that are discordant with those of Burns et al. (34). They showed that backpack-implanted hybridomas secreting IgA against the external capsid VP4 protein, but not against the internal VP6 protein, were protective against RV-induced diarrhea in a neonatal mouse model of infection (34). The discrepancy of those observations and those of Burns et al. may be explained by biological differences between the adult and the neonatal mouse models, or more likely by the VP6 epitopes recognized by the different IgA-producing hybridomas. However, these works did not address the question of whether the mucosal anti-VP6 antibodies elicited by vaccination play a determining role in protection and whether Ig transcytosis via the pIgR is actually involved in protection.
Mucosal pIgA and pIgM transcytose through epithelial cells after binding to pIgR, which is expressed at the basolateral cellular pole of crypt epithelial cells (2). The pIg-pIgR interaction is strictly dependent on the Ig disulfide-mediated covalent link with the 15-kDa polypeptide J chain (41). The pIg-pIgR complex is then transported via a vesicular pathway inside the epithelial cells. At the luminal cell surface, the pIgR is proteolytically cleaved, with a portion known as secretory component remaining associated with the pIgs in secretions (40). J-chain-deficient (J chain-/-) mice that are impaired in mucosal IgA and IgM transport have been generated. They exhibit serum IgA accumulation and lack pIgA in their intestinal secretions (23) and in feces (17, 19).
In order to demonstrate that pIgR-mediated transcytosis of antibodies directed to the inner capsid (VP2 and VP6) proteins prevents RV infection, we assessed whether the protection conferred by heterologous VLP2/6 is altered in J chain-/- mice. We found that whereas adult wild-type (WT) mice nasally vaccinated with VLP2/6 were protected against infection by a high-dose murine ECw virus challenge, immune J chain-/- mice got readily infected and shed virus for several days. These results indicate that nasal vaccination with heterologous VLP2/6 can protect normal mice against infection with a high-viral-dose challenge, and that transport of nonneutralizing anti-VP2 or -VP6 mucosal Ig via the J chain/pIgR-dependent pathway is mandatory to achieve protection.
|
|
|---|
Recombinant baculoviruses used in this study were BacRF2A, encoding for bovine strain RF VP2 (22), Bac RF6, encoding for bovine strain RF VP6 (39), and DsRedJA16, which expresses the DsRed fluorescent protein fused to VP2 (6).
Spodoptera frugiperda 9 (Sf9) insect cells were maintained in Hink's medium (Gibco-BRL, Grand Island, N.Y.) containing 10% fetal calf serum (FCS).
Animals. The J chain-/- mice were generated as previously described (17) and bred for 10 to 12 generations into the BALB/c background (Taconic Laboratories). Age-matched WT BALB/c mice (Taconic Laboratories) were used as controls. Mice were systematically confirmed to be RV antibody free prior to vaccination by enzyme-linked immunosorbent assay (ELISA) on serum samples. They were housed in microisolator cages within a barrier facility during the vaccination period. The infectious challenge was performed in an A3 animal facility (Unité d'Expérimentation Animale Rongeurs, Jouy-en-Josas, France).
VLP. VLP2/6 were prepared in Sf9 cells coinfected with recombinant baculoviruses (five particle-forming units per cell) expressing the bovine RF VP2 and VP6 genes and purified as previously described (22). Briefly, infected cells were collected 5 days postinfection and then extracted with Freon 113, and the aqueous phase containing VLP was subjected to an isopycnic cesium chloride gradient in a 20 mM PIPES [piperazine-N,N'-bis(2-ethanesulfonic acid)] buffer with 10 µM CaCl2 (pH 6.6). Purified VLP2/6 were then desalted with RPMI medium-equilibrated Sephadex G-25 columns. The desalted VLP2/6 were then subjected to a Bradford assay for protein quantitation using bovine serum albumin as a standard.
DsRed fused to VP2 was coexpressed with VP6 in Sf9 cells to produce particles designated DsRed-VLP2/6 (6).
Nasal immunizations and sample collection. Six-week-old mice were anesthetized by intraperitoneal administration of a mixture of ketamine (100 mg/kg of body weight) and xylazine (10 mg/kg). The immunogen was given in a 20-µl volume by gradual inoculation in the nostrils of the mice on days 0, 21, and 42. Groups of six to eight mice received 9 µg of VLP2/6 mixed with 5 µg of cholera toxin (CT) (Sigma, St. Louis, Mo.) or phosphate-buffered saline (PBS) mixed with 5 µg of CT as a control. Blood was collected 21 days after the last immunization under general anesthesia. Stools were harvested from individual mice placed in individual cages on absorbent paper on the day of the challenge and on days 2, 3, 4, 7, and 8 after the challenge. Both the serum and the feces specimens were kept at -20°C before analysis.
