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Alveolar epithelial integrity is dependent upon the alveolar milieu, yet the milieu of the damaged alveolar epithelial cell type 2 (AEC2) has been little studied. Characterization of its components may offer the potential for ex vivo manipulation of stem cells to optimize their therapeutic potential. We examined the cytokine profile of AEC2 damage milieu, hypothesizing that it would promote endogenous epithelial repair while recruiting cells from other locations and instructing their engraftment and differentiation. Bronchoalveolar lavage and lung extract from hyperoxic rats represented AEC2 in vivo damage milieu, and medium from a scratch-damaged AEC2 monolayer represented in vitro damage. CINC-2 and ICAM, the major cytokines detected by proteomic cytokine array in AEC2 damage milieu, were chemoattractive to normoxic AECs and expedited in vitro wound healing, which was blocked by their respective neutralizing antibodies. The AEC2 damage milieu was also chemotactic for exogenous uncommitted human amniotic fluid stem cells (hAFSCs), increasing migration greater than 20-fold. hAFSCs attached within an in vitro AEC2 wound and expedited wound repair by contributing cytokines migration inhibitory factor and plasminogen activator inhibitor 1 to the AEC2 damage milieu, which promoted wound healing. The AEC2 damage milieu also promoted differentiation of a subpopulation of hAFSCs to express SPC, TTF-1, and ABCA3, phenotypic markers of distal alveolar epithelium. Thus, the microenvironment created by AEC2 damage not only promotes autocrine repair but also can attract uncommitted stem cells, which further augment healing through cytokine secretion and differentiation.
Characterization of the soluble factors in the alveolar epithelial milieu that are required for AEC2 maintenance and repair would have important translational clinical and bioengineering applications. The AEC2 damage milieu is shown to promote endogenous healing of an epithelial wound through autocrine cytokines, and also can attract uncommitted human progenitor cells, which augment healing though secretion of healing cytokines. The AEC2 damage milieu is sufficient to induce differentiation of uncommitted stem cells into epithelial cells expressing SPC, TTF-1 and ABCA3, markers of distal pulmonary alveolar epithelium, thus further establishing its reparative potential.
The functional role of the alveolar epithelial cell type 2 (AEC2) has expanded over the years from a surfactant factory to an immunomodulator of the alveolus (1). AEC2s express toll receptors, which play an important role in innate host defense of the lung (2). Surfactant secreted by AEC2s not only lowers surface tension and promotes compliance but also acts as a barrier and as a source of proteins that modify inflammatory signaling in the alveolus (3). Human lung diseases, such as fatal respiratory distress syndrome in the neonatal period and interstitial lung disease of later onset, have been linked to mutations in surfactant protein–C (SP-C) (4). In addition, AEC2s are the putative resident alveolar progenitors that can replace damaged AEC1s after injury (5). Maintenance of a functional population of AEC2s is therefore critical for normal lung alveolar function, homeostasis, and repair.
The observation that damaged AEC2 monolayers can repair themselves in vitro in the absence of serum or exogenous growth factors (6) suggests that autocrine modification of the AEC2 milieu promotes alveolar epithelial repair. We have shown previously that exogenous human amniotic fluid stem cells (hAFSCs), when delivered by tail vein injection to mice, can target damaged pulmonary alveolar epithelium, where they are induced to express markers of distal pulmonary alveolar epithelium (7). Thus, the AEC2 damage milieu may be capable of recruiting undifferentiated cells from other locations, including the local alveolar bloodstream, and directing their engraftment and differentiation as well as promoting autocrine repair. Bone marrow stem cells also track to damaged alveolar epithelium (8), where they can expedite repair without engrafting (9), perhaps by contributing growth factors or cytokines that augment endogenous epithelial healing. A threshold of lung injury seems to be required for the appearance of significant numbers of stem cell–derived lung epithelia (10), suggesting that cells administered systemically are homing to local chemotactic signals released by alveolar epithelial damage. However, the niche, or milieu, of damaged AEC2 cells and its complement of autocrine and exocrine factors has been little studied.
