In histologically normal (not inflamed) human lung parenchyma there are approximately 1 x 1010 CD3 positive T-cells in residence. These T-cells are mainly tissue resident memory cells enriched for the immune response to local environmental antigens. This suggests that tissue resident T cells are present to maintain peripheral immune defence and that recruitment of memory T cells from blood or lymph nodes may not be necessary to recall an immune response in the lung. Memory T-cells originate from a pool of progenitor cells developing in the thymus that are undergoing a complex differentiation, selection and maturation process then encounter their T cell receptor (TCR)-specific antigen to finally mature into memory cells. However, naive T-cell output progressively decreases with age due to involution of the thymus [28]. Epithelial cells that turn into adipocytes with age in the thymus, can no longer support T-cell differentiation and selection [28]. Consequently, the T-cell pool in the lung parenchyma can no longer be replenished by freshly released naive T-cells reducing fast and effective immune reactions against novel antigens. Although lung-specific analysis of T-cell subtypes awaits further studies, data indicate that intrinsic deficiencies and defects in signalling, such as pathways involving T-cell cytokine production that lead to Th2 differentiation, are seen in old T cells [29].

< Prev   CONTENTS   Source   Next >