Alpha-synuclein levels in blood plasma decline with healthy aging.

There is unequivocal evidence that alpha-synuclein plays a pivotal pathophysiological role in neurodegenerative diseases, and in particular in synucleinopathies. These disorders present with a variable extent of cognitive impairment and alpha-synuclein is being explored as a biomarker in CSF, blood serum and plasma. Considering key events of aging that include proteostasis, alpha-synuclein may not only be useful as a marker for differential diagnosis but also for aging per se. To explore this hypothesis, we developed a highly specific ELISA to measure alpha-synuclein. In healthy males plasma alpha-synuclein levels correlated strongly with age, revealing much lower concentrations in older (avg. 58.1 years) compared to younger (avg. 27.6 years) individuals. This difference between the age groups was enhanced after acidification of the plasmas (p<0.0001), possibly reflecting a decrease of alpha-synuclein-antibody complexes or chaperone activity in older individuals. Our results support the concept that alpha-synuclein homeostasis may be impaired early on, possibly due to disturbance of the proteostasis network, a key component of healthy aging. Thus, alpha-synuclein may be a novel biomarker of aging, a factor that should be considered when analyzing its presence in biological specimens.

Data analysis as a source of variability of the HLA-peptide multimer assay: from manual gating to automated recognition of cell clusters.

Multiparameter flow cytometry is an indispensable method for assessing antigen-specific T cells in basic research and cancer immunotherapy. Proficiency panels have shown that cell sample processing, test protocols and data analysis may all contribute to the variability of the results obtained by laboratories performing ex vivo T cell immune monitoring. In particular, analysis currently relies on a manual, step-by-step strategy employing serial gating decisions based on visual inspection of one- or two-dimensional plots. It is therefore operator dependent and subjective. In the context of continuing efforts to support inter-laboratory T cell assay harmonization, the CIMT Immunoguiding Program organized its third proficiency panel dedicated to the detection of antigen-specific CD8(+) T cells by HLA-peptide multimer staining. We first assessed the contribution of manual data analysis to the variability of reported T cell frequencies within a group of laboratories staining and analyzing the same cell samples with their own reagents and protocols. The results show that data analysis is a source of variation in the multimer assay outcome. To evaluate whether an automated analysis approach can reduce variability of proficiency panel data, we used a hierarchical statistical mixture model to identify cell clusters. Challenges for automated analysis were the need to process non-standardized data sets from multiple centers, and the fact that the antigen-specific cell frequencies were very low in most samples. We show that this automated method can circumvent difficulties inherent to manual gating strategies and is broadly applicable for experiments performed with heterogeneous protocols and reagents.

Promiscuous survivin peptide induces robust CD4+ T-cell responses in the majority of vaccinated cancer patients.

CD4(+) T cells have been shown to be crucial for the induction and maintenance of cytotoxic T cell responses and to be also capable of mediating direct tumor rejection. Therefore, the anticancer therapeutic efficacy of peptide-based vaccines may be improved by addition of HLA class II epitopes to stimulate T helper cells. Survivin is an apoptosis inhibiting protein frequently overexpressed in tumors. Here we describe the first immunological evaluation of a survivin-derived CD4(+) T cell epitope in a multipeptide immunotherapy trial for prostate carcinoma patients. The survivin peptide is promiscuously presented by several human HLA-DRB1 molecules and, most importantly, is naturally processed by dendritic cells. In vaccinated patients, it was able to induce frequent, robust and multifunctional CD4(+) T cell responses, as monitored by IFN-γ ELISPOT and intracellular cytokine staining. Thus, this HLA-DR restricted epitope is broadly immunogenic and should be valuable for stimulating T helper cells in patients suffering from a wide range of tumors.

HLA antibody specification using single-antigen beads--a technical solution for the prozone effect.

BACKGROUND: Substantial progress in human leukocyte antigen antibody specification has been made by the introduction of Luminex single-antigen bead (SAB) assays. This progress was impaired when it turned out that this method is prone to a prozone effect leading to false-negative results in the case of high antibody titers. Testing serum and ethylenediaminetetraacetic acid (EDTA) plasma of one patient in parallel, we observed the prozone effect with the serum sample only. This led us to investigate complement component 1 (C1) as the cause of the prozone in SAB testing. We also found an easy way to overcome the prozone effect. METHODS: Sera with a prozone effect were tested in the SAB assay, applying different methods of serum pretreatment to explore the parameters leading to the prozone. RESULTS: The prozone was not observed when EDTA plasma or serum with EDTA added were tested. Further, addition of dithiothreitol, addition of C1 inhibitor, or heat inactivation of the sera abolished the prozone effect. Adding fresh nonimmune serum to heat-inactivated sera restored the prozone effect. Only beads showing a prozone were found to be covered with C1q. CONCLUSION: Our observations are consistent with the hypothesis that dissociation or destruction of complement C1 eliminates the prozone effect. Addition of EDTA to serum of highly immunized patients is the easiest way to avoid false-negative results in SAB testing caused by a prozone effect.

