Gerold Schuler
University Hospital,
Erlangen, Germany

Use of Monocyte-Derived Dendritic Cells in Cancer Immunotherapy

A novel approach to antigen-specific immunotherapy is to take advantage of dendritic cells (DCs) as “nature’s adjuvant” and to actively immunize cancer patients with a sample of their own DC charged with tumor antigens. DC vaccination is, however, still in an early stage, and most potentially important variables (e.g. type and maturational stage of DCs, type of antigen and loading method, schedule etc.) have yet to be addressed. Despite this need for establishing an optimized DC vaccine even in some of the initial exploratory clinical trials, vaccination with DC loaded with tumor antigens in the form of peptides has induced a) tumor-specific killer and helper T cells (“proof of concept”), and b) occasional regression of metastases even in far-advanced cancer patients. This was reported for DC directly isolated from blood as well as for DC generated ex vivo from either CD34+ or CD14+ precursors (reviewed in 1-3). Most of the studies including informative ones in volunteers have been performed by using DC generated from CD14+ monocytes (so-called Monocyte-derived DC or Mo-DC), which are now considered as a gold standard. These Mo-DC can be reproducibly generated within a few days in large numbers (300-800 million mature DC per apheresis) from precursors in blood without any need for pretreating the patients with cytokines like GM-CSF or Flt3-L. Importantly, it is possible to obtain populations of immature Mo-DC by exposing monocytes to GM-CSF + IL-4, which can then be transformed into homogenously mature Mo-DC by various stimuli such as TLR ligands (e.g. microbial products such as LPS or poly I:C), inflammatory cytokines like TNF alpha, monocyte-conditioned medium or its mimic (IL-1beta + TNF alpha + IL-6 + PGE₂), or CD40L. The use of mature Mo-DC is likely critical to induce strong immunity as it has become clear that antigen delivered on immature or incompletely matured Mo-DC can even induce tolerance. Interestingly, it has recently also become evident that in the case of the Mo-DC the choice of maturation stimulus is probably critical for success. Specifically, PGE2 has to be part of the maturation stimulus in order to obtain CCR7 expressing Mo-DC that migrate in response to CCL19 and CCL21 that guide DC into lymphoid organs. Using such DC loaded with tumor peptides we have demonstrated their migratory capacity in vivo, the induction of tumor-specific cytotoxic and helper T cells, and the presence of tumor-antigen specific T cells in situ in regressing metastases. Interestingly, immunizing to Mage-3A1 peptide by DC has been shown to result in polyclonal T-cell responses while other vaccination strategies appear to yield only monoclonal ones. Currently, we are exploring in two-armed trials whether mature Mo-DC exposed to CD40L and / or an unspecific helper protein (4) influence the quantity or quality of induced T-cell responses.

Recently, new approaches to charge DC with antigens have become evident which promote the DC vaccination approach. The observation that DC can take up naked RNA, express antigens encoded by the RNA and induce antigen-specific T cells in vitro as well as in vivo in patients has given an enormous additional momentum to the use of DC as vectors for antigen delivery and cancer vaccination as this approach now allows one to administer to DC both defined antigens, including universal ones such as telomerase or survivin as well as the total antigenic repertoire of a given tumor (as total tumor or PCR amplified RNA). We have optimized protocols for the electroporation of RNA into both immature and mature Mo-DC, and worked out an intracellular staining method that allows a reliable validation of the resulting DC vaccine. A clinical trial in melanoma patients employing mature Mo-DC transfected with RNA encoding for MelanA, Mage-3 and survivin is ongoing.

The use of dying tumor cells, notably antibody-coated ones, is yet another loading technique. Other preclinical research has shown that the delivery of defined antigens as antigen-antibody complexes to DC enhances cross presentation and allows for the potent induction of both CD4+ and CD8+ T-cell responses. Several recent results suggest that DC might even be useful to directly trigger NK, and to mobilize the additional power of the innate immune system to attack tumor cells. Upon loading with alphaGalactosylCeramide DC can also induce IFN gamma producing NKT cells. It appears, therefore, rational and timely to optimize the use of DC as vectors for the delivery of antigens to vaccinate against cancer.

Selected References (Reviews and Editorials only):

1. Schuler G, Schuler-Thurner B, Steinman RM. The use of dendritic cells in cancer immunotherapy. Curr Opin Immunol. 2003 Apr;15(2):138-147.
2. Steinman RM, Dhodapkar M. Active immunization against cancer with dendritic cells: the near future. Int J Cancer. 2001 Nov15;94(4):459-473.
3. Banchereau J, Schuler-Thurner B, Palucka AK, Schuler G. Dendritic cells as vectors for therapy. Cell. 2001 Aug 10;106(3):271-4.
4. Kaech SM, Ahmed R. CD8 T cells remember with a little help. Science. 2003 Apr 11;300(5617):263-265.