AEC2s and other cell types in the alveolus, together with the underlying matrix, contribute to the soluble components of the AEC2 milieu, which is modified by damage as matrix is exposed and infiltrating cells are recruited. In this study, we examined the milieu of AEC2s using two damage models, with an emphasis on cytokine/chemokine production. Hyperoxia was used as an in vivo epithelial damage model because subacute hyperoxia in rodents is a relevant model for human acute lung injury associated with oxidant stress without inflammation (11). BAL and lung extract from hyperoxic rats were used to represent AEC2s in vivo damage milieu and are reflective of autocrine contributions from many cell types. In contrast, conditioned medium from a scratch-damaged AEC2 monolayer reflects AEC2-specific cytokines released after damage, and this model was used as a convenient closed system to examine the AEC2 in vitro damage milieu. A template system that generated wounds with minimal damage was used as a model of passive epithelial denudation to compare with scratch-generated damage denudation. We present novel evidence that the AEC2 damage milieu not only promotes endogenous healing through autocrine cytokines but also induces uncommitted human progenitor cells to actively participate in epithelial repair by contributing healing cytokines into the AEC2 milieu. The AEC2 damage milieu per se is sufficient to induce subsets of these uncommitted stem cells to acquire phenotypic markers of distal alveolar epithelium, thus further establishing its reparative potential.
AEC2s were isolated as described by Dobbs (12). Human amniotic fluid from normal male fetuses (17–18 wk gestation) was obtained from Genzyme Genetics Corporation (Monrovia, CA) and sorted for hAFSCs (c-kit positive cells) using magnetic-activated cell sorting (7). hAFSC details are provided in the online supplement.
Confluent AEC2 monolayers were damaged as described previously (6). Cytokines, BAL, lung extract, exogenous cells, or neutralizing antibodies were added immediately after damage. The inhibitors PD98059 (50 μM) (14) and SB 505124 (1 μM) (15) (Sigma, St. Louis, MO) were incubated with the freshly damaged monolayer for 2 hours before the addition of the test cytokine.
A strip of passive denudation of the same size as the scratch gap, but with minimal cell damage, was generated using a wound template (Cytoselect Wound Healing Assay; Cell Biolabs, San Diego, CA).
Normoxic and hyperoxic rat lungs were lavaged to capacity six times with Hepes-buffered DMEM. The BAL was centrifuged to remove cells, aliquoted, and frozen at –20°C.
Minced, unfiltered, elastase-digested lung (11) was incubated in Hepes-buffered DMEM at 37°C, with gentle agitation, for 3 hours (1 minced lung/50 ml DMEM), filtered through 100-μm nitex, centrifuged to remove cells, aliquoted, and frozen at −20°C.
Conditioned medium (undiluted) and BAL was screened using proteomic cytokine arrays (R&D, Minneapolis, MN). The blots were scanned, and spot densities were compared after correction for internal controls.
CINC-2 and ICAM were measured in cell-conditioned medium and BAL using rat-specific Quantikine Rat ELISAs (R&D Systems) according to the manufacturer's instructions.
AEC2s grown in a separate flask in parallel with the monolayer plate or hAFSCs cultured as described previously (7) were labeled with cell tracker CM-DiI (Invitrogen, Carlsbad, CA) according to the manufacturer's instruction.
Cells were fixed in 4% paraformaldehyde or 1:1 ice cold methanol acetone, depending on the antibodies used (see figure legends).
RNA extraction and RT-PCR details, including human SPC and TTF-1 primer sequences, are provided in the online supplement.
Western blot analysis was performed on cell lysates as previously described (20). Antibodies and cytokines are listed in the online supplement.
Scanned blots were analyzed using Scion Image software (NIH). Students t test was used to compare treated versus untreated groups using Graph Pad software. Data are expressed as mean ± SEM. A P value ≤ 0.05 was considered significant.