Analysis of tumor antigen-specific T cells and antibodies in cancer patients treated with radiofrequency ablation.

Radiofrequency (RF) ablation is a minimally invasive technique routinely applied for the treatment of primary and secondary liver tumors. It induces cell death by thermal coagulative necrosis of tumor tissues, whereas cellular metabolism can still take place in a transition zone surrounding the necrotic area. An increase in heat shock protein expression occurs shortly after treatment, suggesting that the induction of activating signals may stimulate the host immune system. In addition, various effects on immune effectors have also been observed, including stimulation of tumor-directed T lymphocytes. Here, we prospectively assessed the activation of tumor antigen-specific antibodies, as well as antigen-specific CD4(+) and CD8(+) T cells in patients suffering from primary or secondary malignancies and treated by RF ablation with or without concomitant chemotherapy. An increase of antibodies (in 4 patients of 49), CD4(+) T cells or CD8(+) T cells (in 2 patients of 49) could be detected several weeks to months following intervention. These findings suggest that in addition to the local control of tumor growth, RF ablation can provide the appropriate conditions for activating tumor-antigen specific immune responses.

Involvement of CD91 and scavenger receptors in Hsp70-facilitated activation of human antigen-specific CD4+ memory T cells.

Hsp70 plays several roles in the adaptive immune response. Based on the ability to interact with diverse peptides, extracellular Hsp70:peptide complexes exert profound effects both in autoimmunity and in tumor rejection by evoking potent T cell responses to the chaperoned peptide. The interaction with receptors on APC represents the basis for the immunological functions of Hsp70 and a critical point where the immune response can be regulated. Various surface proteins (e.g. CD91, scavenger receptors (SR)) have been implicated in binding of Hsp70. In this study, antigenic peptides from tetanus toxin and influenza hemagglutinin complexed to human stress-inducible Hsp70 were found to enhance the proliferation and cytokine production of human antigen-specific CD4(+) T cells. This was demonstrated in proliferation experiments using human monocytes as APC. Proliferated antigen-specific cells were detected combining HLA-DRB1*0401 or HLA-DRB1*1101 tetramer and CFSE staining. Treating monocytes with CD91 siRNA diminished these effects. Additional blocking of SR by the SR ligand fucoidan completely abolished enhanced proliferation and production of Th1 and Th2 cytokines. Taken together, our data indicate that in the human system, CD91 and members of the SR family efficiently direct Hsp70:peptide complexes into the MHC class II presentation pathway and thus enhance antigen-specific CD4(+) T cell responses.

Ex vivo detection of adenovirus specific CD4+ T-cell responses to HLA-DR-epitopes of the Hexon protein show a contracted specificity of T(HELPER) cells following stem cell transplantation.

Human adenovirus (HAdV) is a cause of significant morbidity and mortality in immunocompromised patients, especially after stem cell transplantation (SCT). Viral clearance has been attributed to CD4(+) T-cell responses against the Hexon-protein, but the frequency of specific T(HELPER) cells is extremely low or not detectable ex vivo and preference for different CD4(+) T-cell epitopes is variable among individuals. We therefore analyzed 44 healthy donors and 6 SCT-recipients for Hexon-specific CD4(+)-responses ex vivo, to identify epitopes which would be broadly applicable. We selected 19 candidate epitopes with predicted restriction to HLA-DR1/DR3/DR4/DR7; 16 were located within the highly conserved regions, indicating cross-reactivity of T cells among HAdV-subspecies. Ten epitopes induced CD4(+)-proliferation in >50% of individuals, confirmed by intracellular IFN-gamma detection. Three SCT recipients who recovered from an infection with HAdV displayed reactivity towards only a single hexon epitope, whereas healthy individuals were responsive to two to eight epitopes (median 3). The ex vivo detection of Hexon-specific CD4(+) T-cells, without any long-term culture in vitro, enables the detection and generation of HAdV-specific CD4(+) T cells for adoptive T-cell transfer against HAdV-infection post SCT.

