Human CD28(-)CD8(+) T cells contain greatly expanded functional virus-specific memory CTL clones

At birth, almost all human peripheral blood CD8(+) T cells express the cost imulatory molecule CD28, With increasing age, the proportion of CD8(+) T ce lls that lack CD28 increases. Because the Ag specificity of CD28(-)CD8(+) T cells has not previously been defined, we studied the contribution of CD28 (-)CD8(+) T cells to the memory CD8(+) CTL response against two human persi stent viruses, human CMV (HCMV) and HIV. From PBMC of healthy virus carrier s we, generated multiple independent CTL clones specific for defined viral peptides and sequenced their TCR beta-chains, We designed clonotypic oligon ucleotides complementary to each P-chain hypervariable sequence and quantif ied the size of individual immunodominant CTL clones in PBMC. Some individu al CTL clones were very large, comprising up to 3.1% of all CD8(+) T cells in PBMC, and were generally maintained at a stable level for months. Indivi dual virus-specific CTL clones were consistently more abundant in purified CD28(-) cells than in the CD8(+) population as a whole. Because CD28(-)CD8( +) cells as a population have been reported to proliferate poorly in respon se to mitogen, we studied the function of these virus-specific CD28(-) CTL clones by quantifying the frequency of peptide-specific CTL precursors usin g limiting dilution analysis. CD28(-)CD8(+) T cells contained high frequenc ies of functional memory CTL precursors specific for peptides of HCMV or HI V, generally higher than in the CD8(+) T cell population as a whole. We con clude that in asymptomatic HCMV and HIV infection, human CD28(-)CD8(+) T ce lls contain high frequencies of functional virus-specific memory CTL clones .