Freshly isolated normoxic AEC2s are essentially nonproliferative, with greater than 90% of the population in G1 phase of the cell cycle, and remain quiescent in culture (20). In contrast, AEC2s isolated from hyperoxic rats are more proliferative, suggesting that the hyperoxic milieu within the lung is conducive to AEC2 growth. We obtained BAL and lung extracts from hyperoxic rats, taken to be representative of the AEC2’s in vivo damage environment, and compared their effects with BAL and lung extract from normoxic animals when added to 24-hour cultured, attached, nonconfluent normoxic AEC2s. The hyperoxic BAL and lung extracts increased the cell numbers by 2-fold in 24 hours (Figures 1A and 1B). In contrast, normoxic BAL and lung extract did not significantly affect AEC2 proliferation. Thus, the AEC2 cell in vivo hyperoxic milieu is sufficient to recapitulate the proliferation induced by in vivo hyperoxia when added to normoxic AEC2s ex vivo.
We used the scratch damage model to determine whether the damage cues present in the microenvironent of damaged AEC2s could potentiate alveolar epithelial repair. Hyperoxic BAL and lung extract promoted significantly faster healing of an AEC2 monolayer scratch wound (Figure 1C). The AEC2 is normally unmigratory (18) but can be induced to migrate by in vivo hyperoxia (19). To determine whether AEC2 cell damage milieu promoted chemotaxis of AEC2s, we used an in vitro Boyden chamber approach (18, 19) to measure migration of AEC2s toward AEC2 damage milieu. In vivo and in vitro AEC2 damage milieu stimulated migration in the normally unmigratory AEC2s (Figure 1D). These migration experiments were also repeated using primary cultures of AEC1s with similar results (data not shown), suggesting that the whole alveolar epithelium is responsive to signals resulting from AEC2 damage.
Because AEC2 in vitro damage milieu stimulated AEC2 migration, we tracked exogenous fluorescent-labeled AEC2s added to a fresh monolayer scratch wound. We added 105 exogenous cells to a damaged monolayer in a 5-cm2 tissue culture well, replacing the estimated 5% loss caused by wounding and washing. A wound generated by growing the monolayer around a removable template served as a passive (minimally damaged) denudation control, and exogenous cells were added to nonwounded confluent monolayers in parallel to assess the pattern of engraftment to nondamaged cells. The size of the wounds generated by scratch and template was equivalent (952 ± 208 μm versus 988 ± 33 μm, respectively; n = 7). After 24 hours, exogenous fluorescent AEC2s had efficiently attached to a scratch wound (Figure 2A, left panel) and to a lesser extent to a template wound (Figure 2A, middle panel), where they were lightly attached and more likely to be removed by washing. Exogenous cells added to undamaged monolayers were diffusely scattered over the monolayer with much less efficient attachment (Figure 2A, right panel). The scratch wound had healed by 24 hours in the presence of exogenous AEC2 cells, in contrast to the template wound, which remained unhealed (Figure 2B, left panels). Endogenous wound healing was also more efficient in the scratch wound than in the template wound (Figure 2B, right panels). To determine whether exposed matrix was responsible for the increased wound healing seen when cells were damaged versus denuded, we compared scratch and template wounds immediately after damaging for protein, fibronectin, laminin, and collagen deposition. There were similar small remnants of protein detected in template and scratch wounds, mainly adjacent to the wound edge. Laminin was detected similarly (Figure 2C), but no fibronectin or collagen staining was detected in the wounds or at the wound edges of either damage model (data not shown). Although it is possible that endogenous and exogenous AEC2s are attracted to traces of laminin or other proteins left in the wound after damage, this is unlikely to account for the difference in healing properties between scratch-damage milieu and template-damage milieu because both wound types showed similar trace amounts of protein staining. Taken together, the data suggest that soluble autocrine factors released by damaged AEC2s are sufficient to expedite endogenous repair of a wounded alveolar epithelial membrane but can also attract exogenous cells to augment endogenous wound healing. In contrast, denudation without damage is a less effective stimulus for repair.
