Immune mechanisms in interstitial lung diseases
Authors: G. Semenzato, F. Adami, N. Maschio, C. Agostini
Journal: Allergy (2000)

Several features have long suggested a strict relationship between the functional behavior of the host immune system and the pathobiology of interstitial lung diseases (ILDs). Most clinical manifestations of ILDs are secondary to the direct effect of the accumulation of activated immunocompetent cells in involved tissues, notably T cells, macrophages, or neutrophils. However, it is also believed that pathophysiologic events leading to the persistence and/or progression of the pulmonary disease are the same in most ILDs: the persistence of the causative agents and/or the imbalance of mechanisms for the removal of inflammatory cells favor a prolongation of inflammatory responses, the formation of pulmonary granulomas, and, in some cases, the development of irreversible fibrosis. Other characteristics which have captured the attention of immunologists are related to the fact that the development of some complications of ILDs, including the progression to fibrosis, are regulated by a complex cascade of interactions among T cells, accessory cells, and cytokines released by relevant immunocompetent cells. Recent data on the complex network of interactions between cells and biologic response modifiers which control the pulmonary immune system have dramatically expanded our knowledge of the pathogenesis of ILDs. Thanks to the cloning of cytokine genes, in situ hybridization methods, and cytokine receptor characterization, the cascade of interacting extracellular signaling molecules which orchestrate the trafficking of immune cells accounting for the alveolitis are beginning to be clarified. In this paper, we will mainly summarize the basic aspects of pathogenetic mechanisms leading to the recruitment, homing, and activity of inflammation in the respiratory tract of patients with ILDs. In the second part of the paper, we will comment on the role of the immunologic evaluations of pulmonary immunocompetence in specific ILDs. The development of alveolitis represents the immunologic hallmark of ILDs and is sustained by an ordered series of events that can be schematically divided into three phases: the triggering of CD4 T cells by local antigen-presenting cells (APCs) the release of cytokines with multiple and overlapping functions the intra-alveolar and interstitial accumulation of alveolar macrophages (AMs), CD4+ T cells, CD8+ T cells, or neutrophils, depending on the type of ILD ( 1, 2). The persistence of the antigenic stimulation and the consequent chronic inflammatory response lead to the organization of the local inflammatory process into a granuloma ( 3) or to the evolution toward fibrosis ( 4, 5) ( Fig. 1). In the next few paragraphs, we will describe the functional activities of cellular ( Table 1) and soluble factors that participate in the development of alveolitis. Pathogenesis of ILDs. Antigenic exposure triggers T cells and local APCs to release cytokines with multiple and overlapping functions. This favors intra-alveolar accumulation of macrophages and lymphocytes (alveolitis). After removal of antigenic exposure, alveolitis disappears. In contrast, persistence of antigenic stimulation leads to organization of local inflammatory process into granuloma and/or evolution toward tissue injury and fibrosis. The lymphoid tissue of the lung can be functionally subdivided into lymphoid cells arranged in organized aggregates or follicles, which, located throughout the bronchial tree, represent a site of secondary lymphoid differentiation the diffuse lymphoid tissue consisting of scattered lymphocytes which interact with AMs and dendritic cells in the induction of effector functions related to both cellular and humoral immunity ( 6). Full credit for advances in the knowledge of the pathophysiologic role of T cells in ILDs must be given to the bronchoalveolar lavage (BAL) method. By providing access to intra-alveolar cell populations, this method has brought immunologic studies closer to the focus of inflammatory events taking place in the lungs. While lung parenchyma normally contains a few lymphoid elements, lymphocytes are strikingly