New-variant Creutzfeldt-Jakob disease and scrapie are typically initiated by extracerebral exposure to the causative agent, and exhibit early prion replication in lymphoid organs. In mouse scrapie, depletion of B-lymphocytes prevents neuropathogenesis after intraperitoneal inoculation, probably due to impaired lymphotoxin-dependent maturation of follicular dendritic cells (FDCs), which are a major extracerebral prion reservoir. FDCs trap immune complexes with Fc-gamma receptors and C3d/C4b-opsonized antigens with CD21/CD35 complement receptors. We examined whether these mechanisms participate in peripheral prion pathogenesis. Depletion of circulating immunoglobulins or of individual Fc-gamma receptors had no effect on scrapie pathogenesis if B-cell maturation was unaffected. However, mice deficient in C3, C1q, Bf/C2, combinations thereof or complement receptors were partially or fully protected against spongiform encephalopathy upon intraperitoneal exposure to limiting amounts of prions. Splenic accumulation of prion infectivity and PrPSc was delayed, indicating that activation of specific complement components is involved in the initial trapping of prions in lymphoreticular organs early after infection.

Regulatory T cells (Tregs) suppress other immune cells and are critical mediators of peripheral tolerance. Therapeutic manipulation of Tregs is subject to numerous clinical investigations including trials for adoptive Treg transfer. Since the number of naturally occurring Tregs (nTregs) is minute, it is highly desirable to develop a complementary approach of inducing Tregs (iTregs) from naïve T cells. Mouse studies exemplify the importance of peripherally induced Tregs as well as the applicability of iTreg transfer in different disease models. Yet, procedures to generate iTregs are currently controversial, particularly for human cells. Here we therefore comprehensively compare different established and define novel protocols of human iTreg generation using TGF-β in combination with other compounds. We found that human iTregs expressed several Treg signature molecules, such as Foxp3, CTLA-4 and EOS, while exhibiting low expression of the cytokines Interferon-γ, IL-10 and IL-17. Importantly, we identified a novel combination of TGF-β, retinoic acid and rapamycin as a robust protocol to induce human iTregs with superior suppressive activity in vitro compared to currently established induction protocols. However, iTregs generated by these protocols did not stably retain Foxp3 expression and did not suppress in vivo in a humanized graft-versus-host-disease mouse model, highlighting the need for further research to attain stable, suppressive iTregs. These results advance our understanding of the conditions enabling human iTreg generation and may have important implications for the development of adoptive transfer strategies targeting autoimmune and inflammatory diseases.

In the early phase of an immune response, T cells are activated and acquire effector functions. Whereas these short term activated T cells are resistant to CD95-mediated apoptosis, activated T cells in prolonged culture are readily sensitive, leading to activation-induced cell death and termination of the immune response. The translation inhibitor, cycloheximide, partially overcomes the apoptosis resistance of short term activated primary human T cells. Using this model we show in this study that sensitization of T cells to apoptosis occurs upstream of mitochondria. Neither death-inducing signaling complex formation nor expression of Bcl-2 proteins is altered in sensitized T cells. Although the caspase-8 inhibitor c-FLIPlong was only slightly down-regulated in sensitized T cells, c-FLIPshort became almost undetectable. This correlated with caspase-8 activation and apoptosis. These data suggest that c-FLIPshort, rather than c-FLIPlong, confers resistance of T cells to CD95-mediated apoptosis in the context of immune responses.