Although in vivo priming of CD8+ cytotoxic T lymphocytes (CTLs) generally requires the participation of CD4+ T-helper lymphocytes, the nature of the 'help' provided to CTLs is unknown. One widely held view is that help for CTLs is mediated by cytokines produced by T-helper cells activated in proximity to the CTL precursor at the surface of an antigen-presenting cell (APC). An alternative theory is that, rather than being directly supplied to the CTL by the helper cell, help is delivered through activation of the APC, which can then prime the CTL directly. CD40 and its ligand, CD40L, may activate the APC to allow CTL priming. CD40L is expressed on the surface of activated CD4+ T-helper cells and is involved in their activation and in the development of their effector functions. Ligation of CD40 on the surface of APCs such as dendritic cells, macrophages and B cells greatly increases their antigen-presentation and co-stimulatory capacity. Here we report that signalling through CD40 can replace CD4+ T-helper cells in priming of helper-dependent CD8+ CTL responses. Blockade of CD40L inhibits CTL priming; this inhibition is overcome by signalling through CD40. CD40-CD40L interactions are therefore vital in the delivery of T-cell help for CTL priming.

Foxp3 plays a key role in CD4 + CD25 + T reg cell function in mice and represents a specific marker for these cells. Despite the strong association between FOXP3 expression and regulatory function in fresh human T cells, little is known about the dynamics of endogenous FOXP3 expression and its relation to the suppressive function in activated human T cells. Here, we addressed the dynamics of FOXP3 expression during human CD4 + T cell activation by plate-bound anti-CD3 Ab as well as the relationship between its expression and regulatory function at the single-cell level. Our data show that FOXP3 is expressed in a high percentage of activated T cells after in vitro stimulation of human CD4 + CD25 -cells. FOXP3 expression is strongly associated with hyporesponsiveness of activated T cells, but is not directly correlated with their suppressive capabilities, as we demonstrate that it is also expressed in activated nonsuppressive T cells. However, in this nonsuppressive T cell population, FOXP3 expression is transient, while it is stably expressed in activated T cells that do display suppressive function, and in natural CD4 + CD25 ++ T reg cells. These data indicate that expression of endogenous FOXP3, in humans, is not sufficient to induce regulatory T cell activity or to identify T reg cells.

Adequate spontaneous activation of tumor-specific T lymphocytes in tumor-bearing hosts is rare, despite the expression of tumor antigens that are potentially highly immunogenic. For example, failure of the immune system to raise competent responses against established tumors expressing the human adenovirus E1A-antigen allows this tumor to grow in immunocompetent mice. We show that systemic in vivo administration of agonistic anti-CD40 antibodies into tumor-bearing mice results in tumor eradication mediated by CD8 ؉ T cells. Treatment resulted in a strong expansion and systemic accumulation of E1A-specific CTL and depended on CD40 expression on host cells, as the tumor was CD40 ؊ , and therapy failed in CD40-deficient mice. Local intratumoral administration of anti-CD40 mAb is equally effective in licensing strong, systemic CTL immunity, resulting in the clearance of distant tumor nodules. Our data indicate that the immune response after cancerhost interactions can be directed toward competence, leading to the cure of established tumors merely by delivery of a CD40-dependent ''license to kill'' signal.M ost solid tumors express MHC class I molecules but lack costimulatory molecules essential for appropriate CTL activation (1, 2). Therefore, presentation of tumor-derived antigens by professional antigen-presenting cells (APCs) is most likely required for optimal tumor-specific T cell induction (3-6). Such activation of naïve T cells is called cross-priming and was first demonstrated by Bevan (7). As naïve T cells are thought to recirculate within the lymphoid system, cross-presentation provides the immune system with a means to detect and respond to antigens that are expressed only in the periphery.An important factor determining the outcome of immune responses is the level of antigen expressed in the periphery (8). In the case of relatively low levels of antigen, antigen is not presented at sufficient levels to activate naïve T cells. This situation is associated with ignorance of the antigen by the immune system. In the case of higher antigen-expression levels, antigen will be (cross-)presented in sufficient quantities to be detected by naïve T cells. In this case, antigen-recognition can either lead to tolerance or immunity (9, 10). The outcome of antigen recognition by naïve T cells, i.e., tolerance or immunity, is thought to be the consequence of the activation state of professional APCs that (cross-)present the antigen. This activation state is strongly influenced by inflammatory stimuli as well as the action of CD4 ϩ T helper (Th) cells.Studies on the requirement of CD4 ϩ Th cells in cross-priming of cytotoxic T lymphocytes (CTL) showed that both Th cells and CTLs must recognize antigens presented on the same APC (11,12). The interaction between Th cell and APC is sufficient to convert the APC to a state that allows priming of antigen-specific CTL (13, 14), which explains the observation that infusion of antigen-specific Th cells can rescue autoreactive CTL from deletion, resulting in CTL-mediated autoimmun...

The well defined, immature murine dendritic cell (DC) line D1 was used to study the role of DC maturation in CTL induction in vitro and in vivo. Maturation of D1 cells, characterized by markedly increased expression of MHC and costimulatory molecules, was induced by incubation with lipopolysaccharide, agonistic CD40 antibody, or specific CD4 ϩ T helper (Th) cells. Activated, but not immature, D1 cells efficiently primed alloreactive T cell responses in vitro. Similarly, priming of CTL immunity in vivo in CD4-depleted mice was only observed if these mice were immunized with activated D1 cells. This study provides formal evidence that activation of DCs, induced by Th-independent as well as Th-dependent stimuli, is essential for efficient induction of CTL responses.

The tumor suppressor protein p53 is overexpressed in close to 50% of all human malignancies. The p53 protein is therefore an attractive target for immunotherapy. Cytotoxic T lymphocytes (CTLs) recognizing a murine wild-type p53 peptide, presented by the major histocompatibility complex class I molecule H-2Kb, were generated by immunizing p53 gene deficient (p53 −/−) C57BL/6 mice with syngeneic p53-overexpressing tumor cells. Adoptive transfer of these CTLs into tumor-bearing p53 +/+ nude mice caused complete and permanent tumor eradication. Importantly, this occurred in the absence of any demonstrable damage to normal tissue. When transferred into p53 +/+ immunocompetent C57BL/6 mice, the CTLs persisted for weeks in the absence of immunopathology and were capable of preventing tumor outgrowth. Wild-type p53-specific CTLs can apparently discriminate between p53-overexpressing tumor cells and normal tissue, indicating that widely expressed autologous molecules such as p53 can serve as a target for CTL-mediated immunotherapy of tumors.