REGULATION OF T-CELL ACTIVATION IN THE LUNG - ISOLATED LUNG T-CELLS EXHIBIT SURFACE PHENOTYPIC CHARACTERISTICS OF RECENT ACTIVATION INCLUDING DOWN-MODULATED T-CELL RECEPTORS, BUT ARE LOCKED INTO THE G(0) G(1) PHASE OF THE CELL-CYCLE/

Peripheral lung tissue contains large numbers of T cells, strategicall y located for immune surveillance at the blood-air interface. Given th e intensity of antigenic exposure at this site, it is clear that local T-cell activation events require strict control, in order to maintain tissue homeostasis. How this control is achieved in this unique tissu e microenvironment is unknown, and the present study sought to elucida te the process via detailed analysis of the surface phenotypic charact eristics of freshly isolated lung T cells. We report below that these cells display typical characteristic of 'postactivation', notably elev ated basal Ca2+ concentrations, down-modulated T-cell receptors, expre ssion of Ia and 'late' activation antigens and concomitant CD4/CD8. Ho wever, levels of interleukin-2 receptor and CD2 expression were below those expected of 'activated' T-cell populations, and virtually all of the cells were found to be in the G(0)/G(1) phases of the cell cycle. These properties bear a remarkable similarity to those of T cells act ivated in the presence of endogenous tissue (alveolar) macrophages fro m the lung (see accompanying paper). We hypothesize that they reflect the in vivo operation of an endogenous macrophage-mediated T-cell aner gy-induction process, the function of which is to limit the local clon al expansion of T cells in peripheral lung tissue after in situ activa tion.