Plasmacytoid dendritic cells are present in lymphoid and nonlymphoid tissue and contribute substantially to both innate and adaptive immunity. Recently, we have described several monoclonal antibodies that recognize a plasmacytoid dendritic cell-specific antigen, which we have termed BDCA-2. Molecular cloning of BDCA-2 revealed that BDCA-2 is a novel type II C-type lectin, which shows 50.7% sequence identity at the amino acid level to its putative murine ortholog, the murine dendritic cell–associated C-type lectin 2. Anti–BDCA-2 monoclonal antibodies are rapidly internalized and efficiently presented to T cells, indicating that BDCA-2 could play a role in ligand internalization and presentation. Furthermore, ligation of BDCA-2 potently suppresses induction of interferon α/β production in plasmacytoid dendritic cells, presumably by a mechanism dependent on calcium mobilization and protein-tyrosine phosphorylation by src-family protein-tyrosine kinases. Inasmuch as production of interferon α/β by plasmacytoid dendritic cells is considered to be a major pathophysiological factor in systemic lupus erythematosus, triggering of BDCA-2 should be evaluated as therapeutic strategy for blocking production of interferon α/β in systemic lupus erythematosus patients.

CD11b+/Gr‐1+ myeloid‐derived suppressor cells (MDSC) contribute to tumor immune evasion by restraining the activity of CD8+ T‐cells. Two major MDSC subsets were recently shown to play an equal role in MDSC‐induced immune dysfunctions: monocytic‐ and granulocytic‐like. We isolated three fractions of MDSC, i.e. CD11b+/Gr‐1high, CD11b+/Gr‐1int, and CD11b+/Gr‐1low populations that were characterized morphologically, phenotypically and functionally in different tumor models. In vitro assays showed that CD11b+/Gr‐1int cell subset, mainly comprising monocytes and myeloid precursors, was always capable to suppress CD8+ T‐cell activation, while CD11b+/Gr‐1high cells, mostly granulocytes, exerted appreciable suppression only in some tumor models and when present in high numbers. The CD11b+/Gr‐1int but not CD11b+/Gr‐1high cells were also immunosuppressive in vivo following adoptive transfer. CD11b+/Gr‐1low cells retained the immunosuppressive potential in most tumor models. Gene silencing experiments indicated that GM‐CSF was necessary to induce preferential expansion of both CD11b+/Gr‐1int and CD11b+/Gr‐1low subsets in the spleen of tumor‐bearing mice and mediate tumor‐induced tolerance whereas G‐CSF, which preferentially expanded CD11b+/Gr‐1high cells, did not create such immunosuppressive environment. GM‐CSF also acted on granulocyte–macrophage progenitors in the bone marrow inducing local expansion of CD11b+/Gr‐1low cells. These data unveil a hierarchy of immunoregulatory activity among MDSC subsets that is controlled by tumor‐released GM‐CSF.

Vaccination against cancer by using dendritic cells has for more than a decade been based on dendritic cells generated ex vivo from monocytes or CD34 þ progenitors. Here, we report on the first clinical study of therapeutic vaccination against cancer using naturally occurring plasmacytoid dendritic cells (pDC). Fifteen patients with metastatic melanoma received intranodal injections of pDCs activated and loaded with tumor antigen-associated peptides ex vivo. In vivo imaging showed that administered pDCs migrated and distributed over multiple lymph nodes. Several patients mounted antivaccine CD4 þ and CD8 þ T-cell responses. Despite the limited number of administered pDCs, an IFN signature was observed after each vaccination. These results indicate that vaccination with naturally occurring pDC is feasible with minimal toxicity and that in patients with metastatic melanoma, it induces favorable immune responses. Cancer Res; 73(3); 1063-75. Ó2012 AACR.