The role of CD3+, CD4−, and CD8− T cells in mucosal immunoregulation
A large number of studies concerning recognition of antigen by T cells have been conducted during the past 5 years. T cells express a heterodimer T-cell receptor (TCR) that corecognizes antigen processed and a gene product of the major histocompatibility complex (MHC). Further, T cells can be divided into two subsets according to their expression of these disulfidelinked heterodimer proteins of TCR, e.g., those expressing ot (38 kD) 13 (43 kD) or ",/(35 kD) ~ (45 kD) heteroglycoproteins. Both forms of TCR are noncovalently associated with a cluster of proteins, the CD3 molecules (2). This cluster of CD3 molecules consists of four proteins, namely CD3--,/(21 kD), CD3-8 (28 kD), CD3-e (25 kD), and CD3-~ (16 kD), which are involved in mediation of signal transduction during T-cell activation (4). CD4 ÷ T helper (Th) or CD8 ÷ T cytotoxic (Tctx) cells use the or13 TCR heterodimer for recognition of antigens in association with Class II or Class I MHC molecules, respectively. On the other hand, T cells bearing TCR'y~ have been identified as a subset of T cells that do not possess CD4 or CD8 molecules (2, 4) and have been commonly termed double-negative (DN) cells. Although intensive investigation has resulted in a partial characterization of TCR~/bbearing T cells, the biologic function of CD3 ÷ DN T cells in the immune system is still not clear. In this article, we provide evidence that CD3 ÷ DN T cells do possess regulatory function; namely, they may mediate the balance between CD4 ÷ Th and CD8 ÷ T suppressor (Ts) cells in the mucosal immune system.