Christophe Pannetier, Jos Even and Philippe Kourilsky lmproved polymerase chailr reaction (RX)-based methods now permit a mare in-depth analysis of the repertoire of T cells recovered m biological samples from mice and humans. At a certain level of resolution, the diversity of the T-cell repertoire can be readily estimated and clonal expansions become easily detectable. As discussed here by Christophe Pannetier, Jos Even and Philippe Kourilsky, these improvements allow a better appreciation of the degree of reproducibility of immune responses, both in mice and humans, and should harle a significant impact on clinical investigations.

With millions of potential specificities, T-cell popu lations are probably as complex as B-cell populations. The analysis of the B-cell repertoire and specific B-cell responses has been aided by the isolation of antibodies and anti-idiotypic antibodies. However, clonotypic antibodies have proven difficult to obtain for T cells and, until recently, this has hampered the analysis of T-cell repertoires. Major progress has been achieved recently through the studies of T-cell receptor (TCR) genes and their transcripts. TCRs are heterodimers comprising either c+ or y6 chains, each encoded, like immunoglobulin(Ig), by rearranged V (D) J-gene segments and a C region. The approximate numbers of V and J regions in mice and humans are now known (Table 1). Compared with the (Y and y families, the p and S families are easier to analyse by polymerase chain reaction (PCR) techniques as a consequence of their smaller number of V and J segments. The following discussion therefore focuses on p rearrangements.