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The Journal of Immunology, 2002, 168: 1796-1803.
Copyright © 2002 by The American Association of Immunologists

Nasal-Associated Lymphoid Tissue Is a Mucosal Inductive Site for Virus-Specific Humoral and Cellular Immune Responses1

Adrian W. Zuercher*, Susan E. Coffin{dagger}, M. Christine Thurnheer*, Petra Fundova{ddagger} and John J. Cebra2,*

Departments of * Biology and {dagger} Pediatrics, University of Pennsylvania School of Medicine, Philadelphia, PA 19104; and {ddagger} Division of Immunology and Gnotobiology, Institute of Microbiology, Czech Academy of Science, Prague, Czech Republic


    Abstract
 Top
 Abstract
 Introduction
 Materials and Methods
 Results
 Discussion
 References
 
Peyer’s patches are known as mucosal inductive sites for humoral and cellular immune responses in the gastrointestinal tract. In contrast, functionally equivalent structures in the respiratory tract remain elusive. It has been suggested that nasal-associated lymphoid tissue (NALT) might serve as a mucosal inductive site in the upper respiratory tract. However, typical signs of mucosal inductive sites like development of germinal center reactions after Ag stimulation and isotype switching of naive B cells to IgA production have not been directly demonstrated. Moreover, it is not known whether CTL can be generated in NALT. To address these issues, NALT was structurally and functionally analyzed using a model of intranasal infection of C3H mice with reovirus. FACS and histological analyses revealed development of germinal centers in NALT in parallel with generation and expansion of IgA+ and IgG2a+ B cells after intranasal reovirus infection. Reovirus-specific IgA was produced in both the upper respiratory and the gastrointestinal tract, whereas production of reovirus-specific IgG2a was restricted to NALT, submandibular, and mesenteric lymph nodes. Moreover, virus-specific CTL were detected in NALT. Limiting dilution analysis showed a 5- to 6-fold higher precursor CTL frequency in NALT compared with a cervical lymph node. Together these data provide direct evidence that NALT is a mucosal inductive site for humoral and cellular immune responses in the upper respiratory tract.


    Introduction
 Top
 Abstract
 Introduction
 Materials and Methods
 Results
 Discussion
 References
 
The mucosal immune system consists of two functionally distinct types of tissue: 1) inductive sites, where naive B and T cells are clonally selected and expanded upon Ag contact; and 2) effector sites, where activated B and T cells relocate after Ag-priming in inductive sites to express their effector functions. This concept is best established for the components of the intestinal mucosal immune system. They are organized in the so-called gut-associated lymphoid tissue (GALT),3 and include Peyer’s patches (PP), mesenteric lymph nodes (LN), and dispersed lymphoid cells in the epithelial layer and in the gut lamina propria (1). PP are inductive sites and have been described as the major location for Ag-specific B cell activation and isotype switching to IgA (2) and generation of IgA+ memory B cells (3), as well as for the induction of Ag-specific CTL (4). Generally, these primed B and T cells emigrate from PP, undergo terminal differentiation, and eventually home to the lamina propria and the intra-epithelial lymphocyte compartment of the gut (5, 6, 7). In contrast, little is known about the anatomic location and functional potential of inductive and effector sites in the respiratory tract. However, the known compartmentalization of mucosal immune responses depending on the route of Ag administration (8, 9) suggests that structures other than PP, e.g., in the respiratory tract, may function as mucosal inductive sites.

In the murine upper respiratory tract, nasal-associated lymphoid tissue (NALT) is believed to be the equivalent of the Waldeyer’s ring of humans (10). It consists of a paired lymphoid tissue located at the floor of the nasal cavity lined by ciliated respiratory epithelium, and has been postulated in different studies as a possible functional equivalent in the upper respiratory tract to PP in the gut (reviewed in Ref. 11). Several findings support this hypothesis. First, the cellular composition of NALT is similar to PP (12, 13, 14); both tissues contain a major population of naive B cells as well as naive (CD45RBhigh) T cells (12). Second, it has been shown in rats that both NALT and PP have overlaying epithelium containing M cells (15, 16) and follicle-associated epithelium (17) that may serve as entry-sites for different pathogens (18, 19). Third, upon stimulation with Ag, the major isotype of Ab produced by NALT B cells is IgA (12, 13, 20, 21). Salivary glands and tear glands were suggested as possible effector sites for IgA production (22, 23), which is reflected by the presence of IgA Abs in saliva and tear fluid. Despite these similarities, differences between PP and NALT, such as a markedly diverging expression and function of homing receptors (24), have been reported. Other hallmarks of mucosal inductive sites, like formation of germinal centers as well as isotype-switching and expansion of surface IgA+ B cells upon Ag-stimulation have not been directly demonstrated in NALT. Moreover, even though specific CTLs have been observed in cervical or mediastinal LN after intranasal (i.n.) immunization (25, 26), it is unknown whether CTLs are induced in NALT.

