Noël E.C. Schartz

Current immunization protocols in cancer patients involve CTL-defined tumor peptides. Mature dendritic cells (DC) are the most potent APCs for the priming of naive CD8(+) T cells, eventually leading to tumor eradication. Because DC can secrete MHC class I-bearing exosomes, we addressed whether exosomes pulsed with synthetic peptides could subserve the DC function consisting in MHC class I-restricted, peptide-specific CTL priming in vitro and in vivo. The priming of CTL restricted by HLA-A2 molecules and specific for melanoma peptides was performed: 1) using in vitro stimulations of total blood lymphocytes with autologous DC pulsed with GMP-manufactured autologous exosomes in a series of normal volunteers; 2) in HLA-A2 transgenic mice (HHD2) using exosomes harboring functional HLA-A2/Mart1 peptide complexes. In this study, we show that: 1). DC release abundant MHC class I/peptide complexes transferred within exosomes to other naive DC for efficient CD8(+) T cell priming in vitro; 2). exosomes require nature's adjuvants (mature DC) to efficiently promote the differentiation of melanoma-specific effector T lymphocytes producing IFN-gamma (Tc1) effector lymphocytes in HLA-A2 transgenic mice (HHD2). These data imply that exosomes might be a transfer mechanism of functional MHC class I/peptide complexes to DC for efficient CTL activation in vivo.

Ideal vaccines should be stable, safe, molecularly defined, and out-of-shelf reagents efficient at triggering effector and memory Ag-specific T cell-based immune responses. Dendritic cell-derived exosomes could be considered as novel peptide-based vaccines because exosomes harbor a discrete set of proteins, bear functional MHC class I and II molecules that can be loaded with synthetic peptides of choice, and are stable reagents that were safely used in pioneering phase I studies. However, we showed in part I that exosomes are efficient to promote primary MHC class I-restricted effector CD8(+) T cell responses only when transferred onto mature DC in vivo. In this work, we bring evidence that among the clinically available reagents, Toll-like receptor 3 and 9 ligands are elective adjuvants capable of triggering efficient MHC-restricted CD8(+) T cell responses when combined to exosomes. Exosome immunogenicity across species allowed to verify the efficacy of good manufactory procedures-manufactured human exosomes admixed with CpG oligonucleotides in prophylactic and therapeutic settings of melanoma in HLA-A2 transgenic mice. CpG adjuvants appear to be ideal adjuvants for exosome-based cancer vaccines.

Dendritic cell-derived exosomes (DEX) are nanomeric vesicles harboring MHC/peptide complexes capable of promoting primary T cell responses and tumor rejection in the presence of adjuvants. In this study, we show that, in the absence of adjuvants, DEX mediate potent Ag-dependent antitumor effects against preestablished tumors in mice pretreated with immunopotentiating dosing of cyclophosphamide. Cyclophosphamide could 1) abolish the suppressive function of CD4+CD25+Foxp3+ regulatory T cells, 2) markedly enhance the magnitude of secondary but not primary CTL responses induced by DEX vaccines, 3) synergize with DEX in therapy but not prophylaxis tumor models. Therefore, therapeutic vaccines such as DEX aimed at boosting tumor-primed effector T cells could benefit procedures that minimize the effects of CD4+CD25+ regulatory T cells.

BACKGROUND: Four epidemiologic forms of Kaposi's sarcoma have been described, all of which are associated with the human herpesvirus-8. In western countries, human herpesvirus-8 is more prevalent in homosexual men than in the general population, and anecdotal cases of Kaposi's sarcoma in HIV-negative homosexual men have been reported. PATIENTS AND METHODS: We included HIV-negative homosexual and bisexual male patients with histologically proven Kaposi's sarcoma in a retrospective study. Clinical data were collected using a standardized form. Risk factors for human herpesvirus-8 infection and for the development of Kaposi's sarcoma were systematically recorded. RESULTS: Between 1995 and 2007, 28 men met the defined inclusion criteria. Mean age at first symptoms of Kaposi's sarcoma was 53 years. Clinical presentation resembled classical Kaposi's sarcoma, with limited disease in most patients. No cellular or humoral immunodeficiency was observed. Serologic tests for human herpesvirus-8 (latent immunofluorescence assay) were positive in 88% of patients, and only two patients displayed human herpesvirus-8 viremia at the time of Kaposi's sarcoma diagnosis. Three patients developed lymphoproliferative disorders (Castleman disease, follicular lymphoma and Burkitt lymphoma). In this population, alpha-interferon was well tolerated and gave a complete response, but most patients require only local treatment, if any. CONCLUSION: Kaposi's sarcoma may develop in homosexual or bisexual men without HIV infection. This type of Kaposi's sarcoma has clinical features in common with classical Kaposi's sarcoma but occurs in younger patients. Its prognosis is good, as Kaposi's sarcoma is generally limited, but clinicians should be aware of the association with lymphoproliferative diseases, which may affect prognosis.