To examine cytokine/chemokine release into the AEC2 milieu after a breach of alveolar epithelial integrity, we screened conditioned medium from undamaged versus scratch-damaged versus template-damaged AEC2s using a proteomic cytokine array of 30 rat-specific cytokines. CINC-2 and ICAM were the only cytokines significantly elevated in the medium collected from scratch-damaged cells at levels greater than 1.5-fold higher than in control medium (Figure 3A), and this difference had resolved by 48 hours (data not shown). A template wound with minimal cell damage generated 25% less ICAM and 66% less CINC-2 than scratch-damaged wounds. The damage-mediated CINC-2 and ICAM release was confirmed using rat-specific ELISAs (Figures 3A and 3B). CINC-2 was increased 1.5-fold during the first 24 hours of healing. Basal levels of ICAM were relatively high in control AEC2 conditioned medium (CM) (4,942 ± 50 pg/ml), with a 1.6-fold increase in ICAM secretion in the first 3 hours after damage, although we were not able to detect a significant difference by 24 hours after damage. Up-regulation of these cytokines has been reported previously in models of acute lung injury in vivo (21, 22). Indeed, we detected a 3.4-fold increase in CINC-2 and a 1.8-fold increase in ICAM-1 in hyperoxic BAL versus normoxic BAL (Figures 3B and 3C). To demonstrate that CINC-2 and ICAM-1 secretion was a response to damage and not due to proliferation per se, conditioned medium from undamaged proliferating and nonproliferating AEC2s was screened by protein microarray (see Figure E2 in the online supplement). Proliferation was induced by 25 ng/ml KGF treatment for 24 hours and confirmed by PCNA Western blot. There was no difference in CINC-2 expression between proliferating and nonproliferating AEC2s, and ICAM was barely detectable in CM from either population. This is in contrast to the elevated CINC-2 and ICAM levels secreted by damaged AEC2s shown in Figure 3A and confirms that release of CINC-2 and ICAM reflects an autocrine response to cell damage.
We measured in vitro migration of AEC2s through porous 8-μm filters toward DMEM ± CINC-2 and ICAM at concentrations at and above the level seen in damage milieu and found significantly increased migration at 5 ng/ml or greater (Figure 3D). Both cytokines also expedited in vitro wound healing, whereas endogenous wound healing was delayed by their respective neutralizing antibodies (Figures 4A and 4B), confirming a functional role for both cytokines in AEC2 autocrine repair in vitro. CINC-2 and ICAM neutralizing antibodies also delayed wound healing stimulated by AEC2 in vivo damage milieu since delayed wound healing was also seen in the presence of hyperoxic BAL (Figure E3). CINC-2 or ICAM did not promote proliferation of AEC2s when added at the time of plating or to already attached cells for a period of 24–hours, as measured by cell counts. However, when added to freshly damaged AEC2s, both cytokines stimulated proliferation significantly by 24 hours (CINC-2: 1.4 ± 0.1-fold versus untreated [P < 0.03]; ICAM: 1.6 ± 0.3-fold versus untreated [P < 0.04; n = 4]).