Using i.n. infection with reovirus serotype 1/Lang, an established mucosal pathogen that infects and elicits mucosal immune responses in both the gastrointestinal (4, 27, 28, 29) and respiratory tract (30, 31), we have structurally and functionally analyzed murine NALT. Germinal centers, as well as IgA+ and IgG2a+ B cells, were induced and expanded in NALT, and potent reovirus-specific IgA responses were detected in the upper respiratory tract (NALT, palatine salivary glands, and submandibular LN) and gastrointestinal tract (PP, small intestine, and mesenteric LN). Importantly, virus-specific CTLs were detected in NALT as well as in mediastinal, submandibular, and cervical LN. Limiting dilution analysis revealed a 5- to 6-fold higher precursor-CTL frequency in NALT compared with cervical LN. Thus, NALT is a mucosal inductive site in the upper respiratory tract for specific humoral and cellular immune responses.


    Materials and Methods
 Top
 Abstract
 Introduction
 Materials and Methods
 Results
 Discussion
 References
 
Mice and media

Male C3HeB/FeJ (referred to as C3H), BALBc/ByJ, and DBA/2 mice were purchased from The Jackson Laboratory (Bar Harbor, ME). C.B-17 (C.B-Igh1b/IcrTac) mice were from Taconic Farms (Germantown, NY). All mice were used at the age of 8–14 wk. Stock mice were housed in the animal facility of the Department of Biology, University of Pennsylvania (Philadelphia, PA). After infection with reovirus, mice were kept physically separated from naive stock mice in a Trexler plastic isolator (Standard Safety, McHenry, IL).

L-929 fibroblasts were grown in M199 medium (Life Technologies, Grand Island, NY) containing 5% FCS (Life Technologies), 2 mM L-glutamine (Life Technologies), 1000 U/ml penicillin, and 0.1 mg/ml streptomycin (Life Technologies). CTLs were grown in RPMI-1640 (Life Technologies) containing 10% FCS, L-glutamine, penicillin, streptomycin, 50 µg/ml gentamicin (Life Technologies), and 50 µM 2-ME (Sigma Aldrich, St. Louis, MO). For organ fragment cultures, Kennett’s HY medium (Life Technologies) supplemented with 10% FBS, L-glutamine, penicillin, streptomycin, and gentamicin was used.

Histology, immunohistochemistry, and immunofluorescence

NALT was frozen in OCT compound (TissueTek; EMSCO, Philadelphia, PA), horizontal 5-µm cryosections were cut on a Cryocut 1800 cryotome (Leica; Dolbey-Jamison, Norristown, PA) and after air-drying fixed inice-cold acetone. Some slides were stained with hematoxylin (Mayer’s hematoxylin; Sigma Aldrich) and eosin (Sigma Aldrich). For immunohistochemical staining, slides were incubated in 0.3% H2O2 for 30 min, then blocked with Superblock (Pierce, Rockford, IL) for 30 min, and with biotin/avidin block (Vector Laboratories, Burlingame, CA) following the manufacturer’s instructions. Slides were incubated for 60 min with 50 µl of 2.5 µg/ml biotinylated anti-IgD (clone AMS9.1; BD PharMingen, San Diego, CA), 2.5 µg/ml biotinylated anti-CD4 (clone GK1.5; BD PharMingen), 2.5 µg/ml biotinylated anti-CD8 (clone 53.6-72; BD PharMingen), or 4 µg/ml biotinylated Ulex Europaeus agglutinin I (UEA; Vector Laboratories), followed by an incubation with HRP-conjugated streptavidin (1/1000; BD PharMingen). All reagents were diluted in a solution of 10% Superblock in PBS. 3,3-Diaminobenzidine (Sigma Aldrich) was used as substrate according to the manufacturer’s instructions, followed by counterstaining with hematoxylin (Gill’s hematoxylin; Fisher, Pittsburgh, PA), and overlaid with Permount (Fisher).

For immunofluorescence, slides were incubated for 30 min with 50 µl of the following FITC-conjugated reagents: PNA (Pierce, coupled to FITC in our laboratory as described in Ref. 32), 10 µg/ml anti-IgD (clone 11-26c.2a; BD PharMingen), 10 µg/ml anti-IgA (Southern Biotechnology Associates, Birmingham, AL), and 10 µg/ml anti-IgG2a (Southern Biotechnology Associates). Sections were overlaid with Vectashield containing 4',6'-diamidino-2-phenylindole (Vector Laboratories).

