Editorials
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Airway Eosinophils and Lymphocytes in Asthma Birds of a Feather?
Although advances in immunologic sciences in the past ten years have yet to delineate the basic defect in asthma, continuing investigations are yielding insights into the immunologic components of asthma. Studies have documented the presence of inflammatory cell infiltrates in the airways of asthmatic patients and have linked this inflammation to airway epithelial damage and the resultant airway smooth muscle hyperreactivity that is characteristic of the disease (1). The inflammatory cellular infiltrates, identified both by lavageand by biopsy,include T lymphocytes, neutrophils, and eosinophils. Although abundant evidence implicates the eosinophil and its granule components, such as major basic protein, in epithelial damage other inflammatory cells, particularly lymphocytes, also appear to be involved in the pathogenesis of asthma (1, 2). Asthma is thus likely to be the result of a complex interplay of inflammatory cellular events involving lymphocytes, eosinophils, epithelial cells, smooth muscle cells, and, finally, fibroblasts. The study by Ohashi and colleagues (3) in this issue of the REVIEW sheds some light on this complex cellular ecology. Ohashi and colleagues examined a cohort of 19 individuals each with hyperreactive airways and asthma of varying severity and 10control subjects evaluated for cough, who did not have airway hyperreactivity. From bronchial biopsies performed on all patients during a period of disease quiescence, the authors quantitated ongoing epithelial damage by assessingseparation of epithelial junctions at the electron microscopic leveland evaluated the inflammatory cells in the subjacent airway tissue. With this measure of epithelial cell damage, there was a strong link between morphologically altered epithelial junctions and clinically evident airways hyperreactivity as measured by the PC zo to acetylcholine. In addition, these investigators found statistical correlations between the presence of eosinophils in the subepithelial region and loss of epithelial junctions; 1246
and between the presence of subepithelial lymphocytes and the presence of eosinophils. In contrast, no direct statistical link was noted between the presence of subepithelial lymphocytes and epithelial junction spaces, and there were no differences in numbers of airway lymphocytes between the asthmatic and the non-asthmatic, albeit bronchitic, subjects. These findings, derived from ultrastructural characterizations of bronchial biopsies of subjects with clinically quiescent disease, provide further correlative evidence for the relationship between eosinophils and airway damage and an impetus to evaluate the mechanisms responsible for lymphocyte and eosinophil recruitment in asthma. Lymphocyte and Eosinophil Recruitment
What is the link between the presence of lymphocytes and eosinophils? Recruitment of all inflammatory cells from the blood initially involvesadherence of leukocytes to endothelial cells, with subsequent migration of the adherent cells through the endothelium and into tissue (4). Eosinophils, cells derived from a granulocyte lineage, transit through the circulation and migrate rapidly to tissues principally adjacent to mucosal surfaces as well as to sites of specific varieties of inflammation. T lymphocytes, derived from a different lineage in the bone marrow, relocate to the thymus for maturation and then continuously recirculate from blood to lymphoid tissue and into inflammatory foci. Although lymphocytes and eosinophils are thus very different types of cells, these two leukocytes appear to be birds of a feather as far as certain migration patterns are concerned. While normal "resting" endothelial cellscan interact with inflammatory cells, the adhesive capacity of the vascular endothelium can be markedly increased by exposure to inflammatory cytokines such as tumor necrosis factor (TNF)-a, interleukin (IL)-I, interferon (lFN)-y, or IL-4. This increase in adhesion is mediated by increased expression on the endotheli-
um of specialized adhesion molecules such as intercellular adhesion molecule [ICAM]-I, endothelial cell-leukocyte adhesion molecule [ELAM]-I, and vascular cell adhesion molecule [VCAM]. These adhesion molecules bind to complementary adhesion molecules expressed on blood leukocytes (4, 5). Because not all inflammatory infiltrates contain the same types of infiltrating cells, selectivity exists in the process of leukocyte recruitment. This selectivity reflects specific events occurring at several levels, including alterations in the nature, amounts, and activities of the specific adhesion molecules separately expressible by endothelium and by blood leukocytes, as well as the subsequent responsiveness to locally produced chemotactic factors which promote tissue migration of adherent cells (4). Recent findings demonstrate that lymphocytes and eosinophils use a number of common mechanisms important in their recruitment.