To determine mechanisms of CINC and ICAM-mediated AEC2 wound healing, we examined the effect of CINC and ICAM on the activation of common signaling pathways in AEC2s. CINC-2 and ICAM were added to fresh isolates of AEC2 for 5 minutes or 1 hour or added at the time of plating for 24-hour culture. The dosage used was 5 ng/ml, which was previously shown to promote migration and expedite wound healing. The cells were lysed after 5 minutes, 1 hour, or 24 hours of exposure to the cytokines. Although short-term exposure to CINC-2 and ICAM did not induce ERK or Smad2 activation (data not shown), both cytokines induced sustained ERK and Smad2 phosphorylation after 24 hours of treatment (Figure 4C). The delayed activation of both pathways may be secondary to cytokine-mediated attachment per se or to induction and release of growth factors or cytokines. In contrast, we could not detect CINC-2– or ICAM-mediated changes in the phosphorylation of the other key signaling members of the MAP kinase family, p38 and JUNK (data not shown). Inhibition of MAP kinase kinase (MAPKK) activation by PD98059 or TGF-β1 receptor kinase activity by SB505124 slowed endogenous wound healing and decreased CINC-2 and ICAM-mediated wound healing to endogenous levels (Figure 4D). These data suggest that CINC- and ICAM-mediated epithelial repair is mediated through indirect stimulation of MAPK and TGF-β signaling.
We have previously shown that in systemically delivered uncommitted human amniotic fluid stem cells, c-kit + hAFSCs can be detected in murine lung epithelium after hyperoxic injury in vivo, albeit in low numbers (7). To see if the milieu of damaged AEC2 per se could provide an attractive environment for uncommitted stem cells, we added CM-DiI–labeled hAFSCs to AEC2 monolayers freshly damaged by scratch or by template. The AEC2 milieu generated by the damage model was more attractive to engraftment of hAFSCs than the passive denudation model since hAFSCs attached more efficiently to a scratch wound than to a template wound (Figure 5A, upper panels). Wound healing at 24 hours in the presence of hAFSCs was also more efficient in the scratch wound versus the template wound (Figure 5A, lower panels).
To confirm that the microenvironment of damaged AEC2s was attractive to hAFSCs, we compared the migration of hAFSCs toward AEC2-conditioned medium from undamaged and in vitro scratch-damaged AEC2s as well as BAL from control and hyperoxic animals (Figure 5B, upper panels). hAFSCs migrated toward conditioned medium from in vitro damaged AEC2s with an approximately 5-fold increase over basal medium. The migratory response of hAFSCs toward hyperoxic BAL had a greater than 20-fold increase in migration compared with basal medium and was approximately 10-fold higher than the response to BAL from normoxic animals, which was also stimulatory. The cells migrating toward hyperoxic BAL had a predominantly fibroblast-like phenotype, in contrast to the cells migrating toward damaged AEC2 CM, which had an epithelial phenotype, or control BAL, where tight spheres of small cells were seen, only a few of which had recognizably epithelial and fibroblast morphology (Figure 5B, lower panels). Unlike rat AEC2s, hAFSCs did not migrate significantly toward CINC-2 or ICAM, the major cytokines released by AEC2 damage (data not shown). Thus, the chemotactic signals originating from damaged AEC2 milieu that are attractive to hAFSCs remain to be identified.
hAFSCs can ameliorate tissue damage by modulating cytokines in the damage milieu, as seen in a rodent model of acute tubular necrosis (23). Therefore, we screened for human cytokines released when hAFSCs were incubated in the presence of damaged AEC2 milieu using a human-specific cytokine proteomic screen of 36 cytokines (Figure 6A). Although no human cytokines were detected in damaged AEC2 milieu in the absence of hAFSCs, five human cytokines were seen at readily detectable levels when hAFSCs were cultured with AEC2 in vivo damage milieu: growth regulated protein-α, IL-6 and IL-8, macrophage migration inhibitory factor (MIF), and plasminogen activator inhibitor 1 (PAI-1), whereas MIF and PAI-1, and to a lesser extent IL-6, were detected after culture of hAFSCs with AEC2 in vitro scratch damage medium. PAI-1 and MIF were endogenously secreted by hAFSCs that were cultured in the absence of damage milieu but were strongly induced by AEC2 damage. MIF, a lymphokine involved in cell-mediated immunity, immunoregulation, and inflammation, plays a role in the regulation of macrophage function in host defense and may be involved in integrin signaling pathways. AEC2s proliferate in response to MIF and express high surface levels of CD74, which binds MIF (24). PAI-1 is a major negative regulator of pericelllular plasmin and accompanies wound repair in vitro and in vivo (25), including alveolar epithelial wound healing (26). Because MIF and PAI-1 were likely candidates to promote wound healing and were strongly induced by in vivo and in vitro damage milieu, we focused on these cytokines.