Flow cytometry

Single cell suspensions (2 x 105–106/sample) of NALT or submandibular LN were stained for 20 min at 4°C with FITC-conjugated anti-CD19 (clone 1D3; BD PharMingen), PE-conjugated anti-CD4 (GK1.5; BD PharMingen), FITC-CD8 (53-6.72; BD PharMingen), FITC-PNA, PE-{kappa} L chain (Southern Biotechnology Associates), FITC-IgA (Southern Biotechnology Associates), and FITC-IgG2a (Southern Biotechnology Associates). Cells were washed and fixed in 1% paraformaldehyde in PBS and analyzed on a FACScan flow cytometer (BD Biosciences, Mountain View, CA). WinMDI2.8 (The Scripps Research Institute, La Jolla, CA) software was used for evaluation.

Virus preparation and infection

Third passage stocks of reovirus type 1/Lang (33) were produced and purified as described (34). For i.n. infection, mice were lightly anesthetized with 0.1 mg/g body weight Avertin (2,2,2,-tribromoethanol; Aldrich, Milwaukee, WI) and 1–2.5 x 107 PFU reovirus was applied to both nostrils with a micropipette in a total volume of 25 µl saline/0.5% gelatin.

Virus titration

For determination of viral titers in various tissues, C3H mice (n = 3–4 per time point) were sacrificed by CO2 asphyxiation and cervical dislocation on days 2, 5, 7, and 14 after i.n. infection. After perfusion through the right ventricle with 20 ml PBS, the upper right lobe of the lungs, the entire trachea, one palatine salivary gland, and a 1-cm piece of terminal ileal small intestine were removed, washed, and weighed. The tissues were homogenized in 3 ml saline/0.5% gelatin and serial dilutions incubated on monolayers of L-929 fibroblasts in 6-well tissue culture plates (Costar, Cambridge, MA) for 45 min at 37°C, and thereafter overlaid with 3 ml of 1% Agar in complete M199 medium and cultured at 34°C. Cultures were overlaid with more Agar/M199 after 3 and 6 days. Plaques were counted after 7 days incubation.

Analysis of Ab production in organ fragment cultures

On days 0, 4, 7, 11, and 14 after i.n. infection, C3H mice (n = 3–4 per time point) were sacrificed and blood collected by heart puncture for serum isolation. After perfusion with 20 ml PBS, the entire small intestine and mesenteric LN were surgically removed. PP were visually detected and excised from the small intestine. After decapitation, submandibular LN were removed. NALT was isolated after removal of the mandible as described elsewhere (13). Palatine salivary glands were isolated after removal of the hard palate.

For organ fragment culture (35), tissues were sterilized by sequential washes as described in detail elsewhere (36). Individual PP or LN, 3 x 3 mm pieces of small intestine from jejunum, individual palatine salivary glands, and NALT from one mouse (a pair of NALT still attached to a small piece of nasal epithelium overlaying the palate) were incubated in wells of 24-well tissue culture plates (Costar) in 1 ml complete Kennett’s HY medium for 7 days under a 90% O2/10% CO2 atmosphere at 37°C.

Reovirus-specific IgM, IgA, and IgG2a Abs were measured by RIA. For this purpose, flexible polyvinyl plates (Serocluster; Costar) were coated with 2.5 x 109 particles of reovirus per well in 50 µl PBS overnight at 4°C. Plates were blocked with 1% BSA in PBS and incubated with organ fragment culture supernatant fluid overnight at 4°C. Thereafter, plates were incubated for 6 h at room temperature with 125I-labeled anti-IgA, anti-IgG2a, or anti-IgM Abs (all from Southern Biotechnology Associates). Radioactivity of individual wells was measured using a 1272 Clinigamma gamma counter (Wallac, Gaithersburg, MD). Total IgA Abs were measured by RIA as described earlier (35), and a standard curve of purified, monoclonal IgA was used to convert cpm to nanograms per milliliter.

In vitro restimulation and analysis of cytotoxic lymphocytes

Seven days after i.n. infection of C3H mice with reovirus, single cell suspensions of NALT, mediastinal, cervical, and submandibular LN pooled from 16–20 mice were restimulated in vitro by incubation in 96-well round-bottom microtiter plates (2 x 105/well; Costar) in the presence of 5 x 104 virus-pulsed, irradiated, thioglycolate-elicited peritoneal exudate cells (4). After 24 h, ConA-conditioned medium (5% (v/v)) mixed with methyl-a-D-mannopyranosid (100 µM; Sigma Aldrich) was added. Effector cells were restimulated the same way after 7 days of culture, and after a total of 13 days of culture, cellular cytotoxicity was assessed in a standard 51Cr-release cytotoxicity assay (37). L-929 fibroblasts infected with reovirus (or uninfected for control) were labeled with 100 µCi 51Cr (NEN, Boston, MA) and then incubated with effector cells at different E:T ratios (3000 target cells/well). After 5 h of incubation in V-bottom microtiter plates (Costar), 100 µl supernatant fluids were collected, mixed with 1 ml scintillation fluid (Cytoscint; ICN Pharmaceuticals, Costa Mesa, CA), and {beta}-emission measured on a LS6500 multipurpose scintillation counter (Beckman Coulter, Fullerton, CA).

