Sabina Müller

Human regulatory T cells inhibit polarization of T helper cells toward antigen-presenting cells via a TGF-β-dependent mechanism

The molecular mechanisms used by regulatory T cells (Treg) to inhibit the effector phase of adaptive immune responses are still elusive. In the present work, we investigated the possibility that Treg may interfere with a basic biological function of T helper cells (TH): polarization of secretory machinery for dedicated help delivery. To address this question, we visualized by confocal microscopy different parameters of activation in TH and Treg cells interacting simultaneously with individual antigen-presenting cells (APC). Our results show that, although productive TCR engagement in TH/APC conjugates was unaffected by the presence of adjacent Treg, the reorientation of TH secretory machinery toward APC was strongly inhibited. Blocking TGF-β completely reverted Treg induced inhibition of TH polarization. Our results identify a previously undescribed mechanism by which Treg inhibit effector T cells. TGF-β produced by adjacent Treg interferes with polarization of TH secretory machinery toward APC, thus affecting a crucial step of TH-mediated amplification of the immune response.

Melanoma cell lysosome secretory burst neutralizes the CTL-mediated cytotoxicity at the lytic synapse

Human melanoma cells express various tumour antigens that are recognized by CD8+ cytotoxic T lymphocytes (CTLs) and elicit tumour-specific responses in vivo. However, natural and therapeutically enhanced CTL responses in melanoma patients are of limited efficacy. The mechanisms underlying CTL effector phase failure when facing melanomas are still largely elusive. Here we show that, on conjugation with CTL, human melanoma cells undergo an active late endosome/lysosome trafficking, which is intensified at the lytic synapse and is paralleled by cathepsin-mediated perforin degradation and deficient granzyme B penetration. Abortion of SNAP-23-dependent lysosomal trafficking, pH perturbation or impairment of lysosomal proteolytic activity restores susceptibility to CTL attack. Inside the arsenal of melanoma cell strategies to escape immune surveillance, we identify a self-defence mechanism based on exacerbated lysosome secretion and perforin degradation at the lytic synapse. Interfering with this synaptic self-defence mechanism might be useful in potentiating CTL-mediated therapies in melanoma patients.