To confirm a functional role for MIF and PAI-1 in alveolar epithelial wound healing, we added human recombinant MIF and PAI-1 (5 ng/ml) to freshly scratch-damaged monolayers of AEC2s in the absence of hAFSCs. After 24 hours, the scratch wounds treated with MIF and PAI-1 were healed (Figure 6B), in contrast to the control scratch. Proliferation of damaged AEC2s, as measured by cell numbers, was increased 2.1 ± 0.1-fold by MIF and 1.6 ± 0.1-fold by PAI-1, suggesting that expedited wound closure in the presence of the cytokines was due to increased cell proliferation. In contrast, AEC2s did not migrate toward MIF or PAI-1 at doses that promoted wound healing (data not shown). Inhibition of PAI-1 or MIF, using MIF antagonist ISO-1 (50 μM) or an excess of PAI-1 neutralizing antibody, delayed hAFSC-mediated AEC2 wound healing (Figure 6C), confirming that the major cytokines secreted by hAFSCs in the presence of damaged AEC2s contribute to a prohealing milieu.
Cytokeratin staining of cells within an AEC2 wound, 24 hours after wounding and the addition of DiI-labeled hAFSC, showed islands of cytokeratin-positive rat AEC2s in intimate proximity to cytokeratin-negative, DiI-labeled hAFSCs (Figure 7A, upper panels). The hAFSC-modified milieu also attracted cytokeratin-negative cells from the monolayer into the wound. These cells are likely fibroblasts, which are present as contaminants in the AEC2 cultures (usually ~ 2%). Although SPC protein was not detected in hAFSC (DiI+) cells at this time (24 h), it was detected after 72 hours of culture of hAFSCs with damaged AEC2s by RT-PCR using human-specific primers (Figure 7A, bottom panel).
To assess phenotypic changes mediated by AEC2 damage milieu in uncommitted hAFSCs, we cultured hAFSCs on collagen 1 with AEC2 control and damage milieu for various times and analyzed for SPC, a protein manufactured by AEC2s and a marker of distal alveolar epithelium, by immunostaining. Cells that stained for pro-SPC were not seen until after 4 days of culture with damage milieu (Figure 7B, upper panels). No SPC-staining cells were seen in hAFSCs cultured in basal medium (data not shown) or in cytospins of c-kit–positive hAFSCs assayed before each experiment (Figure E1). Human SPC was detected using RT-PCR with human primers in hAFSCs grown with AEC2 damage milieu, confirming that pro-SPC–positive immunostaining was not due to endocytosis of rat surfactant from the damage milieu, whereas coexpression of human TTF-1 reaffirmed an epithelial phenotype in SPC+ hAFSCs (Figure 7B, lower panel). The differentiation rate, as measured by SPC immunostaining, was 13 ± 1.4% (range, 8–27%; n = 11) and was similar between the various types of AEC2 damage milieu tested (damaged AEC2 CM, hyperoxic BAL, and hyperoxic lung extract). SPC-staining hAFSCs were seen in 1 of 11 undamaged AEC2 milieu tested (a sample of normoxic BAL, where approximately 2% SPC + hAFSCs were detected). ABCA3, a transporter protein that plays an essential role in surfactant lipid metabolism and lamellar body biogenesis, was also induced in hAFSCs treated with AEC2 damage milieu (Figure E4 in the online supplement). SPC expression, together with TTF-1 and ABCA3 expression, confirms induction of a phenotype consistent with distal lung alveolar epithelium in hAFSCs treated with AEC2 cell damage milieu.
