Although the contribution of CD8+ T cells to the pathogenesis of noncommunicable lung diseases has become increasingly appreciated, our knowledge about the mechanisms controlling self-reactive CD8+ T cells in the respiratory tract remains largely elusive. The outcome of the encounter between pulmonary self-antigen and naive CD8+ T cells, in the presence or absence of inflammation, was traced after adoptive transfer of fluorescence- labeled CD8+ T cells specific for the neo–self-antigen influenza A hemagglutinin into transgenic mice expressing hemagglutinin specifically in alveolar type II epithelial cells in order: to study the outcome of alveolar antigen encounter in the steady state and under inflammatory conditions; to define the phenotype and fate of CD8+ T cells primed in the respiratory tract; and, finally, to correlate these findings with the onset of autoimmunity in the lung. We found that CD8+ T cells remain ignorant in the steady state, whereas transient proliferation of self-reactive CD8+ T cells is induced by forced maturation or licensing of dendritic cells, increases in the antigenic threshold, and targeted release of alveolar self-antigen by epithelial injury. However, these cells fail to acquire effector functions, lack the expression of the high-affinity IL-2 receptor CD25, and do not precipitate autoimmunity in the lung. We conclude that inadvertent activation of CD8+ T cells in the lung is prevented in the absence of “danger signals,” whereas tissue damage after infection or noninfectious inflammation creates an environment that allows the priming of previously ignorant T cells. Failure in effector cell differentiation after abortive priming, however, precludes the establishment of self-perpetuating autoimmunity in the lung.