[AB0-incompatible kidney transplantation--case 13/2009]. / AB0-inkompatible Nierentransplantation - Fall 13/2009.

UNLABELLED: PATIENT HISTORY AND CLINICAL FINDINGS: A 49-year old patient with chronic kidney disease was referred for evaluation of living donor kidney transplantation from his spouse in a blood group incompatible setting. CLINICAL INVESTIGATIONS: Patient blood group was 0, donor blood group was A, subtype A. recipient isoagglutinin titer against donor erythrocytes was 1:64. Crossmatch (CDC) and antibody screening were negative. There were no contraindications for living donor kidney transplantation from donor and recipient side. DIAGNOSIS, TREATMENT AND CLINICAL COURSE: For AB0-incompatible living donor kidney transplantation, recipient preconditioning using rituximab and selective immunoadsorption was initiated to obtain reduction of isoagglutinin titers < or = 1:4. Transplantation was performed without complications. With only moderate increases in titer, no immunoadsorption was required postoperatively. Monitoring of isoagglutinin titers was discontinued after day 14, since increasing titers do not result in organ loss any more (accomodation). 18 months after transplantation, renal function is excellent under corticosteroid-free maintenance immunosuppression. CONCLUSION: Recipient preconditioning nowadays allows successful blood group incompatible kidney transplantation in most cases, increasing the number of patients eligible for living donor kidney transplantation by up to 20%. No intensified maintenance immunosuppression is required and renal allograft function after AB0-incompatible transplantation is comparable to blood group compatible living donor kidney transplantation.

Simultaneous infiltration of polyfunctional effector and suppressor T cells into renal cell carcinomas.

Renal cell carcinoma is frequently infiltrated by cells of the immune system. This makes it important to understand interactions between cancer cells and immune cells so they can be manipulated to bring clinical benefit. Here, we analyze subsets and functions of T lymphocytes infiltrating renal cell tumors directly ex vivo following mechanical disaggregation and without any culture step. Subpopulations of memory and effector CD4(+) Th1, Th2, and Th17 and CD8(+) Tc1 cells were identified based on surface phenotype, activation potential, and multicytokine production. Compared with the same patient's peripheral blood, T lymphocytes present inside tumors were found to be enriched in functional CD4(+) cells of the Th1 lineage and in effector memory CD8(+) cells. Additionally, several populations of CD4(+) and CD8(+) regulatory T cells were identified that may synergize to locally dampen antitumor T-cell responses.

Novel multi-peptide vaccination in Hla-A2+ hormone sensitive patients with biochemical relapse of prostate cancer.

BACKGROUND: A phase I/II trial was conducted to assess feasibility and tolerability of tumor associated antigen peptide vaccination in hormone sensitive prostate carcinoma (PC) patients with biochemical recurrence after primary surgical treatment. METHODS: Nineteen HLA-A2 positive patients with rising PSA without detectable metastatic disease or local recurrence received 11 HLA-A*0201-restricted and two HLA class II synthetic peptides derived from PC tumor antigens subcutaneously for 18 months or until PSA progression. The vaccine was emulgated in montanide ISA51 and combined with imiquimod, GM-CSF, mucin-1-mRNA/protamine complex, local hyperthermia or no adjuvant. PSA was assessed, geometric mean doubling times (DT) calculated and clinical performance monitored. RESULTS: PSA DT of 4 out of 19 patients (21%) increased from 4.9 to 25.8 months during vaccination. Out of these, two patients (11%) exhibited PSA stability for 28 and 31 months which were still continuing at data cut-off. One patient showed no change of PSA DT during vaccination but decline after the therapy. Three patients had an interim PSA decline or DT increase followed by DT decrease compared to baseline PSA DT. Three of the responding patients received imiquimod and one the mucin-1-mRNA/protamine complex as adjuvant; both are Toll-like receptor-7 agonists. Eleven (58%) patients had progressive PSA values. The vaccine was well tolerated, and no grade III or IV toxicity occurred. CONCLUSION: Multi-peptide vaccination stabilized or slowed down PSA progress in four of 19 cases. The vaccination approach is promising with moderate adverse events. Long-term stability delayed androgen deprivation up to 31 months. TLR-7 co-activation seems to be beneficial.

HLA ligand profiles of primary renal cell carcinoma maintained in metastases.