Adherence to Endothelium That eosinophils and lymphocytes might use common pathways for adherence to endothelial cells is suggested by microscopic observations of infiltrating cells in lesions of patients with the disease leukocyte adhesion deficiency (LAD), in which there are genetic deficiencies in the CD18 subunit of the ~-2 integrin adherence molecules, such as LFA-l (6). Neutrophils from patients with LAD do not adhere normally to activated endothelial cells, resulting in impaired neutrophil migration into tissues. In contrast, tissue migration of lymphocytes and eosinophils is relativelyunimpaired in these patients (6). A molecular mechanism for this was recently described by several groups (7-11). Eosinophils were shown to bind to cytokine-activated endothelial cells by adherence to VCAM, a ligand earlier recognized to be involved in mediating lymphocyte adherence to endothelium. Moreover, eosinophil adhesion to VCAM was mediated by the a-4 integrin adhesion molecule, which is expressed on eosinophils and lymphocytes but not AM REV RESPIR DIS 1992; 145:1246-1246
EDITORIAL
HLA-DR (16), eosinophils are not activated by LCF as judged by degranulation, superoxide production, release of leukotriene C 4 , and HLA-DR expression (20). The role of the histamine- and serotonin-induced lymphokine LCF in enhancing eosinophil and CD4lymphocyte migration in allergic disease, particularly asthma, remains to be ascertained; but the fact that LCF at 10-11 to 10-12 M is a potent chemoattractant for CD4+ Transmigration and Chemotaxis eosinophils and lymphocytes would make Adherence to the endothelium is the first it a candidate to participate in the constep to migration into tissue, after which comitant recruitment of these two types emigrating leukocytes must also breach of leukocytes. the endothelial barrier and move with IL-2 is the T-cell growth factor (22). amoeboid motion through the interstiti- IL-2 exerts its effect through binding to um. This motile behavior is responsive multimeric high-affinity receptors exto stimulation by chemotactic factors pressed on activated T cells (22). This (14). Multiple substances have been de- lymphokine exhibits true chemotactic acscribed, which alter the motile response tivity for activated T cells in physiologic of eosinophils (15)and lymphocytes (14). concentrations (17). In this regard, IL-2 For eosinophils, the complement frag- exemplifies the relation between chemoment C5a and the lipid mediator platelet taxis and growth in lymphocytes (14). Reactivating factor (PAP) are potent chemo- cently, IL-2 was also found to be a poattractants, but both of these are also tent eosinophil chemoattractant, active equally active as neutrophil chemoattrac- at about 10-12 M (18). Interestingly, there tants (15). Recent studies have identified was evidence for functional high affinitwo lymphokines, which are 100-to 1000- ty IL-2 receptors on eosinophils as evifold more potent as eosinophil chemoat- denced by the molar potency of IL-2 as tractants than either C5a or PAF, and an eosinophil chemoattractant and by the which stimulate eosinophil but not neu- capacities of monoclonal antibodies to trophil migration. These two lympho- either the p55 or p75 components of the kines, lymphocyte chemoattractant fac- IL-2 receptor to block IL-2 elicited eotor (LCF) (16)and interleukin-2 (17, 18), sinophil migration. Local release of IL-2, again represent common means by which therefore, may contribute to recruitment eosinophils and lymphocytes may be of eosinophils as well as lymphocytes. Thus, given the capabilities of eorecruited. LCF is a 14kD protein that is produced sinophils and lymphocytes to use comas a 56kD tetramer in cell culture (16); mon adherence pathways for binding to LCF has recently been cloned (unpub- endothelial cells and to respond in comlished data). LCF appears to be produced mon to two potent lymphokine stimuli by CD8 + T cellsstimulated by mitogens, for cellular migration, it may not be suras wellas the amines histamine and sero- prising that eosinophils and lymphocytes tonin. All available evidence indicates appear to be birds of a feather cothat the activities of LCF are limited to migrating into the airways of asthmatic cells that express CD4 (19)and that CD4 patients as noted in the studies of Ohashi serves as the cellular receptor for LCE and colleagues. The interactions, howIn addition to CD4+ T lymphocytes, ever, that occur among the diversity of CD4+ cells include both monocytes (16) cells that are components of the airways and eosinophils (20), and both of these and of the infiltrating inflammatory excell types migrate in response to LCE udate are dynamic and complex. One levLCF has no effect on motility of neutro- el of complexity that could not be asphils, which lack CD4 expression. Other sessed solely by ultrastructural observaCD4-binding ligands, including anti- tions is the heterogeneity that exists CD4 antibodies and the HIV-l gp120 among populations of lymphocytes. T envelope protein, also stimulate eosino- lymphocytes are extremelyheterogeneous phil and lymphocyte motility (21). The in their functional capabilities, as illuseffects of gp120, and anti-CD4, and LCF trated by their usual separation into helpare all blocked by Fab fragments of anti- er cells (lymphokine production) and CD4, suggesting that these ligands act cytotoxic cells (lymphokine production by binding to CD4. Although LCF ap- and cytolytic activity). T helper cells are pears to activate a subset of CD4 + lym- likely to be even further specialized in phocytes to express IL-2 receptors and their repertoire of lymphokine producneutrophils, and it mediates lymphocyte adherence to VCAM and may be important in lymphocyte localization to sites of inflammation, particularly rheumatoid synovium (12, 13). Thus, while eosinophils like neutrophils can adhere to ICAM and ELAM, eosinophils share with lymphocytes a common capacity to adhere to VCAM expressed on the activated vascular endothelium.
