Separate roles for antigen recognition and lymph node inflammation in CD8+ memory T cell formation.

Priming of naive CD8(+) T cells by pathogens or vaccines generally involves their interaction with Ag-loaded dendritic cells (DCs) in the context of an inflamed lymph node. Lymph node activation fosters DC and T cell encounters and subsequently provides newly primed T cells with nurturing conditions. We dissected these two aspects by infusing in vitro primed CD8(+) T cells into naive recipient mice harboring a single activated lymph node and comparing the fate of these T cells with those infused into control recipients. Brief (20 h) in vitro priming empowered the T cells to expand in vivo without further Ag stimulation. This primary response was not affected by the presence or absence of a nonspecifically activated lymph node. In contrast, in vivo antigenic challenge after contraction of the primary response resulted in significantly stronger secondary T cell responses in mice harboring activated lymph nodes, demonstrating that the availability of an activated lymph node supported the generation of T cell memory in an Ag-unrelated manner. The presence of an activated lymph node during the expansion and contraction phase of the primary response did not endow T cells with an instructional program for increased survival or secondary expansion, but primarily served to conserve increased numbers of T cells.

Immunization with mannosylated peptide induces poor T cell effector functions despite enhanced antigen presentation.

In this study, we investigated the development of T cell responses in mice after administration of a mannosylated ovalbumin peptide (M-OVA(323-339)). Immunization with M-OVA(323-339) in complete adjuvant resulted in enhanced antigen presentation in draining lymph nodes. Monitoring the fate of CFSE-labeled ovalbumin peptide-specific TCR transgenic CD4(+) T cells revealed that immunization with M-OVA(323-339) induced normal clonal expansion, recirculation and CD62L expression of antigen-specific T cells in vivo. However, these T cells developed only poor effector functions, reflected by minimal IFN-gamma production, low IgG2a levels in serum and poor peptide-specific delayed-type hypersensitivity (DTH) responses. This diminished inflammatory response was associated with decreased infiltration of T cell blasts and macrophages. Importantly, also mice with functional effector T cells did not mount a robust DTH response after a challenge with M-OVA(323-339) in the ear, although their T cells responded normally to M-OVA(323-339) in vitro. In conclusion, mannosylated peptide induces proliferation of T cells with impaired T(h)1 cell effector functions and additionally abrogates the activity of pre-existing effector T cells.

Peptide vaccination after T-cell transfer causes massive clonal expansion, tumor eradication, and manageable cytokine storm.

Adoptive T-cell transfer (ACT) is successfully applied as a cancer treatment that is based on the activation and effector functions of tumor-specific T cells. Here, we present results from a mouse model in which ACT is combined with a long peptide-based vaccine comprising gp100 T-cell epitopes. Transferred CD8(+) T cells expanded up to 1,000-fold after peptide vaccination, leading to a 3-fold increase in white blood cell count and a very high frequency in the generation of antigen-specific memory T cells, the generation of which tended to correlate with effective antitumor responses. An enormous pool of effector T cells spread widely to different tissues, including the skin and the immune-privileged eye, where they mediate tumor eradication. Importantly, these striking T-cell dynamics occurred in immunocompetent mice without prior hematologic conditioning. Continued activation of the specific T-cell pool by vaccination led to strong T-cell-mediated cytokine storm and lethality due to multi-organ failure. However, this immunopathology could be prevented by controlling the rapid biodistribution of the peptide or by using a weakly agonistic peptide. Together, these results identify a peptide vaccination strategy that can potently accentuate effective ACT in non-lymphodepleted hosts.

Design of agonistic altered peptides for the robust induction of CTL directed towards H-2Db in complex with the melanoma-associated epitope gp100.

Immunogenicity of tumor-associated antigens (TAA) is often weak because many TAA are autoantigens for which the T-cell repertoire is sculpted by tolerance mechanisms. Substitutions at main anchor positions to increase the complementarity between the peptide and the MHC class I (MHC-I) binding cleft constitute a common procedure to improve binding capacity and immunogenicity of TAA. However, such alterations are tailored for each MHC-I allele and may recruit different CTL specificities through conformational changes in the targeted peptides. Comparative analysis of substituted melanoma-differentiation antigen gp100 in complex with H-2D(b) revealed that combined introduction of glycine and proline residues at the nonanchor positions 2 and 3, respectively, resulted in an agonistic altered peptide with dramatically enhanced binding affinity, stability, and immunogenicity of this TAA. Peptide vaccination using the p2Gp3P-altered peptide version of gp100 induced high frequencies of melanoma-specific CTL in the endogenous CD8+ repertoire. Crystal structure analysis of MHC/peptide complexes revealed that the conformation of the modified p2Gp3P-peptide was similar to the wild-type peptide, and indicated that this mimotope was stabilized through interactions between peptide residue p3P and the tyrosine residue Y159 that is conserved among most known MHC-I molecules throughout mammalian species. Our results may provide an alternative approach to enhance MHC stabilization capacity and immunogenicity of low-affinity peptides for induction of robust tumor-specific CTL.

Development of CTL memory despite arrested clonal expansion.

Activation of a cytotoxic T lymphocyte (CTL) response in an antigen-exposed lymph node involves a great diversity of encounters between naive CTL and APC that differ in both duration and quality. This broad spectrum of priming events instigates a complex blend of CTL developmental pathways. Using an experimental system that allows tight control over CTL priming, we have singled out defined priming events and analyzed the impact of the resulting instructional program on the effector and memory phases of the CTL response. As expected, prolonged antigenic stimulation induces potent CTL expansion, effector function and CTL memory. In contrast, CTL that have received suboptimal stimulation fail to undergo extensive expansion. Nevertheless these arrested CTL persist long term and acquire memory function. Thus, our data demonstrate that CTL memory can develop as a result of a suboptimal stimulation that causes arrested clonal expansion.

CD40L blockade prevents autoimmune diabetes by induction of bitypic NK/DC regulatory cells.

Systemic treatment with antibody to CD40 ligand (aCD40L) can prevent autoimmunity and transplant rejection in several animal models and is currently under evaluation in clinical trials. While it is known that aCD40L administration inhibits expansion and effector functions of aggressive T cells, it is still unclear whether additional regulatory mechanisms are operative. Here we demonstrate that a single episode of CD40L blockade during development of the autoaggressive immune response completely prevented autoimmune disease in the RIP-LCMV mouse model for virally induced type 1 diabetes. Interestingly, protection could be transferred by a highly potent, bitypic cell population sharing phenotypic and functional properties of both natural killer (NK) and dendritic cells (DC). Furthermore, protection of prediabetic recipients was autoantigen specific and did not result in generalized immunosuppression. The origin, function, and therapeutic potential of these bitypic NK/DC regulatory cells is discussed.

Dynamic programming of CD8+ T lymphocyte responses.

The initial encounter with an antigen-presenting cell (APC) is the primary force behind the expansion, differentiation and survival of naive T cells. Using an APC that permits temporal control of priming, we examined whether the duration of antigenic stimulation can influence the functional development of CD8+ cytotoxic T lymphocytes (CTLs) in vivo. Whereas CTLs given a 4-h stimulus underwent an abortive clonal expansion with transient surface CD25 expression, those given a 20-h stimulus sustained CD25 up-regulation, proliferated extensively, and efficiently mediated destruction of peripheral target tissues. Our results show that an instructional program preceding the first cell division integrates differences in signal strength into the decision to activate versus tolerize specific CTL clones.