CD4 antigen loss in bronchoalveolar lavage fluid with neutrophilia and infectious organisms: A diagnostic pitfall when analyzing T-cell subsets by flow cytometry

CD4 antigen loss on CD3+ T cells may be seen in neutrophil-predominant BAL fluid specimens that contain infectious microorganisms. The underlying mechanism for CD4 antigen loss is unclear but may involve proteolytic cleavage by protease and/or elastase of either neutrophilic or microbial origin, or possibly chronic antigenic stimulation. Awareness of this diagnostic pitfall is important in clinical practice to avoid misinterpretation of a falsely low CD4/CD8 ratio or a population of T cells lacking expression of CD4 and CD8 in BAL samples.