Predictive biochemical assays for late radiation effects

Overlapping Science in Radiation and Sulfur Mustard Exposures of Skin and Lung: Consideration of Models, Mechanisms, Organ Systems, and Medical Countermeasures: Overlapping science in radiation and sulfur mustard injuries to lung and skin

PURPOSE

To summarize presentations and discussions from the 2022 trans-agency workshop titled "Overlapping science in radiation and sulfur mustard (SM) exposures of skin and lung: Consideration of models, mechanisms, organ systems, and medical countermeasures."

METHODS

Summary on topics includes: (1) an overview of the radiation and chemical countermeasure development programs and missions; (2) regulatory and industry perspectives for drugs and devices; 3) pathophysiology of skin and lung following radiation or SM exposure; 4) mechanisms of action/targets, biomarkers of injury; and 5) animal models that simulate anticipated clinical responses.

RESULTS

There are striking similarities between injuries caused by radiation and SM exposures. Primary outcomes from both types of exposure include acute injuries, while late complications comprise chronic inflammation, oxidative stress, and vascular dysfunction, which can culminate in fibrosis in both skin and lung organ systems. This workshop brought together academic and industrial researchers, medical practitioners, US Government program officials, and regulators to discuss lung-, and skin- specific animal models and biomarkers, novel pathways of injury and recovery, and paths to licensure for products to address radiation or SM injuries.

CONCLUSIONS

Regular communications between the radiological and chemical injury research communities can enhance the state-of-the-science, provide a unique perspective on novel therapeutic strategies, and improve overall US Government emergency preparedness.

Animal models and medical countermeasures development for radiation-induced lung damage: report from an NIAID Workshop

Since 9/11, there have been concerns that terrorists may detonate a radiological or nuclear device in an American city. Aside from several decorporation and blocking agents for use against internal radionuclide contamination, there are currently no medications within the Strategic National Stockpile that are approved to treat the immediate or delayed complications resulting from accidental exposure to radiation. Although the majority of research attention has focused on developing countermeasures that target the bone marrow and gastrointestinal tract, since they represent the most acutely radiosensitive organs, individuals who survive early radiation syndromes will likely suffer late effects in the months that follow. Of particular concern are the delayed effects seen in the lung that play a major role in late mortality seen in radiation-exposed patients and accident victims. To address these concerns, the National Institute of Allergy and Infectious Diseases convened a workshop to discuss pulmonary model development, mechanisms of radiation-induced lung injury, targets for medical countermeasures development, and end points to evaluate treatment efficacy. Other topics covered included guidance on the challenges of developing and licensing drugs and treatments specific to a radiation lung damage indication. This report reviews the data presented, as well as key points from the ensuing discussion.

Molecular markers of radiation-related normal tissue toxicity

Over the past five decades, those interested in markers of radiation effect have focused primarily on tumor response. More recently, however, the view has broadened to include irradiated normal tissues-markers that predict unusual risk of side-effects, prognosticate during the prodromal and therapeutic phases, diagnose a particular toxicity as radiation-related, and, in the case of bioterror, allow for tissue-specific biodosimetry. Currently, there are few clinically useful radiation-related biomarkers. Notably, levels of some hormones such as thyroid-stimulating hormone (TSH) have been used successfully as markers of dysfunction, indicative of the need for replacement therapy, and for prevention of cancers. The most promising macromolecular markers are cytokines: TGFbeta, IL-1, IL-6, and TNFalpha being lead molecules in this class as both markers and targets for therapy. Genomics and proteomics are still in nascent stages and are actively being studied and developed.

Simultaneous measurements of KL-6 and SP-D in patients undergoing thoracic radiotherapy

PURPOSE

Radiation pneumonitis (RP) is a serious complication in patients undergoing thoracic radiotherapy (TRT). Serum KL-6 and SP-D have been shown to increase in several kinds of interstitial pneumonia. To evaluate their clinical usefulness in detecting RP, we serially measured them in patients receiving TRT.

MATERIALS AND METHODS

Thirty-nine patients, who received TRT for lung cancer between July 1999 and April 2004, were prospectively studied. Serum levels of KL-6 and SP-D were measured using enzyme-linked immunosorbent assays. Patients were followed up until August 2004 or their deaths.