Virulent murine RV challenge. WT and J chain-/- mice (eight mice per group) were challenged at the same time, 21 days after the last immunization. They were orally gavaged with 104 SD50 of ECw murine virus after receiving 100 µl of 1.33% sodium bicarbonate to neutralize stomach acid.
Detection of RV antigens. Shedding of RV in feces samples was examined. Feces were suspended as a 10% suspension in 10 mM Tris (pH 7.4)-140 mM NaCl-10 mM CaCl2-25 mM EDTA-0.05% Tween 20-1% protease inhibitor cocktail (Complete Mini; Roche, Switzerland). After 45 min of incubation on ice, the feces suspension was frozen at -80°C, thawed, and spun down at 13,000 x g, and the crude supernatant was tested for the presence of RV antigens. Two monoclonal mouse anti-RV VP6 antibodies (R50 and 138) were used to coat an ELISA 96-well plate (Probind 3915; Becton Dickinson, San Jose, Calif.) at a 2-µg/ml dilution in carbonate buffer (pH 8.3) overnight at 4°C. The plates were washed twice in PBS containing 0.05% Tween 20 (PBS-T) and saturated in PBS plus 5% FCS (saturation buffer) for 2 h at 37°C. Undiluted feces supernatant was added to plates and incubated for 2 h at 20°C. After six washes in PBS-T, a 1:1,000 dilution of rabbit anti-RV serum diluted in saturation buffer was applied onto the well for 1 h at 20°C, washed, and followed by a peroxidase-conjugated goat anti-rabbit serum (Biosys S.A., Compiègne, France). After six washes, the Sigma Fast o-phenylenediamine dihydrochloride (OPD) (Sigma Chemical Co.) was used for color development, which was stopped by HCl. The plates were read at 492 nm by a microplate reader (Dynex, Chantilly, Va.). Stools from noninfected mice were included as control samples and gave a mean optical density at 492 nm (OD492) value of 0.08. Samples with OD492 above 0.2 were considered positive for RV shedding. The total fecal virus shedding for each mouse was calculated by integrating the area under the curve defined by the OD492 of each feces sample (y axis) over the 8-day period of feces collection (x axis) with the Microcal Origin 5.0 software. The day of viral clearance was deduced from a mathematical formula derived from the viral shedding curve and corresponded to the time point at which the OD492 value reached 0.2 (11).
Detection of RV-specific fecal Ig. ELISA wells were coated overnight with 200 ng of VLP2/6 in carbonate buffer (pH 8.3) at 4°C. They were washed with PBS-T and saturated for 2 h at 37°C in saturation buffer. Feces dilutions in saturation buffer were incubated for 2 h at 20°C. Peroxidase-conjugated anti-mouse IgA (Caltag), IgG (Jackson Laboratories), or IgM (Jackson Laboratories) was added to the plate for 1 h and washed with PBS-T, and the OPD reagent was used for revelation. Antigen-specific IgA, IgM, or IgG titers in fecal samples were expressed as the reciprocal of the highest feces dilution giving an OD492 reading above 0.2. The mean OD492 value obtained with the lower fecal dilution (1:5) from unvaccinated control mice was 0.1.
Detection of RV-specific serum Ig. ELISA wells were coated overnight with 200 ng VLP2 and VLP6 in carbonate buffer pH 8.3 at 4°C. They were washed with PBS-T and saturated for 2 h at 37°C in saturation buffer. Serum dilutions in saturation buffer were incubated for 2 h at 20°C. A peroxidase conjugated anti mouse heavy and light chain antibody (Biosys, Compiègne, France) or peroxidase conjugated anti mouse IgA, G or M were added to the plate for one hour, washed with PBS-T and the OPD reagent was used for revelation. Antigen-specific Ig titers were expressed as the reciprocal of the highest serum dilution giving a OD492 reading above 0.15. The mean OD492 value obtained with the lower serum dilution (1:100) from unvaccinated control mice was 0.075.