Limiting dilution of precursor cytotoxic lymphocytes

Replicate cultures (n = 17–24) containing varying numbers of either NALT or cervical LN cells (4.7 x 102–1.2 x 105 for NALT, 3.9 x 103–5 x 105 for cervical LN) obtained from C3H mice 7 days after i.n. infection were restimulated in vitro for 7 days as described above. Thereafter, the contents of individual microtiter wells were harvested and equally divided into two wells of V-bottom microtiter plates containing either reovirus-infected or -uninfected L-929 cells (3000 target cells/well, labeled with 51Cr) for a 5 h standard 51Cr-release cytotoxicity assay as described above. Cultures were considered to demonstrate cytotoxicity if the resulting 51Cr release was at least three times more than background of spontaneous release. Cytotoxicity was considered virus-specific if the release from infected targets was at least 20% higher from infected than from uninfected targets. The 95% confidence intervals of the linear regression curves were calculated using the Sigmaplot 2.00 software (Jandel Corporation, SSPS, Chicago, IL).


    Results
 Top
 Abstract
 Introduction
 Materials and Methods
 Results
 Discussion
 References
 
Structure and cellular composition of naive murine NALT

We analyzed the structure of NALT by immunohistochemistry. As shown in Fig. 1Go, NALT in the naive C3H mouse is organized in distinct B and T cell areas (Fig. 1Go). As in PP, B cells reside in follicular areas, whereas T cells, particularly CD8+ T cells, predominantly occupy the parafollicular spaces between the B cell follicles. NALT is lined with an epithelium that binds the lectin UEA (Fig. 1Go). UEA is specific for {alpha}-L-fucose residues which are typically present on M cells and follicle-associated epithelium (16, 38). Staining with labeled UEA was restricted to the epithelium on the luminal side of NALT (Fig. 1Go). A detailed view of the luminal epithelium revealed staining of individual cells within the epithelium. These data demonstrate that NALT displays the typical structure and organization of a secondary lymphoid tissue and contains epithelial cells that potentially enable it to efficiently take up Ag from the nasal cavity.



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FIGURE 1. Histological analysis of naive murine NALT. Serial horizontal sections of naive mouse NALT were stained with biotinylated anti-IgD and HRP-labeled streptavidin, biotinylated anti-CD4, and HRP-streptavidin, or biotinylated anti-CD8 and HRP-streptavidin. Original magnification x100. Sections were counterstained with Gill’s hematoxylin. Serial horizontal sections of paired NALT (N) were stained with H&E (x40) or biotinylated UEA and HRP-streptavidin or HRP-streptavidin alone (x200). Staining of the epithelium lining the luminal side of NALT is indicated by the arrows (x40). Staining of individual cells at higher magnifications is shown (x200 and x400). NC, nasal cavity. Sections were counterstained with Mayer’s hematoxylin.

 
As shown in Fig. 2Go, B cells were the most abundant cell population in NALT comprising 47–78% of lymphocytes (Fig. 2Go, upper panel) depending on the strain of mice. The ratio of B:T cells varied from 3.8:1 in C.B17 to 0.9:1 in C3H mice, whereas the CD4:CD8 ratio varied from 3:1 in C.B17 to 1.6:1 in C3H mice (Fig. 2Go, lower panel). These data reveal that the cellular composition of NALT is similar to PP. As most studies using reovirus have been done in C3H mice, this strain was used for all further analyses.



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FIGURE 2. Lymphocyte composition of NALT. Single cell suspension of NALT cells (2 x 105–106/sample) isolated from naive DBA/2, C.B-17, BALB/c, or C3H mice were stained with FITC-labeled anti-CD19, PE-anti-CD4, or FITC-anti-CD8. Numbers represent the percentage of positive cells among gated lymphocytes. One of two identical experiments is shown.

 
Induction of germinal centers and generation of IgA+ and IgG2a+ B cells in NALT after infection with reovirus

To address the function of NALT, C3H mice were infected i.n. with 1–2.5 x 107 PFU of reovirus serotype 1/Lang in a volume of 25 µl. Replicating virus was detected in the respiratory tract (NALT, palatine salivary gland, lung, and trachea), and to a lesser extent in the small intestine, and was cleared within 7–14 days after infection (Fig. 3Go). These data demonstrate that the respiratory tract is the major site of viral infection after i.n. inoculation, but that coincidental swallowing of virus leads to infection and possible cross-priming of the gastrointestinal tract.