1247
tion, if the data derived from characterization of limited lymphokine gene cluster expression by mouse helper T cell clones hold true in humans (23). Several recent papers (24, 25) have shown that a particular subset of T helper cells (Th2 cells) are present in greater numbers in the asthmatic airway as compared with control. These cellsexpress a limited cluster of lymphokines, including IL-4 and IL-5. Hence, heterogeneity of lymphocytes, not appreciable by electron microscopy, may have confounded some of the analyses of Ohashi and coworkers. Thus, although lymphocytes wereas prominent in the airways of control subjects as in asthmatics, differences may have existed between the nature of lymphocyte subpopulations and their levels of cellular activation. The products of recruited lymphocytes are likely to affect the functioning and subsequent recruitment of additional eosinophils and lymphocytes. As noted above, LCF, the lymphokine product of CD8 + lymphocytes, potentially elicited by histamine stimulation, could contribute to the recruitment of CD4 + lymphocytes and eosinophils. CD8 + lymphocytes can be present in the airways early following experimental antigen challenges (26). Thus, activation of small numbers of CD8 + lymphocytes, even by mast-cell derived histamine, may help recruit additional CD4+ lymphocytes and eosinophils commonly encountered as major later components of infiltrating cells in asthma and following airway allergen challenge. In addition, cytokine products of the Th2lymphocytes found in the airwaysof asthmatic patients may also contribute to further cellular infiltration. IL-4, a product of Th2 cells, is notably effectivein promoting VCAM expression on the endothelium (27), which would facilitate eosinophil and lymphocyte infiltration. Thus, mechanisms involved in the adherence and migration of eosinophils are likely to contribute to the recruitment of circulating T cells,especially memory T cells, which are potent sources of lymphokine secretion (28). With recruitment and activation of CD4+ lymphocytes of the Th2 subtype, the generation of Th2-derived cytokines, including IL-3, IL-5, and OM-CSF, would promote eosinophilopoiesis, eosinophil recruitment, prolong eosinophil survival, and enhance eosinophil functioning (29-32). Thus, the studies of Ohashi and coworkers, as well as work of other investigators, are helping to identify the major cellular constituents present within the tissues or airways of patients with asth-
1248
EDITORIAL
rna and to correlate these cellular constituents with evidence oftissue damage in the airways. These studies provide data based on single points in time in what is clearly a dynamic and chronic ongoing disease process. A complexity of cellular interactions is likely to be involved throughout the course of asthma. Cells that may not be quantitatively dominant may contribute to the initiation of pathways of inflammatory cell recruitment and activation. Recruited cells may then contribute to further cellular recruitment as well to the activation of cells leading to the release of cytokines, cell constituents, such as eosinophil granule proteins, and lipid and peptide mediators. It is in this "hurricane" of chronic inflammation that eosinophils and lymphocytes are flocked together as birds of a feather contributing to acute airway tissue damage, bronchial hyperreactivity, and even possibly later airway fibrosis. JEFFREY
S.
BERMAN,
M.D.
Pulmonary Center Boston University School of Medicine Boston, MA PETER
F.
WELLER,
M.D.