In recent years, several approaches have been taken in the peptide-based immunotherapy of metastatic renal cell carcinoma (RCC), although little is known about HLA presentation on metastases compared to primary tumor and normal tissue of RCC. In this study we compared primary tumor, normal tissue and metastases with the aim of identifying similarities and differences between these tissues. We performed this comparison for two RCC patients on the level of the HLA ligandome using mass spectrometry and for three patients on the level of the transcriptome using oligonucleotide microarrays. The quantitative results show that primary tumor is more similar to metastasis than to normal tissue, both on the level of HLA ligand presentation and mRNA. We were able to characterize a total of 142 peptides in the qualitative analysis of HLA-presented peptides. Six of them were significantly overpresented on metastasis, among them a peptide derived from CD151; fourteen were overpresented on both primary tumor and metastasis compared to normal tissue, among them an HLA ligand derived from tumor protein p53. Thus, we could demonstrate that peptide-based immunotherapy might affect tumor as well as metastasis of RCC, but not healthy kidney tissue. Furthermore we were able to identify several peptides derived from tumor-associated antigens that are suitable for vaccination of metastatic RCC.

Essential differences in ligand presentation and T cell epitope recognition among HLA molecules of the HLA-B44 supertype.

Human leukocyte antigens (HLA) have long been grouped into supertypes to facilitate peptide-based immunotherapy. Analysis of several hundreds of peptides presented by all nine antigens of the HLA-B44 supertype (HLA-B*18, B*37, B*40, B*41, B*44, B*45, B*47, B*49 and B*50) revealed unique peptide motifs for each of them. Taking all supertype members into consideration only 25 out of 670 natural ligands were found on more than one HLA molecule. Further direct comparisons by two mass spectrometric methods--isotope labeling as well as a label-free approach--consistently demonstrated only minute overlaps of below 3% between the ligandomes of different HLA antigens. In addition, T cell reactions of healthy donors against immunodominant HLA-B*44 and HLA-B*40 epitopes from EBV lacked promiscuous T-cell recognition within the HLA-B44 supertype. Taken together, these results challenge the common paradigm of broadly presented epitopes within this supertype.

Identification of HLA-A*01- and HLA-A*02-restricted CD8+ T-cell epitopes shared among group B enteroviruses.

Acute enteroviral infections ranging from meningitis, pancreatitis to myocarditis are common and normally well controlled by the host immune system comprising virus-specific CD8+ cytotoxic T lymphocytes (CTL). However, in some patients enteroviruses and especially coxsackieviruses of group B are capable of inducing severe chronic forms of diseases such as chronic myocarditis. Currently, it is not known whether divergences in the CTL-related immune response may contribute to the different outcome and course of enterovirus myocarditis. A pre-requisite for the study of CTL reactions in patients with acute and chronic myocarditis is the identification of CTL epitopes. In order to define dominant enterovirus CTL epitopes, we have screened, by using gamma interferon (IFN-gamma) ELISPOT, 62 HLA-A*01- and 59 HLA-A*02-positive healthy blood donors for pre-existing CTL reactions against 12 HLA-A*01 and 20 HLA-A*02 predicted CTL epitopes derived from coxsackieviruses of group B. Positive CTL reactions were verified by FACS analysis in a combined major histocompatibility complex-tetramer IFN-gamma staining. A total of 14.8% of all donors reacted against one of the three identified epitopes MLDGHLIAFDY, YGDDVIASY or GIIYIIYKL. The HLA-A*02-restricted epitope ILMNDQEVGV was recognized by 25% of all tested blood donors. For this peptide, we could demonstrate specific granzyme B secretion, a strong cytolytic potential and endogenous processing. All four epitopes were homologous in 36-92% of group B enteroviruses, providing a strong basis for monitoring the divergence of T-cell-based immune responses in enterovirus-induced acute and chronic diseases.

A cryptic vascular endothelial growth factor T-cell epitope: identification and characterization by mass spectrometry and T-cell assays.