The local environment of a cell dictates cell fate, and bioengineering of complex organs like the lung will require a detailed knowledge of regional microenvironments to faithfully recapitulate regeneration. Similarly, the effectiveness of stem cell therapy to ameliorate tissue damage could likely be optimized if the specific damage niche was well characterized. After distal epithelial damage, the milieu of the damaged AEC2 will arguably determine the rate and nature of alveolar epithelial repair and is therefore of great interest.
Subacute hyperoxia was used as an in vivo damage model in this study because it elicits well documented and reproducible responses in rodent AEC2s and can serve as a model for human acute lung injury associated with oxidant stress without inflammation (11). Our data herein show that the AEC2 in vivo hyperoxic milieu is sufficient to recapitulate the proliferation induced by in vivo hyperoxia when added to normoxic AEC2 ex vivo and can expedite alveolar epithelial wound healing through increased migration and proliferation. A prohealing milieu was specifically related to AEC2 cell damage rather than denudation because in vitro AEC2 wounding models showed that epithelial denudation without damage healed at a slower rate than damage denudation, was less attractive to exogenous cells, and induced less CINC and ICAM release. These differences are unlikely to arise from matrix exposed by damage because protein residue within wounds was minimal and was similar in composition between scratch and template wounds. These data, combined with the CINC-2/ICAM neutralization studies, suggest that soluble autocrine factors secreted by AEC2s in response to cell damage play a functional role in alveolar epithelial healing.
CINC-2 is a proinflammatory CXC chemokine that is a potent chemoattractant for neutrophils. AEC2s express CINC-2 and its receptor, CXCR2, and both are up-regulated after lung injury (21). Two variants of CINC-2 arise from alternative splicing, CINC-2α and CINC-2β, which differ in only three amino acids at the C-terminus (27). CINC-2β is regulated differently from CINC-2α and the other CXC chemokines in that its expression appears to be restricted to differentiated AEC2 cells and it is not increased by cytokines (28). In our studies, the proteome screen, ELISA, and neutralizing antibody recognized both CINC variants.
ICAM was also expressed at high levels in damaged AEC2 milieu, promoted migration of AEC2, and expedited in vitro AEC2 wound healing, which is consistent with the observation that ICAM−/− mice have delayed excisional wound healing (29). ICAM is a cell surface glycoprotein that is typically expressed on endothelial cells and cells of the immune system and is a member of the IgG superfamily. ICAM provides adhesion between endothelial cells and leukocytes after stress or injury and increases planar chemotaxis of alveolar macrophages over the AEC surface, making phagocytosis more efficient (30). It is constitutively expressed in alveolar epithelium, particularly AEC1. In AEC2s, ICAM is associated with the cytoskeleton and provides an interface between mobile inflammatory cells and the alveolar surface (31). ICAM expression is induced during alveolarization as AEC1 differentiation takes place (32), and the increase of ICAM detected in the BAL after hyperoxia is consistent with recapitulation of this developmental process. Damage-induced soluble ICAM shedding by AEC has also been reported after FITC-mediated inflammatory lung injury, mechanical stress, and LPS/TNF-α stimulation (22, 33, 34). We also found significant ICAM levels in the BAL from normoxic animals, suggesting a physiological role for ICAM in alveolar homeostasis.
Postdamage decreases in the cytokines TMP-1, LIX, CINC-1, and CXC3CL1, detected by proteomic cytokine microarray in AEC2 damage milieu, are also consistent with a prohealing environment (19, 35). Taken together, the cytokine microarray data suggest that the AEC2s can contribute to a milieu conducive to healing through a balanced repertoire of autocrine cytokines while recruiting systemic cells to the site of damage. We speculate that the milieu of damaged AEC2s would be attractive to many cell types due to the inflammatory potential of the cytokines released. Because the rat cytokine screen was limited to only 30 cytokines or chemokines, healing of the alveolar epithelium is likely to involve other cell types that have yet to be unidentified.
