|
|
||||||||

* Department of Biology, University of Pennsylvania, Philadelphia, PA 19104; and
Department of Microbiology, Immunology and Cell Biology, West Virginia University, Morgantown, WV 26506
| Abstract |
|---|
|
|
|---|
| Introduction |
|---|
|
|
|---|
However, the mechanisms underlying cross-protection of distant mucosal sites after local priming remain largely elusive. It is, for example, not clear whether this protection is due to emigration of locally primed cells or whether passage/transportation of Ag from the site of entry to distant mucosal inductive sites is responsible for cross-protection. Furthermore, it was recently shown that after systemic immunization, effector memory T cells (11) emigrate and stably populate nonlymphoid compartments, including mucosal sites such as the intestinal LP and lung or nonmucosal sites such as the liver and kidneys (12). However, it has not been ascertained to date whether this generalized distribution of effector memory T cells to nonlymphoid sites also occurs after local mucosal priming and, thus, whether these relocation patterns also apply for cell migration within the common mucosal immune system.
In this study we sought to elucidate mechanisms providing for a common mucosal immune system. Using models of local mucosal reovirus infection, we show that cross-protection of the upper and lower respiratory tracts after intestinal priming is achieved through relocation of locally primed B and T lymphocytes to distant mucosal compartments. However, differences in the quality and kinetics of the immune responses in the upper vs the lower respiratory tract after intestinal priming were observed.
| Materials and Methods |
|---|
|
|
|---|
Male C3HeB/FeJ (referred to as C3H) and BALB/c/ByJ were purchased from The Jackson Laboratory (Bar Harbor, ME). Conventionally reared C.B-17 SCID mice were bred in the germfree animal facility of Department of Biology, University of Pennsylvania (Philadelphia, PA). All mice were used at the age of 814 wk. All animal experiments were conducted in accordance with the guidelines of University Laboratory Animal Resources, University of Pennsylvania.
L-929 fibroblasts were grown in medium 199 (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). For organ fragment cultures, Kennetts HY medium (Life Technologies) supplemented with 10% FCS, L-glutamine, penicillin, streptomycin, and gentamicin was used.
Virus and infection
Third-passage stocks of reovirus serotype 1, strain Lang, were produced by expanding a single plaque through three passages in L-929 fibroblasts, followed by extraction of virus with freon and purification by CsCl gradient centrifugation (13). For i.n. infection mice were lightly anesthetized with 0.1 mg/g body weight avertin (2,2,2-tribromoethanol; Aldrich, Milwaukee, WI), and 12.5 x 107 PFU reovirus was applied to both nostrils in a total volume of 25 µl saline/0.5% gelatin. For intraduodenal (i.du.) and intratracheal (i.t.) infection, mice were deeply anesthetized with 0.4 mg/g body weight avertin, and 50 µl saline/0.5% gelatin containing 12.5 x 107 PFU reovirus was directly injected into the duodenum or trachea, respectively, by a small surgical procedure.
Virus titration
Viral titers in various tissues were determined in a standard virus plaque assay. Mice were sacrificed and perfused through the right ventricle with 20 ml PBS. Thereafter, tissues were homogenized in 3 ml saline/0.5% gelatin, and serial dilutions were incubated on monolayers of L-929 fibroblasts in six-well tissue culture plates (Costar, Cambridge, MA) for 45 min at 37°C and thereafter overlaid with 3 ml 1% agar in complete medium 199 and cultured at 34°C. Plaques were counted after 7 days of incubation.
Ab production in organ fragment cultures and serum
C3H mice (n = 34/time point) were sacrificed and perfused with 20 ml PBS. Thereafter, the entire small intestine, mesenteric lymph node (LN), mediastinal LN, and upper right lung lobe were surgically removed. PP were visually detected and excised from small intestine. Submandibular LN, which probably drain nasal-associated lymphoid tissue (NALT) (14), and NALT were isolated after removal of the mandible. Palatine salivary glands (SG), which are located above the maxilla and are a major effector site in the upper respiratory tract, were isolated after removal of the hard palate. Organ fragment cultures were established, and reovirus-specific IgA and IgG2a Ab were measured by RIA using reovirus-coated polyvinyl plates and 125I-labeled anti-IgA or anti-IgG2a Ab (both from Southern Biotechnology Associates, Birmingham, AL) as described in detail recently (14).
