Th1/Th2 subsets: distinct differences in homing and chemokine receptor expression?

The functional specialization of T effector cells according to cytokine secretion patterns has been recognized as an important parameter shaping local immune responses. Here we discuss evidence that T cell subsets might also develop distinctive properties related to homing and trafficking into inflamed sites. First, ligands for the inflammation-induced endothelial selectins were found to be induced by IL-12, and hence selectively expressed on Th1 cells generated in vitro. However, their expression on effector cells occurring in vivo is less well correlated with the Th subset. Second, a variety of receptors for and responses towards chemokines have been found to be differentially associated with Th subsets. Notably CCR5 and, to a lesser degree CXCR3 were preferentially found on Th1 cells, CCR4, CCR8 and, more controversial, CCR3 and CXCR4 on Th2 cells. Although many points, such as stability of the phenotype versus dependency on inducing cytokines and activation stages remain to be clarified, it appears that this field provides new insights into the regulation of locally balanced activities of Th subsets and might constitute a promising field for the development of new immunosuppressive drugs.