| HOME | HELP | FEEDBACK | SUBSCRIPTIONS | ARCHIVE | SEARCH | TABLE OF CONTENTS |
Guest Access | Sign In via User Name/Password
|
|
|
|||||||
Guest Access | Sign In via User Name/Password |
|||||||||
* From the Lung Biology Laboratory, St. Luke’s-Roosevelt Hospital Center, and Department of Physiology & Cellular Biophysics, College of Physicians and Surgeons, Columbia University, New York, NY.
Correspondence to: Jahar Bhattacharya, MD, DPhil, St. Luke’s-Roosevelt Hospital Center, 1000 10th Ave, AJA 509, New York, NY 10019; e-mail: jb39{at}columbia.edu
Key Words: alveolus • capillary • cross-talk • imaging • inflammation
Host defense mechanisms fall in the categories of innate and adaptive immunity. The innate immune system deploys rapid-defense mechanisms, while the adaptive immune system activates a slower antigen-specific defense. In pulmonary alveoli that are continually exposed to airborne antigens, innate immune mechanisms are well developed and rapidly instituted. A large number of pathogenic substances including viruses, bacteria, and lipopolysaccharide, when introduced in the airway, induce leukocyte migration into the alveolus within 3 to 4 h, attesting to the efficacy of alveolar innate immunity.1 This time course is consistent with secretion of chemokines such as tumor necrosis factor (TNF)-
from alveolar macrophages, which provide the chemotactic signal for leukocyte recruitment. Ample evidence attests to this chemotactic effect, in that chemokines either directly instilled in the airspace or released on secretion by alveolar macrophages123 induce leukocyte influx into alveoli. It is clear that the chemotactic signal must vectorially cross the alveolar barrier to the site of leukocyte recruitment in adjoining capillaries. However, little attention has been paid to the role played by the alveolar wall epithelium in this inflammatory process.
Several reports145 implicate airway receptors in lung leukocyte recruitment. For example, mice lacking the type 1 receptor for TNF- (TNFR-1) or the receptor (CCR2) for macrophage inflammatory protein-2 have been challenged, respectively, by aerosolized lipopolysaccharide, ozone inhalation, or by viral infection. In every case, mice lacking the chemokine receptors had fewer leukocytes in the BAL than control animals. Similar results were obtained for bacterial challenge when CCR2 was blocked with a neutralizing antibody.6 Despite this evidence, the relevant receptor-mediated intracellular mechanisms in alveolar epithelium remain unidentified.
We have addressed this problem through real-time optical imaging of the alveolocapillary region in situ in the isolated blood-perfused rat lung.7 To activate vectorial signaling, we selected TNF- as the model agonist. To quantify cross-talk, we imaged the cytosolic Ca2+ (Ca2+cyt) in epithelial cells of the alveolar wall (AECs) of a selected alveolus, and in endothelial cells (ECs) of a capillary adjoining the alveolus. A 1-min alveolar microinfusion of TNF- increased Ca2+cyt in both AECs and ECs (Fig 1
), revealing the presence of vectorial cross-talk signaling from alveolus to capillary.
|
remains in the alveolus for only 3 to 5 s, thereby abolishing concentration gradients that might otherwise drive transalveolar transport of the chemokine.
Although the alveolar membrane is leak proof for proteins the size of TNF-, leakage could result from accidental microinfusion in the extra-alveolar space. However, definitive evidence against alveolar leak of the microinjected TNF- came from our studies with a blocking monoclonal antibody (mAb) against TNFR-1. These experiments indicated that TNFR-1 immunofluorescence was well expressed in the apical epithelial membrane, that a prior alveolar injection of the mAb blocked EC Ca2+cyt responses to alveolar TNF-, and that crosslinking the mAb-ligated receptor with a secondary antibody-induced AEC and EC Ca2+cyt responses similar to those of alveolar TNF-. Further, in the presence of alveolar blockade with the mAb, capillary injection of TNF- induced Ca2+cyt increases in ECs but not AECs. These results ruled out nonspecific factors as being responsible for the Ca2+cyt responses to alveolar TNF-.