Vascular endothelial growth factor (VEGF) is involved in various physiologic processes, such as angiogenesis or wound healing, but is also crucial in pathologic events, such as tumor growth. Thus, clinical anti-VEGF treatments have been developed that could already show beneficial effects for cancer patients. In this article, we describe the first VEGF-derived CD8(+) T-cell epitope. The natural HLA ligand SRFGGAVVR was identified by differential mass spectrometry in two primary renal cell carcinomas (RCC) and was significantly overpresented on both tumor tissues. SRFGGAVVR is derived from a cryptic translated region of VEGF presumably by initiation of translation at the nonclassic start codon CUG(499). SRFGGAVVR-specific T cells were generated in vitro using peptide-loaded dendritic cells or artificial antigen-presenting cells. SRFGGAVVR-specific CD8(+) T cells, identified by HLA tetramer analysis after in vitro stimulation, were fully functional T effector cells, which were able to secrete IFN-gamma on stimulation and killed tumor cells in vitro. Additionally, we have quantitatively analyzed VEGF mRNA and protein levels in RCC tumor and normal tissue samples by gene chip analysis, quantitative reverse transcription-PCR, in situ hybridization, and bead-based immunoassay. In the future, T cells directed against VEGF as a tumor-associated antigen may represent a possible way of combining peptide-based anti-VEGF immunotherapy with already existent anti-VEGF cancer therapies.

Potent costimulation of human CD8 T cells by anti-4-1BB and anti-CD28 on synthetic artificial antigen presenting cells.

The in vitro generation of cytotoxic T lymphocytes (CTLs) for anticancer immunotherapy is a promising approach to take patient-specific therapy from the bench to the bedside. Two criteria must be met by protocols for the expansion of CTLs: high yield of functional cells and suitability for good manufacturing practice (GMP). The antigen presenting cells (APCs) used to expand the CTLs are the key to achieving both targets but they pose a challenge: Unspecific stimulation is not feasible because only memory T cells are expanded and not rare naïve CTL precursors; in addition, antigen-specific stimulation by cell-based APCs is cumbersome and problematic in a clinical setting. However, synthetic artificial APCs which can be loaded reproducibly with MHC-peptide monomers and antibodies specific for costimulatory molecules could resolve these problems. The purpose of this study was to investigate the potential of complex synthetic artificial APCs in triggering the costimulatory molecules CD28 and 4-1BB on the T cell. Anti-4-1BB antibodies were added to an established system of microbeads coated with MHC-peptide monomers and anti-CD28. Triggering via CD28 and 4-1BB resulted in strong costimulatory synergy. The quantitative ratio between these signals determined the outcome of the stimulation with optimal results when anti-4-1BB and anti-CD28 were applied in a 3:1 ratio. Functional CTLs of an effector memory subtype (CD45RA(-) CCR7(-)) were generated in high numbers. We present a highly defined APC platform using off-the-shelf reagents for the convenient generation of large numbers of antigen-specific CTLs.

Is there a predisposition for the development of autoimmune diseases in patients with fibromyalgia? Retrospective analysis with long term follow-up.

The objectives of the study were to evaluate the prevalence of antinuclear antibodies (ANA) in patients with fibromyalgia (FM) and the probability of the development of clinically overt connective tissue diseases. Four hundred and fifty FM patients were compared to 129 healthy matched blood donors. ANA testing was performed by immunofluorescence on rat tissue sections; in case of highly positive results, ANA were specified further by ELISA and immunodiffusion. All ANA positive FM patients were invited for a control examination. The ANA negative patients received a questionnaire, which was designed to identify those patients with possible connective tissue diseases (CTD). There was no significant difference in the frequency of ANA or thyroid antibodies between patients and controls (11.6% vs. 7%). Two patients had developed SLE: one was already ANA/anti-dsDNA positive at time of first diagnosis of FM; in the other, specific antibodies and SLE-related symptoms developed after 4.5 years. The probability for FM patients to develop CTD (SLE) within one year is 0.0027%, which is comparable to the incidence of SLE in the general population (0.005%). The risk of CTD is not increased in FM. The detection of ANA does not predict the development of CTD. However, in individual cases, FM may be an early sign of an autoimmune disease.

HAGE, a cancer/testis antigen with potential for melanoma immunotherapy: identification of several MHC class I/II HAGE-derived immunogenic peptides.

There remains a need to identify novel epitopes of potential tumour target antigens for use in immunotherapy of cancer. Here, several melanoma tissues and cell lines but not normal tissues were found to overexpress the cancer-testis antigen HAGE at the mRNA and protein level. We identified a HAGE-derived 15-mer peptide containing a shorter predicted MHC class I-binding sequence within a class II-binding sequence. However, only the longer peptide was found to be both endogenously processed and immunogenic for T cells in transgenic mice in vivo, as well as for human T cells in vitro. A different class I-binding peptide, not contained within a longer class II sequence, was subsequently found to be both immunogenic and endogenously processed in transgenic mice, as was a second class II epitope. These novel HAGE-derived epitopes may contribute to the range of immunotherapeutic targets for use in cancer vaccination programs.

