In situ activation of helper T cells in the lung

To better understand the lung and systemic responses of helper T cells medi ating memory immunity to Mycobacterium tuberculosis, we used three- and fou r-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)-infec ted subjects, 8 purified-protein-derivative-negative subjects, and 7 purifi ed-protein-derivative-positive subjects. In marked contrast to CD4(+) lymph ocytes from PB (9% 5% expressing CD45RA and CD29), the majority (55% +/- 16 %) of CD4(+) lymphocytes in BAL (ALs) simultaneously expressed CD45RA, a na ive T-cell marker, and CD29, members of the very late activation family. Fu rther 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 B AL fluid (83% +/- 9%) compared to PB (1% +/- 1%), whereas no significant di fference 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 CD69(bright) CD25(bright) CD4(+) lymphocytes in BAL (10% +/- 6%) that were consistently absent from PB (1% 1%). Thus, CD4(+) ly mphocytes in the lung paradoxically coexpress surface molecules characteris tic of naive and memory helper T cells as well as surface molecules commonl y associated with early and late stages of activation. No difference was ob served for ALs obtained from TB-infected and uninfected lung segments in th is regard. It remains to be determined if these surface molecules are induc ed by the alveolar environment or if CD4(+) lymphocytes coexpressing this u nusual combination of surface molecules are selectively recruited from the circulation. Our data suggest that ex vivo experiments on helper T-cell sub sets that display distinctive phenotypes may be pivotal to studies on the h uman immune response to potential TB vaccines.