Collection, Separation, and Activation of Human T Lymphocytes

There is increasing evidence that T lymphocytes play a central role in regulating both the initial and chronic inflammatory cascades of allergic asthma (1 ,2 ) and can also regulate baseline airway responsiveness in mouse models of asthma (3 ). In particular, activation of antigen-specific CD4⁺ T cells of the Th2 subset in the lungs, resulting in interleukin (IL)-5 secretion, plays a major role in allergic inflammation of the airways (4 ). T-cell activation requires two signals, one provided by interaction between the TCR and specific antigen in association with major histocompatability antigens (MHC) class II molecules, the other provided by costimulatory molecules (5 ,6 ). CD28 is the best-characterized costimulatory molecule and is constitutively expressed on the surface of both CD4⁺ and CD8⁺ T lymphocytes (5 ,6 ). CD28 and its homolog CTLA-4 bind the natural ligands B7.1 and B7.2 (also known as CD80 and CD86, respectively), which are present on antigen presenting cells (APCs ) (5 ,6 ). CD28 ligation provides cyclosporin A-resistant biochemical signals to T cells, which are an absolute requirement to drive proliferation and IL-2 production from CD3-stimulated T cells, as well as enhance cell survival (5 ,6 ). In contrast, CTLA-4 negatively regulates the immune response (7 ,8 ). Costimulation through B7.2 has been demonstrated to be required for the induction of lung mucosal Th2 immune response and altered airway responsiveness (4 ), whereas B7.1 costimulation has been reported to be essential for the maintenance or amplification of lung inflammatory responses (9 ). Hence, better understanding of the biochemical and functional effects of CD28 and CTLA-4 is essential to formulating better strategies aimed at inhibition of CD28 interaction with its ligands as these may represent a possible therapeutic target for the treatment of lung mucosal allergic inflammation.