Dendritic cells matured with TLR4, CD40L and CD70 can save CD8+ T lymphocyte proliferation from suppression by CD4+CD25+Foxp3+ regulatory T cells

Naturally occuring CD4+CD25+Foxp3+ regulatory T cells (Treg) have been shown to suppress the function of both CD4+ and CD8+ effector T cells in several types of cancer, including melanoma. Moreover, higher numbers of Treg in blood or tumor have been associated with reduced prognosis. Therefore, cancer vaccination strategies should not only induce and enhance tumor-specific effector T cell responses but should at the same time either protect effector T cells from subsequent Treg suppression or actively reduce Treg proliferation or function. So far, most dendritic cell (DC) vaccines used for cancer vaccination are made up of monocyte-derived DC that are matured in a mixture of inflammatory cytokines (CM). However, recently, it has been shown that these DC can induce Treg from naive CD4+ T cells, whereas this is not the case for DC matured in the presence of Toll like receptor agonists. We previously showed that immature DC that are electroporated with a constitutively active TLR4 (caTLR4) in combination with CD40 ligand and CD70 (trimix) can induce strong antigen-specific T cell responses, both in vitro and in vivo.
In this study, we wanted to investigate the effect of differentially modified DC on Treg suppression and induction.
First, we investigated the capacity of differentially matured DC on Treg induction. DC were left immature or matured with a mixture of inflammatory cytokines (CM), by electroporation with mRNA encoding caTLR4 and CD40L (dimix) or caTLR4, CD40L and CD70 (trimix) or caTLR4, CD40L and 4-1BBL (modified trimix) and subsequently co-cultured with autologous CD4+CD25- selected T cells. We observed no difference in Treg induction capacity between the different maturation stimuli.
Next, we compared the effect of differentially matured DC on Treg suppression. DC were matured using the above described conditions and subsequently co-cultured in an autologous system with sorted Treg and naive CFSE labeled CD8+ T lymphocytes. We noted that Treg suppression of CD8+ T cells was reduced in the presence of trimix-DC compared to CM-DC. These results also applied in an allogenic setting.
Finally, we wanted to assess to what extent DC matured with different maturation stimuli yield a protective effect for CD8+ T cells against Treg. The same DC were used to pre-stimulate allogenic CD8+ T cells, after which Treg were added to the cultures. Treg suppression of proliferation was markedly reduced in all cultures.
Our preliminary results suggest that CD8+ T cells stimulated with trimix-DC are less prone to Treg suppression than T cells stimulated with CM matured DC. This may prove to be an advantage in cancer treatments involving DC vaccination.