SllmmaryAntigen-specific T cell activation requires the engagement of the T cell receptor (TCR) with antigen as well as the engagement of appropriate costimulatory molecules. The most extensively characterized pathway of costimulation has been that involving the interaction of CD28 and CTLA4 on the T cell with B7 (now termed B7-1) on antigen presenting cells. Recently, B7-2 a second costimulatory ligand for CTLA4, was described, demonstrating the potential complexity of costimulatory interactions. This report examines and compares the expression and function of B7-1 and B7-2. Overall these results indicate that (a) B7-1 and B7-2 can be expressed by multiple cell types, including B cells, T cells, macrophages, and dendritic cells, all of which are therefore candidate populations for delivering costimulatory signals mediated by these molecules; (b) stimulating B cells with either LPS or anti-IgD-dextran induced expression of both B7-1 and B7-2, and peak expression of both costimulatory molecules occurred after 18-42 h of culture. Expression of B7-2 on these B cell populations was significantly higher than expression of B7-1 at all times assayed after stimulation; (c) blocking of B7-2 costimulatory activity inhibited TCgdependent T cell proliferation and cytokine production, without affecting early consequences of TCR signaling such as induction of CD69 or interleukin 2 receptor ot (IL-2Ro~); and (d) expression of B7-1 and of B7-2 can be regulated by a variety of stimuli. Moreover, expression of B7-1 and B7-2 can be independently regulated by the same stimulus, providing an additional complexity in the mechanisms available for regulating costimulation and hence immune response.

The present study has assessed the replicative history and the residual replicative potential of human naive and memory T cells. Telomeres are unique structures at the ends of chromosomes that consist of long hexameric repeats, (TTAGGG)n, and appear to be involved in chromosomal integrity and cellular proliferative capacity (7-9). Due to constraints imposed by the template requirement for semiconservative chromosomal DNA replication, each cycle of cell division and DNA replication results in a loss of 50-100 terminal nucleotides from each chromosome. The length of telomeres has been observed to decrease with cell division in vitro and with increased age in vivo for normal human somatic cells (10)(11)(12)(13)(14)(15). Thus, telomere length has been considered to be a record of a somatic cell's mitotic history and was used here to analyze the replicative history of human naive and memory CD4+ T cells from a panel of donors of diverse age. MATERIALS AND METHODSIsolation of Human CD4+ Naive and Memory T Cells from Peripheral Blood. Naive and memory CD4+ T cells were isolated with immunomagnetic beads as described (16). In brief, mononuclear cells were isolated from buffy-coat blood and incubated with a panel of mouse monoclonal antibodies (mAbs) against CD8 and antigens expressed on B cells, macrophages, natural killer cells, platelets, and erythrocytes. In addition, an anti-CD45 RO (UCHL-1) mAb was added to remove memory cells for isolation of naive T cells (CD4+ CD45

SummaryTelomerase, a ribonucleoprotein that is capable of synthesizing telomeric repeats,.is expressed in germline and malignant cells, and is absent in most normal human somatic cells. The selective expression of telomerase has thus been proposed to be a basis for the immortality of the germline and of malignant cells. In the present study, telomerase activity was analyzed in normal human T lymphocytes. It was found that telomerase is expressed at a high level in thymocyte subpopulations, at an intermediate level in tonsil T lymphocytes, and at a low to undetectable level in peripheral blood T lymphocytes. Moreover, telomerase activity is highly inducible in peripheral T lymphocytes by activation through CD3 with or without CD28 costimulation, or by stimulation with phorbol myristate acetate (PMA)/ionomycin. The induction of telomerase by anti-CD3 plus anti-CD28 (anti-CD3/CD28) stimulation required ILNA and protein synthesis, and was blocked by herbimycin A, an inhibitor of Src protein tyrosine kinases. The immunosuppressive drug cyclosporin A selectively inhibited telomerase induction by PMA/ionomycin and by anti-CD3, but not by anti-CD3/CD28. Although telomerase activity in peripheral T lymphocytes was activation dependent and correlated with cell proliferation, it was not cell cycle phase restricted. These results indicate that the expression oftelomerase in normal human T lymphocytes is both developmentally regulated and activation induced. Telomerase may thus play a permissive role in T cell development and in determining the capacity of lymphoid cells for cell division and clonal expansion.