Understanding the lineage differentiation of memory T cells is a central question in immunology. We investigated this issue by analysing the expression of the chemokine receptor CCR7, which defines distinct subsets of naive and memory T lymphocytes with different homing and effector capacities and antiviral immune responses to HIV and cytomegalovirus. Ex vivo analysis of the expression of CD45RA and CCR7 antigens, together with in vitro analysis of the cell-division capacity of different memory CD8+ T-cell populations, identified four subsets of HIV- and CMV-specific CD8+ T lymphocytes, and indicated the following lineage differentiation pattern: CD45RA+ CCR7+ --> CD45RA- CCR7+ --> CD45RA- CCR7- --> CD45RA+ CCR7-. Here we demonstrate through analysis of cell division (predominantly restricted to the CCR7+ CD8+ T-cell subsets) that the differentiation of antigen-specific CD8+ T cells is a two-step process characterized initially by a phase of proliferation largely restricted to the CCR7+ CD8+ cell subsets, followed by a phase of functional maturation encompassing the CCR7- CD8+ cell subsets. The distribution of these populations in HIV- and CMV-specific CD8+ T cells showed that the HIV-specific cell pool was predominantly (70%) composed of pre-terminally differentiated CD45RA- CCR7- cells, whereas the CMV-specific cell pool consisted mainly (50%) of the terminally differentiated CD45RA+ CCR7- cells. These results demonstrate a skewed maturation of HIV-specific memory CD8+ T cells during HIV infection.

The low frequency of self-peptide–specific T cells in the human preimmune repertoire has so far precluded their direct evaluation. Here, we report an unexpected high frequency of T cells specific for the self-antigen Melan-A/MART-1 in CD8 single–positive thymocytes from human histocompatibility leukocyte antigen-A2 healthy individuals, which is maintained in the peripheral blood of newborns and adults. Postthymic replicative history of Melan-A/MART-1–specific CD8 T cells was independently assessed by quantifying T cell receptor excision circles and telomere length ex vivo. We provide direct evidence that the large T cell pool specific for the self-antigen Melan-A/MART-1 is mostly generated by thymic output of a high number of precursors. This represents the only known naive self-peptide–specific T cell repertoire directly accessible in humans.

Effective adjuvants capable of inducing strong cytotoxic T cell responses in humans are lacking. In this study, we tested 4-1BBL as an adjuvant for activation of human memory antiviral CD8 T cell responses ex vivo. A recombinant replication-defective 4-1BBL adenovirus was used to convert autologous monocytes into efficient antigen-presenting cells after overnight incubation, bypassing the need to generate dendritic cells. Together with viral peptides, 4-1BBL led to robust memory responses of human EpsteinBarr virus-and influenza virus-specific cytotoxic T cells, with expansion of peptide-specific CD8 effector cells; up-regulation of Bcl-x L, granzyme A, and perforin; enhanced cytotoxic activity; and increased cytokine production. The response was significant even at a 100-fold lower peptide dose, compared with responses obtained with control adenovirus. Adenovirus-delivered B7.1 also expanded and activated virus-specific CD8 T cells, but 4-1BBL was more effective in driving the T cells toward a more fully differentiated CD27 ؊ effector state. Thus, 4-1BBL is a promising adjuvant for human memory CD8 T cells and will likely be most effective in the boost phase of a prime-boost strategy.E fficient activation of the cellular arm of the immune system requires a specific T cell antigen receptor signal delivered upon recognition of peptide͞MHC together with costimulatory signals. It has now been well established that dendritic cells are potent antigen-presenting cells (APC) for initiation of immune responses (1). Upon maturation, these cells up-regulate costimulatory molecules required for T cell activation. As a result, there is now considerable interest in the use of dendritic cells loaded with antigens as adjuvants for therapeutic vaccines (2, 3). A limitation of this approach is the need to derive the syngeneic dendritic cells in culture, a process that takes 7 days. In this report we describe the conversion of monocytes into efficient APC for activation of T cell memory responses by overnight incubation with recombinant adenoviruses expressing costimulatory molecules.Although the best characterized costimulatory molecule is CD28, recently other costimulatory molecules have been characterized (4-6). The emerging picture is that CD28 is important for the initial activation of an immune response and that other costimulatory ligand-receptor pairs act later to help sustain and diversify the response (4-6).4-1BB is an inducible costimulatory member of the tumor necrosis factor receptor family expressed on activated CD4 and CD8 T cells. Its ligand, 4-1BBL, is expressed on activated APC (6, 7). 4-1BB can enhance both the proliferation and the survival of murine CD4 and CD8 T cells (8)(9)(10)(11)(12)(13)(14). Recent evidence in mouse models suggests that 4-1BB͞4-1BBL interaction plays an important role in the memory CD8 T cell response to viruses (15-18).In humans, several studies have looked at the role of 4-1BBL or anti-4-1BB in polyclonal activation of T cells, but the specific role of 4-1BBL in activation of antigen-s...