Beth Israel Hospital Harvard Medical School Boston, MA References 1. Djukankovic R, Roche WR, Wilson JW, Beasley CRW, 1\ventyman OP, Howarth PH, Holgate ST. State of the art: Mucosal inflammation in asthma. Am Rev Respir Dis 1990; 142:434-57. 2. Rochester CL, Rankin JA. Is asthma T-cell mediated? Am Rev Respir Dis 1991; 144:1005-7. 3. Ohashi Y, Motojima S, Fukuda T, Makino S. Airway hyperresponsiveness, increased intercellular spaces of bronchial epithelium and increased infiltration of eosinophils and lymphocytesin bronchial mucosa in asthma. Am Rev Respir Dis 1992; 145:1469-76. 4. Butcher EC. Leukocyte-endothelial cell recognition: Three(or more) stepsto specificityand diversity. Cell 1991; 67:1033-6. 5. Haskard DO, Lee T. The role of leukocyteendothelial interactions in the accumulation of leukocytes in allergic inflammation. Am Rev Respir Dis 1992; 145:SIO-3. 6. Anderson DC, Schmalsteig FC, Finegold MJ, et al. The severeand moderate phenotypes of herita-
ble Mac-l, LFA-I deficiency: Their quantitative definition and relation to leukocyte dysfunction and clinical features. J Infect Dis 1985; 152:668-89. 7. Weller PF, Rand TH, Goelz SE, Chi-Rosso G, Lobb RR. Human eosinophil adherence to vascular endothelium mediated by binding to vascular cell adhesion molecule I and endothelial leukocyte adhesion molecule 1.Proc Nat! Acad Sci USA 1991; 88:7430-3. 8. Walsh GM, Mermod J, Hartnell A, Kay AB, Wardlaw AJ. Human eosinophil, but not neutrophil, adherence to IL-I-stimulated human umbilical vascular endothelial cells is a4f31 (very late antigen-4) dependent. J Immunol1991; 146:3419-23. 9. Bochner BS, Luscinskas FW, Gimbrone MA, Newman W, Sterbinsky S, Derse-Anthony CP, Klunk D, Schleimer RP. Adhesion of human basophils, eosinophils, and neutrophils to interleukin-l-activated human vascular endothelial cells: Contributions of endothelial cell adhesion molecules. J Exp Med 1991; 173:1553-6. 10. Dobrina A, MenegazziR, Carlos TM, Nardon E, Cramer R, Zacchi T, Harlan JM, Patriarca P. Mechanisms of eosinophil adherence to cultured vascular endothelial cells. Eosinophils bind to the cytokine-induced endothelial ligand vascular cell adhesion molecule-l via the very late activation antigen-s integrin receptor. J Clin Invest 1991; 88:20-6. II. Kyan-Aung U, Haskard DO, Lee TH. Vascular cell adhesion molecule-l and eosinophil adhesion to cultured human umbilical vein endothelial cells in vitro. Am J Respir Cell Mol BioI 1991; 5:445-50. 12. Issekutz TB. Inhibition of an in vivo lymphocytemigration to inflammation and homing to lymphoid tissues by the TA-2 monoclonal antibody. A likely role for VLA-4 in vivo. J Immunol 1991; 147:4178-84. 13. van Dinther-Janssen ACHM, Horst E, Koopman G, Newman W, Scheper RJ, Meijer CJLM, Pals ST. The VLA-4/VCAM-I pathway is involved in lymphocyte adhesion to endothelium in rheumatoid synovium. J Immunol 1991; 147:4207-10. 14. Berman JS, Beer DJ, Theodore AC, Kornfeld H, Bernardo J, Center DM. State of the Art: Lymphocyte recruitment to the lung. Am Rev Respir Dis 1990; 142:238-57. 15. WardlawAJ, Moqbel R, Cromwell 0, KayAB. Platelet activating factor. Potent chemotactic and chemokinetic factor for human eosinophils. J Clin Invest 1986; 78:1701-6. 16. Cruikshank WW, Berman JS, Theodore AC, Bernardo J, Center DM. Lymphokine activation of T4+ lymphocytes and monocytes. J Immunol 1987; 138:3817-23. 17. KornfeldH, BermanJS, Beer DJ, CenterDM. Induction of human T lymphocyte motility by interleukin 2. J Immunol 1985; 134:3887-90. 18. Rand TH, Silberstein DS, Kornfeld H, Weller PF. Human eosinophils express functional interleukin 2 receptors. J Clin Invest 1991; 88:825-32. 19. Berman JS, Cruikshank WW, Center DM,