Ex vivo CTL analysis
C3H mice were sacrificed 10 and 30 days after i.du. or i.n. infection and perfused with 20 ml PBS. Thereafter, cells were isolated from spleen, PP, cervical LN, or mesenteric LN by mechanical disruption. Intraepithelial lymphocytes (IEL) and intestinal LP cells were isolated by elution from small intestinal epithelium with PBS/1 mM EDTA (for IEL) and subsequent collagenase (1 mg/ml) digestion (for intestinal LP cells), followed by discontinuous Percoll gradient centrifugation at 600 x g for 20 min using 70 and 40% Percoll. Cells from lung interstitium, liver, submandibular SG, and palatine SG were isolated by collagenase digestion and Percoll gradient centrifugation as described above. Ex vivo CTL activity was determined in a standard 51Cr release cytotoxicity assay. 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 cell ratios (3000 target cells/well). After 5 h of incubation in V-bottom microtiter plates (Costar, Cambridge, MA) at 37°C, 100 µl supernatant fluids were collected and mixed with 1 ml scintillation fluid (Cytoscint; ICN, Costa Mesa CA), and beta emission was measured on an LS6500 multipurpose scintillation counter (Beckman Coulter, Fullerton, CA).
One lytic unit is defined as the number of CD8+ cells required to achieve 25% lysis and was calculated for all cell types used, considering the different E:T cell ratios and percentages of CD8+ cells as determined by FACS.
Cell transfers to SCID mice
Donor BALB/c mice were primed i.du. as described above. On day 7
after infection, donor mice were sacrificed and perfused with 20 ml
PBS, and single-cell suspensions of PP and mesenteric LN were injected
i.p. into recipient SCID mice (107/mouse)
infected with reovirus i.n. or i.t. 2 days before transfer of cells.
For some experiments subsets of cells were depleted by MACS using
biotinylated anti-CD8
(clone 53.6-7; BD PharMingen, San
Diego, CA) and streptavidin microbeads (Miltenyi Biotec, Auburn, CA)
according to the manufacturers instructions.
Statistical analysis
Two-tailed unpaired Students t test was applied for all statistical analyses.
| Results |
|---|
|
|
|---|
We tested whether local gastrointestinal priming with reovirus
would protect from a subsequent respiratory challenge. C3H mice were
infected i.du. with reovirus serotype 1, strain Lang. Primed mice were
challenged via the respiratory route 30 days later with the same virus
and compared with an unprimed group of mice that received only a
respiratory challenge. After primary i.du. infection (priming),
reovirus was present in the gastrointestinal tract from where it was
cleared within 714 days (Fig. 1
, A and B). Importantly, after i.du. infection no
replicating virus was found in either the upper or the lower
respiratory tract, demonstrating a localized gut infection. Upon upper
respiratory challenge by i.n. infection, viral titers were clearly
lower, and clearance was achieved more rapidly in i.du. primed mice
compared with unprimed, i.n. infected control mice (Fig. 1
C). Similarly, i.du. priming conferred complete protection
from lower respiratory challenge by i.t. reovirus infection (Fig. 1
D).
|
To determine whether B cells might contribute to this
cross-protection, production of reovirus-specific IgA and IgG2a in the
respiratory tract was monitored by kinetic analysis of organ fragment
culture supernatant fluid. Intraduodenal priming did not induce
appreciable production of reovirus-specific IgA in NALT, submandibular
LN, or palatine SG over a period of 30 days (Fig. 2
A). This lack of Ab
production was not due to failed i.du. priming, since fragment cultures
of PP, mesenteric LN, and small intestine showed induction of robust
amounts of reovirus-specific IgA (Fig. 2
C). Similarly, only
weak IgG2a responses were detected in the upper respiratory tract, even
though late in the course of i.du. infection some anti-reovirus
IgG2a was induced in NALT and palatine SG (Fig. 2
A).
Surprisingly, i.du. infection resulted in the production of some IgA
and significant amounts of IgG2a in the mediastinal LN that drains the
lower respiratory tract (Fig. 2
B). In the lungs, however,
intestinal priming did not lead to the production of any IgA and only
minute amounts of IgG2a (Fig. 2
B).
|
|
Next we analyzed the distribution of ex vivo active effector
memory CTL (11, 12) after local intestinal or upper
respiratory infection of C3H mice. As shown in Fig. 4
A, after i.du. infection,
reovirus-specific CTL displaying strong ex vivo killing activity were
detectable in the lungs over a period of 30 days. In contrast, no CTL
were found in palatine SG at any time after i.du priming, even though
i.n. priming induced strong ex vivo active CTL in palatine SG (Fig. 4
A). To accommodate differences due to varying E:T cell
ratios and percentages of CD8+ cells, lytic units
in individual tissues were calculated (Fig. 4
B). After i.n.
infection the strongest CTL responses were observed in the respiratory
tract (lung and palatine SG), and some activity was found in the gut,
whereas after i.du. priming the highest killing activities were found
in the gastrointestinal tract and lungs.
|
These findings were supported using an adoptive transfer model.
