The Role of CD68+ Cells in Bronchoalveolar Lavage Fluid for the Diagnosis of Respiratory Diseases

Addressing the critical challenge in the differential diagnosis of severe inflammatory lung diseases, we propose a novel methodology for the analysis of macrophage surface receptors, CD68 and CD206, using specific non-antibody ligands. We developed a non-antibody alternative for the fluorometric detection of CD68+ cells, focusing on macrophages as key functional markers in inflammatory processes. Our marker based on dioleylphosphatidylserine (DOPS), a specific ligand to CD68, was incorporated into a liposomal delivery system. The specificity of this DOPS-based ligand can be precisely modulated by the liposome’s composition and the polyvalent presentation of the ligand. We synthesized a series of fluorescently-labeled DOPS-based ligands and developed a liposome-based sandwich fluorometric assay. This assay enables the isolation and quantification of CD68 receptor presence from bronchoalveolar lavage fluid (BALF). The results confirmed the specific binding of DOPS/lecithin liposomes to CD68+ cells compared to control lecithin systems. Furthermore, the incorporation of PEGylated ‘stealth’ liposomes significantly enhanced binding specificity and facilitated the generation of distinct binding profiles, which proved valuable in differentiating various inflammatory conditions. This approach yielded unique binding profiles of PS-based ligands to CD68+ cells, which varied significantly among a broad range of respiratory conditions, including primary ciliary dyskinesia, bronchial asthma, bronchitis, bacterial infection, pneumonia, and bronchiectasis. Confocal Laser Scanning Microscopy demonstrated selective binding and intracellular localization of the DOPS-based marker within CD68+ macrophages from BALF samples of patients with bronchitis or asthma. The binding parameters of this multivalent composite ligand with the CD68 receptor are comparable to those of antibodies. The inherent binding specificity of phosphatidylserine may offer a sufficient and viable alternative to conventional antibodies. Our results demonstrate the remarkable potential of this novel DOPS-based assay as a complementary tool for the developing non-antibody-based systems for the differential diagnosis of the respiratory diseases, warranting further investigation in larger clinical studies.