Theodore AC, Beer DJ. Chemoattractant lymphokines specific for the helper/inducer T-Iymphocyte subset. Cell Immunol 1985; 95:105-12. 20. Rand TH, Cruikshank WW, Center DM, Weller PF. CD4-mediated stimulation of human eosinophils: Lymphocyte chemoattractant factor and other CD4-binding ligands elicit eosinophil migration. J Exp Med 1991; 173:1521-8. 21. Kornfeld H, Cruikshank WW, Pyle SW, Berman JS, Center DM. Lymphocyte activation by HIV-I envelope protein. Nature 1988; 335:445-8. 22. Smith KA. The interleukin 2 receptor. Annu Rev Cell Biology 1989; 5:397-425. 23. Mossman TR, Coffman RL. THI and TH2 cells: Different patterns of Iymphokine secretion lead to different functional properties. Ann Rev Immunol 1989; 7:145-73. 24. Hamid Q, AzzawiM, YingS, Moqbel R, Wardlaw AJ, et al. Expression of mRNA for interleukin 5 in mucosal bronchial biopsies from asthma. J Clin Invest 1991; 87:1541-6. 25. Robinson DS, Hamid Q, Ying S, Tsicopoulos A, Barkans J, et al. Predominant Th2-like bronchoalveolarT-Iymphocyte population in atopic asthma. N Engl J Med 1992; 326:298-304. 26. Gonzalez MC, Diaz P, Galleguillos FR, Ancic P, Cromwell 0, Kay AB. Allergen-induced recruitment of bronchoalveolar helper (OKT4) and suppressor (OKT8) T-cells in asthma. Am Rev Respir Dis 1987; 136:600-4. 27. Thornhill MH, Kyan-aung U, Haskard D. IL-4 increases human endothelial cell adhesiveness for T cellsbut not for neutrophils. J ImmunoI1990; 144:3060-5. 28. Salmon M, Kitas GD, Bacon PA. Production of Iymphokine mRNA by CD45R + and CD45Rhelper T cells from human peripheral blood and by human CD4+ T cell clones. J Immunoll989; 143:907-11. 29. Lopez AF, To LB, YangY, Gamble JR, Shannon MF, Burns GF, Dyson PG, Juttner CA, Clark S, Vadas MA. Stimulation of proliferation, differentiation and function of human cells by primate interleukin 3. Proc Nat! Acad Sci USA 1987; 84:2761-5. 30. Yamaguchi Y, Hayashi Y, Sugama Y, Miura Y, Kasahara T, Kitamura S, Torisu M, Mita S, Tominaga A, Takatsu K, Suda T. Highly purified murine interleukin 5 stimulates eosinophil function and prolongs in vitro survival. IL-5 as an eosinophil chemotactic factor. J Exp Med 1988; 167:1737-42. 31. Lopez AF, Sanderson CJ, Gamble JR, Campbell HD, Young IG, Vadas MA. Recombinant human interleukin-S is a selectiveactivator of human eosinophil function. J Exp Med 1988;167:219-24. 32. Owen WF, Rothenberg ME, Silberstein DS, Gasson JC, Stevens RL, Austen KF,Soberman RJ. Regulation of human eosinophil viability, density, and function by granulocyte/macrophage colonystimulating factor in the presenceof 3T3 fibroblasts. J Exp Med 1987; 166:129-41.
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Airway Eosinophils and Lymphocytes in Asthma Birds of a Feather?