Immunocompetent BALB/c mice were primed i.du. with reovirus and
single-cell suspensions of PP transferred to SCID mice infected i.n. or
i.t. 2 days before cell transfer. Successful repopulation of the SCID
recipients was confirmed by FACS analysis and the appearance of
specific IgG Ab in serum (data not shown). As shown in Fig. 5
A, PP cells readily cleared
reovirus from the upper and lower respiratory tracts of infected SCID
mice (Fig. 5
A). Transfer of mesenteric LN cells yielded
similar results (data not shown). To assess the role of B cells vs T
cells in clearing reovirus from the respiratory tract, subsets of cells
were depleted from PP by MACS, and viral titers were determined 10 days
after transfer. CD8-depleted PP cells cleared reovirus from palatine SG
or soft palate tissue of i.n. infected SCID mice with similar efficacy
(Fig. 5
B) and kinetics (data not shown) as unfractionated
cells. In marked contrast, depletion of CD8+
cells from PP significantly delayed the clearance of reovirus from lung
and trachea of i.t. infected SCID recipients (Fig. 5
B).
Nevertheless, after 21 days recipients of CD8-depleted lymphocytes also
had successfully eradicated reovirus from lungs and trachea (data not
shown). Likewise, depletion of CD8+ cells from
mesenteric LN delayed clearance of virus from the lower respiratory
tract. In contrast, depletion of B220+ cells did
not have any effect (data not shown).
|
| Discussion |
|---|
|
|
|---|
However, we found significant differences in the kinetics and quality
of immune responses in the upper vs lower respiratory tracts after
intestinal priming. First, the kinetics of Ab production were markedly
different. Intestinal priming did not elicit any appreciable Ab
secretion in the upper respiratory tract; in contrast, significant
production of IgG2a was induced in mediastinal LN by this route of
infection. After respiratory challenge, however, strong and rapid
memory responses developed in the upper respiratory tracts whereas no
accelerated kinetics and only a slight boost of responses were observed
in the lower respiratory tract. This suggests an important role for B
cells in protection of the upper respiratory tract. We propose that
gut-primed (memory) B cells migrated to the upper respiratory tract and
accounted for the rapid production of specific IgA and IgG2a.
Alternatively, it might be speculated that the rapid recall responses
were due to recirculating, rather than residing, memory B cells in the
upper respiratory tract. Second, different predominating isotypes of Ab
were observed in the upper vs the lower respiratory tract. All tissues
from the upper respiratory tract, notably also the submandibular LN,
produced both specific IgA and IgG2a; in contrast, IgA was a minor
isotype in the lower respiratory tract. This is in line with findings
presented by Sangster et al. (15) showing a dichotomy in
Ab responses in LN draining the upper vs lower respiratory tract after
i.n. Sendai virus infection. Importantly, Palladino et al.
(16) demonstrated a more important functional role for IgG
compared with IgA Ab in protection of the lungs from influenza
infection. Transudation of IgG Ab from the central circulation to
the lungs is known to contribute a major proportion of specific Ab at
this site, and consequently, even parenteral immunization leads to
significant protection of the lower respiratory tract (reviewed in Ref.
17). To more clearly address the role of IgG2a Ab in the
upper vs the lower respiratory tract we are currently studying the
potential of passively transferred monoclonal, neutralizing
reovirus-specific IgG2a Ab to prevent or resolve reovirus infection in
SCID mice. Third, the homing of effector memory CTL to the upper vs the
lower respiratory tract after intestinal priming was significantly
different. CTL effector activity was found in the lungs as early as 10
days after i.du. priming despite the absence of infection at this site.
By contrast, effector memory CTL only appeared in infected palatine SG.
