The effector T-cell lineage shows great plasticity. Th17 cells are acknowledged to be instrumental in the response against microbial infection, but are also associated with autoimmune inflammatory processes. Here, we report that human regulatory T cells ( IntroductionIn mice, IL-17A (IL-17)-producing T cells have been established as an important T-helper (Th) effector lineage, clearly distinct from the Th1 or Th2 lineage. 1,2 Infectious disease mouse models indicate that IL-17-producing cells (Th17) mediate protection against extracellular pathogens. 3-5 However, on the downside, Th17 cells also appear to be the driving force in the pathogenesis of several autoimmune diseases. 2,6 Furthermore, mouse studies showed that differentiation of Th17 cells from naive CD4 T cells requires the concomitant activity of IL-6 and transforming growth factor-␤ (TGF␤). [7][8][9] The key transcription factor driving Th17 cell differentiation is the orphan nuclear receptor ROR␥t, which is needed for constitutive expression of In in vivo studies in mice, IL-17-and ROR␥t-expressing cells were shown to be present in the lung and digestive mucosal compartments, 11 and especially throughout the intestinal lamina propria. 10 In humans, IL-17 is associated with many inflammatory disorders such as rheumatoid arthritis, asthma, multiple sclerosis, lupus, Crohn disease, and allograft rejection. 3,[12][13][14][15] Recently, human ROR␥t-positive IL-17-producing T cells were identified under physiologic conditions in peripheral blood and tonsils, [15][16][17] being contained within the CCR6 ϩ (CCR4 ϩ )CD4 ϩ memory T-cell population. 15,16 Most recent data show that human Th17 differentiation, distinct from mouse Th17 development, is under control of IL-1␤, 19 In mice, the development of Th17 cells was described to be linked to that of regulatory T cells (Tregs) in a reciprocal fashion, whereby under the influence of TGF␤, IL-6 levels determine the outcome. 8,9 In vitro, activated Tregs promoted Th17 cell differentiation from CD4 T cells, 9,20 likely through their production of TGF␤.In addition, in vivo the transfer of Treg enhanced IL-17 production in a mouse model of systemic autoimmune disease. 21 Next, to this reciprocal relationship, it was recently demonstrated that IL-17-producing cells directly develop from mouse Tregs. 20 This is a remarkable finding, because Tregs are typically associated with T-cell tolerance and immune homeostasis. 22 Different Treg subsets have been described, and even within the naturally occurring CD4 pos CD25 high Foxp3 pos population heterogeneity is evident. This heterogeneity reflects differences in differentiation or developmental stage, trafficking properties, and suppressor capacity. [23][24][25] Although the lineage differentiation of helper T cells is very well accepted, that of Tregs is only scarcely recognized. In humans, naive and memory-like CD4 pos CD25 pos Foxp3 pos Treg have been defined in peripheral blood, discriminated by the expression of CD45 isoforms CD45RA and CD45RO. [25][26][27][28] Al...

Respiratory syncytial virus (RSV) is a leading cause of pneumonia and bronchiolitis in young children and the elderly. Therapeutic small molecules have been developed that bind the RSV F glycoprotein and inhibit membrane fusion, yet their binding sites and molecular mechanisms of action remain largely unknown. Here we show that these inhibitors bind to a three-fold-symmetric pocket within the central cavity of the metastable prefusion conformation of RSV F. Inhibitor binding stabilizes this conformation by tethering two regions that must undergo a structural rearrangement to facilitate membrane fusion. Inhibitor-escape mutations occur in residues that directly contact the inhibitors or are involved in the conformational rearrangements required to accommodate inhibitor binding. Resistant viruses do not propagate as well as wild-type RSV in vitro, indicating a fitness cost for inhibitor escape. Collectively, these findings provide new insight into class I viral fusion proteins and should facilitate development of optimal RSV fusion inhibitors.

