T he migration of T cells is a highly regulated process, serving to distribute functional subsets of cells to the appropriate tissue or microenvironment. The role of adhesion molecules in this process is well appreciated (1, 2) however an additional class of molecules, the chemoattractant cytokines (chemokines) and their receptors, are now receiving considerable attention, because of their fundamental role not only in leukocyte migration, but also in hematopoiesis, T cell activation, and leukocyte degranulation (3). In addition, a recent flurry of papers has demonstrated that chemokine receptors act as cofactors for HIV-1 entry into cells (reviewed in references 4, 5). Two papers in the current issue of this journal add significantly to our understanding of the role of chemokines and their receptors in T cell traffic.

There are two major families of chemokines, termed CXC and CC according to the presence or absence of an amino acid between a pair of cysteine residues near the NH 2 terminus. One of the first members of the chemokines to be identified was IP-10, in 1985 (6), and now 40 or more are known to exist. The first receptors for chemokines were identified several years ago (7, 8), and were found to be seven transmembrane spanning receptors (7TMP,.), that signal through G protein interactions (3, 9, 10). 7TMR also function for the classical chemoattractants, such as fmlp and C5a (11), and also for the chemotaxis of primitive species such as Dictyostelium (12). The human chemokine receptors that have been defined to date are listed in Table 1. The precise expression of many of these receptors is not yet known, because specific mAbs are not available. For T cells, PCI: k or northern blotting indicates that the five known receptors for CC chemokines, defined as CCR1 to CCR5 (A new nomenclature for CC and CXC chemokine receptors was adopted at the 1996 Gordon Research conference on chemotactic cytokines. See Table 1.), are expressed on subsets of T cells. Delineating exactly which subsets is an area of intense study, because chemokine receptor expression may explain the localization or migration of various cell types, such as TH1 or TH2 T cells, or tissue homing subsets. It may also determine which T cells are infected with different strains of HIV-1. A surprising finding is that, within the T lineage, many of the known chemokine receptors are restricted to activated or memory (CD45RO+) type cells. The exact nature of CD45RO+ T cells is still unclear (13), however they do show major differences from CD45RA+ T ceils with respect to adhesion properties in vitro, and migration patterns in vivo (13, 14). The MCP-1 receptor (CCP,. 2) is