Isolation of splenic and mesenteric lymph node cells. Spleen and mesenteric nodes were aseptically removed from three mice per group. Single-cell suspensions were prepared by mechanical dissociation and filtered on 70-µm-pore-size nylon meshes and counted. Two million cells were stained with a fluorescein isothiocyanate-conjugated anti B220 antibody (Becton Dickinson) that specifically labels B cells in mice and with 3 µg of DsRed-VLP2/6 per ml. After an incubation in PBS containing 4% FCS for 30 min at 4°C, the cells were washed twice and analyzed on a FACScan with the CELLQuest software (Becton Dickinson). DsRed-VLP are excited by a 488-nm argon laser, and the emitted fluorescence is detected in the FL2 channel. A control of background fluorescence without DsRed-VLP2/6 was included and attested for the specificity of the labeling. A total of 2 x 105 viable cells were analyzed per sample.
Statistical analyses. A paired two-tailed Student t test was done to estimate statistically significant differences between the experimental values obtained from groups of mice, such as total viral shedding and antibody titers.
|
|
|---|
![]() View larger version (16K): [in a new window] |
FIG. 1. Detection of IgA in feces (A) and serum (B) samples from vaccinated WT and J chain-/- (KOJ) mice. VLP2/6 (9 µg) and CT (5 µg) were intranasally administered to six WT ( ) and J chain-/- ( ) mice on days 0, 21, and 42. Mock-treated WT( ) and J chain-/- ( ) groups of mice were intranasally inoculated with 5 µg of CT in PBS. Feces and serum specimens were collected on day 63. Feces (A) and serum (B) samples were tested by ELISA for specific IgA against VLP2/6; the reverse dilutions are reported on the x axis, and the corresponding OD492 mean values (± standard errors of the means) are shown on the y axis. The cutoff OD492 for positive value is 0.2 (A) and 0.15 (B).
|
|
View this table: [in a new window] |
TABLE 2. Serum and fecal anti-RV Ig in WT and J chain-/- micea
|
![]() View larger version (21K): [in a new window] |
FIG. 2. Fecal viral antigen shedding curves of control WT ( ) and J chain-/- mice ( ). Control mice were infected with 104 SD50 ECw. Feces were collected, solubilized, and tested as crude supernatants for RV antigens by ELISA; the results are expressed as OD492 readings, and the cutoff for positive value is 0.2. Each time point represents the mean OD492 from eight mice ± standard error of the mean. The experiment was done twice with similar results. The results of one experiment are presented.
|
|
View this table: [in a new window] |
TABLE 1. Total viral antigen shedding over 8 days
|
![]() View larger version (19K): [in a new window] |
FIG. 3. J chain-/- mice are not protected against an RV ECw challenge by nasal vaccination with VLP2/6. Eight WT( ) and J chain-/- mice ( ) were nasally inoculated with 9 µg of VLP2/6 plus 5 µg of CT on days 0, 21, and 42. They were challenged with 104 SD50 of ECw on day 63. Feces were collected at the indicated days, solubilized, and tested as crude supernatants for RV antigen detection by ELISA. (A) Mean shedding of RV antigens (expressed as OD492); the cutoff for positive value is 0.2. (B) Percentages of mice with detectable RV shedding. , WT; , J chain-/-. The experiment was done twice with similar results. The results of one experiment are presented.
|
![]() View larger version (46K): [in a new window] |
FIG. 4. B-cell reactivity with VLP2/6 in nasally vaccinated WT and J chain-/- mice in spleen and mesenteric nodes. (A) Dot plot representations of FACS analyses from 2 x 105 splenic and mesenteric cells labeled with fluorescein isothiocyanate-conjugated anti B-220 and DsRed-VLP2/6 from representative mice. Nine micrograms of VLP2/6 containing 5 µg of CT was instilled into WT (WT2/6) and J chain-/- (KOJ 2/6) via the nasal route on days 0, 21, and 42. WT and J chain mice that had been inoculated with PBS and CT were used as controls (WT PBS, KOJ PBS). Ten days after the last injection, cells from the spleen and mesenteric nodes were harvested and labeled. (B) Graphic representation of the number of B220-positive cells specific for VLP2/6 out of 2 x 105 total cells (y axis) from three individual mice per group.
