Ellen I. H. van der Voort

T-cell help for cytotoxic T lymphocytes is mediated by CD40-CD40L interactions.

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.

Transient expression of FOXP3 in human activated nonregulatory CD4+ T cells

Foxp3 plays a key role in CD4+CD25+ Treg 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++ Treg cells. These data indicate that expression of endogenous FOXP3, in humans, is not sufficient to induce regulatory T cell activity or to identify Treg cells.

CD40 stimulation leads to effective therapy of CD40(-) tumors through induction of strong systemic cytotoxic T lymphocyte immunity.

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 cancer–host interactions can be directed toward competence, leading to the cure of established tumors merely by delivery of a CD40-dependent “license to kill” signal.

Activation of Dendritic Cells That Cross-Present Tumor-Derived Antigen Licenses CD8+ CTL to Cause Tumor Eradication

The fate of naive CD8+ T cells is determined by the environment in which they encounter MHC class I presented peptide Ags. The manner in which tumor Ags are presented is a longstanding matter of debate. Ag presentation might be mediated by tumor cells in tumor draining lymph nodes or via cross-presentation by professional APC. Either pathway is insufficient to elicit protective antitumor immunity. We now demonstrate using a syngeneic mouse tumor model, expressing an Ag derived from the early region 1A of human adenovirus type 5, that the inadequate nature of the antitumor CTL response is not due to direct Ag presentation by the tumor cells, but results from presentation of tumor-derived Ag by nonactivated CD11c+ APC. Although this event results in division of naive CTL in tumor draining lymph nodes, it does not establish a productive immune response. Treatment of tumor-bearing mice with dendritic cell-stimulating agonistic anti-CD40 mAb resulted in systemic efflux of CTL with robust effector function capable to eradicate established tumors. For efficacy of anti-CD40 treatment, CD40 ligation of host APC is required because adoptive transfer of CD40-proficient tumor-specific TCR transgenic CTL into CD40-deficient tumor-bearing mice did not lead to productive antitumor immunity after CD40 triggering in vivo. CpG and detoxified LPS (MPL) acted similarly as agonistic anti-CD40 mAb with respect to CD8+ CTL efflux and tumor eradication. Together these results indicate that dendritic cells, depending on their activation state, orchestrate the outcome of CTL-mediated immunity against tumors, leading either to an ineffective immune response or potent antitumor immunity.

In Vivo Triggering Through 4-1BB Enables Th-Independent Priming of CTL in the Presence of an Intact CD28 Costimulatory Pathway

Triggering of 4-1BB, a member of the TNFR family, through in vivo administration of agonistic anti-4-1BB Ab delivers a powerful costimulatory signal to CTL. We found this signal to effectively replace the need for CD4+ T cell help in the cross-priming of tumor-specific CTL immunity. Furthermore, 4-1BB Ab can convert an otherwise tolerogenic peptide vaccine into a formulation capable of efficient CTL priming. Initial activation of naive CTL can occur in the absence of 4-1BB costimulation, but this signal permits increased survival of Ag-stimulated CTL. Because naive CTL do not express 4-1BB at their surface, susceptibility to 4-1BB triggering depends on prior up-regulation of this receptor. We show that this requires both stimulation of the TCR and CD28-dependent costimulation. Accordingly, blockade of the CD28-costimulatory pathway abrogates the capacity of agonistic anti-4-1BB Ab to trigger Th-independent CTL immunity. In conclusion, our data reveal that the 4-1BB-mediated survival signal is positioned downstream of Ag-specific TCR triggering and CD28-dependent costimulation of naive CTL. The powerful effects of 4-1BB triggering on the induction, amplification, and persistence of CTL responses provide a novel strategy for increasing the potency of vaccines against cancers.

Immunomodulatory Dendritic Cells Inhibit Th1 Responses and Arthritis via Different Mechanisms

Dendritic cells (DCs) are professional APCs which have the unique ability to present both foreign and self-Ags to T cells and steer the outcome of immune responses. Because of these characteristics, DCs are attractive vehicles for the delivery of therapeutic vaccines. Fully matured DCs are relatively well-defined and even used in clinical trials in cancer. DCs also have the potential to influence the outcome of autoimmunity by modulating the underlying autoimmune response. To gain a better appreciation of the abilities and mechanisms by which immunomodulatory DCs influence the outcome of T cell responses, we studied several immunomodulatory DCs (TNF-, IL-10-, or dexamethasone-stimulated bone marrow-derived DCs) side by side for their ability to modulate T cell responses and autoimmune diseases. Our data show that these differentially modulated DCs display a different composition of molecules involved in T cell activation. Although, all DC subsets analyzed were able to inhibit the induction of collagen-induced arthritis, the modulation of the underlying immune response was different. Vaccination with TNF- or IL-10-modulated DCs altered the Th1/Th2 balance as evidenced by the induction of IL-5- and IL-10-secreting T cells and the concomitant reduction of the IgG2a-IgG1 ratio against the immunizing Ag. In contrast, DCs modulated with dexamethasone did not affect the ratio of IL-5-producing vs IFN-γ-producing T cells and tended to affect the Ab response in a nonspecific manner. These data indicate that distinct mechanisms can be used by distinct DC subsets to change the outcome of autoimmunity.

Identification of citrullinated vimentin peptides as T cell epitopes in HLA-DR4-positive patients with rheumatoid arthritis.

