Using Signaling Pathways to Overcome Immune Tolerance to Tumors

Murine melanoma-infiltrating dendritic cells are defective in antigen presenting function regardless of the presence of CD4CD25 regulatory T cells

Tumor-infiltrating dendritic cells are often ineffective at presenting tumor-derived antigen in vivo, a defect usually ascribed to the suppressive tumor environment. We investigated the effects of depleting CD4⁺CD25⁺ “natural” regulatory T cells (Treg) on the frequency, phenotype and function of total dendritic cell populations in B16.OVA tumors and in tumor-draining lymph nodes. Intraperitoneal injection of the anti-CD25 monoclonal antibody PC61 reduced Treg frequency in blood and tumors, but did not affect the frequency of tumor-infiltrating dendritic cells, or their expression of CD40, CD86 and MHCII. Tumor-infiltrating dendritic cells from PC61-treated or untreated mice induced the proliferation of allogeneic T cells in vitro, but could not induce proliferation of OVA-specific OTI and OTII T cells unless specific peptide antigen was added in culture. Some proliferation of naïve, OVA-specific OTI T cells, but not OTII T cells, was observed in the tumor-draining LN of mice carrying B16.OVA tumors, however, this was not improved by PC61 treatment. Experiments using RAG1^−/− hosts adoptively transferred with OTI and CD25-depleted OTII cells also failed to show improved OTI and OTII T cell proliferation in vivo compared to C57BL/6 hosts. We conclude that the defective presentation of B16.OVA tumor antigen by tumor-infiltrating dendritic cells and in the tumor-draining lymph node is not due to the presence of “natural” CD4⁺CD25⁺ Treg.

Development of an MHC class I L(d)-restricted PSA peptide-loaded tetramer for detection of PSA-specific CD8+ T cells in the mouse

Objectives

We set out to develop a prostate specific antigen (PSA) peptide-loaded tetramer for enumeration of PSA-specific CD8⁺ T cells in the Balb/c mouse model.

Methods

A candidate MHC class I PSA peptide (HPQKVTKFML_188–197) was selected based on its ability to restimulate PSA-specific CD8⁺ T cells to secrete IFN-γ in our assays. Next, H-2L^d-restricted peptide-loaded and fluorescently labeled tetramers were produced in conjunction with the NIH Tetramer Core Facility. This tetramer was then tested for staining specificity and optimized for detection of PSA-specific CD8⁺ T cells induced by our PSA-encoding adenovirus tumor vaccine.

Results

The MHC class I PSA peptide demonstrated successful restimulation of CD8⁺ T cells isolated from mice previously vaccinated with a PSA-encoding adenovirus tumor vaccine, with no restimulation observed in control vaccinated mice. The peptide-loaded H-2L^d tetramer exhibited the desired binding specificity and allowed for detection and frequency determination of PSA-specific CD8⁺ T cells by flow cytometry.

Conclusions

We have successfully designed and validated a PSA peptide tetramer for use in the Balb/c mouse model that can be used to test PSA-based prostate cancer vaccines. Until now, PSA-specific CD8⁺ T cells in the mouse have only been detectable via cytotoxic T lymphocyte (CTL) assays or intracellular cytokine staining, which primarily assess Ag-specific functional activity, not their absolute number. This research tool provides laboratories the ability to directly quantitate CD8⁺ T cells elicited by PSA-specific immunotherapies and cancer vaccines that are tested in mouse models.

B7-H1 on myeloid-derived suppressor cells in immune suppression by a mouse model of ovarian cancer

Myeloid-derived suppressor cells (MDSCs) accumulate in tumor-bearing hosts and are associated with immune suppression. Here, we described high level of expression of B7-H1 (CD274), PD-1 (CD279) and CTLA4 (CD152) by Gr-1(+)CD11b(+) MDSCs obtained from both ascites and spleens of mice bearing the 1D8 ovarian carcinoma, whereas B7-DC (CD273), CD40 and CD86 were absent. In contrast, B7-H1, PD-1 and CTLA-4 expression was not detected on Gr-1(+)CD11b(+) cells from naive mice. Expression of B7-H1 by Gr-1(+)CD11b(+) cells from naive mice could be induced by co-culture with 1D8 ovarian carcinoma cells. Gr-1(+)CD11b(+) cells derived from 1D8 tumor-bearing mice markedly suppressed antigen-specific immune responses, whereas Gr-1(+)CD11b(+) cells from naive mice did not. siRNA-mediated knockdown of B7-H1 in Gr-1(+)CD11b(+) cells of 1D8 tumor-bearing mice alleviated suppression of antigen-specific immune responses. Suppression of antigen-specific immune responses via B7-H1 on Gr-1(+)CD11b(+) myeloid cells was mediated by CD4(+)CD25(+) Foxp3(+) T regulatory cells and required PD-1. Antibody blockade of either B7-H1 or PD-1 retarded the growth of 1D8 tumor in mice. This suggests that expression of B7-H1 on Gr-1(+)CD11b(+) myeloid cells triggered by the 1D8 mouse model of ovarian carcinoma suppresses antigen-specific immunity via interaction with PD-1 on CD4(+)CD25(+) Foxp3(+) regulatory T cells.