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FIGURE 3. Clearance of reovirus after i.n. infection. Lightly anesthetized naive C3H mice were infected i.n. with 1–2.5 x 107 PFU reovirus serotype 1/Lang in a volume of 25 µl. Two, 5, 7, and 14 days postinfection, groups of mice (n = 3–4) were sacrificed and perfused with PBS. Palatine salivary glands, one submandibular salivary gland, the upper left lobe of lung, trachea, and a 1-cm piece of terminal ileum were isolated, washed, weighed, and homogenized in PBS/0.5% gelatin. Serial dilutions were used to infect monolayers of L-929 fibroblasts for a standard virus titration assay. PFU were counted after 7 days incubation. Detection limit was ~100 PFU/g tissue. One of four similar experiments is shown.

 
Next, we examined the effect of the virus infection on NALT. The average number of cells recovered from NALT increased 54% from 1.3 x 105 (n = 12 mice) before infection to 2.0 x 105 (n = 91) at day 7 (p = 0.003), and decreased to initial levels by day 14 of infection (1.2 x 105; n = 14). In contrast, cell expansion was more dramatic and prolonged in submandibular LNs where cell numbers increased 11-fold within the first 7 days after infection (from 2.5 x 105/mouse in naive mice, n = 16, to 3.0 x 106/mouse at day 7, n = 19) and further to 1.8 x 106/mouse (n = 6) by day 14 postinfection). FACS analysis showed that the percentage of total (CD19+) B cells remained stable during the course of infection (data not shown). Similarly, the CD4:CD8 ratio did not change during the course of infection. Induction of germinal center B cells (PNA-binding, {kappa} L chainlow, Ref. 3) was observed in NALT on day 7 and increased through day 14, in parallel with expansion of both surface IgAlow and IgG2alow germinal center B cells, as well as surface IgAhigh and IgG2ahigh memory type B cells (Ref. 3 ; Fig. 4Go). Staining with anti-IgG2b Ab gave similar results (data not shown). A similar expansion of PNA+, IgA+, and IgG2a+ B cells was observed in submandibular LNs, although with slightly delayed kinetics compared with NALT (Fig. 4Go).



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FIGURE 4. Generation of germinal center (PNA-binding), sIgA+, and sIgG2a+ B cells in NALT and submandibular LN after i.n. infection with reovirus: FACS analysis. Lightly anesthetized naive C3H mice were infected i.n. with 1–2.5 x 107 PFU reovirus serotype 1/Lang in a volume of 25 µl. At 0, 5, 7, or 14 days postinfection, groups of mice (n = 3–4) were sacrificed and single cell suspensions of NALT cells (2 x 105–106/sample) were stained with PE-anti-mouse {kappa} L chain, FITC-PNA, FITC-anti-IgA, or FITC-anti-IgG2a. Numbers represent the percentage of positive cells among gated lymphocytes. One of four similar experiments is shown.

 
These findings were confirmed by immunofluorescence staining of histological sections of NALT. NALT of naive mice was characterized by the presence of distinct areas of IgD+ B cells (Fig. 5Go). No PNA-binding, IgA+, or IgG2a+ cells were detected before infection. Seven days after inoculation with reovirus, germinal centers characterized by the presence of PNA-binding cells were observed preferentially at the periphery of NALT. These PNA-positive follicles colocalized with IgA-stained and IgG2a-stained areas, whereas IgD+ cells predominantly occupied the PNA-negative regions. By day 14 of infection, PNA-binding cells were concentrated at the luminal side of NALT and strongly stained individual IgA+ and IgG2a+ cells were scattered throughout NALT. These findings demonstrate that germinal center reactions with generation and expansion of IgA- and IgG2a-switched B cells are induced in NALT after i.n. infection with reovirus.



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FIGURE 5. Generation of germinal center (PNA-binding), IgA+, and IgG2a+ B cells in NALT after i.n. infection with reovirus: histological analysis. Lightly anesthetized naive C3H mice were infected i.n. with 1–2.5 x 107 PFU reovirus serotype 1/Lang in a volume of 25 µl. At 0, 7, or 14 days after infection, NALT of individual mice was isolated and frozen. Serial horizontal sections were stained with FITC-labeled anti-IgD, FITC-PNA, FITC-anti-IgA, or FITC-anti-IgG2a. Luminal side of NALT is at the bottom of the picture. Original magnification x100.

 
Production of reovirus-specific Abs in organ fragment cultures

Organ fragment cultures (35) of mucosal and lymphoid tissues of the respiratory and gastrointestinal tract were performed to address production of total and virus-specific Abs after i.n. reovirus infection. Marginal amounts of total IgA were produced by NALT and palatine salivary glands of naive mice, whereas the components of GALT produced substantial quantities of IgA before stimulation with reovirus (Fig. 6GoA). After infection, total IgA production was stimulated in NALT and palatine salivary glands, and simultaneously a slight increase was observed in PP and small intestine. However, the total output of IgA was 4- to 150-fold higher (on day 7 or 0, respectively) from PP compared with NALT. Similarly, production of total IgA was generally higher in small intestine and mesenteric LN compared with palatine salivary glands and submandibular LN, respectively.