OBJECTIVE Antibodies directed against citrullinated proteins (ACPAs) are highly specific for rheumatoid arthritis (RA). The production of ACPAs is most likely dependent on the presence of T cells, since ACPAs undergo isotype switching and are associated with the shared epitope (SE)-containing HLA-DRB1 alleles. Vimentin is a likely candidate protein for T cell recognition, since >90% of patients positive for ACPAs that are reactive with (peptides derived from) citrullinated vimentin carry SE-containing HLA-DRB1 alleles. The aim of this study was to identify citrullinated vimentin peptides that are presented to HLA-DRB1*0401-restricted T cells. METHODS HLA-DR4-transgenic mice were immunized with all possible citrulline-containing peptides derived from vimentin, and T cell reactivity was analyzed. Peptides recognized in a citrulline-specific manner by T cells were selected and analyzed for their ability to be processed from the entire vimentin protein. A first inventory of the selected epitopes recognized by T cells was performed using peripheral blood mononuclear cells (PBMCs) from ACPA+, HLA-DR4+ patients with RA. RESULTS A citrulline-specific response was observed for 2 of the peptides analyzed in DR4-transgenic mice. These peptides were found to be naturally processed from the vimentin protein, since citrullinated vimentin was recognized by peptide-specific T cells. T cell reactivity against these peptides was also observed in cultures of PBMCs from RA patients. CONCLUSION This study identifies, for the first time, 2 naturally processed peptides from vimentin that are recognized by HLA-DRB1*0401-restricted T cells in a citrulline-specific manner. These peptides can be recognized by T cells in ACPA+, HLA-DR4+ patients with RA, as shown in a first inventory.

Longevity of Antigen Presentation and Activation Status of APC Are Decisive Factors in the Balance Between CTL Immunity Versus Tolerance

Encounter of Ag by naive T cells can lead to T cell priming as well as tolerance. The balance between immunity and tolerance is controlled by the conditions of Ag encounter and the activation status of the APC. We have investigated the rules that govern this balance in case an environment that normally induces tolerance is reverted into a milieu that promotes T cell priming, using a minimal CTL epitope derived from human adenovirus type 5 E1A. Vaccination of mice s.c. with E1A peptide in IFA readily induces CTL tolerance, resulting in the inability to control E1A-expressing tumors. The present study shows that efficient CTL priming is achieved when this peptide vaccine is combined with systemic administration of APC-activating compounds like agonistic anti-CD40 mAb or polyriboinosinate-polyribocytidylate. Surprisingly, this CTL response is not long-lasting and therefore fails to protect against tumor outgrowth. Disappearance of CTL reactivity was strongly associated with systemic persistence of the peptide for >200 days. In contrast, peptide administered in PBS does not persist and generates long term CTL immunity capable of rejecting Ad5E1A-positive tumors, when combined with CD40 triggering. Thus, presentation of CTL epitopes in an appropriate costimulatory setting by activated APC, although being essential and sufficient for CTL priming, eventually results in tolerance when the Ag persists systemically for prolonged times. These observations are important for the development of immune intervention schemes in autoimmunity and cancer.

The ACPA isotype profile reflects long-term radiographic progression in rheumatoid arthritis

Background The presence of anti-citrullinated protein antibodies (ACPA) is a powerful predictive factor for the development and progression of rheumatoid arthritis (RA). The ACPA response has been shown to consist of various isotypes, but the consequences of differences in isotype distribution have not been extensively investigated. Objective To investigate the relationship between ACPA isotypes, disease progression and radiological outcome. Methods ACPA isotypes were determined in sera of anti-cyclic citrullinated peptide 2-positive patients by enzyme-linked immunosorbent assay (ELISA). To investigate whether the ACPA response continues to evolve during disease development, the ACPA isotype profile during progression of undifferentiated arthritis (UA) to RA was studied. The association of disease progression with ACPA isotype use was assessed using long-term radiographic follow-up data from patients with RA in two independent cohorts. Results The ACPA isotype distribution did not expand during disease progression from UA to RA, but was relatively stable over time. In both RA cohorts, the baseline ACPA isotype profile was a significant predictor of disease severity, with more isotypes indicating a higher risk of radiographic damage (odds ratio for every additional isotype: 1.4 (95% CI 1.1 to 1.9) p<0.001). ACPA isotypes supplied additional prognostic information to ACPA status alone, even after correction for other predictive factors. Conclusions The magnitude of the ACPA isotype profile at baseline reflects the risk of future radiographic damage. These results indicate that the presence and the constitution of the ACPA response are relevant to the disease course of RA.

Dendritic cells, but not macrophages or B cells, activate major histocompatibility complex class II-restricted CD4 + T cells upon immune-complex uptake in vivo

Professional antigen‐presenting cells (APC) are able to process and present exogenous antigen leading to the activation of T cells. Antigen–immunoglobulin (Ig)G complexes (IC) are much more efficiently processed and presented than soluble antigen. Dendritic cells (DC) are known for their ability to take up and process immune complex (IC) via FcγR, and they have been shown to play a crucial role in IC‐processing onto major histocompatibility complex (MHC) class I as they contain a specialized cross‐presenting transport system required for MHC class I antigen‐processing. However, the MHC class II‐antigen‐processing pathway is distinct. Therefore various other professional APC, like macrophages and B cells, all displaying FcγR, are thought to present IC‐delivered antigen in MHC class II. Nonetheless, the relative contribution of these APC in IC‐facilitated antigen‐presentation for MHC class II in vivo is not known. Here we show that, in mice, both macrophages and DC, but not B cells, efficiently capture IC. However, only DC, but not macrophages, efficiently activate antigen‐specific MHC class II restricted CD4+ T cells. These results indicate that mainly DC and not other professional APC, despite expressing FcγR and MHC class II, contribute significantly to IC‐facilitated T cell activation in vivo under steady‐state conditions.