compartmentalized in the airways of patients with ILDs ( 1, 2). In a normal, nonsmoking individual, more than 90% of the cells retrieved from the BAL are macrophages, while lymphocytes account for 5–10% of the entire BAL cell population. Fewer than 1×106 lymphocytes are usually recovered from a normal BAL; they are CD45R0 T "memory" cells which coexpress the α/β T-cell receptor (TCR); both CD4+ and CD8+ T cells are present in approximately the same proportions as in peripheral blood (pulmonary ratio CD4/CD8=2). In most patients with ILDs, lymphocytes dramatically increase both in percentage and in absolute number. The equivalent of 25×106 lymphocytes can easily be recovered from the BAL of patients with sarcoidosis, hypersensitivity pneumonitis (HP), HIV infection, and berylliosis; they are commonly CD45R0 T "memory" coexpressing the α/β T-cell receptor, but γ/δ TCR cells may increase in some patients. From a pathogenetic point of view, two mechanisms account for the increased number of lymphocytes within the alveolar spaces; i.e., cellular redistribution from the peripheral blood to the lung and in situ proliferation. Macrophage-derived cytokines (such as interleukin [IL]-1, IL-15, interferon [IFN]-γ, and tumor necrosis factor [TNF]-α) can upregulate the expression of mRNA for adhesion molecules on endothelial cells ( 7). Therefore, it is possible that macrophages and other accessory cells (dendritic and endothelial cells) favor T-cell-endothelium adhesion at sites of inflammation ( 8). Furthermore, macrophage-derived chemoattractant cytokines and chemokines are likely to cooperate in the expansion of T cells in the lung of patients with ILD. These molecules will be discussed in detail in the next paragraphs. A second mechanism responsible for the accumulation of T-cell subsets in ILDs is in situ proliferation. Various studies have shown that cytokines, particularly IL-2 and IL-15, act as local growth factors for T cells infiltrating inflamed tissues in ILDs ( 9–12). AMs arise from circulating monocytes ( 13). The daily contribution rate of monocyte traffic to the homeostatic maintenance of the AM population characteristically increases in ILDs. Local epithelial regulatory factors with the potential to regulate leukocyte homing and production of chemokines are involved in this phenomenon. Furthermore, the release of macrophage growth factors may also increase the self-renewal of the resident macrophage pool in ILDs. It is remarkable that, as a result of the increased recruitment and proliferation, macrophagic alveolitis occurs in most individuals. The phenomenon, as in patients with scleroderma, may be so strong that macrophage alveolitis rather than T-cell alveolitis represents the hallmark of the ILD which is associated with this disease. Surface phenotypic analysis reveals that AMs are quite heterogeneous in ILDs ( 14). In general terms, macrophages from patients with ILDs bear adhesion receptors (CD11c, CD29, CD36, CD44, CD49e, and CD54), molecules involved in signal transduction and/or inflammation (CD13, CD45, and CD53), and other markers (CD9, CD52, CD71, and CD98). In addition, a massive monocyte influx takes place during the development of the alveolitis, as shown by the increased expression of CD14 and by the type-IV collagenase production of AMs ( 15, 16). The fact that freshly recruited monocytes have accessory capabilities and can produce cytokines which influence the functional and proliferative activities of lung T cells explains why in sarcoidosis, berylliosis, HP, or HIV infection pulmonary macrophages act as professional APCs ( 12, 17, 18). Acting as accessory cells, they release various biologic mediators of the immune response, such as IL-1, IL-6, IL-15, and granulocyte-macrophage colony-stimulating factor (GM-CSF), that, as reported above, may enhance local effector cell functions ( 19). The molecules that allow AMs to act as professional APCs are now being identified. T-cell/macrophage interaction depends on the presence of costimulatory molecules, including members of the B7 family (CD80 and CD86), some molecules of the TNF-receptor superfamily (CD40 and CD27), and the CD5 coligand (CD72). The pattern of CD80 and CD86 expression shown by pulmonary