The findings of the mAb experiments lead to the major conclusion that the TNF-–induced cross-talk responses were receptor mediated. Characteristically, the Ca2+cyt response in AEC consisted of high-amplitude Ca2+ oscillations that were notably absent in ECs (Fig 1). This difference could be attributable to different patterns of Ca2+ mobilization in the two cell types. Thus, while the AEC response was consistent with sequential release and uptake of Ca2+ from endoplasmic stores—a pattern characteristic of receptor-mediated Ca2+ mobilization—the relatively damped EC response pointed to direct Ca2+ entry as being the mechanism that relayed the cross-talk signal to capillaries.
Accordingly, we considered paracrine factors that might be released basolaterally by AECs to induced direct Ca2+ entry in ECs. One such factor is arachidonate, which induces Ca2+ entry by opening plasma membrane Ca2+ channels.9 The possible role of this lipid mediator was indicated, in that inhibitors of the arachidonate precursor, cPLA2, blocked the TNF-–induced cross-talk.7 Two other sets of findings also supported this result, namely, first, an in situ immunofluorescence assay indicated the presence of TNF-–induced activation of cPLA2 in AECs of intact alveoli; and second, microinjections of arachidonate in saponin-permeablized alveoli induced oscillation-damped Ca2+cyt increases in ECs.7
An important consequence of cross-talk signaling in capillaries is the surface expression of the leukocyte adhesion receptor, P-selectin, in ECs. P-selectin is normally held in EC in intracellular vesicles. Increase of Ca2+cyt or H2O2 causes surface expression of P-selectin, which therefore provides a marker of proinflammatory activation in EC.10 Alveolar TNF- increased capillary P-selectin expression in adjoining capillaries within 5 min (Fig 2
),7 attesting to the considerable rapidity of this cross-talk response. The P-selectin expression was also inhibited by intra-alveolar preinfusion of either the anti-TNFR-1 mAb or cPLA2 blockers, indicating that the expression resulted from receptor-mediated processes.
|
–induced alveolocapillary cross-talk, a role also recognized in the context of acid-induced lung inflammation,11 the post-cPLA2 factor that relays the cross-talk signal remains unidentified. The possibilities are as follows: cPLA2 activation causes arachidonate release; hence, basolateral release of arachidonate from AECs might relay the signal to ECs, causing Ca2+cyt-induced P-selectin expression. Also, arachidonate increases AEC reactive oxygen species formation through nicotinamide adenine dinucleotide phosphate oxidase activation12; hence, diffusible reactive oxygen species such as H2O2 could be the relay factor. H2O2 directly induces P-selectin expression in capillary ECs.10 Hence, H2O2 released basolaterally by AECs may induce proinflammatory responses in capillaries. Although these mechanisms require further consideration, our findings provide the hitherto unrecognized role of the alveolar epithelium as an inducer of the inflammatory response of the lung. Clearly, this role is central in conveying proinflammatory signals from alveoli to adjoining blood vessels. Further research is required to elucidate the role of AECs, not only in arming the innate immune response of the lung, but also in shaping lung pathology under conditions associated with alveolar remodeling.
 |
Footnotes |
|---|
This research was supported by National Institutes of Health grants HL64896, HL57556, and HL69514.
|
References |
|---|
 TOP References |
|---|
mediates recruitment of neutrophils and eosinophils during airway inflammation. J Immunol 1995;154,5411-5417[Abstract] receptors. Am J Physiol Lung Cell Mol Physiol 2001;280,L537-L546 and its receptor CCR1 control pulmonary inflammation and antiviral host defense in paramyxovirus infection. J Immunol 2000;165,2677-2682
| ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| HOME | HELP | FEEDBACK | SUBSCRIPTIONS | ARCHIVE | SEARCH | TABLE OF CONTENTS |