|
|
|
|---|
4ß7high B cells, indicating that local B cells have a potent role in the control of RV infection (43). However, the mechanisms by which B cells act against RV are unclear. Among unsolved issues is whether and how the immunodominant antibody response against the inner capsid VP2 and VP6 proteins plays a determining role in the defense against RV. Several experimental studies suggest that anti-VP6 and/or -VP2 antibodies can be involved in protection. Indeed, double-layer EDIM virus, i.e., EDIM virus without the external capsid, induces better protection in WT mice than in B-cell-deficient µMT mice (28). Furthermore, in the mouse backpack model, a hybridoma producing anti-VP6 IgA protected the mice against an RV ECw challenge (4). In this report, we further show that adult mice with an inherent block in IgA and IgM transcytosis via the pIgR demonstrate a delay in clearing RV and are not protected against a murine RV challenge by immunization with heterologous bovine VLP2/6. The J chain-/- mice's failure to control RV infection cannot be attributed to known altered B- or T-cell responses. In fact, we verified that the anti-RV-specific B-cell distributions in spleen and mesenteric lymph node of the two types of mice were similar. In addition, anti-RV IgA-secreting cells were detected by the enzyme-linked spot assay in both immune WT and J chain-/- mouse intestine (data not shown). FACS analysis of splenocytes from the J chain-/- mice using monoclonal antibodies to CD3, CD4, and CD8 revealed staining patterns indistinguishable from those of WT littermates (17). In addition, T-cell-dependent IgG responses were comparable in J chain-/- mice and WT controls (unpublished data). The significant delay in clearance of RV infection in J chain-/- mice in the context of preserved B-cell responses supports the role of mucosal immunoglobulins transported by the pIgR in the control of RV primary infection. This finding suggests that secretion of mucosal IgA and IgM directed to the whole viral particle participates in the clearance of RV infection, either in the lumen of the intestine or possibly inside the epithelial cells, as suggested for human immunodeficiency virus (1), influenza virus (25), and Sendai virus (26) infections. Previous works using genetically engineered mice have shown that B cells are major players in the protection against RV reinfection but minor actors in the resolution of infection (14, 27). However, these findings were obtained with mice bred on a C57BL/6 x 129 (14) or on a C57BL/6 (26) background that probably emphasizes the impact of CD8 T cells on viral clearance, as C57BL/6 mice are genetically biased towards potent cytotoxic responses.
VLP2/6 has been shown to induce protection against RV infection in mouse (30) and rabbit models (7) using intranasal and parenteral vaccination routes, respectively. In this study, nasally delivered VLP2/6 induced protection in the adult mouse against infection with a 104 SD50 challenge dose of ECw virus, which is 103 higher than the dose previously used by O'Neal et al. (30). By contrast, immunization with VLP2/6 did not protect J chain-/- mice under these challenge conditions. This finding strongly suggests that transcytosis of IgA and IgM directed to VLP2/6 is required to fully protect mice against a high virus dose challenge. As for other viruses (1, 25, 26), intracellular neutralization of RV has been proposed by Burns et al. (4). Conceivably, transcytosis of anti-VP2 or -VP6 polymeric Ig associated to pIgR may allow intracellular interaction between the Ig and the corresponding structural protein VP6 or VP2, inhibiting a crucial step of the viral cycle such as transcription or assembly. It has been recently shown that some protective IgA in a backpack mouse model inhibits the assembly of the RV shell by preventing association of the outer capsid VP7 protein (15). The subcellular compartment where the transcytosing IgA interacts with the RV particle has yet to be identified.
Whereas the intracellular neutralization is an attractive mechanistic hypothesis, it still remains possible that the mucosal secretory anti-VP6 antibodies act inside the intestinal lumen, after their intracellular transit. Although monoclonal and polyclonal anti-VP6 antibodies have never been found to be neutralizing in in vitro assays, some secretory antibodies to VP6 may be able to prevent viral infection in vivo. Actually, secretory IgA exhibits higher stability and avidity as well as an increased neutralization potential compared to monomeric IgA or IgG (33, 38). In addition, due to a high sugar moiety content, immune complexes made of secretory IgA are highly hydrophilic and interact efficiently with mucins, leading to an efficient elimination of undesired antigens through the mucus layer (40). Thus, another possible explanation of our data is that secretory IgA (or IgM) directed to VP6-accessible epitopes may actually bind to infectious RV particles efficiently enough to lead to the viral immune exclusion within the mucus layer. This mechanism would be particularly efficient in rodents, as huge amounts of secretory IgA are found in the intestinal lumen due to the biliary excretion of IgA.