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FIGURE 6. Production of total IgA and reovirus-specific Abs in the upper respiratory and the gastrointestinal tract after i.n. infection with reovirus. Lightly anesthetized naive C3H mice were infected i.n. with 1–2.5 x 107 PFU reovirus serotype 1/Lang in a volume of 25 µl. At day 0, 4, 7, 11 and 14 days postinfection, groups of mice (n = 3–4) were sacrificed and perfused with PBS. NALT, PP, palatine salivary glands, a 1-cm piece of jejunum, and submandibular and mesenteric LN were isolated and sterilized by sequential washing. Organ fragment cultures of the tissues were incubated for 7 days. A, Total IgA was measured by RIA using anti-Fab coated polyvinyl plates. Serial dilutions of organ fragment culture supernatant fluid were incubated, and bound Abs detected with polyclonal 125I-labeled anti-IgA Ab. Cpm were converted to ng/ml using a standard curve of monoclonal-purified IgA. B, Supernatant fluids were assessed for reovirus-specific IgA (top panels), IgG2a (middle panels), and IgM (bottom panels) by RIA. Polyvinyl plates coated with 2.5 x 109 particles of reovirus per well were incubated with organ fragment culture supernatant fluid and bound Abs were detected with polyclonal 125I-labeled anti-IgA, anti-IgG2a, and anti-IgM Ab. Results from one of four similar experiments are shown.

 
Maximal amounts of reovirus-specific IgA were produced by NALT 7 days after infection (Fig. 6GoB, top row). Whereas production rapidly declined from NALT, secretion of virus-specific IgA by palatine salivary glands was sustained for at least 2 wk. Only marginal amounts of virus-specific IgA were detected in other mucosal tissues of the oral cavity such as lip or tongue (data not shown). Large amounts of virus-specific IgA were detected in supernatant fluid of PP and small intestine fragment cultures indicating cross-priming during i.n. inoculation. Virus-specific IgA was also produced by mesenteric and to a lesser extent by submandibular LN which drain the gastrointestinal and upper respiratory tract, respectively. Compared with IgA, the production of virus-specific IgG2a was delayed in NALT, but by day 14 postinfection, NALT was producing considerable amounts of virus-specific IgG2a (Fig. 6GoB, middle row). Similarly, virus-specific IgG2a responses were induced in submandibular and mesenteric LN. In contrast, no appreciable production of virus-specific IgG2a occurred in PP, palatine salivary glands, or the small intestine. Virus-specific IgM was detected in PP, submandibular, and mesenteric LN as early as 4 days postinfection, whereas no virus-specific IgM Abs were detected in supernatant fluids of NALT, palatine salivary gland, and small intestine fragment cultures (Fig. 6GoB, bottom row). The major isotype of virus-specific Ab induced in serum was IgG2a concomitant with production of virus-specific IgM, but only trace amounts of virus-specific IgA (data not shown).

Generation of reovirus-specific CTL in NALT

It is not known to date whether cytotoxic cellular immune responses can be induced in NALT. To address this, 7 days after i.n. infection with reovirus, cells from NALT, mediastinal, submandibular, and cervical LN from 16–20 mice were isolated and pooled. After in vitro restimulation for 13 days, the presence of virus-specific CTL was assessed in a standard 51Cr-release cytotoxicity assay. Potent virus-specific CTL were obtained from NALT, as well as from the LN draining the respiratory tract (Fig. 7Go). Identical restimulation of NALT and LN cells isolated from naive mice did not result in the outgrowth of virus-specific CTL (Fig. 7Go).



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FIGURE 7. CTL in NALT after i.n. infection with reovirus. Lightly anesthetized naive C3H mice were infected i.n. with 1–2.5 x 107 PFU reovirus serotype 1/Lang in a volume of 25 µl. After 7 days, mice (n = 16–20) were sacrificed and perfused with PBS. Single cell suspensions were cultured in microcultures in the presence of virus-pulsed, irradiated APCs and 5% (v/v) ConA-conditioned medium. Cells were restimulated after 7 days of culture with fresh APCs and ConA-conditioned medium. As a control, cells from naive mice were cultured under identical conditions (squares). After 13 days of culture, cells were used in a 5Cr-release cellular cytotoxicity assay with 51Cr-labeled, reovirus-infected (filled symbols) or uninfected (open symbols) L-929 fibroblasts as targets. After 5 h of incubation, {beta}-emission of 100 µl supernatant fluids was measured on a scintillation-counter. Results show the percentage of specific lysis after subtraction of spontaneous release. Spontaneous release was never above 18% of total release. One of three similar experiments is shown.