Although advances in immunologic sciences in the past ten years have yet to delineate the basic defect in asthma, continuing investigations are yielding insights into the immunologic components of asthma. Studies have documented the presence of inflammatory cell infiltrates in the airways of asthmatic patients and have linked this inflammation to airway epithelial damage and the resultant airway smooth muscle hyperreactivity that is characteristic of the disease (1). The inflammatory cellular infiltrates, identified both by lavageand by biopsy,include T lymphocytes, neutrophils, and eosinophils. Although abundant evidence implicates the eosinophil and its granule components, such as major basic protein, in epithelial damage other inflammatory cells, particularly lymphocytes, also appear to be involved in the pathogenesis of asthma (1, 2). Asthma is thus likely to be the result of a complex interplay of inflammatory cellular events involving lymphocytes, eosinophils, epithelial cells, smooth muscle cells, and, finally, fibroblasts. The study by Ohashi and colleagues (3) in this issue of the REVIEW sheds some light on this complex cellular ecology. Ohashi and colleagues examined a cohort of 19 individuals each with hyperreactive airways and asthma of varying severity and 10control subjects evaluated for cough, who did not have airway hyperreactivity. From bronchial biopsies performed on all patients during a period of disease quiescence, the authors quantitated ongoing epithelial damage by assessingseparation of epithelial junctions at the electron microscopic leveland evaluated the inflammatory cells in the subjacent airway tissue. With this measure of epithelial cell damage, there was a strong link between morphologically altered epithelial junctions and clinically evident airways hyperreactivity as measured by the PC zo to acetylcholine. In addition, these investigators found statistical correlations between the presence of eosinophils in the subepithelial region and loss of epithelial junctions; 1246
and between the presence of subepithelial lymphocytes and the presence of eosinophils. In contrast, no direct statistical link was noted between the presence of subepithelial lymphocytes and epithelial junction spaces, and there were no differences in numbers of airway lymphocytes between the asthmatic and the non-asthmatic, albeit bronchitic, subjects. These findings, derived from ultrastructural characterizations of bronchial biopsies of subjects with clinically quiescent disease, provide further correlative evidence for the relationship between eosinophils and airway damage and an impetus to evaluate the mechanisms responsible for lymphocyte and eosinophil recruitment in asthma. Lymphocyte and Eosinophil Recruitment
What is the link between the presence of lymphocytes and eosinophils? Recruitment of all inflammatory cells from the blood initially involvesadherence of leukocytes to endothelial cells, with subsequent migration of the adherent cells through the endothelium and into tissue (4). Eosinophils, cells derived from a granulocyte lineage, transit through the circulation and migrate rapidly to tissues principally adjacent to mucosal surfaces as well as to sites of specific varieties of inflammation. T lymphocytes, derived from a different lineage in the bone marrow, relocate to the thymus for maturation and then continuously recirculate from blood to lymphoid tissue and into inflammatory foci. Although lymphocytes and eosinophils are thus very different types of cells, these two leukocytes appear to be birds of a feather as far as certain migration patterns are concerned. While normal "resting" endothelial cellscan interact with inflammatory cells, the adhesive capacity of the vascular endothelium can be markedly increased by exposure to inflammatory cytokines such as tumor necrosis factor (TNF)-a, interleukin (IL)-I, interferon (lFN)-y, or IL-4. This increase in adhesion is mediated by increased expression on the endotheli-
um of specialized adhesion molecules such as intercellular adhesion molecule [ICAM]-I, endothelial cell-leukocyte adhesion molecule [ELAM]-I, and vascular cell adhesion molecule [VCAM]. These adhesion molecules bind to complementary adhesion molecules expressed on blood leukocytes (4, 5). Because not all inflammatory infiltrates contain the same types of infiltrating cells, selectivity exists in the process of leukocyte recruitment. This selectivity reflects specific events occurring at several levels, including alterations in the nature, amounts, and activities of the specific adhesion molecules separately expressible by endothelium and by blood leukocytes, as well as the subsequent responsiveness to locally produced chemotactic factors which promote tissue migration of adherent cells (4). Recent findings demonstrate that lymphocytes and eosinophils use a number of common mechanisms important in their recruitment.
