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For acute self-limiting infections a vaccine is successful if it elicits memory at least as good as the natural experience; however, for persistent and chronic infections such as HIV, hepatitis C virus (HCV), human papillomavirus (HPV), and human herpes viruses, this paradigm is not applicable. At best, during persistent virus infection the person must be able to maintain the integrity of the immune system in equilibrium with controlling replicating virus. New vaccine strategies are required that elicit both potent high-avidity CD8(+) T-cell effector/memory and central memory responses that can clear the nidus of initial virus-infected cells at mucosal surfaces to prevent mucosal transmission or significantly curtail development of disease. The objective of an HIV-1 T-cell vaccine is to generate functional CD8(+) effector memory cells at mucosal portals of virus entry to prevent viral transmission. In addition, long-lived CD8(+) and CD4(+) central memory cells circulating through secondary lymphoid organs and resident in bone marrow, respectively, are needed to provide a concerted second wave of defense that can contain virus at mucosal surfaces and prevent systemic dissemination. Further understanding of factors which can influence long-lived effector and central memory cell differentiation will significantly contribute to development of effective T-cell vaccines. In this review we will focus on discussing mechanisms involved in T-cell memory and provide promising new approaches toward expanding current vaccine strategies to enhance antiviral memory.

IL-15 is critical for natural killer (NK)-cell development and function and for memory CD8(+) T-cell homeostasis. The IL-15 receptor consists of IL-15Ralpha, IL-2Rbeta, and the common cytokine receptor gamma chain (gamma(c)). IL-15Ralpha is known to "trans-present" IL-15 to an IL-2Rbeta/gamma(c) heterodimeric receptor on responding cells to initiate signaling. To investigate the importance of the IL-15Ralpha cytoplasmic domain, we generated a chimeric receptor consisting of the extracellular domain of IL-15Ralpha and intracellular domain of IL-2Ralpha (IL-15Ralpha(ext)/IL-2Ralpha(int)) and examined its function in 32D cells, in knock-in (KI) mice, and in adoptive-transfer experiments. The chimeric protein exhibited decreased cell-surface expression, and KI mice exhibited diminished NK, NKT, and CD8(+) T-cell development and defects in T-cell functional responses. However, 32D cells expressing the chimeric receptor had less IL-15-induced proliferation than wild-type (WT) transfectants with similar levels of IL-15Ralpha expression, indicating a signaling role for the IL-15Ralpha cytoplasmic domain beyond its effect on expression, and demonstrating that the IL-2Ralpha and IL-15Ralpha cytoplasmic domains are functionally distinct. Interestingly, adoptive-transfer experiments indicated that the chimeric IL-15Ralpha(ext)/IL-2Ralpha(int) receptor still supports trans-presentation. These experiments collectively indicate that IL-15Ralpha can act in cis in addition to acting in trans to present IL-15 to responding cells.

Natural HIV transmission occurs through mucosa, but it is debated whether mucosal cytotoxic T lymphocytes (CTLs) can prevent or reduce dissemination from the initial mucosal site to the systemic circulation. Also, the role of CTL avidity in mucosal AIDS viral transmission is unknown. To address these questions, we used delay in acute-phase peak viremia after intrarectal challenge as an indicator of systemic dissemination. We found that a peptide-prime/poxviral boost vaccine inducing high levels of high-avidity mucosal CTLs can have an impact on dissemination of intrarectally administered pathogenic SHIV-ku2 in macaques and that such protection correlates better with mucosal than with systemic CTLs and particularly with levels of high-avidity mucosal CTLs.

DNA immunizations with glycoprotein 120 (gp120) of human immunodeficiency virus-1 (HIV-1) usually require boosting with protein or viral vaccines to achieve optimal efficacy. Here, we demonstrate for the first time that mice immunized with DNA encoding gp120 fused with proinflammatory chemoattractants of immature dendritic cells, such as beta-defensin 2, monocyte chemoattractant protein-3 (MCP-3/CCL7) or macrophage-derived chemokine (MDC/CCL22), elicited anti-gp120 antibodies with high titers of virus-neutralizing activity. The immunogenicity was further augmented with the use of chemokine fusion constructs with gp140, gp120 linked to the extracellular domain of gp41 via a 14-amino acid spacer peptide sequence. This construct elicited antibodies with more effective neutralizing activity than corresponding constructs expressing gp120. Responses were dependent on physical linkage with chemokine moiety, as no immunity was detected following immunization of mice with DNA encoding a free mixture of chemokine and gp120. Although the route of immunization was inoculation into skin, both systemic and mucosal CD8(+) cytolytic immune responses were elicited in mice immunized with DNA expressing MCP-3 or beta-defensin 2 fusion constructs. In contrast, no cytotoxic T lymphocyte activity (CTL) was detected in mice immunized with DNA encoding gp120 either alone or as fusion with MDC. Therefore, the potential for broad application of this approach lies in the induction of mucosal CTL and neutralizing antibodies to HIV-1 envelope, both key requirements for prevention of viral transmission and clearance of pathogenic HIV from mucosal reservoirs.