macrophages of patients with ILDs is consistent with that of conventional APCs ( 20–23). In fact, AMs increase their expression of molecules which endow macrophages with accessory function, including CD80, CD86, CD40, and CD72. Interestingly, recent results obtained in an animal model show that blockade of CD28/B7 interactions by CTLA4-Ig inhibits lung inflammation and immune response in mice inoculated with Saccharoployspora rectivirgula, a major causative agent of HP ( 24). Polymorphonucleates (PMNs) can be found in excess in the BAL of patients with ILDs who show radio-graphic evidence of lung damage and fibrosis ( 25). Compartmentalization of PMNs involves the inter-action of homing receptors on the surface of rolling neutrophils with organ-specific molecules expressed by cytokine-stimulated endothelial cells of the pulmonary tissue (CD54, CD62P, and CD106) ( 26), and it can be initiated and driven by AMs, which release chemotactic factors for granulocytes (CXCL8/IL-8 and other chemokines) ( 27–30). Interestingly, activated neutrophils are in turn able to release cytokines and chemoattractant molecules, such as CXCL8, which further heighten the ongoing inflammatory response. Neutrophils are equipped with a powerful respiratory burst and granules which contain proteases capable of degrading many components of the extracellular matrix. It is therefore widely believed that neutrophils participate in the oxidant or protease-mediated damage of lung tissue that can be observed in biopsy specimens obtained from most patients with ILDs. In fact, lung damage is consistently associated with persistent neutrophilic alveolitis, and an increased concentration of neutrophil granule enzymes can be found in the BAL of patients with ILDs and fibrotic changes ( 4, 5). Endothelial and epithelial cells contribute to immune events taking place in ILDs expressing adhesion molecules, cooperating with accessory cells in the presentation of antigen to pulmonary effector cells. They may also produce immunomodulatory molecules, including chemokines and other chemoattractant molecules, which are involved in the pathogenesis of ILDs. Airway epithelial cells are also able to produce and interact with products of both Th1 (IFN-γ) and Th2 cells (IL-4), modulating the release of chemokines and, thus, local inflammatory responses taking place in the lungs ( 31, 32). IL-9 and TNF-α are additional molecules which can induce the expression of CC-type chemokines in epithelial cells ( 33, 34). A possible role for epithelial-derived cytokines and chemokines in the regulation of immunologic and fibrotic responses in the lung is suggested by the fact that type II epithelial cells in HP and sarcoidosis show an upregulation of immunoreactivity for both IL-4 and IFN-γ, while only IL-4 is detectable in the alveolar epithelial cells of lung specimens obtained from patients with idiopathic pulmonary fibrosis (IPF) ( 35). Lung fibroblasts represent another cell type which actively participates in the cascade of interactions involved in the pathogenesis of ILD. These cells release a number of cytokines and chemokines into the surrounding microenvironment, mediating tissue injury and fibroproliferation. For instance, recent data suggest that lung fibroblasts may be altered in their expression of monocyte chemoattractant protein-1 (CCL2/MCP-1), transforming growth factor (TGF)-β, procollagen, and chemokine receptors (CCR2) after their participation in pulmonary inflammatory processes, and that these changes may be important in fibrosis ( 36). Probably, the Th1/Th2 balance in the lung tissue plays a key regulatory role in these changes since the release of Th1 or Th2 cytokines can polarize lung fibroblasts to produce chemokines such as regulated on activation normal T-cell expressed and secreted (RANTES)/CCL5 or eotaxin/CCL11 ( 37). Recent studies have added to our understanding of the role of cytokine and cytokine receptors in the generation of pulmonary inflammatory responses. In the lung, the production of cytokines and expression of cytokine receptors is under complex biologic control, including negative and positive feedback by the cytokines themselves ( 19, 38). In most cases, receptors for cytokines