RESULTS

During the period, RP occurred in 19 patients. In five patients with diffuse RP extended outside the radiation field, serum KL-6 levels increased, reaching more than 1,000 U/mL. Serum KL-6 levels at 40 Gy in patients who developed RP were higher than those without it (p = 0.0363, Mann-Whitney U test). In addition, serum KL-6 levels at 40 Gy in patients who developed RP were higher than those of pretreatment (p = 0.0126, Wilcoxon signed rank test). On the other hand, there were no statistical differences between sp-d at 40 Gy and those before TRT (P = 0.1165).

CONCLUSIONS

Increased KL-6 at 40 Gy compared with those before treatment in patients undergoing TRT may be of clinical significance. KL-6 proved to be a useful indicator for estimating RP, while usefulness of SP-D was not confirmed in this study.

Strain dependent differences in a histological study of CD44 and collagen fibers with an expression analysis of inflammatory response-related genes in irradiated murine lung

Using a mouse model, we investigated the mechanisms of heterogeneity in response to ionizing radiation in this research. C57BL/6J and C3H/HeMs mice were irradiated with gamma rays at 10 and 20 Gy. The animals were sacrificed at times corresponding to the latent period, the pneumonic phase, and the start of the fibrotic phase for histological investigation. Small areas of fibrosis initially appeared in C57BL/6J mice at 4 weeks postirradiation with 20 Gy, whereas small inflammatory lesions appeared at 4 and 8 weeks after 20 and 10 Gy, respectively. The alveoli septa were thickened by an infiltration of inflammatory cells, and alveoli were obliterated in lungs from C57BL/6J mice after 20 Gy irradiation. At 24 hours and from 2 to 4 weeks postirradiation, fourfold more CD44 positive cells had accumulated in the lungs of C3H/HeMs than in C57BL/6J mice. Hyaluronan accumulated 12 hours after irradiation, and the rapid resolution was achieved within 2 weeks in the lungs in both strains of mice. C57BL/6J mice lungs accumulated dense collagen at 8 weeks. Quantitative RT-PCR assay was performed for several genes selected by cDNA microarray analysis. The expression of several genes, such as Cap1, Il18, Mmp12, Per3, Ltf, Ifi202a, and Rad51ap1 showed strain-dependent variances. In conclusion, a histological investigation suggested that C3H/HeMs mice were able to induce a more rapid clearance of matrix after irradiation than C57BL/6J mice. The expression analysis showed that the several genes are potentially involved in interstrain differences in inflammatory response causing radiation-induced lung fibrosis.

Time dependence of the activity concentration ratio of red marrow to blood and implications for red marrow dosimetry

BACKGROUND

The method for red marrow dosimetry in radioimmunotherapy, in the absence of specific activity uptake in red marrow, is based on the activity measured in the blood or plasma. The activity concentration ratio of red marrow to blood is then assumed to be constant. The aim of the current study was to determine whether this ratio varies with time after injection.

METHODS

Measurements were carried out with both animals and patients.Tumor-bearing rats were intravenously injected with iodine-131-, iodine-125-, indium-111-, or rhenium-188-labeled BR96, a chimeric immunoglobulin G1 monoclonal antibody. (All were chelate-labeled, except for iodine-131, which was iodogen-labeled.) Measurements were made of the activity concentration in blood and bone marrow at different points in time after injection, and the ratio of activity concentration in red marrow and blood as a function of time postinjection (RMBLR[t)]) was calculated. For patients treated with iodine-131-labeled monoclonal antibody (LL2, Immunomedics Inc., Morris Plains, NJ; anti-CD22; immunoglobulin G2 isotype of mouse origin), blood samples were drawn and scintillation camera images taken at different times after injection. The red marrow activity concentration in the sacrum was determined by activity quantification from regions of interest. The activity concentration in blood was also measured. The RMBLR(t) was calculated based on these data.

RESULTS

For both patients and rats, the RMBLR(t) was increased 72 hours after injection. Furthermore, it was found that the use of a constant RMBLR can lead to an over- or underestimation of the absorbed dose in bone marrow.

CONCLUSIONS

These data demonstrate the difficulty in using fixed values of the activity concentration ratio of red marrow to blood for dosimetry.

Clinical significance of serum pulmonary surfactant proteins a and d for the early detection of radiation pneumonitis

PURPOSE

Radiation pneumonitis (RP) is one of the most serious complications for patients who receive thoracic irradiation. To avoid this, early diagnosis of radiation pneumonitis is extremely important. The purpose of the present study is to investigate whether serum pulmonary surfactant proteins A and D (SP-A and SP-D, respectively) could be useful markers for RP.