Breaking self-tolerance to tumor-associated antigens by in vivo manipulation of dendritic cells

Dendritic cells (DC) are extremely potent antigen-presenting cells, which can prime both naive CD4+ and CD8+ T lymphocytes. In their immature state, DC continuously sample and process antigens from the surrounding environment, but only mature DC express sufficient levels of costimulatory molecules to activate naive T cells. DC present in tumors are functionally immature owing to the immunosuppressive actions of tumor-derived factors and regulatory T cells, and such immature DC promote immune tolerance to the tumor. Recent studies from animal models suggest that Toll-like receptor (TLR) agonists such as CpG can reverse the tolerogenic state of tumoral DC. Strategies that allow DC to gain access to both tumor antigens and TLR agonists, in situ, can overcome tumor tolerance leading to the induction of potent systemic antitumor immunity.

Systemic antitumor effect of intratumoral injection of dendritic cells in combination with local photodynamic therapy

PURPOSE

Photodynamic therapy (PDT), which is used clinically for the palliative treatment of cancer, induces local tumor cell death but has no effect on tumors in untreated sites. The purpose of this study was to determine if local PDT followed by intratumoral injection of naïve dendritic cells (IT-DC) induces systemic antitumor immunity that can inhibit the growth of untreated as well as PDT + IT-DC-treated tumors.

EXPERIMENTAL DESIGN

BALB/c or C57Bl/6 mice were injected s.c. with CT26 colorectal carcinoma cells and B16 melanoma cells, respectively, and following 10 to 12 days of tumor growth, the tumors were treated with PDT alone or PDT followed by IT-DC or IT-PBS. In other studies, tumors were established simultaneously in both lower flanks or in one flank and in the lungs, but only one flank was treated.

RESULTS

Whereas neither PDT nor IT-DC alone was effective, PDT + IT-DC eradicated both CT26 and B16 tumors in a significant proportion of animals and prolonged the survival of mice of which the tumors were not cured. The spleens of mice treated with PDT + IT-DC contained tumor-specific cytotoxic and IFN-gamma-secreting T cells whereas the spleens of control groups did not. Moreover, adoptive transfer of splenocytes from successfully treated CT26 tumor-free mice protected naïve animals from a subsequent challenge with CT26, and this was mediated mainly by CD8 T cells. Most importantly, PDT plus IT-DC administered to one tumor site led to tumor regression at distant sites, including multiple lung metastases.

CONCLUSIONS

PDT + IT-DC induces potent systemic antitumor immunity in mice and should be evaluated in the treatment of human cancer.

Matrix protein mutant of vesicular stomatitis virus stimulates maturation of myeloid dendritic cells

Matrix (M) protein mutants of vesicular stomatitis virus have recently been used as oncolytic viruses for tumor therapies and are being developed as vaccine vectors for heterologous antigens. Because dendritic cell (DC) maturation is an important correlate of tumor immunosurveillance and vaccine efficacy, we sought to determine the ability of a recombinant M protein mutant virus (rM51R-M virus) to mature DC in vitro. We have previously shown that rM51R-M virus is defective at inhibiting host gene expression in several cell lines compared to its recombinant wild-type counterpart, rwt virus. Therefore, rM51R-M virus allows the expression of genes involved in antiviral responses, such as the type I interferon (IFN) gene. Our results demonstrate that, in contrast to the rwt virus, rM51R-M virus induced the maturation of myeloid DC (mDC) populations, as indicated by an increase in the surface expression of CD40, CD80, and CD86 as well as the secretion of interleukin-12 (IL-12), IL-6, and type I IFN. In addition, mDC infected with rM51R-M virus effectively activated naïve T cells in vitro, whereas rwt virus-infected mDC were defective in antigen presentation. The inability of rwt virus to induce mDC maturation was correlated with the inhibition of host gene expression in rwt virus-infected cells. Our studies also indicated that the production of costimulatory molecules on mDC by rM51R-M virus was dependent on the type I IFN receptor, while maturation induced by this virus was largely independent of MyD88. These data indicate that rM51R-M virus effectively stimulates the maturation of mDC and has the potential to promote effective T-cell responses to vector-expressed antigens, activate DC at tumor sites during therapy, and aid in tumor immunosurveillance and destruction.