Adherence to Endothelium That eosinophils and lymphocytes might use common pathways for adherence to endothelial cells is suggested by microscopic observations of infiltrating cells in lesions of patients with the disease leukocyte adhesion deficiency (LAD), in which there are genetic deficiencies in the CD18 subunit of the ~-2 integrin adherence molecules, such as LFA-l (6). Neutrophils from patients with LAD do not adhere normally to activated endothelial cells, resulting in impaired neutrophil migration into tissues. In contrast, tissue migration of lymphocytes and eosinophils is relativelyunimpaired in these patients (6). A molecular mechanism for this was recently described by several groups (7-11). Eosinophils were shown to bind to cytokine-activated endothelial cells by adherence to VCAM, a ligand earlier recognized to be involved in mediating lymphocyte adherence to endothelium. Moreover, eosinophil adhesion to VCAM was mediated by the a-4 integrin adhesion molecule, which is expressed on eosinophils and lymphocytes but not AM REV RESPIR DIS 1992; 145:1246-1246
EDITORIAL
HLA-DR (16), eosinophils are not activated by LCF as judged by degranulation, superoxide production, release of leukotriene C 4 , and HLA-DR expression (20). The role of the histamine- and serotonin-induced lymphokine LCF in enhancing eosinophil and CD4lymphocyte migration in allergic disease, particularly asthma, remains to be ascertained; but the fact that LCF at 10-11 to 10-12 M is a potent chemoattractant for CD4+ Transmigration and Chemotaxis eosinophils and lymphocytes would make Adherence to the endothelium is the first it a candidate to participate in the constep to migration into tissue, after which comitant recruitment of these two types emigrating leukocytes must also breach of leukocytes. the endothelial barrier and move with IL-2 is the T-cell growth factor (22). amoeboid motion through the interstiti- IL-2 exerts its effect through binding to um. This motile behavior is responsive multimeric high-affinity receptors exto stimulation by chemotactic factors pressed on activated T cells (22). This (14). Multiple substances have been de- lymphokine exhibits true chemotactic acscribed, which alter the motile response tivity for activated T cells in physiologic of eosinophils (15)and lymphocytes (14). concentrations (17). In this regard, IL-2 For eosinophils, the complement frag- exemplifies the relation between chemoment C5a and the lipid mediator platelet taxis and growth in lymphocytes (14). Reactivating factor (PAP) are potent chemo- cently, IL-2 was also found to be a poattractants, but both of these are also tent eosinophil chemoattractant, active equally active as neutrophil chemoattrac- at about 10-12 M (18). Interestingly, there tants (15). Recent studies have identified was evidence for functional high affinitwo lymphokines, which are 100-to 1000- ty IL-2 receptors on eosinophils as evifold more potent as eosinophil chemoat- denced by the molar potency of IL-2 as tractants than either C5a or PAF, and an eosinophil chemoattractant and by the which stimulate eosinophil but not neu- capacities of monoclonal antibodies to trophil migration. These two lympho- either the p55 or p75 components of the kines, lymphocyte chemoattractant fac- IL-2 receptor to block IL-2 elicited eotor (LCF) (16)and interleukin-2 (17, 18), sinophil migration. Local release of IL-2, again represent common means by which therefore, may contribute to recruitment eosinophils and lymphocytes may be of eosinophils as well as lymphocytes. Thus, given the capabilities of eorecruited. LCF is a 14kD protein that is produced sinophils and lymphocytes to use comas a 56kD tetramer in cell culture (16); mon adherence pathways for binding to LCF has recently been cloned (unpub- endothelial cells and to respond in comlished data). LCF appears to be produced mon to two potent lymphokine stimuli by CD8 + T cellsstimulated by mitogens, for cellular migration, it may not be suras wellas the amines histamine and sero- prising that eosinophils and lymphocytes tonin. All available evidence indicates appear to be birds of a feather cothat the activities of LCF are limited to migrating into the airways of asthmatic cells that express CD4 (19)and that CD4 patients as noted in the studies of Ohashi serves as the cellular receptor for LCE and colleagues. The interactions, howIn addition to CD4+ T lymphocytes, ever, that occur among the diversity of CD4+ cells include both monocytes (16) cells that are components of the airways and eosinophils (20), and both of these and of the infiltrating inflammatory excell types migrate in response to LCE udate are dynamic and complex. One levLCF has no effect on motility of neutro- el of complexity that could not be asphils, which lack CD4 expression. Other sessed solely by ultrastructural observaCD4-binding ligands, including anti- tions is the heterogeneity that exists CD4 antibodies and the HIV-l gp120 among populations of lymphocytes. T envelope protein, also stimulate eosino- lymphocytes are extremelyheterogeneous phil and lymphocyte motility (21). The in their functional capabilities, as illuseffects of gp120, and anti-CD4, and LCF trated by their usual separation into helpare all blocked by Fab fragments of anti- er cells (lymphokine production) and CD4, suggesting that these ligands act cytotoxic cells (lymphokine production by binding to CD4. Although LCF ap- and cytolytic activity). T helper cells are pears to activate a subset of CD4 + lym- likely to be even further specialized in phocytes to express IL-2 receptors and their repertoire of lymphokine producneutrophils, and it mediates lymphocyte adherence to VCAM and may be important in lymphocyte localization to sites of inflammation, particularly rheumatoid synovium (12, 13). Thus, while eosinophils like neutrophils can adhere to ICAM and ELAM, eosinophils share with lymphocytes a common capacity to adhere to VCAM expressed on the activated vascular endothelium.