METHODS AND MATERIALS

Eighty-six patients (lung cancer: 42 [primary: 39, metastatic: 3], breast cancer: 23, esophageal cancer: 21) who underwent radiation therapy were prospectively studied. Radiation doses ranged from 30-76 Gy (median, 58 Gy). Serum SP-A and SP-D levels were evaluated sequentially by a sandwich enzyme-linked immunosorbent assay (ELISA) method before, during, and throughout the follow-up period until the development of symptomatic RP or until one year after completion of radiotherapy. Specificity of the ELISA results was confirmed by Western blot analysis. Patients symptomatic for RP were graded according to the Common Toxicity Criteria.

RESULTS

RP occurred in 19 patients. Serum SP-D levels of patients with RP were sequentially higher than those in patients without RP. In the monitoring, serum SP-D levels at 50-60 Gy showed greater sensitivity and positive predictive values for RP detection (74% and 68%, respectively) than SP-A (26% and 21%, respectively). Western blot analysis showed that the development of RP was due to overproduction, but not proteolysis of surfactant proteins.

CONCLUSION

We confirm that serum SP-A and SP-D monitoring is a practical and useful diagnostic method for the early detection of RP.

A perpetual cascade of cytokines postirradiation leads to pulmonary fibrosis

PURPOSE

Radiation-induced pulmonary reactions have classically been viewed as distinct phases--acute pneumonitis and, later, fibrosis--occurring at different times after irradiation and attributed to different target cell populations. We prefer to view these events as a continuum, with no clear distinction between the temporal sequence of the different pulmonary reactions; the progression is the result of an early activation of an inflammatory reaction, leading to the expression and maintenance of a cytokine cascade. In the current study, we have examined the temporal and spatial expression of cytokine and extracellular matrix messenger ribonucleic acid (mRNA) abundance in fibrosis-sensitive mice after thoracic irradiation.

METHODS AND MATERIALS

Radiation fibrosis-prone (C57BL/6) mice received thoracic irradiation of 5 and 12.5 Gy. At Day 1, and 1, 2, 8, 16, and 24 weeks after treatment, animals were killed and lung tissue processed for light microscopy and isolation of RNA. Expression of cytokine and extracellular matrix mRNA abundance was evaluated by slot-blot analysis and cellular localization by in situ hybridization and immunochemistry.

RESULTS

One of the cytokines responsible for the inflammatory phase (IL-1 alpha) is elevated at 2 weeks, returns to normal baseline values, then increases at 8 weeks, remaining elevated until 26 weeks when lung fibrosis appears. Transforming growth factor-beta (TGF beta), a proliferative cytokine, is elevated at 2 weeks, persists until 8 weeks, and then returns to baseline values. In parallel with the cytokine cascade, the fibrogenic markers for CI/CIII/IV (collagen genes) correlate by showing a similar early and then later elevation of activity. For instance, the collagen gene expression of CI/CIII is a biphasic response with an initial increase at 1-2 weeks that remits at 8 weeks, remains inactive from 8 to 16 weeks, and then becomes elevated at 6 months when collagen deposition is recognized histopathologically.

CONCLUSION

These studies clearly demonstrate the early and persistent elevation of cytokine production following pulmonary irradiation. The temporal relationship between the elevation of specific cytokines and the histological and biochemical evidence of fibrosis serves to illustrate the continuum of response, which, we believe, underlies pulmonary radiation reactions and supports the concept of a perpetual cascade of cytokines produced immediately after irradiation, prompting collagen genes to turn on, and persisting until the expression of late effects becomes apparent pathologically and clinically.

Injury to the lung from cancer therapy: clinical syndromes, measurable endpoints, and potential scoring systems

Toxicity of the respiratory system is a common side effect and complication of anticancer therapy that can result in significant morbidity. The range of respiratory compromise can extend from acute lethal events to degrees of chronic pulmonary decompensation, manifesting years after the initial cancer therapy. This review examines the anatomic-histologic background of the lung and the normal functional anatomic unit. The pathophysiology of radiation and chemotherapy induced lung injury is discussed as well as the associated clinical syndromes. Radiation tolerance doses and volumes are assessed in addition to chemotherapy tolerance and risk factors and radiation-chemotherapy interactions. There are a variety of measurable endpoints for detection and screening. Because of the wide range of available quantitative tests, it would seem that the measurement of impaired lung function is possible. The development of staging systems for acute and late toxicity is discussed and a new staging system for Late Effects in Normal Tissues (LENT) is proposed.

