Aspirin-treated human DCs up-regulate ILT-3 and induce hyporesponsiveness and regulatory activity in responder T cells

Mature dendritic cells (mDCs) are potent antigen presenting cells, but immature DCs (iDCs) have been shown to have reduced antigen stimulatory capacity. Different strategies have been investigated to augment the tolerogenic capacity of dendritic cells (DCs). We demonstrate that in aspirin-treated human DCs, there is reduced expression of CD1a, HLA-DR and CD86, up-regulation of ILT-3 expression and marginal increases in PDL-1. Aspirin-treated DCs are partially resistant to phenotypic changes following maturational stimuli, such as lipopolysaccharide (LPS) or TNFalpha, IL-1alpha and PGE2. Aspirin-treated DCs demonstrate normal endocytic function, but have a reduced ability to stimulate allogeneic T cells, which is comparable to iDCs. Furthermore, they induce hyporesponsiveness and regulatory activity in responder naïve and memory T cells; for naïve T cells this is achieved more quickly and efficiently than with iDCs. We investigated the mechanism of this regulatory activity and found that both cell-cell contact and inhibitory cytokine activity are involved, although no one cytokine predominates in importance. Blocking ILT-3 or IL-12 does not diminish the capacity of these DCs to induce regulation or Foxp3 expression on the regulatory T cells. Results demonstrate that aspirin-treated DCs display tolerogenic potential, which is of interest in their therapeutic potential in reducing chronic allograft rejection.

Differential expression of CTLA-4 among T cell subsets

CTLA-4 (CD152), the CD28 homologue, is a costimulatory molecule with negative effects on T cell activation. In addition to its role in the termination of activation, CTLA-4 has been implicated in anergy induction and the function of regulatory cells. As an intracellular molecule, it must first relocate to the cell surface and be ligated, in order to inhibit activation. Although some studies have investigated CTLA-4 expression on CD4(+) T cells, evidence is lacking regarding the kinetics of expression, and expression on T cell subpopulations. We have investigated CTLA-4 kinetics on human purified peripheral CD4(+), naïve, memory, CD4(+)CD25(-), CD4(+)CD25(+) regulatory T cells, and T cell clones. Intracellular stores of CTLA-4 were shown to be very low in naïve T cells, whilst significant amounts were present in memory T cells and T cell clones. Cell surface CTLA-4 expression was then investigated on CD4(+)CD45RA(+) (naïve), CD4(+)CD45RO(+) (memory), CD4(+)CD25(-), and CD4(+)CD25(+) T cells. CD25 and CD45RO are both expressed by regulatory T cells. On naïve and CD4(+)CD25(-) T cells, CTLA-4 expression declined after four hours. In contrast, on memory and CD4(+)CD25(+) T cells, high levels of expression were maintained until at least 48 hours. In addition, significant CTLA-4 expression was observed on T cell clones following anergy induction, indicating the potential involvement of CTLA-4 also in this form of tolerance.