which are expressed by pulmonary cells are made up of two or more different subunits that are shared with receptors for other cytokines. For example, the γ chain of the IL-2 receptor (IL-2R) is also a component of the receptors for IL-4, IL-7, IL-9, and IL-15. The resulting system is a network of staggering complexity that has begun to be appreciated only in the last few years. In the next few paragraphs we will describe the most relevant cytokines involved in various ILDs. The role of IL-2, which is actively released by pulmonary Th1 cells, in the pulmonary immune system is to expand activated T-cell populations; its receptor is formed by three different subunits: α (CD25), β (CD122), and γ (CD132) chains. IL-2 acts as a local growth factor for T cells infiltrating lung tissues of patients with sarcoidosis, HP, HIV infection, tuberculosis, berylliosis, and other ILDs ( 9, 10, 12). This classic Th1 cytokine is also involved in the regulation of immunoglobulin production and in the enhancement of the potential capabilities of pulmonary cytotoxic T lymphocytes (CTL). The range of pulmonary cellular targets of IL-2 is broader than originally appreciated. Inasmuch as some AMs normally express the β/γ IL-2R at low density, and in view of the fact that the addition of IL-2 to activated AMs increases GM-CSF expression, it is believed that IL-2 may play a role in the activation of some functional capabilities of AMs. The presence of binding sites for IL-2 was also demonstrated on human lung fibroblasts with mAbs specific for p55 IL-2Rα, p75 IL-2Rβ, and p64 IL-2Rγ chains ( 39). The addition of IL-2 to fibroblasts leads to an enhanced expression of the gene coding for the chemokine MCP-1/CCL2, which is involved in fibrosis through the regulation of profibrotic cytokine generation and matrix deposition ( 36, 39). IL-2 may thus serve to integrate fibroblasts and tissue macrophages into a coordinated response of the connective tissue initiated by Th1 cells. This lymphokine, released by most cell types, including Th2 cells, eosinophils, and mast cells, is a cofactor for proliferation of fibroblasts. Inducing the expression of class II major histocompatibility complex (MHC) antigens on the surface membrane of accessory cells, it acts in synergism with IL-2 in stimulating the growth of T cells. The IL-4/IL-13 axis is also involved in the triggering and maintaining of the recruitment, homing, and activation of inflam-matory cells during remodeling processes of the airways ( 40). Finally, IL-4 induces the release of chemokines, including CXCL8/IL-8, from human bronchial epithelial cells. This effect is thought to be of particular importance in attracting neutrophils and monocytes to sites of inflammation ( 31). The production of IL-4 during pulmonary inflammation has been related to the development of lung fibrosis complicating the clinical course of ILDs ( 35, 41). IL-9 is a multifunctional cytokine produced by activated Th2 cells in vitro and during Th2-like T-cell responses in vivo. Data obtained in animal models indicate that IL-9 promotes inflammation and airway hyperresponsiveness ( 42). Lung expression of IL-9 in transgenic mice causes massive airway inflammation with eosinophils and lymphocytes as the main infiltrating cell types ( 43). An additional striking finding is the presence of increased numbers of mast cells within the airway epithelium. Other impressive pathologic changes in the airways are epithelial cell hypertrophy associated with accumulation of mucus-like material within nonciliated cells and increased subepithelial deposition of collagen. Since human fibroblasts express the IL-9 receptor ( 39), it is believed that this cytokine is involved in the fibroproliferative responses taking place in some ILDs. It is believed that activated Th2 cells represent the main cell source of this molecule in the pulmonary microenvironment. Nonetheless, Th0 CD4+ T cells, CD8+ T cells, LPS-activated macro-phages, and mast cells may also produce IL-10. There are data on the involvement of IL-10 in the pathophysiology of HIV-associated ILD ( 28, 44), as well as in the regulation of fibroproliferative responses in the lung. This cytokine has partial inhibitory effects on T-cell-mediated