Induction of CD4+ T cell apoptosis as a consequence of impaired cytoskeletal rearrangement in UVB-irradiated dendritic cells.

Low dose UVB irradiation of dendritic cells (DC) dose-dependently decreases their allostimulatory capacity and inhibits alloreactive T cell proliferation. The reduction of the stimulatory capacity is not associated with a perturbation of CD28 costimulation. To examine the underlying mechanism, cell cycle analysis of T cells from cocultures with UVB-irradiated DC (UVB-DC) was performed, revealing no cell cycle arrest, but an increased number of apoptotic T cells in sub-G(0) phase. We confirmed T cells to undergo apoptosis after coincubation with UVB-DC by TUNEL staining and DNA laddering. To analyze whether T cell apoptosis requires the Fas/Fas ligand (FasL) pathway, MLRs were performed with Fas-, FasL-deficient, and wild-type DC and T cells. No differences were found on comparison of wild-type DC with Fas-/FasL-deficient DC or T cells. Likewise, addition of a neutralizing anti-TNF-alpha mAb to cocultures could not overcome inhibition of T cell proliferation by UVB-DC, excluding involvement of the TNF-alpha/TNF-alphaR pathway. FACS analysis of CD69 and CD25 revealed no up-regulation on T cells cocultured with UVB-DC, suggesting a perturbation of early T cell activation. Analysis of UVB-DC by confocal microscopy demonstrated impaired filamentous actin bundling, a process critical for T cell stimulation. To investigate the functional relevance of these observations, time lapse video microscopy was performed. Indeed, calcium signaling in CD4(+) T cells was significantly diminished after interaction with UVB-DC. In conclusion, UVBR of DC impairs their cytoskeletal rearrangement and induces apoptosis in CD4(+) T cells by disruption of early DC-T cell interaction, resulting in a reduced Ca(2+) influx in T cells.

Targeting dendritic cells with CD44 monoclonal antibodies selectively inhibits the proliferation of naive CD4+ T-helper cells by induction of FAS-independent T-cell apoptosis.

CD44 is a multifunctional adhesion molecule that has been shown to be a costimulatory factor for T-cell activation in vitro and in vivo. The aim of the present study was to expand these findings by characterizing the role of CD44 during dendritic cell (DC) antigen presentation to naive, resting T cells. Certain monoclonal antibodies (mAbs) directed against all CD44 isoforms (pan CD44), or against the epitope encoded by the alternatively spliced exon v4 (CD44v4), dose-dependently inhibited the capacity of murine DC to induce proliferation of naive alloreactive T cells. Preincubation of the T cells or DC with these CD44 mAbs revealed that the effect was dependent upon mAb binding to DC, but not to T cells. DC treated with anti-pan CD44 and anti-CD44v4 mAbs induced CD4+ T-cell apoptosis, as shown by annexin V staining and TdT-mediated biotin-dUTP nick-end labelling (TUNEL) assays. However, CD4+ T-cell apoptosis was not dependent on the Fas/Fas ligand (Fas/FasL) system, as DC from FasL-deficient (Gld) mice and T cells from Fas-deficient (Lpr) mice were still susceptible to apoptosis induced by CD44-treated DC. To investigate whether CD44 treatment of DC affects early T-cell/DC interactions, time-lapse video microscopy was performed using peptide-specific T cells from T-cell receptor (TCR) transgenic mice. Interestingly, calcium signalling in CD4+ T cells was significantly diminished following interaction with CD44 mAb-treated DC, but this was not observed in CD8+ T cells. Taken together, we found that perturbation of distinct epitopes of CD44 on DC interfere with early Ca2+ signalling events during the activation of CD4+ T cells, resulting in T-cell apoptosis.