Background: Accumulating preclinical data indicate that targeting the SIRPα/CD47 axis alone or in combination with existing targeted therapies or immune checkpoint inhibitors enhances tumor rejection. Although several CD47-targeting agents are currently in phase I clinical trials and demonstrate activity in combination therapy, high and frequent dosing was required and safety signals (acute anemia, thrombocytopenia) were recorded frequently as adverse events. Based on the restricted expression pattern of SIRPα we hypothesized that antibodies targeting SIRPα might avoid some of the concerns noted for CD47-targeting agents. Methods: SIRPα-targeting antibodies were generated and characterized for binding to human SIRPα alleles and blockade of the interaction with CD47. Functional activity was established in vitro using human macrophages or neutrophils co-cultured with human Burkitt's lymphoma cell lines. The effect of SIRPα versus CD47 targeting on human T-cell activation was studied using an allogeneic mixed lymphocyte reaction and a Staphylococcus enterotoxin B-induced T-cell proliferation assay. Potential safety concerns of the selected SIRPα-targeting antibody were addressed in vitro using a hemagglutination assay and a whole blood cytokine release assay, and in vivo in a single-dose toxicity study in cynomolgus monkeys. Results: The humanized monoclonal IgG2 antibody ADU-1805 binds to all known human SIRPα alleles, showing minimal binding to SIRPβ1, while cross-reacting with SIRPγ, and potently blocking the interaction of SIRPα with CD47. Reduced FcγR binding proved critical to retaining its function towards phagocyte activation. In vitro characterization demonstrated that ADU-1805 promotes macrophage phagocytosis, with similar potency to anti-CD47 antibodies, and enhances neutrophil trogocytosis. Unlike CD47-targeting agents, ADU-1805 does not interfere with T-cell activation and is not expected to require frequent and extensive dosing due to the restricted expression of SIRPα to cells of the myeloid lineage. ADU-1805 is cross-reactive to cynomolgus monkey SIRPα and upon singledose intravenous administration in these non-human primates (NHPs) did not show any signs of anemia, thrombocytopenia or other toxicities. Conclusions: Blocking the SIRPα-CD47 interaction via SIRPα, while similarly efficacious in vitro, differentiates ADU-1805 from CD47-targeting agents with respect to safety and absence of inhibition of T-cell activation. The data presented herein support further advancement of ADU-1805 towards clinical development.

APRIL (A proliferation-inducing ligand) is a TNF family member that binds two TNF receptor family members, TACI and BCMA. It shares these receptors with the closely related TNF family member, B-cell activating factor (BAFF). Contrary to BAFF, APRIL binds heparan sulfate proteoglycans (HSPGs), which regulates cross-linking of APRIL and efficient signaling. APRIL was originally identified as a growth promoter of solid tumors, and more recent evidence defines APRIL also as an important survival factor in several human B-cell malignancies, such as chronic lymphocytic leukemia (CLL). To target APRIL therapeutically, we developed two anti–human APRIL antibodies (hAPRIL.01A and hAPRIL.03A) that block APRIL binding to BCMA and TACI. Their antagonistic properties are unique when compared with a series of commercially available monoclonal anti–human APRIL antibodies as they prevent in vitro proliferation and IgA production of APRIL-reactive B cells. In addition, they effectively impair the CLL-like phenotype of aging APRIL transgenic mice and, more importantly, block APRIL binding to human B-cell lymphomas and prevent the survival effect induced by APRIL. We therefore conclude that these antibodies have potential for further development as therapeutics to target APRIL-dependent survival in B-cell malignancies.

†A. M. H. Boots and I. Joosten contributed equally to this study.Mycophenolic acid is the active ingredient of the immunosuppressant mycophenolate mofetil that is widely used in transplantation medicine and autoimmunity. Mycophenolic acid inhibits inosine monophosphate dehydrogenase, an enzyme involved in biosynthesis of guanine nucleotides required for lymphocyte clonal expansion. Here, we present novel insights into the mechanisms underlying mycophenolic acid-mediated suppression of human CD4+ T cells. Upon CD3/CD28 stimulation, mycophenolic acid inhibited T cell IL-17, IFN-c and TNF-a production but not IL-2 production. Phenotypic analysis showed that drug treatment enhanced the expression of negative co-stimulators PD-1, CTLA-4 and the transcription factor FoxP3 and decreased the expression of positive costimulators CD27 and CD28, whereas CD25 was unaffected. Mycophenolic acid-treated cells were anergic, but not suppressive, and at the same time proved hyperblastoid with high metabolic activity. Moreover, a reduced Akt/mTOR and STAT5 signaling was observed. Interestingly, the co-stimulatory molecule CD70 was uniquely and dose-dependently upregulated on mycophenolic acid-treated T cells and found to be directly linked to target enzyme inhibition. CD70 on mycophenolic acid-treated cells proved functional: an anti-CD70 agonist was found to restore both STAT5 and Akt/mTOR signaling and may thereby prevent apoptosis and promote survival. These novel insights may contribute to optimization of protocols for MPA-based immunosuppressive regimens.