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

Abstract

Flow cytometric T-cell subtyping is an important component of bronchoalveolar lavage (BAL) analysis with diagnostic value in routine practice. BAL fluid was collected for differential and standard 4-color panel T-cell subtype analysis using flow cytometry, followed by a 10-color comprehensive T-cell panel to confirm CD4 T-cell antigen loss. Here, we report 4 BAL cases that were incidentally identified during our daily practice over the past 5 years and showed apparent loss of detectable surface CD4 on CD3+ T cells by 2 different anti-CD4 clones. Each of these BAL fluids was rich in neutrophils and had infectious microorganisms identified on microbiological examination, including Pseudomonas aeruginosa, Haemophilus influenzae, and Mycobacterium avium complex. The dual CD4- and CD8-negative CD3+ T cells were confirmed to express α/β but not γ/δ T-cell receptor in 1 case. 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.