Distorted relation between mRNA copy number and corresponding major histocompatibility complex ligand density on the cell surface.

The major histocompatibility complex (MHC) presents peptides derived from degraded cellular proteins to T-cells and is thus crucial for triggering specific immune responses against viral infections or cancer. Up to now, there has been no evidence for a correlation between levels of mRNA (the "transcriptome") and the density of MHC-peptide complexes (the "MHC ligandome") on cells. Because such dependences are of intrinsic importance for the detailed understanding of translation efficiency and protein turnover and thus for systems biology in general and for tumor immunotherapy in practical application, we quantitatively analyzed the levels of mRNA and corresponding MHC ligand densities in samples of renal cell carcinomas and their autologous normal kidney tissues. Relative quantification was carried out by gene chip analysis and by stable isotope peptide labeling, respectively. In comparing more than 270 pairs of gene expression and corresponding peptide presentation ratios, we demonstrate that there is no clear correlation (r = 0.32) between mRNA levels and corresponding MHC peptide levels in renal cell carcinoma. A significant number of peptides presented predominantly on tumor or normal tissue showed no or only minor changes in mRNA expression levels. In several cases, peptides could even be identified despite the virtual absence of the respective mRNA. Thus we conclude that a majority of epitopes from tumor-associated antigens will not be found in approaches based mainly on mRNA expression studies as mRNA expression reflects a distorted picture of the situation on the cell surface as visible for T-cells.

Unexpected abundance of HLA class II presented peptides in primary renal cell carcinomas.

PURPOSE: To elicit a long-lasting antitumor immune response, CD8+ and CD4+ T cells should be activated. We attempted to isolate HLA-DR-presented peptides directly from dissected solid tumors, in particular from renal cell carcinoma, to identify MHC class II ligands from tumor-associated antigens (TAA) for their use in peptide-based immunotherapy. EXPERIMENTAL DESIGN: Tumor specimens were analyzed by immunohistochemical staining for their HLA class II expression. HLA class II peptides were subsequently isolated and identified by mass spectrometry. Gene expression analysis was done to detect genes overexpressed in tumor tissue. Peptides from identified TAAs were used to induce peptide-specific CD4+ T-cell responses in healthy donors and in tumor patients. RESULTS: In the absence of inflammation, expression of MHC class II molecules is mainly restricted to cells of the immune system. To our surprise, we were able to isolate and characterize hundreds of class II peptides directly from primary dissected solid tumors, especially from renal cell carcinomas, and from colorectal carcinomas and transitional cell carcinomas. Infiltrating leukocytes expressed MHC class II molecules and tumor cells, very likely under the influence of IFNgamma. Our list of identified peptides contains ligands from several TAAs, including insulin-like growth factor binding protein 3 and matrix metalloproteinase 7. The latter bound promiscuously to HLA-DR molecules and were able to elicit CD4+ T-cell responses. CONCLUSIONS: Thus, our direct approach will rapidly expand the limited number of T-helper epitopes from TAAs for their use in clinical vaccination protocols.

The heat shock protein Hsp70 enhances antigen-specific proliferation of human CD4+ memory T cells.

Heat shock proteins (HSP) can interact with a wide variety of peptides and the resulting HSP:peptide complexes are known to be highly immunogenic. The ability of HSP:peptide complexes to elicit CD8+ T cell responses by cross-presentation of exogenous antigen via MHC class I is well known. In contrast, their role in the activation of CD4+ T cells is less clearly defined, although several recent studies in mice and T cell lines suggest an involvement of HSP in the presentation of antigenic peptides via MHC class II. In this study we have investigated the potential of antigenic peptides from tetanus toxin and influenza hemagglutinin complexed to the human stress-inducible Hsp70 to enhance activation and proliferation of human memory CD4+ T cells. Hsp70:peptide complexes were found to amplify the proliferation of antigen-specific CD4+ T cells as confirmed by HLA-DR tetramer staining. Complex formation of the antigenic peptide with Hsp70 was absolutely required to elicit an antigen-specific amplification. This effect was most pronounced at low doses of antigen and decreasing APC/CD4+ T cell ratios. Taken together, we show the potential of Hsp70 to enhance antigen-specific CD4+ T cell proliferation and to increase the immunogenicity of presented peptides in human CD4+ T cells.