IL-1 has an essential role in T-cell activation and is considered a proinflammatory cytokine (1). It is a strong immune adjuvant and contributes to the stimulation of nonspecific host responses besides promoting wound healing (2). It enhances blood flow and the induction of chemoattractants, which bring to the injury sites the key inflammatory cells. The administration of high doses of IL-1 to animals produces a clinical picture of systemic inflammation that mimics septic shock (2). IL-1 is composed of two separate cytokines, IL-1α and IL-1β (3). IL-1α is an immunoregulatory cytokine with an essential role in T-cell activation (3). IL-1β is the predominant form of IL-1 released upon stimulation of human monocytes and macrophages (3). IL-1β induces mesothelial cells to release plasminogen activator inhibitor type 1 (PAI-1) and transforming growth factor beta (TGF-β) (4).

The highest levels of pleural fluid IL-1 (5) and IL-1β (6) are seen with empyema, which is the pleural disease with the most inflammation. Pleural fluid IL-1 and tissue necrosis factor alpha (TNF-α) are significantly correlated (1). The mean levels of IL-1 are significantly higher in tuberculous than in malignant effusions (1,7), but there is sufficient overlap that IL-1 levels are not useful diagnostically. In one paper, the pleural fluid levels of IL-1 were higher than the simultaneous serum levels, suggesting local production (1). In another paper (3) the pleural fluid levels of both IL-1α and IL-1β were lower than the simultaneously obtained serum levels in patients with both malignant and benign pleural effusions. The pleural fluid levels of IL-1β are higher in exudates than in transudates.

Pleural fluid levels of IL-1 appear to be associated with loculated pleural effusions and the development of pleural fibrosis. Patients with tuberculous pleuritis who develop chronic pleural thickening have significantly higher IL-1 levels than those who do not (1). Patients with loculated pleural effusions have much higher pleural fluid levels of IL-1β than patients with nonloculated pleural effusions irrespective of whether the effusions are due to malignancy, pneumonia, or tuberculosis (4). The levels of IL-1β correlate positively with the levels of TGF-β and PAI-1 in the pleural fluid, but negatively with the levels of tissue type plasminogen activator (tPA) (4).

There is a naturally occurring interleukin-1 receptor antagonist (IL-1RA). Marie et al. (8) measured the IL-1RA levels in the pleural fluid and serum of 24 patients. They found that the mean plasma level of
IL-1RA was significantly higher than the mean pleural fluid level and patients with infections did not have higher levels than those without infections (8).

IL-2 plays a crucial role in the mediation of the immune response. The interaction of IL-2 with the IL-2 receptor stimulates a cytokine cascade that includes various interleukins, interferons, and TNF-α (2). IL-2 also induces and maintains the proliferation of T lymphocytes following mitogen or antigen activation and it also induces production of cytotoxic lymphocytes, natural killer (NK) cells, and lymphokine-activated killer cells (2).

The ability of IL-2 to induce the production of these lymphocytes has led to the evaluation of its antitumor effects. Intrapleural IL-2 has been used to treat malignant pleural effusions because of its ability to induce the production of the various lymphocytes (9). When IL-2 is administered intrapleurally, there are increases in the pleural fluid levels of IL-6, but not of TNF-α or IL-1 (9). When IL-2 is injected intrapleurally in patients with positive pleural fluid cytology, the cytology becomes negative within approximately 1 week (10). When mesothelial cells are incubated with IL-2, there is a significant increase in their proliferative response and cytolytic activity against autologous tumors (11). In one study, the intrapleural administration of low-dose IL-2, as an initial treatment, resulted in an objective clinical response in 72 of 100 patients (72%) with a median duration of 5 months (12).

There is one other situation in which IL-2 is relevant to the pleural space. When high-dose intravenous IL-2 is used to treat metastatic disease, a capillary leak syndrome develops in some patients (see Chapter 22) (2). This syndrome is characterized by an increase in vascular permeability which frequently results in pleural effusions (13). In mice, this capillary leak syndrome can be prevented by the oral administration of a nitric oxide inhibitor (14).

As with IL-1, the pleural fluid levels of IL-2 are higher with exudates than with transudates and with tuberculous pleuritis than with malignant pleural effusion, but there is much overlap (7,15). IL-2 levels tend to be lower in the pleural fluid than in the simultaneously obtained serum (3). The pleural fluid levels of IL-2 are significantly correlated with the pleural fluid levels of IL-4, IL-5, IL-10, and TNF-α (16). One of the first events with T-cell activation is the synthesis and surface expression of an interleukin 2 receptor (IL-2R) along with the release of a shorter soluble form of IL-2R. The median levels of the soluble IL-2R are higher in exudates than in transudates and are higher in tuberculous effusions than in malignant pleural effusions, and parapneumonic pleural effusions, but again there is substantial overlap (15,17). The highest pleural fluid IL-2R levels are seen with rheumatoid pleuritis (18).