This along with our results from the adoptive transfer model (Fig. 5
B) suggest a more prominent role for CTL in surveillance of
the lower respiratory tract. Altogether we demonstrate that intestinal
priming leads to protection of both the upper and lower respiratory
tracts, but that distinct cell types and/or different Ab isotypes
account for protection. Our data implicate that the lungs are not a
classical site of the common mucosal immune system, but, rather, are
part of the systemic immune system. In contrast, we have shown recently
that the immune components of the upper respiratory tract share many
characteristics with the mucosal immune system of the gut and can thus
be considered a more typical mucosal site (14).
Consequently, protection of the upper respiratory tract by gut-primed cells may require specific homing of cells to sites in the upper respiratory tract, whereas transition of gut-primed cells into the systemic circulation may be sufficient to protect the lower respiratory tract. It remains to be established what mechanisms are operative in other situations of cross-protection, such as protection of the genital tract after nasal priming (10) or possible protection of the gastrointestinal tract after local nasal priming.
Interestingly, we observed memory IgG2a responses in NALT,
submandibular LN, and palatine SG after i.n. challenge of i.du. primed
mice (Fig. 3
A) even though only marginal IgG2a responses
were induced in the primary mucosal inductive sites of the gut, i.e.,
the PP, by i.du. priming. However, i.du. priming induced significant
production of IgG2a in the mesenteric LN (Fig. 2
C)
(18). Thus, these findings demonstrate that the mesenteric
LN may not only serve their acknowledged function as amplifiers of
responses induced in PP, but, probably due to a distinct connection to
the systemic immune system, may support generation of isotypes that are
more important for B cells emigrating to systemic or distant mucosal
sites rather than for those destined for homing to the intestinal
LP.
It remains largely elusive which homing receptors control cell
trafficking within the common mucosal immune system. It is established
that
4
7 integrins and
CCR9 (reactive with MadCAM-1 and TECK/CCL25, respectively) are
responsible for homing of gut-primed B cells, including IgA-producing B
cells, and T cells to the intestinal LP (19, 20, 21).
Csencsits et al. (22) have reported significant
differences in the expression of integrins in NALT vs PP, suggesting
that different receptors may control homing to mucosal sites distant
from the gut. In line with these findings we did not find preferential
expression of
4
7 or
E
7 by lymphocytes in
NALT or the lungs of immunocompetent mice or SCID mice after transfer
of gut-primed lymphocytes (data not shown). Recently, Pan and
colleagues (23) described a chemokine receptor, termed
MEC/CCL28, that is expressed on epithelial cells of bronchi, salivary
glands, colon, and mammary gland. Intriguingly, CCL28 shares high
sequence homology with TECK/CCL25 and CTACK/CCL27, which are all
involved in homing to mucosal sites (reviewed in Ref. 24).
This suggests a possible network of integrin and chemokine receptors
that may control trafficking of cells within the common mucosal immune
system. It will be particularly important to determine whether the same
receptors control homing of B vs T cells and in this respect to analyze
the expression of homing receptors on effector memory CTL. Having
demonstrated the distinct functional roles and migration
characteristics of gut-primed B and T cells in protection of the upper
and lower respiratory tracts, our model of local reovirus infection may
represent a promising approach to address these open questions.
| Acknowledgments |
|---|
| Footnotes |
|---|
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 ![]()
3 Abbreviations used in this paper: PP, Peyers patch; IEL, intraepithelial lymphocyte; i.du., intraduodenal(ly); i.n., intranasal(ly); i.t., intratracheal(ly); LN, lymph node; LP(L), lamina propria (lymphocytes); NALT, nasal-associated lymphoid tissue; SG, salivary gland. ![]()
Received for publication April 4, 2002. Accepted for publication August 1, 2002.
| References |
|---|
|
|
|---|
4
7 integrin mediates lymphocyte binding to mucosal vascular addressin MAdCAM-1. Cell 74:185.[Medline]
This article has been cited by other articles:
![]() |
J. Q. Jiang, X.-S. He, N. Feng, and H. B. Greenberg Qualitative and Quantitative Characteristics of Rotavirus-Specific CD8 T Cells Vary Depending on the Route of Infection J. Virol., July 15, 2008; 82(14): 6812 - 6819. [Abstract] [Full Text] [PDF] |
||||
![]() |
F.X. Lu and R.S. Jacobson Oral Mucosal Immunity and HIV/SIV Infection Journal of Dental Research, March 1, 2007; 86(3): 216 - 226. [Abstract] [Full Text] [PDF] |
||||
| |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| HOME | HELP | FEEDBACK | SUBSCRIPTIONS | ARCHIVE | SEARCH | TABLE OF CONTENTS |