Effect of pentoxifylline on radiation-induced lung and skin toxicity in rats

PURPOSE

There is currently substantial clinical interest in pentoxifylline as an inhibitor of radiation-related normal tissue injury. To further assess this drug's potential toxicity-sparing effects, pentoxifylline was studied in rats using a radiation-induced lung injury model.

METHODS AND MATERIALS

Adult male rats were exposed to either sham irradiation or a single fraction of 21 Gy delivered to the left hemithorax. Four study groups were defined: those that received neither radiation nor pentoxifylline, those that received pentoxifylline (500 mg/L in drinking water) but no irradiation, those that underwent irradiation without pentoxifylline, and those that received both pentoxifylline and radiation. Lung injury was measured by changes in relative left:right lung perfusion ratios derived from quantitative gamma camera imaging of 99mTechnetium-macroaggregated albumin uptake in the pulmonary circulation. Serial scans were done over a 40-week period following radiation. Skin toxicity was also assessed. After 40 weeks, the animals were killed, and lung tissue was assayed for angiotensin converting enzyme activity as a marker for endothelial cell damage.

RESULTS

Both groups of radiated (with or without pentoxifylline) rats showed equivalent acute sharp decreases in left:right lung perfusion ratios compared to the nonirradiated groups, reaching a mean nadir value of 0.29 at week 4. Irradiated lung perfusion in subsequent weeks in the radiation-only group showed minimal recovery, with a plateau mean ratio of 0.37 (0.36-0.39). However, there was apparent later recovery of lung perfusion in the radiation with pentoxifylline group from weeks 14 through 40, to a mean ratio of 0.47 (0.43-0.52) (p < 0.01 compared to the radiation-only group). Angiotensin converting enzyme activity correlated closely with lung perfusion data. No effect of pentoxifylline on acute or late skin toxicity was detected.

CONCLUSIONS

This study suggests that pentoxifylline does not have any measurable effect on acute lung injury following hemithoracic irradiation in rats, but does result in sparing of later lung toxicity.

Pulmonary changes induced by combined mouse beta-interferon (rMuIFN-beta) and irradiation in normal mice--toxic versus protective effects

This study in normal mice was undertaken to investigate possible enhancement of pulmonary toxicity by interferon-beta (IFN-beta) combined with single doses of irradiation. A pharmacokinetic study preceded the toxicity study to determine the optimal route and timing of IFN administration. Graded single doses of radiation were combined with graded doses of IFN. Pulmonary toxicity was determined using endpoints of alveolar surfactant and procollagen in lung lavage fluid at 7 days, breathing frequency, lethality and histology. Increased lethality was seen when IFN was combined with irradiation at 12.5 Gy vs. irradiation alone. This occurred between 20 and 30 weeks post treatment with no increased breathing frequency or surfactant release, suggesting independent mechanisms of injury. Increased breathing frequency after 40 weeks, usually associated with fibrosis, was less pronounced for IFN treated vs. irradiation only controls. Ultrastructural studies at 72 weeks suggest reduced fibrosis in lungs of IFN treated vs. irradiation only controls. Supporting this was the finding that Procollagen III, a biosynthetic precursor of collagen, was increased in the lavage fluid at 7 days for all radiation doses but decreased with the addition of IFN at 12.5 and 15 Gy. Interferons can act either as sensitizers or radioprotectors, depending on the biological system and type of interferon. Our study suggests that while IFN-beta may increase the acute effects of radiation in the mouse lung, some protection from radiation-induced fibrosis, possibly related to alteration of immune mechanisms, may exist.