IL-3 induces the proliferation of eosinophils in vitro and also prolongs their survival. In patients with eosinophilic pleural effusions, IL-3 appears to help promote the eosinophil proliferation and also prolongs the survival of eosinophils. It appears, however, that IL-3 is less important than IL-5 in promoting these two activities in patients with eosinophilic pleural effusions. Blocking antibodies to IL-5 neutralize more of these activities than do blocking antibodies to IL-3 (19). IL-3 is not detectable by enzyme-linked immunosorbent assay (ELISA) in eosinophilic pleural effusions (20,21).

Human immunity has two major components— cellular and humoral. The T-helper type 1 (Th1) pathway favors cellular immunity, whereas the Th2 pathway favors humoral immunity (22). Early determination toward Th1 and Th2 cells in the immune response is dependent on the balance between IL-12, which favors the Th1 response, and IL-4, which favors the Th2 response. In a murine model of delayed hypersensitivity, the subcutaneous administration of recombinant murine IL-4 significantly blocked cell trafficking into the pleural space (23).

In one study on 21 patients with malignant pleural effusions, IL-4 levels in the pleural fluid were below minimal detectable concentrations (22). In another paper, the IL-4 pleural fluid levels were measurable in at least some patients, but there was no significant difference in the levels in patients with tuberculosis or malignancy (24). However, in a subsequent study, IL-4 levels were detectable by cytometric bead array in almost all the pleural fluids that were tested (16).

IL-5 is a T-helper 2 cytokine, which is important in the trafficking of eosinophils. IL-5 induces the proliferation of eosinophils in vitro and prolongs their survival. Pleural fluid IL-5 levels are elevated in patients with
posttraumatic eosinophilic pleural effusions (20). The pleural fluid from such individuals acts as a stimulus for eosinophil colony formation, and this stimulatory capability is largely blocked by specific antibodies toward IL-5. In like manner, eosinophilic pleural fluid enhances the survival of eosinophils, and this capability is largely blocked by specific antibodies toward IL-5.

When air is introduced into the pleural space of mice, there is a brisk eosinophilic response with a 100-fold increase in pleural eosinophils by 12 hours, which peaks at 48 hours (25). When IL-5 knockout mice are injected with air, the number of eosinophils in the pleural lavage increases, but only approximately 10% as much as they did in the wild-type mice (25).

In 40 patients with pleural fluid eosinophilia, including 30 with more than 10% eosinophils, there was a significant relationship between the number of eosinophils in the pleural fluid and the pleural fluid IL-5 level (r = 0.55) (21). In a subsequent paper, the pleural fluid and serum IL-5 levels were measured in 38 patients with pleural effusions occurring after coronary artery bypass graft surgery including 13 with eosinophilic pleural effusions (26). In this study, the pleural fluid IL-5 levels significantly correlated with the pleural fluid eosinophil counts and the serum IL-5 levels significantly correlated with the number of blood eosinophils (26). The pleural fluid IL-5 levels were significantly higher than the serum IL-5 levels (26). In patients with paragonimiasis, the pleural fluid IL-5 levels are markedly elevated and correlate with the number of eosinophils in the pleural fluid (27). Therefore, it appears that IL-5 is one of the primary factors responsible for eosinophilic pleural effusions.

It appears that pleural fluid IL-5 is important in the pathogenesis of malignant pleural effusions at least in mice. When Lewis Lung Cancer cells or adenocarcinoma cells are injected directly into the pleural space of mice, host derived IL-5 promotes the formation of malignant pleural fluid (28). Knockout mice for IL-5 produce much less pleural fluid and have less tumor growth than wild type mice (28).

IL-6, also called B-cell stimulatory factor-2 or hepatocytestimulating growth factor, is a multifunctional cytokine produced by several different cell types such as monocytes, fibroblasts, and endothelial cells (29). IL-6 has a pivotal role in many regulatory functions including maturation of B-cells to antibody-producing cells and induction of the synthesis of acute phase proteins. When IL-6 is injected 5 minutes before the intrapleural injection of carrageenan in the mouse, the exudation and total and differential leukocyte migration in both the early and late response are reduced in a dose-dependent and significant manner (30). The intrapleural injection of IL-6 antibodies 30 minutes before the intrapleural injection of carrageenan decreases both the total and differential leukocyte influx, but significantly increases the exudation (30).

The levels of IL-6 are much higher in the pleural fluid than they are in the serum (3,29,31) and the mean IL-6 levels are much higher in exudates than in transudates (6,29,32). Tuberculous effusions contain a significantly higher level of IL-6 than do malignant pleural effusions (32) or parapneumonic effusions (6). The pleural fluid from patients with mesothelioma has a significantly higher mean IL-6 level than does the pleural fluid from patients with adenocarcinoma (33). It has been postulated that the thrombocytosis seen in patients with mesothelioma is due to the intrapleural production of large amounts of IL-6 (33). The intrapleural administration of IL-2 results in increased pleural fluid levels of IL-6 in malignant pleural effusions (9). One study reported that the levels of soluble IL-6 receptor in the pleural fluid were lower than those in the serum and were comparable in different diagnostic categories (34).