The role of anti-VP6 antibodies in protection could not be demonstrated with anti-VP6 IgA-producing hybridomas in a neonatal mouse infection model (34). The discordant findings between studies regarding the protective capacities of anti-VP6 hybridomas may be due to different biological conditions between the adult and the neonatal mouse models of infection. The protective mechanisms involved, such as pIgR transcytosis, secretory IgA interaction with the intestinal milieu, or complementation with undefined immune factors, may be modified with age. In addition, the lack of protection observed in the neonatal mouse model of infection with the chosen anti-VP6 IgA-producing hybridomas may have resulted from anti-VP6 IgA unable to interfere with the viral life cycle, in either an extracellular or an intracellular compartment. Actually, the epitopes that are recognized by the anti-VP6 IgA are probably essential to achieving antiviral effects. As vaccination with VLP2/6 elicits a wide panel of anti-VP6 and -VP2 antigenic specificities, it is likely that anti-VP2 or -VP6 IgA (or IgM) of adequate reactivity, efficient at blocking the viral cycle, was produced in our immune mice.
The major role of mucosal Ig in protection against RV that can be deduced from the J chain-/- model is not in accordance with the observation that lack of IgA synthesis in IgA-/- mice does not impair protection against secondary RV infection (31). This discrepancy may result from the fact that J chain-/- mice have a defect in secretion of both IgM and IgA whereas IgA-/- mice can produce secretory IgM. In fact, IgM from intestinal human plasma B cells shows accumulations of somatic mutations in the variable region to the same extent as IgA, implying that IgM could functionally replace IgA in mucosa (13). A compensatory role of IgM in IgA-/- mice immunized against reovirus (37) and against CT (16) has been suggested, as these mice showed increased specific IgM levels in their feces compared to WT mice. However, RV-specific IgM levels could not be detected in the IgA-/- feces (31), nor were they found in our study (Table 2). This lack of detection could result from the stability of secretory IgM being lower than that of secretory IgA, rendering IgM detection in feces unreliable (35). Intestinal IgG secretion was suggested to compensate for IgA absence in the IgA-/- mice (31). However, although immune J chain-/- mice had higher levels of anti-inner core fecal IgG than WT mice, they were unable to efficiently control the viral challenge. Finally, infection of the IgA-/- mice, which were bred onto the C57BL/6 genetic background, could elicit immune effectors that compensate for the lack of IgA in the IgA-/- mice and that may not be elicited in our J chain-/- BALB/c mice. Although CD4 and CD8 T cells did not seem to be involved in the protection of immune IgA-/- mice (31), subpopulations of intraepithelial T cells or
/
T cells could have played a determining compensatory role.
J chain-/- mice have been used in several infectious models to assess whether secretory polymeric immunoglobulins play a dominant role in protection against pathogens. J chain is not required for the cross-protective immunity against influenza A virus (9) or for protection against herpes simplex virus type 2 disease following immunization with an attenuated virus (18). Thus, in some instances, lack of J chain does not interfere with the establishment of an efficient immune response to ensure protection of mucosa. By contrast, lack of J chain was associated with a marked decrease in the resistance to a CT challenge in small intestinal ligated loops of orally immunized mice (23). This finding indicates that epithelial transport of specific IgA and/or IgM can prevent toxin-induced symptoms, i.e., loss of epithelial barrier integrity. In our study, we further show that the epithelial transport of IgM and/or IgA can actually control RV spreading through the intestine.
Other pathways of IgA secretion through the epithelial layer of the intestine have been suggested. J chain-/- mice show large amounts of monomeric IgA in milk and in nasal and intestinal washes (19), although the extent of IgA representation in intestinal secretions was not consistent between studies (23). The accumulation of monomeric IgA in the intestinal lumen may result from passive leakage or from alternative intraepithelial routing of IgA, as several IgA receptors that are not specific for dimeric IgA have been described (29, 36). In any event, the possible alternative pathways of monomeric IgA secretion in the intestine were not efficient at controlling RV infection, nor were they efficient at preventing CT-induced damages.
Overall, our data support that pIgR-mediated transcytosis of IgA and/or IgM directed to the inner capsid proteins plays a major role in protection against RV infection in an adult mouse model. This finding encourages the development of mucosal vaccination strategies to ensure an optimal defense of the intestine through secretory immunoglobulins. Our study also presents additional data in favor of heterotypic virus-based strategies in the field of RV vaccine design.
This work was supported in part by a 5th PCRD grant from UE (QLRT 1999-00634).
|
|
|---|
This article has been cited by other articles:
| |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
Copyright © 2009 by the American Society for Microbiology. For an alternate route to Journals.ASM.org, visit: http://intl-journals.asm.org | More Info»