 
However, limiting dilution analysis revealed a 5-fold higher frequency of precursor CTL in NALT compared with cervical LN (Fig. 8Go). This ratio is similar when the frequencies are adjusted to total numbers of CD8+ cells in the initial NALT or cervical LN populations (NALT: 23.5% CD8+ cells, frequency = 962/106; cervical LN: 27.8% CD8+ cells, frequency = 165/106; ratio = 5.8:1).



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FIGURE 8. Limiting dilution analysis of precursor CTL in NALT (•) and cervical LN ({circ}). Lightly anesthetized naive C3H mice were infected i.n. with 1–2.5 x 107 PFU reovirus serotype 1/Lang in a volume of 25 µl. After 7 days, mice (n = 20) were sacrificed and perfused with PBS. Various dilutions of single cell suspensions were cultured for 7 days as described above. Contents of individual wells were assessed for cytotoxicity against 51Cr-labeled reovirus-infected or uninfected L-929 fibroblasts in a 51Cr-release assay. Results were evaluated as described in Materials and Methods. Frequencies (f) of precursor CTL among total cells are shown.

 

    Discussion
 Top
 Abstract
 Introduction
 Materials and Methods
 Results
 Discussion
 References
 
In this study, we have analyzed the induction of reovirus-specific humoral and cellular immune responses in the upper respiratory tract. We found a number of striking functional similarities between the mucosa-associated lymphoid tissue of the upper respiratory and the gastrointestinal tract. Both sites contain organized secondary lymphoid tissue lined by epithelium which possibly enables them to efficiently absorb Ag. At both sites, germinal center reactions with expansion of specific IgA+ and IgG2a+ B cells ensue from infection with reovirus. Furthermore, both sites contain similar draining LN, i.e., mesenteric LN in the gastrointestinal and submandibular LN in the respiratory tract, that may serve to amplify the responses generated in PP or NALT, respectively.

It is well accepted that mesenteric LN serve to amplify immune responses initiated in PP. Unlike this, it remains elusive which, and whether at all, LN of the respiratory tract fulfill a similar function for NALT. However, some of our findings may implicate such a role for the submandibular LNs. Whereas only modest expansion of cells was observed in NALT after i.n. infection (<2-fold within first 7 days postinfection), a more pronounced expansion of cells occurred in submandibular LN (11-fold) over the same time period. Furthermore, increased numbers of cells were still found in submandibular LN 14 days postinfection, although by that time cell numbers in NALT were not significantly different from naive mice. In contrast, our analysis of precursor CTL frequencies in NALT vs cervical LN (and presuming similar numbers for submandibular LNs) revealed a higher frequency of virus-specific CTL in NALT (Fig. 7Go). Moreover, kinetic analysis of generation of PNA+, IgA+, and IgG2a+ B cells showed a delay in the generation of these cells in submandibular LN compared with NALT (Fig. 4Go), i.e., whereas percentages of PNA+, IgA+, and IgG2a+ cells peaked or reached a plateau after ~7 days in NALT, the respective cells appeared later in submandibular LNs. Taken together, these findings may suggest that submandibular LN can indeed amplify the specific responses generated in NALT, and that these structures are functionally equivalent to PP and mesenteric LN, respectively.

Despite these similarities, the essentially different physiological functions of the respiratory tract (gas exchange) vs the gastrointestinal tract (uptake of food) result in distinct requirements for the immune components present at these sites. GALT largely ignores food Ags to allow uptake of nutrients, and mounts only minimal immune responses to the components of the resident bacterial flora, but at the same time serves to exclude potential pathogens. In contrast, the immune components of the respiratory tract, in particular the lower respiratory tract, are not permissive and attempt to keep the noncolonized lungs sterile. These functional differences among a number of subtle differences might be best reflected in the predominance of different isotypes at these sites. Clearly, IgA is the major isotype secreted in the gut, and its limited inflammatory effector potential is the ideal "low-key" isotype for this site. In contrast, there is increasing evidence that the lower respiratory tract is mainly controlled and protected by IgG Abs that have broader effector potential than IgA. Studies in influenza-infected SCID mice after transfer of neutralizing mAbs of different isotypes (39), and more recently in influenza-infected IgA knockout mice (40) have provided elegant evidence for this notion. IgG Abs may originate from the systemic circulation and enter the lung by transudation (41), or be produced locally in bronchus-associated lymphoid tissue (BALT). Indeed, BALT in rodents has been shown to mount specific immune responses to Ags present in the lung lumen (reviewed in Ref. 42), as opposed to a more discrete function in humans (43). It may be postulated that NALT may serve an "intermediate" function between BALT and GALT; although it is in physical connection with the alimentary tract and not sterile, NALT also serves as an important "gate-keeper" for the well-protected lower respiratory tract. This intermediate status may be reflected in the mixed IgA/IgG2a isotype-pattern prevalent at this site. As shown in Fig. 6Go, NALT not only produces IgA, but also considerable amounts of IgG2a Abs (Fig. 6GoB, middle row). Similarly, IgG2a was produced in larger quantities than IgA in submandibular LN, whereas IgA predominated over IgG2a in mesenteric LN. A more detailed comparison of the functions of BALT vs NALT in comparison to GALT will be particularly interesting. We are currently investigating this in a model of reovirus infection of rats.