IL-7 was originally discovered as a pre—B-cell growth factor (35). Soon thereafter, it was found to be a critical cytokine for normal T and B lymphopoiesis and a mobilizer of pluripotent stem cells and myeloid progenitors. It has also been found to enhance T-cell functioning and induce cytokine expression in monocytes (35). In cell culture, the proliferative response of lymphocytes from malignant pleural effusions is increased significantly more with IL-7 plus IL-2 than with IL-12 plus IL-2. Chen et al (35) concluded that IL-7 in the presence of IL-2 could restore the immunosuppressed cytolytic activity of the lymphocytes of malignant pleural effusion against autologous tumor. To my knowledge, there have been no studies in which the pleural fluid levels of IL-7 have been measured.

IL-8 is a powerful neutrophil chemotaxin that contributes to the influx of neutrophils into the pleural space (36—38). IL-8 is a downstream cytokine to IL-1 and TNF-α. Cultured mesothelial cells produce IL-8 in response to IL-1, TNF-α, or endotoxin (35). In
contrast, antibodies to IL-1 or TNF-α inhibit IL-8 release from mesothelial cells (2). Mesothelial cells produce IL-8 in basal conditions and the production is increased if the mesothelial cells are stimulated with inflammatory stimuli, asbestos fibers, or infective agents (2). The pleural fluid IL-8 levels are higher than the serum IL-8 levels in exudative pleural effusions suggesting that in vivo IL-8 is produced in the pleural space (3). IL-8 can also stimulate the growth of certain tumors. In mesothelioma cell lines, IL-8 causes a dose-dependent increase in proliferating activity. Lastly IL-8 has angiogenic properties (3).

Pleural fluid IL-8 levels are higher in exudates than in transudates (6). Pleural fluid IL-8 levels are most elevated in patients with empyema (37—40). There are also relatively high levels of IL-8 in the pleural fluid from patients with cancer or tuberculosis, but the levels of IL-8 in the pleural fluid from patients with congestive heart failure are low (41,42). There is a significant correlation between the number of neutrophils in empyema fluid and the level of IL-8 in the fluid (36,37). Neutrophil chemotactic activity is correlated with IL-8 activity, and most of the neutrophil chemotactic activity in pleural fluid is neutralized with anti-IL-8 antibodies (36).

IL-8 may also induce lymphocyte chemotaxis for the pleural space (41). Pace et al. (41) demonstrated that in patients with malignant and tuberculous pleural effusions, the lymphocyte count was more closely correlated with the IL-8 level than was the neutrophil count. Moreover, these workers reported that the pleural fluid was chemotactic for lymphocytes and that the chemotactic activity could be eliminated with antibodies to IL-8 (41).

IL-10 is the most important antiinflammatory cytokine found within the human immune response (42). An antiinflammatory cytokine, by definition, is one that can inhibit the synthesis of IL-1, TNF-α, or other major proinflammatory cytokines (42). IL-10 is a potent inhibitor of Th1 cell cytokines, including IL-2 and interferon-gamma (42). In humans, the main sources of IL-10 are the lymphocytes and monocytes, but macrophages, mast cells, and eosinophils also synthesize IL-10 (2). In a mouse model of hypersensitivity pleuritis, the administration of recombinant murine IL-10 before challenge significantly blocked cell trafficking to the pleural cavity (23). IL-4 was more potent than IL-10 in blocking this trafficking in this model (23). In the mouse model of carrageenan pleurisy, the intrapleural injection of IL-10 5 minutes before the injection of carrageenan led to a significant inhibition of the early phase (4 hours) but had no significant effect on the late phase (48 hours) of the response to carrageenan (30). In the same model, the administration of anti-IL-10 antibody caused a graded and marked increase of both total and differential leukocyte influx and also increased fluid leakage in the early phase, but had no effect on the late phase (30).

Chen et al. (22) measured the pleural fluid and serum IL-10 in 21 patients with a malignant pleural effusion. They reported that IL-10 was detectable in 19 of the 21 pleural fluids (90%) and that the levels of IL-10 were higher in the pleural fluid than in the serum (22). However, a second study reported that the IL-10 levels were comparable in the pleural fluid and in the serum (31). Chen et al (22) found no correlation between IL-10 levels and lymphocyte subpopulations. Aoe et al. (16) measured the pleural fluid levels of IL-10 in 93 pleural fluids and found detectable levels in 92. The levels were similar in patients with malignant effusions, tuberculous pleuritis, and other miscellaneous effusions (16). In this study, the pleural fluid IL-10 levels were significantly correlated with the levels of IL-2 and IL-4 (16).

IL-12 is a heterodimeric cytokine composed of two subunits of molecular masses of 40 kd (p40) and 35 kd (p35) (43). IL-12 is capable of enhancing cell-mediated and cytotoxic immune responses to intracellular pathogens and tumors. It is produced primarily by antigenpresenting cells and is considered crucial in promoting Th1 responses and subsequent cell-mediated immunity (2). Knockout mice with deficient IL-12 genes have defective cell-mediated immunity, fail to develop granulomatous reactions, and are prone to develop tuberculosis (2). When pleural cells are incubated with heat-killed Mycobacterium tuberculosis, IL-12 is detectable in the supernatants (43). The addition of anti-IL-12 antibodies suppressed proliferative responses of pleural fluid cells to M. tuberculosis by 36% (43).