Abstract
Dendritic cells (DCs) are potent antigen-presenting cells with a promising potential in cancer immunotherapy. Cbl proteins are E3 ubiquitin ligases and have been implicated in regulating the functional activity of various immune cells. As an example, c-Cbl negatively affects DC activation. We here describe that another member of the Cbl-protein family (i.e. Cbl-b) is highly expressed in murine bone-marrow-derived DCs (BMDCs). Differentiation of cblb−/− bone marrow mononuclear cells into classical BMDCs is unaltered, except enhanced induction of DEC-205 (CD205) expression. When tested in mixed-lymphocyte reaction (MLR), cblb−/− BMDCs exhibit increased allo-stimulatory capacity in vitro. BMDCs were next in vitro stimulated by various toll like receptor (TLR)-agonists (LPS, Poly(I:C), CpG) and exposed to FITC-labeled dextran. Upon TLR-stimulation, cblb−/− BMDCs produce higher levels of proinflammatory cytokines (IL-1α, IL-6 and TNF-α) and exhibit a slightly higher level of FITC-dextran uptake. To further characterize the functional significance of cblb−/− BMDCs we tested them in antigen-specific T cell responses against ovalbumin (OVA) protein and peptides, activating either CD8+ OT-I or CD4+ OT-II transgenic T cells. However, cblb−/− BMDCs are equally effective in inducing antigen-specific T cell responses when compared to wildtype BMDCs both in vitro and in vivo. The migratory capacity into lymph nodes during inflammation was similarly not affected by the absence of Cbl-b. In line with these observations, cblb−/− peptide-pulsed BMDCs are equally effective vaccines against OVA-expressing B16 tumors in vivo when compared to wildtype BMDCs. We conclude that in contrast to c-Cbl, Cbl-b plays only a limited role in the induction of Ag-specific T cell responses by murine BMDCs in vitro and in vivo.
Highlights
Dendritic cells (DCs) are a unique class of leukocytes whose primary function is to capture antigens and process them to activate or tolerize T cells [1]
No difference in bone-marrow-derived DCs (BMDCs) cell yields on day seven of in vitro granulocyte-monocyte colony-stimulating factor (GM-CSF) mediated differentiation could be detected between wildtype and cblb2/2 BMDCs
They identified a proinflammatory phenotype of ccbl2/2 versus wildtype BMDCs, which is accompanied with an increased potency as DC-based vaccine against established tumors
Summary
DCs are a unique class of leukocytes whose primary function is to capture antigens and process them to activate or tolerize T cells [1]. The high efficacy of DCs in initiating primary T cell responses inspired tumor immunologists to evaluate the therapeutic potential of DCs as cellular vaccine to induce antigen-specific immune responses targeting cancer cell associated antigens [2]. A randomized trial in hormone-refractory prostate cancer led to the first FDA approval of a DC-based vaccine after observation of an improved overall survival rate of DC-treated patients, whereas numerous other studies (i.e. in melanoma etc.) did not yield positive clinical results [3,4]. At least in the current application schedules DC vaccines do not sufficiently help cancer patients by inducing significant clinical responses, in spite of consistently measurable T cell responses and a virtual lack of therapy-related side-effects [5,6]. Improvement of DC vaccines is a clear unmet medical need
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