The memory cytotoxic T-lymphocyte (CTL) response to human cytomegalovirus infection contains individual peptide-specific CTL clones that have undergone extensive expansion in vivo

Human cytomegalovirus (HCMV)-specific CD8(+) cytotoxic T lymphocytes (CTL) appear to play an important role in the control of virus replication and in protection against HCMV-related disease. We have previously reported high frequencies of memory CTL precursors (CTLp) specific to the HCMV tegument p rotein pp65 in the peripheral blood of healthy virus carriers. In some indi viduals, the CTL response to this protein is focused on only a single epito pe, whereas in other virus carriers CTL recognized multiple epitopes which we identified by using synthetic peptides. We have analyzed the clonal comp osition of the memory CTL response to four of these pp65 epitopes by sequen cing the T-cell receptors (TCR) of multiple independently derived epitope-s pecific CTL clones, which were derived by formal single-cell cloning or fro m clonal CTL microcultures. In all cases, we have observed a high degree of clonal focusing: the majority of CTL clones specific to a defined pp65 pep tide from any one virus carrier use only one or two different TCRs at the l evel of the nucleotide sequence. Among virus carriers who have the same maj or histocompatibility complex (MHC) class I allele, we observed that CTL fr om different donors that recognize the same peptide-MHC complex often used the same V beta segment, although other TCR gene segments and CDR3 length w ere not in general conserved. We have also examined the clonal composition of CTL specific to pp65 peptides in asymptomatic human immunodeficiency vir us-infected individuals. We have observed a similarly focused peptide-speci fic CTL response. Thus, the large population of circulating HCMV peptide sp ecific memory CTLp in virus carriers in fact contains individual CTL clones that have undergone extensive clonal expansion in vivo.