Alveolar macrophage phagocytosis in sarcoidosis. The role of activated lymphocytes

Expression of alveolar macrophage adhesion molecules in pulmonary sarcoidosis

Beta-2-integrins belong to a family of leukocyte surface glycoproteins that are essential for immune functions of bronchoalveolar cells. The expression of three alpha chains designed as CD11a, CD11b, CD11c, a common beta chain CD18, and of a ligand for several integrins CD54 (ICAM-1) was studied on alveolar macrophages of patients with active and inactive sarcoidosis and in control subjects. The percentage of macrophages expressing CD11b (CR3) was significantly increased in patients with active sarcoidosis compared with patients who had inactive disease and control subjects. The adhesion molecule CD54 (ICAM-1) was detected on a higher percentage of alveolar macrophages in patients with active rather than inactive sarcoidosis and in control subjects. Since integrin-mediated adhesion seems to be important in macrophage-lymphocyte interactions during the immune response, higher expression of both CD11b and CD54 on sarcoid alveolar macrophages may be related to several immune abnormalities reported in pulmonary sarcoidosis.

Phagocytosis of agarose beads by receptors for C3b (CR1) and iC3b (CR3) on alveolar macrophages from patients with sarcoidosis

Alveolar macrophages (AM) from sarcoidosis patients exhibit no detectable defect in their potential to synthesize the functional alternative and terminal pathway of complement. They also synthesize more C9 than AM from healthy controls. Various authors have suggested that sarcoid AM have decreased phagocytic ability. In the present work we studied whether there was any difference in C3 receptor-mediated phagocytosis of serum-treated and native agarose beads by AM recovered from patients with active sarcoidosis compared with controls. AM from seven patients with active sarcoidosis and seven healthy controls were cultured under serum-free conditions for 2, 12, 24, and 48 h. We found a significantly increased CR1 and CR3 receptor-mediated phagocytosis of native agarose beads by AM from the seven patients. CR1 and CR3 were also detected on AM directly recovered from bronchoalveolar lavage fluid using fluorescein-conjugated monoclonal anti-receptor antibodies. The percentage of AM expressing CR appeared to be increased in sarcoidosis. The reason for the enhanced phagocytosis of agarose beads by the sarcoid AM is probably the result of both increased synthesis and receptors of complement. Altered complement production and complement receptors may be important for the pathogenesis of this granulomatous disorder.

Immune aspects of sarcoidosis

Although the initiating factor(s) is unknown, it is now accepted that pulmonary sarcoidosis develops as a result of an over-stimulated local cellular immune response. Starting as a lymphocytic alveolitis, there is a progression to granuloma formation within the interstitium as stimulated T lymphocytes release mediators capable of attracting and activating monocytes to differentiate into macrophages and epithelioid cells. We are also aware that macrophage-like cells must act as antigen presenters to initiate T cell stimulation within the immune response. To date, interest in the alveolar macrophages of patients with sarcoidosis has focused more on their passive role as responders of the soluble T cell products released as the disease progresses. This paper explores the active role of mononuclear non-lymphoid cells as inducers of immune responses, by taking advantage of monoclonal antibodies capable of discriminating between phenotypically distinct subsets of macrophages. Recent results are presented that suggest a central role for these cells in controlling the course of this disease, focusing specifically on the mechanisms underlying the failure in some patients to resolve the interstitial inflammation and subsequently progressing to fibrosis. A new hypothesis proposes that aberrations in the functional capacity of macrophages may prohibit the emergence of a granuloma-resolving mechanism in some sarcoid patients.