Several other studies have described humoral immune responses in murine NALT (12, 13, 20, 21). In these studies, Ab production was assessed using the ELISPOT technique. In general, the results correlate well with our findings using the organ fragment culture technique. Both IgA- and IgG-producing cells were observed in NALT after i.n. infection with live influenza virus (21, 21, 44). Our results show local production of both IgA and IgG2a Abs in NALT after i.n. infection with reovirus. This is in line with the reported production of a mixed Th1/Th2-type cytokine profile after respiratory infection with reovirus (45). Others described IgA as the clearly predominating local isotype in NALT (12, 13). In these studies, cholera toxin subunit B was used either as Ag or as an adjuvant in combination with bacterial Ag, which may somehow mediate this skewing of the B cell response toward IgA. However, the use of the fragment culture technique may allow for a more quantitative analysis of Ab production than ELISPOT, as it integrates the number of specific cells with the actual secretion of Ab, yielding information about total Ab production capacity of an entire organ or tissue. For example, we show that 7 days postinfection, NALT produces ~4 times less total IgA than PP (Fig. 6GoA), which is probably due to the smaller number of total cells present in NALT. Nevertheless, equal amounts of virus-specific IgA are produced, which likely indicates a higher frequency of Ag-specific IgA-producing B cells in NALT than in PP.

In this study, we report the first direct demonstration of CTL generation in NALT. NALT CTL appear to kill infected targets more efficiently at lower E:T ratios than CTL from the draining LNs of the same animals (Fig. 7Go). More strikingly, the 5- to 6-fold higher precursor CTL frequency in NALT compared with cervical LN (Fig. 8Go) clearly shows that NALT is a potent inductive site for specific CTL responses upon i.n. infection. A similarly increased precursor CTL frequency in PP over peripheral LN was observed after local gastrointestinal reovirus infection (4). It will be interesting to pinpoint effector sites of these CTL to establish whether they specifically emigrate to mucosal effector sites of the respiratory tract through a homing-receptor mediated process, or whether a more random distribution to different nonlymphoid tissues occurs, as shown recently by Masopust et al. (46) in systemically and orally primed mice. Furthermore, we demonstrate the generation of germinal centers in NALT after i.n. infection along with expansion of IgA+ and IgG2a+ B cells, and local production of reovirus-specific IgA and IgG2a Abs. Altogether, these findings provide direct evidence that NALT is an inductive site for humoral mucosal immune responses. Thus, tracking patterns of distribution of CTL and specific B lymphoblasts after induction in local mucosal inductive sites (e.g., in NALT vs PP) may represent a promising approach to address cross-priming and communication between distant mucosal sites, and a way to experimentally assess questions concerning the regulation and functional characteristics of the common mucosal immune system.


    Acknowledgments
 
We thank Haruka Hishiki for help with NALT isolation, and Alec McKay for running the FACS and preparing radiolabeled Abs. We also thank Dr. Chris Cuff for his continuous support and helpful comments.


    Footnotes
 
1 This work was supported by Grant AI-23970 from the National Institutes of Health (to J.J.C.), and a fellowship from the Swiss Foundation for Medical-Biological Grants (Schweizerische Stiftung fuer medizinisch-biologische Studien; to A.W.Z.). The Flow Cytometry Facility of the Cancer Center at the University of Pennsylvania is supported by the Lucille P. Markey Trust. Back

2 Address correspondence and reprint requests to: Dr. John J. Cebra, Department of Biology, University of Pennsylvania, 415 South University Avenue, Philadelphia, PA, 19104-6018. E-mail address: jcebra{at}sas.upenn.edu Back

3 Abbreviations used in this paper: GALT, gut-associated lymphoid tissue; NALT, nasal-associated lymphoid tissue; i.n., intranasal(ly); PP, Peyer’s patch; LN, lymph node; UEA, Ulex Europaeus agglutinin I; BALT, bronchus-associated lymphoid tissue. Back

Received for publication June 27, 2001. Accepted for publication December 4, 2001.


    References
 Top
 Abstract
 Introduction
 Materials and Methods
 Results
 Discussion
 References
 

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