immune responses taking place in beryllium disease ( 44). Finally, it has been hypothesized that the reduced production of IL-10 might be involved in the pathogenesis of alveolitis observed in the lung of patients with HP ( 45). From a functional point of view, IL-10 shows inhibitory activity on the release of IFN-γ and IL-2 by lung Th1 cells, stimulates mast-cell growth, and regulates the accessory function of APCs resident in the pulmonary microenvironment. IL-13 is expressed in activated Th0 cells, Th1-like cells, Th2-like cells, and T cells expressing CD8. This molecule strongly inhibits cytokine secretion induced by LPS in monocyte-macrophages. In particular, the pulmonary release of IL-1, IL-6, TNF-α, and IL-8 may be influenced by the local release of IL-13. IL-13 is also a monocyte chemoattractant. IL-13 is believed to be involved in the regulation of inflammatory responses during ILD. Recent data indicate that IL-13 mRNA is not only a product of pulmonary T cells, but is also expressed in both normal AMs and those from subjects with pulmonary fibrosis, and that at least some of the IL-13 mRNA is translated into protein and secreted in subjects with pulmonary fibrosis ( 46). In addition, during lung inflammation, endogenous IL-13 regulates NF-κB activation and related cytokine/chemokine generation, determining the intensity of the inflammatory response ( 47). IL-13 also has effects on fibrogenesis, since it increases adhesion molecule and inflammatory cytokine expression in human lung fibroblasts ( 40) and is critical for recruitment of inflammatory cells ( 32). In fact, the pulmonary expression of IL-13 causes a mononuclear and eosinophilic inflammatory response, mucus cell metaplasia, airway fibrosis, eotaxin production, airway obstruction, and nonspecific airway hyperresponsiveness ( 48). This is the prototype of Th1 cytokines and is a key factor in the events that favor local immune responses in the lung. IFN-γ enhances the accessory function of AMs, increases the cytotoxic function of lung macrophages and lymphocytes, and regulates the secretion of an array of lymphokines, cytokines, and chemokines into the surrounding micro-environment. In addition, this Th1 cytokine activates pulmonary macrophages to phagocytose pathogens. IFN-γ is typically expressed by T cells infiltrating the lung in most ILDs, including sarcoidosis, HP, tuberculosis, and HIV infection ( 20, 21, 45, 49, 50). Some data suggest that monocytes/macrophages may represent a cell source of IFN-γ in the lungs, but this is disputed. Through its pleiotropic effects on cytokine production, IFN-γ modulates mucosal immune responses in ILDs ( 49, 51). IFN-γ upregulates the expression of the costimulatory molecules on pulmonary accessory cells, including CD80 and CD86 ( 21). It influences cell-mediated mechanisms of cytotoxicity and modulates T-cell growth and functional differentiation. However, by inducing non-ERL chemokines (CXCL9, CXCL10, and CXCL11; see below), this cytokine plays a major role in the recruitment of activated CXCR3+ T cells into the inflamed tissues of patients with ILDs ( 52). IFN-γ also has crucial antifibrotic effects, since it inhibits the proliferation of endothelial cells and the synthesis of collagens by fibroblasts. Data on the pattern of lymphokine production in ILDs are here summarized in the context of the Th1/Th2 paradigm ( 53). Lymphokine-producing T cells are subdivided into Th1 and Th2 cells according to the lymphokine production pattern. Th1 cells secrete IL-2, IFN-γ, and TNF-β, while Th2 cells produce IL-4, IL-5, IL-6, IL-9, and IL-10; both cell types produce GM-CSF and IL-3. From a functional point of view, the two T-cell subpopulations are quite different. Th1 cells elicit delayed-type hypersensitivity reactions and help in IgG synthesis; they play a critical role in combating microbial and viral infections, but can also cause tissue damage. Th2 cells promote IgE synthesis, activate eosinophils, and exert anti-inflammatory effects since they negatively regulate Th1-cell-mediated functional effects through the release of IL-10. There are data in experimental and human ILD suggesting that a Th1 CD4+ or Tc1 T-cell profile predominates during the