1247
tion, if the data derived from characterization of limited lymphokine gene cluster expression by mouse helper T cell clones hold true in humans (23). Several recent papers (24, 25) have shown that a particular subset of T helper cells (Th2 cells) are present in greater numbers in the asthmatic airway as compared with control. These cellsexpress a limited cluster of lymphokines, including IL-4 and IL-5. Hence, heterogeneity of lymphocytes, not appreciable by electron microscopy, may have confounded some of the analyses of Ohashi and coworkers. Thus, although lymphocytes wereas prominent in the airways of control subjects as in asthmatics, differences may have existed between the nature of lymphocyte subpopulations and their levels of cellular activation. The products of recruited lymphocytes are likely to affect the functioning and subsequent recruitment of additional eosinophils and lymphocytes. As noted above, LCF, the lymphokine product of CD8 + lymphocytes, potentially elicited by histamine stimulation, could contribute to the recruitment of CD4 + lymphocytes and eosinophils. CD8 + lymphocytes can be present in the airways early following experimental antigen challenges (26). Thus, activation of small numbers of CD8 + lymphocytes, even by mast-cell derived histamine, may help recruit additional CD4+ lymphocytes and eosinophils commonly encountered as major later components of infiltrating cells in asthma and following airway allergen challenge. In addition, cytokine products of the Th2lymphocytes found in the airwaysof asthmatic patients may also contribute to further cellular infiltration. IL-4, a product of Th2 cells, is notably effectivein promoting VCAM expression on the endothelium (27), which would facilitate eosinophil and lymphocyte infiltration. Thus, mechanisms involved in the adherence and migration of eosinophils are likely to contribute to the recruitment of circulating T cells,especially memory T cells, which are potent sources of lymphokine secretion (28). With recruitment and activation of CD4+ lymphocytes of the Th2 subtype, the generation of Th2-derived cytokines, including IL-3, IL-5, and OM-CSF, would promote eosinophilopoiesis, eosinophil recruitment, prolong eosinophil survival, and enhance eosinophil functioning (29-32). Thus, the studies of Ohashi and coworkers, as well as work of other investigators, are helping to identify the major cellular constituents present within the tissues or airways of patients with asth-
1248
EDITORIAL
rna and to correlate these cellular constituents with evidence oftissue damage in the airways. These studies provide data based on single points in time in what is clearly a dynamic and chronic ongoing disease process. A complexity of cellular interactions is likely to be involved throughout the course of asthma. Cells that may not be quantitatively dominant may contribute to the initiation of pathways of inflammatory cell recruitment and activation. Recruited cells may then contribute to further cellular recruitment as well to the activation of cells leading to the release of cytokines, cell constituents, such as eosinophil granule proteins, and lipid and peptide mediators. It is in this "hurricane" of chronic inflammation that eosinophils and lymphocytes are flocked together as birds of a feather contributing to acute airway tissue damage, bronchial hyperreactivity, and even possibly later airway fibrosis. JEFFREY
S.
BERMAN,
M.D.
Pulmonary Center Boston University School of Medicine Boston, MA PETER
F.
WELLER,
M.D.
Beth Israel Hospital Harvard Medical School Boston, MA References 1. Djukankovic R, Roche WR, Wilson JW, Beasley CRW, 1\ventyman OP, Howarth PH, Holgate ST. State of the art: Mucosal inflammation in asthma. Am Rev Respir Dis 1990; 142:434-57. 2. Rochester CL, Rankin JA. Is asthma T-cell mediated? Am Rev Respir Dis 1991; 144:1005-7. 3. Ohashi Y, Motojima S, Fukuda T, Makino S. Airway hyperresponsiveness, increased intercellular spaces of bronchial epithelium and increased infiltration of eosinophils and lymphocytesin bronchial mucosa in asthma. Am Rev Respir Dis 1992; 145:1469-76. 4. Butcher EC. Leukocyte-endothelial cell recognition: Three(or more) stepsto specificityand diversity. Cell 1991; 67:1033-6. 5. Haskard DO, Lee T. The role of leukocyteendothelial interactions in the accumulation of leukocytes in allergic inflammation. Am Rev Respir Dis 1992; 145:SIO-3. 6. Anderson DC, Schmalsteig FC, Finegold MJ, et al. The severeand moderate phenotypes of herita-
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