Procollagen-III in serum, plasminogen activation and fibronectin in bronchoalveolar lavage fluid during and following irradiation of human lung

In the search for predictors of late radiation-induced lung injury we studied procollagen type III peptide concentration (P-III-P) in serum as well as fibronectin and plasminogen activation in bronchoalveolar lavage (BAL) fluid during and following irradiation of human lung. The patients received either high-dose hemithorax irradiation for pleural mesothelioma (11 patients) or high-dose irradiation with individually shaped fields for non-small cell lung cancer (12 patients). The severity of radiation fibrosis was assessed clinically from CT scans 6 months and 12 months after treatment. Four scores were used: severe, moderate, mild, or normal. Radiological lung injury varied from "severe" (9 patients) to near absence of injury-"normal" (6 patients). Serum levels of P-III-P, when measured weekly during the 5-week period of radiotherapy or at several time-points after treatment, did not show consistent changes, nor did the levels correlate with the score for radiation fibrosis as assessed by CT scanning. Changes in fibronectin levels or in markers of plasminogen activation in BAL fluid did not correlate with the development of late lung injury. The levels of BAL fluid plasmin and plasminogen activator as assessed zymographically, but not the free net enzyme values, showed a tendency to be elevated in patients with severe radiation-induced lung injury, suggesting a possible role for inhibitors of the plasminogen activation cascade in the process of radiation-induced lung injury.

Dose and dose-splitting effects of X-rays on lung tumour induction in mice

This paper reports lung-tumour induction 12 months after single or split doses of X-rays in C3H/He male mice. The early proliferative response of lung cells after doses which induced lung tumours was also examined after single X-irradiation. The lung-tumour incidence tended to increase with increasing dose after a single irradiation and peaked at 5 Gy. At more than 10 Gy it decreased sharply to the control level. The mean tumour diameters tended to increase with doses up to 7.5 Gy and then decreased beyond 10 Gy. These results suggested that suppression of tumour growth reduced the tumour incidence at doses of over 10 Gy. The lung-tumour incidence decreased with increasing intervals between two equal doses of 2.5 or 5 Gy. The decrease was thought to be caused by the repair of the tumorigenic injury. However, the tumour incidence after two 2.5 Gy irradiations at 1 day intervals or two 5 Gy irradiations at 6 h intervals was higher than that observed after a single dose. This phenomenon was regarded as a progression of the tumorigenic injury. The labelling indices of the lung cells, determined using tritiated thymidine after a single irradiation, were higher than those of non-irradiated control cells. This response was delayed as the dose increased. The responses versus days after irradiation could be classified into three patterns on the basis of their peaks. The possibility that the larger delay after higher doses had some relation to the reduction of target cells for tumour incidence is suggested.

Pharmacological alteration of the lung vascular response to radiation

The role of endothelial cell damage in the development of radiation injury in the lung was investigated in rats. Vascular permeability-surface area product (PS) was measured as an indicator of the degree of endothelial cell damage in lungs of rats exposed to single dose hemithorax irradiation. Hemithorax irradiation was chosen to simulate clinical radiotherapy, in which only a portion of the lung is irradiated. In addition, it provided a control lung to compare to the irradiated lung. Radiation is postulated to lead to activation of several different biochemical pathways that result in lung injury and fibrosis. Many of these pathways can be specifically blocked with drugs. Thirteen different drugs were studied. Dexamethasone, indomethacin, cromolyn, cyproheptadine, Vitamin D3, theophylline, and diethylcarbamazine were all effective at reducing lung PS on the irradiated side. Dexamethasone, Vitamin D3, and indomethacin also significantly reduced lung PS in the unirradiated lungs and in sham-irradiated rats. Captopril, cobra venom factor, penicillamine, trapidil, epsilon-amino caproic acid, and dapsone had no significant effect on lung PS after hemithorax irradiation. We conclude that the major pathways involved in early post-radiation lung injury involve prostaglandin, leukotriene, and histamine release from macrophages and mast cells. Complement activation, proteolytic enzymes, and neutrophil migration do not seem to be important mediators of early post-radiation lung injury.

A mast cell secretagogue, compound 48/80, prevents the accumulation of hyaluronan in lung tissue injured by ionizing irradiation