The hAFSC is an intermediate type of stem or progenitor cell between embryonic stem cells and adult stem cells resident in differentiated organs and has been well characterized (13). hAFSCs are derived from a noncontroversial source and are pluripotent and nonteratogenic, making them potentially suitable for cell-based therapies. We have previously shown that systemically delivered uncommitted hAFSCs can target hyperoxia-damaged murine lung and integrate into the lung epithelium (7), suggesting that the damaged AEC2 milieu is chemoattractive to hAFSCs. Indeed, hAFSCs migrated toward hyperoxic BAL at rates 20-fold higher than basal migration toward medium alone and expedited endogenous AEC2 wound repair. hAFSCs were attracted to damaged AEC2s and could heal a scratch wound in 24 hours but were less effective in healing a wound generated by passive denudation, suggesting that interaction between the milieu of damaged AEC2s and hAFSCs promoted AEC2 repair. A human proteomic cytokine screen comparing CM from in vitro and in vivo damaged AEC2s, cultured in the presence and absence of hAFSCs, showed that human macrophage migration inhibitory factor (MIF) and plasminogen activator inhibitor-1 (PAI-1) proteins were secreted by hAFSCs and induced by exposure of hAFSCs to AEC2 damage milieu. Although MIF and PAI-1 are considered to be proinflammatory, they are also associated with epithelial wound repair (24, 25) and indeed were shown to expedite in vitro AEC2 wound healing in our study. Inhibition of MIF and PAI-1 slowed hAFSC-mediated AEC2 wound healing, which suggests that hAFSCs augment endogenous AEC2 repair by modifying the cytokine milieu as well as by their physical presence within the wound. The other human cytokines detected in CM from damaged AEC2s in the presence of hAFSCs, growth regulated protein-α, IL-6, and IL-8 are also proinflammatory cytokines that can play antiinflammatory or cytoprotective roles under specific conditions (36–38); thus, controlled induction of inflammation may be an important key to prompt alveolar epithelial wound repair. The prerequisite of damage to an organ for homing of exogenous stem cells to that organ (10) is consistent with this concept.
The AEC2 damage milieu could also induce expression of SPC, a protein manufactured by AEC2s and a marker of distal alveolar epithelium, in a subpopulation of hAFSCs. SPC protein was detected in engrafting hAFSCs within an AEC2 damage wound after 96-hour culture and in hAFSCs that had been cultured on collagen 1 in the presence of CM from in vitro damaged AEC2 cells or hyperoxic BAL or lung extract, after 96-hour culture. SPC transcript was also detected, using human-specific primers, suggesting that the protein detected was of human origin and arising from the hAFSCs rather than rat surfactant in the milieu that had been endocytosed by the hAFSCs. The frequency of SPC-staining cells (13 ± 1.4%; range, 8–27%) could potentially be increased by identification of the inducing factors, which would enable optimization of stem cells ex vivo before administration in vivo, by manipulating the milieu accordingly. The characterization of these SPC-expressing cells and the mechanism of differentiation will be the basis of future research.
Extension of this preliminary work into a rigorous proteomic characterization of the optimal cytokine/chemokine and growth factor balance that is required for alveolar epithelial cell maintenance and repair could have important translational clinical and bioengineering applications for patients with alveolar damage or disease.
Supported by NIH/NHLBI grants R01HL44060, RO1HL44997, and PO1HL60231 and CIRM TG (D.W.), R01HL68597 (W.S.), R01HL65352 (B.D.), and NIH/NIDDK grant K08073082 (R.deF.).
This article has an online supplement, which is accessible from this issue's table of contents at www.atsjournals.org
Originally Published in Press as DOI: 10.1165/rcmb.2010-0325OC on June 23, 2011
Author Disclosure: None of the authors has a financial relationship with a commercial entity that has an interest in the subject of this manuscript.