formation of typical alveolitis. Th1 cells producing IFN-γ or IL-2 are responsible for initiating alveolitis in the lungs of patients with most ILDs, including sarcoidosis (Th1), tuberculosis (Th0), HP (Tc1), and HIV infection (Tc1) ( 45, 50, 52). In contrast, the Th2-type pattern is associated with local activation of allergic mechanisms and fibroproliferative responses, or with reduced resistance to intracellular pathogens. In fact, a switch to Th2 cells with concomitant release of IL-10, IL-6, and IL-4 is involved in the pathogenesis of pulmonary hypersensitivity diseases characterized by eosinophilic pneumonia (including asthma, allergic bronchopulmonary aspergillosis, Löffler's syndrome, etc.), as well as in ILDs evolving toward pulmonary fibrosis ( 41, 54). In the lungs of persons with HIV infection, the Th1/Th2 shift favors the development of pulmonary opportunistic infections ( 55). The clinical story of ILDs is also influenced by the Th1/Th2 pattern of cytokine production. It is postulated that in the early phases of ILDs pulmonary T cells produce Th1 cytokines which favor local hypersensitivity reactions (i.e., IL-2) or have suppressive effects on the production of extracellular proteins (i.e., IFN-γ). In some ILDs, such as sarcoidosis, the net effect of the Th1 response is the development of the granuloma, while, in HP or allograft rejection, CTL are generated as a part of an antigen-specific immune response. In general terms, an inhibition of fibrogenetic processes may be observed in these phases. However, depending on the host susceptibility, a switch to Th2 cells may occur in ILDs evolving toward lung fibrosis with concomitant release of chemoattractants for fibroblasts and cytokines, including IL-4, which stimulate the production of extracellular matrix proteins. Human AMs are capable of producing detectable amounts of IL-1α and 1β, as well as IL-1ra, a specific receptor antagonist of IL-1, in response to several inflammatory stimuli in various ILDs ( 56–59). These cytokines are also produced by other cell types that participate in the ongoing inflam-matory response during ILDs, including neutrophils, endothelial cells, and fibroblasts. IL-1 provides accessory growth factor activity for inflammatory lung T cells. In fact, the main biologic activity of IL-1 is the stimulation of T helper cells, which are induced to secrete IL-2 and to express IL-2 receptors. In this way, IL-1 stimulates the formation of ILD-associated granulomas. By promoting the adhesion of neutrophils, monocytes, and T cells through enhancing the expression of adhesion molecules, such as CAM-1 (intercellular adhesion molecule) and ELAM (endothelial leukocyte adhesion molecule), IL-1 regulates the development of alveolitis. Since it promotes the proliferation of fibroblasts and increases collagen production, IL-1 has been involved in the development of lung fibrosis associated with ILD. In the lung, this proinflammatory factor is mainly produced by AMs, but T cells, endothelial cells, and fibroblasts may also produce IL-6. An increased release of IL-6 has been involved in the pathogenesis of various ILDs, including sarcoidosis, tuberculosis, berylliosis, HIV infection, and ILDs associated with autoimmune disorders ( 60–65). In the lung, this cytokine influences antigen-specific immune responses and potentiates inflammatory reactions. IL-6 is also involved in the control of the in situ proliferation of fibroblasts. In fact, overexpression of IL-6 and the IL-6 receptor genes mimics lymphocytic interstitial pneumonia and contrasts with the changes induced by TGF-β1 or human platelet-derived growth factor B (PDGF-B) ( 66). This cytokine, originally reported as a product of B cells, is mainly produced in the lung by macrophages and dendritic cells. It has been reported that neutrophils may also represent a cell source of IL-12. IL-12 is involved in Th1 immune responses and stimulates the proliferation and the lytic activity of activated lung T cells. Specifically, IL-12 induces the Th0 to Th1 shift and stimulates the proliferation and lytic activity of activated T cells and natural killer (NK) cells. In synergy with IL-15, IL-12 favors the contact between activated T cells and APCs. Its