Irradiation with a single dose of 30 Grey on the basal regions of the lungs of Sprague-Dawley rats induced a peribronchial and alveolar inflammation. Infiltration of mast cells in the edematous alveolar interstitial tissue and also in the peribronchial tissue were characteristic features of the lesion. The appearance of mast cells was already seen 4 wk after irradiation and by weeks 6 to 8 there was a heavy infiltration. The staining properties suggested that they were connective tissue-type mast cells. The infiltration of mast cells was paralleled by an accumulation of hyaluronan (hyaluronic acid) in the alveolar interstitial tissue 6 and 8 wk after irradiation. The recovery of hyaluronan (HA) during bronchoalveolar lavage (BAL) of the lungs also increased at this time. Treatment with a mast cell secretagogue, compound 48/80, induced a distinct reduction of granulated mast cells in the alveolar tissue. Regular treatment with compound 48/80 from the time of irradiation considerably reduced the HA recovery during BAL and the HA accumulation in the interstitial tissue but did not affect the interstitial infiltration of mononuclear cells and polymorphonuclear leukocytes. By contrast, an accumulation of HA in the alveolar interstitial space was induced when compound 48/80 was given not until mast cell infiltration of the lung had started. The effects of compound 48/80 indicate that the connective tissue response after lung irradiation is dependent on whether or not mast cell degranulation is induced before or after the mast cell infiltration of the alveolar tissue.(ABSTRACT TRUNCATED AT 250 WORDS)

Physiologic and toxicologic responses of alveolar type II cells

This report summarizes recent data regarding the direct responses of the type II alveolar epithelial cell to agents that are known to produce lung injury. These responses are not limited to cytotoxicity or cell death, but include alterations in the known differentiated functions of this cell type. Among the functions assessed and shown to be altered by toxic agents are: (1) synthesis and secretion of pulmonary surfactant; and (2) proliferation and renewal of the alveolar type I cell population. Agents such as ionizing radiation, CdCl2 and hyperoxia are shown to directly alter pulmonary surfactant phospholipid synthesis and secretion by type II cells in a manner consistent with their known effect at the whole animal level. Changes in protein synthesis are also observed. In addition, information is presented which suggests that pulmonary epithelial proliferation and repair is a complex process mediated, in part, by complex cell-cell interaction in the pulmonary parenchyma. In particular, the alveolar macrophage may play a significant role through its ability to synthesize and secrete potent growth factors that influence type II cell growth.

Serum markers for prediction of pulmonary radiation syndromes. Part I: Surfactant apoprotein

Detection of a biochemical marker indicating radiation lung injury prior to the onset of clinical pathologic events could prove valuable in patient management. An increased level of alveolar surfactant is one of the earliest detectable changes following lung irradiation, starting within hours of irradiation and persisting a maximum of 2-6 weeks. However, because broncho-alveolar lavage is impracticable and endothelial cell damage due to radiation results in changes in permeability of vessel wall with leakage of alveolar proteins into serum, identification of serum markers was sought. A series of experiments in rabbits are described that clearly demonstrate serum surfactant apoprotein is an accurate marker and predictor for later lethal radiation pneumonitis. At 3-7 days after graded single doses to lung, surfactant was found in the serum paralleling the dose response for lethality. Control studies with a physiologic agent such as terbutaline release alveolar surfactant, but no serum surfactant was detected. Monitoring serum surfactant could direct preventive intervention prior to clinicopathologic manifestation of pulmonary radiation syndromes.

The ultrastructure of epithelial cells of the distal lung

This review has focused on the structural and functional characteristics of those epithelial cells that line the walls of the lower respiratory bronchioles, alveolar ducts, and alveoli. In all, five cells types were considered: Clara cells, types I, II, and III pneumocytes, and alveolar macrophages. In addition, a very brief mention of the structure and influence of the basement membrane in alveolar development and repair was included, as well as a brief review of the role of epithelial cells in response to selected deleterious influences. No attempt was made to extend this review to cover the structure and functions of the epithelial lining of the conducting portions of the respiratory system, or the exciting and expanding complexities and interrelationships of the septal stroma. Since the volume of literature encircling this subject has virtually exploded during the last 15 years, it becomes almost impossible to review all reports. However, attempts were made to be selective in citations. Insofar as future developments are concerned, much remains to be understood concerning (1) the responses of all cell types to cytotoxic influences, including their respective abilities to repair induced damage, (2) cell-cell and cell-extracellular matrix relationships in response to injury, (3) the uniqueness of the basement membrane in the lung in controlling permeability and gaseous exchange, (4) the role(s) of alveolar macrophages in response to injury and their relationships to the septal macrophage population, (5) the aberrations in the respective cell types that can give rise to neoplastic growth, and (6) the role of the immune system in responding to the general defense of the lung. Indeed much has been learned in the past 2 decades, and it is expected that a review of this sort 1 or 2 decades hence will elucidate many of the functions and structural modifications of the lung.