In Situ Activation of Helper T Cells in the Lung

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Abstract
To better understand the lung and systemic responses of helper T cells mediating memory immunity toMycobacterium tuberculosis, we used three- and four-color flow cytometry to study the surface phenotype of CD4+lymphocytes. Bronchoalveolar lavage (BAL) fluid and peripheral blood (PB) samples were obtained from a total of 25 subjects, including 10 tuberculosis (TB)-infected subjects, 8 purified-protein-derivative-negative subjects, and 7 purified-protein-derivative-positive subjects. In marked contrast to CD4+lymphocytes from PB (9% ± 5% expressing CD45RA and CD29), the majority (55% ± 16%) of CD4+lymphocytes in BAL (ALs) simultaneously expressed CD45RA, a naı̈ve T-cell marker, and CD29, members of the very late activation family. Further evaluation revealed that CD4+ALs expressed both CD45RA and CD45RO, a memory T-cell marker. In addition, the proportion of CD4+lymphocytes expressing CD69, an early activation marker, was drastically increased in BAL fluid (83% ± 9%) compared to PB (1% ± 1%), whereas no significant difference was seen in the expression of CD25, the low-affinity interleukin 2 receptor (34% ± 15% versus 40% ± 16%). More importantly, we identified a minor population of CD69brightCD25brightCD4+lymphocytes in BAL (10% ± 6%) that were consistently absent from PB (1% ± 1%). Thus, CD4+lymphocytes in the lung paradoxically coexpress surface molecules characteristic of naı̈ve and memory helper T cells as well as surface molecules commonly associated with early and late stages of activation. No difference was observed for ALs obtained from TB-infected and uninfected lung segments in this regard. It remains to be determined if these surface molecules are induced by the alveolar environment or if CD4+lymphocytes coexpressing this unusual combination of surface molecules are selectively recruited from the circulation. Our data suggest that ex vivo experiments on helper T-cell subsets that display distinctive phenotypes may be pivotal to studies on the human immune response to potential TB vaccines.