involvement in sarcoidosis, HP, and tuberculosis has been clearly demonstrated ( 19, 50, 67–70). The cytokine acts by interacting with specific receptors (IL-12Rβ) expressed by lymphocytes accumulating in the lungs of patients with Th1-driven ILDs, including sarcoidosis and HP ( 45, 71). This pleiotropic lymphokine shares biologic activities and components of its receptor with IL-2 (β/γ IL-2R). In the lung, IL-15 is mainly produced by resident macrophages. The role of IL-15 in the pathogenesis of T-cell alveolitis is suggested by data obtained from patients with sarcoidosis, tuberculosis, HP, and HIV infection ( 11, 12, 20, 72). In these patients, IL-15 is produced by AMs and supports the growth and chemotaxis of T cells, favoring the development of T-cell alveolitis. It also acts as a costimulatory factor for the production of GM-CSF, IFN-γ, and TNF-α, and for the expression of a number of molecules that are involved in the APC capability of AMs (CD80/CD86). Its role in HIV infection and in the immune response to intracellular pathogens suggests that the cytokine may support T-cell alveolitis even in the absence of IL-2-producing CD4+ T cells. Furthermore, the finding that IL-15 downmodulates the apoptosis rate of lung T cells supports a role for IL-15 as a possible inhibitor of the death-inducing effects of physiologic apoptotic stimuli during ILD. Previously known as IFN-γ-inducing factor (IGIF), IL-18 has activity roughly similar to, although distinct from, that of IL-1. Mainly produced by monocytes and macrophages, it induces expression of IFN-γ and CSFs, while it inhibits the production of IL-10. In addition to stimulating IFN-γ synthesis, IL-18 also exerts inflammatory effects by inducing synthesis of the proinflammatory cytokines TNF and IL-1β and the chemokines IL-8/CXCL8 and monocyte inflammatory protein-1α (MIP-1α)/CCL3. Interestingly, IL-18 and IL-12 act on Th1 cells synergistically to induce IFN-γ, suggesting that both cytokines cooperate in the development of a Th1-type immune response in ILD, including sarcoidosis ( 73). GM-CSF, granulocyte-CSF (G-CSF), monocyte-CSF (M-CSF), and IL-3 can induce the growth and differentiation of myeloid progenitors, facilitating the accumulation of macrophages in the lungs of patients with ILD ( 74–77). Furthermore, GM-CSF modulates cytokine production and enhances the APC ability and growth of AMs. TGF-β and the family of TGF-related cytokines are secreted by monocyte-macrophages and activated lymphocytes, including pulmonary lymphocytes. TGF-β is a potent immuno-suppressive molecule that exerts chemotactic effects on monocytes. It also modulates the synthesis and the effect of several molecules, including IL-1, IL-2, IL-3, GM-CSF, IFN-γ, and TNF-α. TGF-β, which is constitutively released in the respiratory tract, is involved in the pathogenesis of fibrotic processes associated with most ILDs ( 19, 78–80). TNF-α, a pleiotropic factor predomin-antly produced by activated cells belonging to the monocyte-macrophage lineage, activates both neutro-phils and macrophages, leading to protease release, stimulation of the respiratory burst, and the induct-ion of vascular adhesion molecule expression essential for cell recruitment at sites of inflammation. In the lung of patients with ILDs, TNF-α is actively produced by pulmonary macrophages ( 81, 82). It plays a critical role in pulmonary injury and in the regulation of fibroblast growth via the induction of IL-6. Furthermore, TNF-α stimulates and regulates the synthesis and release of other lymphokines (IL-1, GM-CSF, platelet-activating factor, and IL-6) and increases prostaglandin (PG)E2 production. TNF-α is a member of an emerging family of soluble molecules with several complex immunoregulatory properties (TNF-α, LT-α and its membrane-associated form LT-β, FasL/CD95L, CD40L/CD154, CD30L/CD153, and CD27L/CD70) that interact with specific receptors (TNF-R). TNF-α and other ligands of the TNF superfamily (TNF-L) have a conflicting role in modulating apoptotic mechanisms at sites of inflammation. Some data suggest that the chronic overexpression of TNF-α and IFN-γ and the dysregulation of TNF-R/TNF-L set the stage for the persistence and the progression of the inflammatory even